Sema.zig

//! Semantic analysis of ZIR instructions.
//! Shared to every Block. Stored on the stack.
//! State used for compiling a ZIR into AIR.
//! Transforms untyped ZIR instructions into semantically-analyzed AIR instructions.
//! Does type checking, comptime control flow, and safety-check generation.
//! This is the the heart of the Zig compiler.

pt: Zcu.PerThread,
/// Alias to `zcu.gpa`.
gpa: Allocator,
/// Points to the temporary arena allocator of the Sema.
/// This arena will be cleared when the sema is destroyed.
arena: Allocator,
code: Zir,
air_instructions: std.MultiArrayList(Air.Inst) = .{},
air_extra: std.ArrayListUnmanaged(u32) = .empty,
/// Maps ZIR to AIR.
inst_map: InstMap = .{},
/// The "owner" of a `Sema` represents the root "thing" that is being analyzed.
/// This does not change throughout the entire lifetime of a `Sema`. For instance,
/// when analyzing a runtime function body, this is always `func` of that function,
/// even if an inline/comptime function call is being analyzed.
owner: AnalUnit,
/// The function this ZIR code is the body of, according to the source code.
/// This starts out the same as `sema.owner.func` if applicable, and then diverges
/// in the case of an inline or comptime function call.
/// This could be `none`, a `func_decl`, or a `func_instance`.
func_index: InternPool.Index,
/// Whether the type of func_index has a calling convention of `.naked`.
func_is_naked: bool,
/// Used to restore the error return trace when returning a non-error from a function.
error_return_trace_index_on_fn_entry: Air.Inst.Ref = .none,
comptime_err_ret_trace: *std.ArrayList(LazySrcLoc),
/// When semantic analysis needs to know the return type of the function whose body
/// is being analyzed, this `Type` should be used instead of going through `func`.
/// This will correctly handle the case of a comptime/inline function call of a
/// generic function which uses a type expression for the return type.
/// The type will be `void` in the case that `func` is `null`.
fn_ret_ty: Type,
/// In case of the return type being an error union with an inferred error
/// set, this is the inferred error set. `null` otherwise. Allocated with
/// `Sema.arena`.
fn_ret_ty_ies: ?*InferredErrorSet,
branch_quota: u32 = default_branch_quota,
branch_count: u32 = 0,
/// Populated when returning `error.ComptimeBreak`. Used to communicate the
/// break instruction up the stack to find the corresponding Block.
comptime_break_inst: Zir.Inst.Index = undefined,
/// These are lazily created runtime blocks from block_inline instructions.
/// They are created when an break_inline passes through a runtime condition, because
/// Sema must convert comptime control flow to runtime control flow, which means
/// breaking from a block.
post_hoc_blocks: std.AutoHashMapUnmanaged(Air.Inst.Index, *LabeledBlock) = .empty,
/// Populated with the last compile error created.
err: ?*Zcu.ErrorMsg = null,

/// The temporary arena is used for the memory of the `InferredAlloc` values
/// here so the values can be dropped without any cleanup.
unresolved_inferred_allocs: std.AutoArrayHashMapUnmanaged(Air.Inst.Index, InferredAlloc) = .empty,

/// Links every pointer derived from a base `alloc` back to that `alloc`. Used
/// to detect comptime-known `const`s.
/// TODO: ZIR liveness analysis would allow us to remove elements from this map.
base_allocs: std.AutoHashMapUnmanaged(Air.Inst.Index, Air.Inst.Index) = .empty,

/// Runtime `alloc`s are placed in this map to track all comptime-known writes
/// before the corresponding `make_ptr_const` instruction.
/// If any store to the alloc depends on a runtime condition or stores a runtime
/// value, the corresponding element in this map is erased, to indicate that the
/// alloc is not comptime-known.
/// If the alloc remains in this map when `make_ptr_const` is reached, its value
/// is comptime-known, and all stores to the pointer must be applied at comptime
/// to determine the comptime value.
/// Backed by gpa.
maybe_comptime_allocs: std.AutoHashMapUnmanaged(Air.Inst.Index, MaybeComptimeAlloc) = .empty,

/// Comptime-mutable allocs, and any comptime allocs which reference it, are
/// stored as elements of this array.
/// Pointers to such memory are represented via an index into this array.
/// Backed by gpa.
comptime_allocs: std.ArrayListUnmanaged(ComptimeAlloc) = .empty,

/// A list of exports performed by this analysis. After this `Sema` terminates,
/// these are flushed to `Zcu.single_exports` or `Zcu.multi_exports`.
exports: std.ArrayListUnmanaged(Zcu.Export) = .empty,

/// All references registered so far by this `Sema`. This is a temporary duplicate
/// of data stored in `Zcu.all_references`. It exists to avoid adding references to
/// a given `AnalUnit` multiple times.
references: std.AutoArrayHashMapUnmanaged(AnalUnit, void) = .empty,
type_references: std.AutoArrayHashMapUnmanaged(InternPool.Index, void) = .empty,

/// All dependencies registered so far by this `Sema`. This is a temporary duplicate
/// of the main dependency data. It exists to avoid adding dependencies to a given
/// `AnalUnit` multiple times.
dependencies: std.AutoArrayHashMapUnmanaged(InternPool.Dependee, void) = .empty,

/// Whether memoization of this call is permitted. Operations with side effects global
/// to the `Sema`, such as `@setEvalBranchQuota`, set this to `false`. It is observed
/// by `analyzeCall`.
allow_memoize: bool = true,

/// The `BranchHint` for the current branch of runtime control flow.
/// This state is on `Sema` so that `cold` hints can be propagated up through blocks with less special handling.
branch_hint: ?std.builtin.BranchHint = null,

const RuntimeIndex = enum(u32) {
    zero = 0,
    comptime_field_ptr = std.math.maxInt(u32),
    _,

    pub fn increment(ri: *RuntimeIndex) void {
        ri.* = @enumFromInt(@intFromEnum(ri.*) + 1);
    }
};

const MaybeComptimeAlloc = struct {
    /// The runtime index of the `alloc` instruction.
    runtime_index: RuntimeIndex,
    /// Backed by sema.arena. Tracks all comptime-known stores to this `alloc`. Due to
    /// RLS, a single comptime-known allocation may have arbitrarily many stores.
    /// This list also contains `set_union_tag`, `optional_payload_ptr_set`, and
    /// `errunion_payload_ptr_set` instructions.
    /// If the instruction is one of these three tags, `src` may be `.unneeded`.
    stores: std.MultiArrayList(struct {
        inst: Air.Inst.Index,
        src: LazySrcLoc,
    }) = .{},
};

const ComptimeAlloc = struct {
    val: MutableValue,
    is_const: bool,
    src: LazySrcLoc,
    /// `.none` indicates that the alignment is the natural alignment of `val`.
    alignment: Alignment,
    /// This is the `runtime_index` at the point of this allocation. If an store
    /// to this alloc ever occurs with a runtime index greater than this one, it
    /// is behind a runtime condition, so a compile error will be emitted.
    runtime_index: RuntimeIndex,
};

/// `src` may be `null` if `is_const` will be set.
fn newComptimeAlloc(sema: *Sema, block: *Block, src: LazySrcLoc, ty: Type, alignment: Alignment) !ComptimeAllocIndex {
    const idx = sema.comptime_allocs.items.len;
    try sema.comptime_allocs.append(sema.gpa, .{
        .val = .{ .interned = try sema.pt.intern(.{ .undef = ty.toIntern() }) },
        .is_const = false,
        .src = src,
        .alignment = alignment,
        .runtime_index = block.runtime_index,
    });
    return @enumFromInt(idx);
}

pub fn getComptimeAlloc(sema: *Sema, idx: ComptimeAllocIndex) *ComptimeAlloc {
    return &sema.comptime_allocs.items[@intFromEnum(idx)];
}

const std = @import("std");
const math = std.math;
const mem = std.mem;
const Allocator = mem.Allocator;
const assert = std.debug.assert;
const log = std.log.scoped(.sema);

const Sema = @This();
const Value = @import("Value.zig");
const MutableValue = @import("mutable_value.zig").MutableValue;
const Type = @import("Type.zig");
const Air = @import("Air.zig");
const Zir = std.zig.Zir;
const Zcu = @import("Zcu.zig");
const trace = @import("tracy.zig").trace;
const Namespace = Zcu.Namespace;
const CompileError = Zcu.CompileError;
const SemaError = Zcu.SemaError;
const LazySrcLoc = Zcu.LazySrcLoc;
const RangeSet = @import("RangeSet.zig");
const target_util = @import("target.zig");
const Package = @import("Package.zig");
const crash_report = @import("crash_report.zig");
const build_options = @import("build_options");
const Compilation = @import("Compilation.zig");
const InternPool = @import("InternPool.zig");
const Alignment = InternPool.Alignment;
const AnalUnit = InternPool.AnalUnit;
const ComptimeAllocIndex = InternPool.ComptimeAllocIndex;
const Cache = std.Build.Cache;
const LowerZon = @import("Sema/LowerZon.zig");
const arith = @import("Sema/arith.zig");

pub const default_branch_quota = 1000;

pub const InferredErrorSet = struct {
    /// The function body from which this error set originates.
    /// This is `none` in the case of a comptime/inline function call, corresponding to
    /// `InternPool.Index.adhoc_inferred_error_set_type`.
    /// The function's resolved error set is not set until analysis of the
    /// function body completes.
    func: InternPool.Index,
    /// All currently known errors that this error set contains. This includes
    /// direct additions via `return error.Foo;`, and possibly also errors that
    /// are returned from any dependent functions.
    errors: NameMap = .{},
    /// Other inferred error sets which this inferred error set should include.
    inferred_error_sets: std.AutoArrayHashMapUnmanaged(InternPool.Index, void) = .empty,
    /// The regular error set created by resolving this inferred error set.
    resolved: InternPool.Index = .none,

    pub const NameMap = std.AutoArrayHashMapUnmanaged(InternPool.NullTerminatedString, void);

    pub fn addErrorSet(
        self: *InferredErrorSet,
        err_set_ty: Type,
        ip: *InternPool,
        arena: Allocator,
    ) !void {
        switch (err_set_ty.toIntern()) {
            .anyerror_type => self.resolved = .anyerror_type,
            .adhoc_inferred_error_set_type => {}, // Adding an inferred error set to itself.

            else => switch (ip.indexToKey(err_set_ty.toIntern())) {
                .error_set_type => |error_set_type| {
                    for (error_set_type.names.get(ip)) |name| {
                        try self.errors.put(arena, name, {});
                    }
                },
                .inferred_error_set_type => {
                    try self.inferred_error_sets.put(arena, err_set_ty.toIntern(), {});
                },
                else => unreachable,
            },
        }
    }
};

/// Stores the mapping from `Zir.Inst.Index -> Air.Inst.Ref`, which is used by sema to resolve
/// instructions during analysis.
/// Instead of a hash table approach, InstMap is simply a slice that is indexed into using the
/// zir instruction index and a start offset. An index is not present in the map if the value
/// at the index is `Air.Inst.Ref.none`.
/// `ensureSpaceForInstructions` can be called to force InstMap to have a mapped range that
/// includes all instructions in a slice. After calling this function, `putAssumeCapacity*` can
/// be called safely for any of the instructions passed in.
pub const InstMap = struct {
    items: []Air.Inst.Ref = &[_]Air.Inst.Ref{},
    start: Zir.Inst.Index = @enumFromInt(0),

    pub fn deinit(map: InstMap, allocator: mem.Allocator) void {
        allocator.free(map.items);
    }

    pub fn get(map: InstMap, key: Zir.Inst.Index) ?Air.Inst.Ref {
        if (!map.contains(key)) return null;
        return map.items[@intFromEnum(key) - @intFromEnum(map.start)];
    }

    pub fn putAssumeCapacity(
        map: *InstMap,
        key: Zir.Inst.Index,
        ref: Air.Inst.Ref,
    ) void {
        map.items[@intFromEnum(key) - @intFromEnum(map.start)] = ref;
    }

    pub fn putAssumeCapacityNoClobber(
        map: *InstMap,
        key: Zir.Inst.Index,
        ref: Air.Inst.Ref,
    ) void {
        assert(!map.contains(key));
        map.putAssumeCapacity(key, ref);
    }

    pub const GetOrPutResult = struct {
        value_ptr: *Air.Inst.Ref,
        found_existing: bool,
    };

    pub fn getOrPutAssumeCapacity(
        map: *InstMap,
        key: Zir.Inst.Index,
    ) GetOrPutResult {
        const index = @intFromEnum(key) - @intFromEnum(map.start);
        return GetOrPutResult{
            .value_ptr = &map.items[index],
            .found_existing = map.items[index] != .none,
        };
    }

    pub fn remove(map: InstMap, key: Zir.Inst.Index) bool {
        if (!map.contains(key)) return false;
        map.items[@intFromEnum(key) - @intFromEnum(map.start)] = .none;
        return true;
    }

    pub fn contains(map: InstMap, key: Zir.Inst.Index) bool {
        return map.items[@intFromEnum(key) - @intFromEnum(map.start)] != .none;
    }

    pub fn ensureSpaceForInstructions(
        map: *InstMap,
        allocator: mem.Allocator,
        insts: []const Zir.Inst.Index,
    ) !void {
        const start, const end = mem.minMax(u32, @ptrCast(insts));
        const map_start = @intFromEnum(map.start);
        if (map_start <= start and end < map.items.len + map_start)
            return;

        const old_start = if (map.items.len == 0) start else map_start;
        var better_capacity = map.items.len;
        var better_start = old_start;
        while (true) {
            const extra_capacity = better_capacity / 2 + 16;
            better_capacity += extra_capacity;
            better_start -|= @intCast(extra_capacity / 2);
            if (better_start <= start and end < better_capacity + better_start)
                break;
        }

        const start_diff = old_start - better_start;
        const new_items = try allocator.alloc(Air.Inst.Ref, better_capacity);
        @memset(new_items[0..start_diff], .none);
        @memcpy(new_items[start_diff..][0..map.items.len], map.items);
        @memset(new_items[start_diff + map.items.len ..], .none);

        allocator.free(map.items);
        map.items = new_items;
        map.start = @enumFromInt(better_start);
    }
};

/// This is the context needed to semantically analyze ZIR instructions and
/// produce AIR instructions.
/// This is a temporary structure stored on the stack; references to it are valid only
/// during semantic analysis of the block.
pub const Block = struct {
    parent: ?*Block,
    /// Shared among all child blocks.
    sema: *Sema,
    /// The namespace to use for lookups from this source block
    namespace: InternPool.NamespaceIndex,
    /// The AIR instructions generated for this block.
    instructions: std.ArrayListUnmanaged(Air.Inst.Index),
    // `param` instructions are collected here to be used by the `func` instruction.
    /// When doing a generic function instantiation, this array collects a type
    /// for each *runtime-known* parameter. This array corresponds to the instance
    /// function type, while `Sema.comptime_args` corresponds to the generic owner
    /// function type.
    /// This memory is allocated by a parent `Sema` in the temporary arena, and is
    /// used to add a `func_instance` into the `InternPool`.
    params: std.MultiArrayList(Param) = .{},

    label: ?*Label = null,
    inlining: ?*Inlining,
    /// If runtime_index is not 0 then one of these is guaranteed to be non null.
    runtime_cond: ?LazySrcLoc = null,
    runtime_loop: ?LazySrcLoc = null,
    /// Non zero if a non-inline loop or a runtime conditional have been encountered.
    /// Stores to comptime variables are only allowed when var.runtime_index <= runtime_index.
    runtime_index: RuntimeIndex = .zero,
    inline_block: Zir.Inst.OptionalIndex = .none,

    comptime_reason: ?BlockComptimeReason = null,
    is_typeof: bool = false,

    /// Keep track of the active error return trace index around blocks so that we can correctly
    /// pop the error trace upon block exit.
    error_return_trace_index: Air.Inst.Ref = .none,

    /// when null, it is determined by build mode, changed by @setRuntimeSafety
    want_safety: ?bool = null,

    /// What mode to generate float operations in, set by @setFloatMode
    float_mode: std.builtin.FloatMode = .strict,

    c_import_buf: ?*std.ArrayList(u8) = null,

    /// If not `null`, this boolean is set when a `dbg_var_ptr`, `dbg_var_val`, or `dbg_arg_inline`.
    /// instruction is emitted. It signals that the innermost lexically
    /// enclosing `block`/`block_inline` should be translated into a real AIR
    /// `block` in order for codegen to match lexical scoping for debug vars.
    need_debug_scope: ?*bool = null,

    /// Relative source locations encountered while traversing this block should be
    /// treated as relative to the AST node of this ZIR instruction.
    src_base_inst: InternPool.TrackedInst.Index,

    /// The name of the current "context" for naming namespace types.
    /// The interpretation of this depends on the name strategy in ZIR, but the name
    /// is always incorporated into the type name somehow.
    /// See `Sema.createTypeName`.
    type_name_ctx: InternPool.NullTerminatedString,

    /// Create a `LazySrcLoc` based on an `Offset` from the code being analyzed in this block.
    /// Specifically, the given `Offset` is treated as relative to `block.src_base_inst`.
    pub fn src(block: Block, offset: LazySrcLoc.Offset) LazySrcLoc {
        return .{
            .base_node_inst = block.src_base_inst,
            .offset = offset,
        };
    }

    fn isComptime(block: Block) bool {
        return block.comptime_reason != null;
    }

    fn builtinCallArgSrc(block: *Block, builtin_call_node: std.zig.Ast.Node.Offset, arg_index: u32) LazySrcLoc {
        return block.src(.{ .node_offset_builtin_call_arg = .{
            .builtin_call_node = builtin_call_node,
            .arg_index = arg_index,
        } });
    }

    pub fn nodeOffset(block: Block, node_offset: std.zig.Ast.Node.Offset) LazySrcLoc {
        return block.src(LazySrcLoc.Offset.nodeOffset(node_offset));
    }

    fn tokenOffset(block: Block, tok_offset: std.zig.Ast.TokenOffset) LazySrcLoc {
        return block.src(.{ .token_offset = tok_offset });
    }

    const Param = struct {
        /// `none` means `anytype`.
        ty: InternPool.Index,
        is_comptime: bool,
        name: Zir.NullTerminatedString,
    };

    /// This `Block` maps a block ZIR instruction to the corresponding
    /// AIR instruction for break instruction analysis.
    pub const Label = struct {
        zir_block: Zir.Inst.Index,
        merges: Merges,
    };

    /// This `Block` indicates that an inline function call is happening
    /// and return instructions should be analyzed as a break instruction
    /// to this AIR block instruction.
    /// It is shared among all the blocks in an inline or comptime called
    /// function.
    pub const Inlining = struct {
        call_block: *Block,
        call_src: LazySrcLoc,
        func: InternPool.Index,

        /// Populated lazily by `refFrame`.
        ref_frame: Zcu.InlineReferenceFrame.Index.Optional = .none,

        /// If `true`, the following fields are `undefined`. This doesn't represent a true inline
        /// call, but rather a generic call analyzing the instantiation's generic type bodies.
        is_generic_instantiation: bool,

        has_comptime_args: bool,
        comptime_result: Air.Inst.Ref,
        merges: Merges,

        fn refFrame(inlining: *Inlining, zcu: *Zcu) Allocator.Error!Zcu.InlineReferenceFrame.Index {
            if (inlining.ref_frame == .none) {
                inlining.ref_frame = (try zcu.addInlineReferenceFrame(.{
                    .callee = inlining.func,
                    .call_src = inlining.call_src,
                    .parent = if (inlining.call_block.inlining) |parent_inlining| p: {
                        break :p (try parent_inlining.refFrame(zcu)).toOptional();
                    } else .none,
                })).toOptional();
            }
            return inlining.ref_frame.unwrap().?;
        }
    };

    pub const Merges = struct {
        block_inst: Air.Inst.Index,
        /// Separate array list from break_inst_list so that it can be passed directly
        /// to resolvePeerTypes.
        results: std.ArrayListUnmanaged(Air.Inst.Ref),
        /// Keeps track of the break instructions so that the operand can be replaced
        /// if we need to add type coercion at the end of block analysis.
        /// Same indexes, capacity, length as `results`.
        br_list: std.ArrayListUnmanaged(Air.Inst.Index),
        /// Keeps the source location of the rhs operand of the break instruction,
        /// to enable more precise compile errors.
        /// Same indexes, capacity, length as `results`.
        src_locs: std.ArrayListUnmanaged(?LazySrcLoc),
        /// Most blocks do not utilize this field. When it is used, its use is
        /// contextual. The possible uses are as follows:
        /// * for a `switch_block[_ref]`, this refers to dummy `br` instructions
        ///   which correspond to `switch_continue` ZIR. The switch logic will
        ///   rewrite these to appropriate AIR switch dispatches.
        extra_insts: std.ArrayListUnmanaged(Air.Inst.Index) = .empty,
        /// Same indexes, capacity, length as `extra_insts`.
        extra_src_locs: std.ArrayListUnmanaged(LazySrcLoc) = .empty,

        pub fn deinit(merges: *@This(), allocator: Allocator) void {
            merges.results.deinit(allocator);
            merges.br_list.deinit(allocator);
            merges.src_locs.deinit(allocator);
            merges.extra_insts.deinit(allocator);
            merges.extra_src_locs.deinit(allocator);
        }
    };

    pub fn makeSubBlock(parent: *Block) Block {
        return .{
            .parent = parent,
            .sema = parent.sema,
            .namespace = parent.namespace,
            .instructions = .{},
            .label = null,
            .inlining = parent.inlining,
            .comptime_reason = parent.comptime_reason,
            .is_typeof = parent.is_typeof,
            .runtime_cond = parent.runtime_cond,
            .runtime_loop = parent.runtime_loop,
            .runtime_index = parent.runtime_index,
            .want_safety = parent.want_safety,
            .float_mode = parent.float_mode,
            .c_import_buf = parent.c_import_buf,
            .error_return_trace_index = parent.error_return_trace_index,
            .need_debug_scope = parent.need_debug_scope,
            .src_base_inst = parent.src_base_inst,
            .type_name_ctx = parent.type_name_ctx,
        };
    }

    fn wantSafeTypes(block: *const Block) bool {
        return block.want_safety orelse switch (block.sema.pt.zcu.optimizeMode()) {
            .Debug => true,
            .ReleaseSafe => true,
            .ReleaseFast => false,
            .ReleaseSmall => false,
        };
    }

    fn wantSafety(block: *const Block) bool {
        if (block.isComptime()) return false; // runtime safety checks are pointless in comptime blocks
        return block.want_safety orelse switch (block.sema.pt.zcu.optimizeMode()) {
            .Debug => true,
            .ReleaseSafe => true,
            .ReleaseFast => false,
            .ReleaseSmall => false,
        };
    }

    pub fn getFileScope(block: *Block, zcu: *Zcu) *Zcu.File {
        return zcu.fileByIndex(getFileScopeIndex(block, zcu));
    }

    pub fn getFileScopeIndex(block: *Block, zcu: *Zcu) Zcu.File.Index {
        return zcu.namespacePtr(block.namespace).file_scope;
    }

    fn addTy(
        block: *Block,
        tag: Air.Inst.Tag,
        ty: Type,
    ) error{OutOfMemory}!Air.Inst.Ref {
        return block.addInst(.{
            .tag = tag,
            .data = .{ .ty = ty },
        });
    }

    fn addTyOp(
        block: *Block,
        tag: Air.Inst.Tag,
        ty: Type,
        operand: Air.Inst.Ref,
    ) error{OutOfMemory}!Air.Inst.Ref {
        return block.addInst(.{
            .tag = tag,
            .data = .{ .ty_op = .{
                .ty = Air.internedToRef(ty.toIntern()),
                .operand = operand,
            } },
        });
    }

    fn addBitCast(block: *Block, ty: Type, operand: Air.Inst.Ref) Allocator.Error!Air.Inst.Ref {
        return block.addInst(.{
            .tag = .bitcast,
            .data = .{ .ty_op = .{
                .ty = Air.internedToRef(ty.toIntern()),
                .operand = operand,
            } },
        });
    }

    fn addNoOp(block: *Block, tag: Air.Inst.Tag) error{OutOfMemory}!Air.Inst.Ref {
        return block.addInst(.{
            .tag = tag,
            .data = .{ .no_op = {} },
        });
    }

    fn addUnOp(
        block: *Block,
        tag: Air.Inst.Tag,
        operand: Air.Inst.Ref,
    ) error{OutOfMemory}!Air.Inst.Ref {
        return block.addInst(.{
            .tag = tag,
            .data = .{ .un_op = operand },
        });
    }

    fn addBr(
        block: *Block,
        target_block: Air.Inst.Index,
        operand: Air.Inst.Ref,
    ) error{OutOfMemory}!Air.Inst.Ref {
        return block.addInst(.{
            .tag = .br,
            .data = .{ .br = .{
                .block_inst = target_block,
                .operand = operand,
            } },
        });
    }

    fn addBinOp(
        block: *Block,
        tag: Air.Inst.Tag,
        lhs: Air.Inst.Ref,
        rhs: Air.Inst.Ref,
    ) error{OutOfMemory}!Air.Inst.Ref {
        return block.addInst(.{
            .tag = tag,
            .data = .{ .bin_op = .{
                .lhs = lhs,
                .rhs = rhs,
            } },
        });
    }

    fn addStructFieldPtr(
        block: *Block,
        struct_ptr: Air.Inst.Ref,
        field_index: u32,
        ptr_field_ty: Type,
    ) !Air.Inst.Ref {
        const ty = Air.internedToRef(ptr_field_ty.toIntern());
        const tag: Air.Inst.Tag = switch (field_index) {
            0 => .struct_field_ptr_index_0,
            1 => .struct_field_ptr_index_1,
            2 => .struct_field_ptr_index_2,
            3 => .struct_field_ptr_index_3,
            else => {
                return block.addInst(.{
                    .tag = .struct_field_ptr,
                    .data = .{ .ty_pl = .{
                        .ty = ty,
                        .payload = try block.sema.addExtra(Air.StructField{
                            .struct_operand = struct_ptr,
                            .field_index = field_index,
                        }),
                    } },
                });
            },
        };
        return block.addInst(.{
            .tag = tag,
            .data = .{ .ty_op = .{
                .ty = ty,
                .operand = struct_ptr,
            } },
        });
    }

    fn addStructFieldVal(
        block: *Block,
        struct_val: Air.Inst.Ref,
        field_index: u32,
        field_ty: Type,
    ) !Air.Inst.Ref {
        return block.addInst(.{
            .tag = .struct_field_val,
            .data = .{ .ty_pl = .{
                .ty = Air.internedToRef(field_ty.toIntern()),
                .payload = try block.sema.addExtra(Air.StructField{
                    .struct_operand = struct_val,
                    .field_index = field_index,
                }),
            } },
        });
    }

    fn addSliceElemPtr(
        block: *Block,
        slice: Air.Inst.Ref,
        elem_index: Air.Inst.Ref,
        elem_ptr_ty: Type,
    ) !Air.Inst.Ref {
        return block.addInst(.{
            .tag = .slice_elem_ptr,
            .data = .{ .ty_pl = .{
                .ty = Air.internedToRef(elem_ptr_ty.toIntern()),
                .payload = try block.sema.addExtra(Air.Bin{
                    .lhs = slice,
                    .rhs = elem_index,
                }),
            } },
        });
    }

    fn addPtrElemPtr(
        block: *Block,
        array_ptr: Air.Inst.Ref,
        elem_index: Air.Inst.Ref,
        elem_ptr_ty: Type,
    ) !Air.Inst.Ref {
        const ty_ref = Air.internedToRef(elem_ptr_ty.toIntern());
        return block.addPtrElemPtrTypeRef(array_ptr, elem_index, ty_ref);
    }

    fn addPtrElemPtrTypeRef(
        block: *Block,
        array_ptr: Air.Inst.Ref,
        elem_index: Air.Inst.Ref,
        elem_ptr_ty: Air.Inst.Ref,
    ) !Air.Inst.Ref {
        return block.addInst(.{
            .tag = .ptr_elem_ptr,
            .data = .{ .ty_pl = .{
                .ty = elem_ptr_ty,
                .payload = try block.sema.addExtra(Air.Bin{
                    .lhs = array_ptr,
                    .rhs = elem_index,
                }),
            } },
        });
    }

    fn addCmpVector(block: *Block, lhs: Air.Inst.Ref, rhs: Air.Inst.Ref, cmp_op: std.math.CompareOperator) !Air.Inst.Ref {
        const sema = block.sema;
        const pt = sema.pt;
        const zcu = pt.zcu;
        return block.addInst(.{
            .tag = if (block.float_mode == .optimized) .cmp_vector_optimized else .cmp_vector,
            .data = .{ .ty_pl = .{
                .ty = Air.internedToRef((try pt.vectorType(.{
                    .len = sema.typeOf(lhs).vectorLen(zcu),
                    .child = .bool_type,
                })).toIntern()),
                .payload = try sema.addExtra(Air.VectorCmp{
                    .lhs = lhs,
                    .rhs = rhs,
                    .op = Air.VectorCmp.encodeOp(cmp_op),
                }),
            } },
        });
    }

    fn addReduce(block: *Block, operand: Air.Inst.Ref, operation: std.builtin.ReduceOp) !Air.Inst.Ref {
        const sema = block.sema;
        const zcu = sema.pt.zcu;
        const allow_optimized = switch (sema.typeOf(operand).childType(zcu).zigTypeTag(zcu)) {
            .float => true,
            .bool, .int => false,
            else => unreachable,
        };
        return block.addInst(.{
            .tag = if (allow_optimized and block.float_mode == .optimized) .reduce_optimized else .reduce,
            .data = .{ .reduce = .{
                .operand = operand,
                .operation = operation,
            } },
        });
    }

    fn addAggregateInit(
        block: *Block,
        aggregate_ty: Type,
        elements: []const Air.Inst.Ref,
    ) !Air.Inst.Ref {
        const sema = block.sema;
        const ty_ref = Air.internedToRef(aggregate_ty.toIntern());
        try sema.air_extra.ensureUnusedCapacity(sema.gpa, elements.len);
        const extra_index: u32 = @intCast(sema.air_extra.items.len);
        sema.appendRefsAssumeCapacity(elements);

        return block.addInst(.{
            .tag = .aggregate_init,
            .data = .{ .ty_pl = .{
                .ty = ty_ref,
                .payload = extra_index,
            } },
        });
    }

    fn addUnionInit(
        block: *Block,
        union_ty: Type,
        field_index: u32,
        init: Air.Inst.Ref,
    ) !Air.Inst.Ref {
        return block.addInst(.{
            .tag = .union_init,
            .data = .{ .ty_pl = .{
                .ty = Air.internedToRef(union_ty.toIntern()),
                .payload = try block.sema.addExtra(Air.UnionInit{
                    .field_index = field_index,
                    .init = init,
                }),
            } },
        });
    }

    pub fn addInst(block: *Block, inst: Air.Inst) error{OutOfMemory}!Air.Inst.Ref {
        return (try block.addInstAsIndex(inst)).toRef();
    }

    pub fn addInstAsIndex(block: *Block, inst: Air.Inst) error{OutOfMemory}!Air.Inst.Index {
        const sema = block.sema;
        const gpa = sema.gpa;

        try sema.air_instructions.ensureUnusedCapacity(gpa, 1);
        try block.instructions.ensureUnusedCapacity(gpa, 1);

        const result_index: Air.Inst.Index = @enumFromInt(sema.air_instructions.len);
        sema.air_instructions.appendAssumeCapacity(inst);
        block.instructions.appendAssumeCapacity(result_index);
        return result_index;
    }

    /// Insert an instruction into the block at `index`. Moves all following
    /// instructions forward in the block to make room. Operation is O(N).
    pub fn insertInst(block: *Block, index: Air.Inst.Index, inst: Air.Inst) error{OutOfMemory}!Air.Inst.Ref {
        return (try block.insertInstAsIndex(index, inst)).toRef();
    }

    pub fn insertInstAsIndex(block: *Block, index: Air.Inst.Index, inst: Air.Inst) error{OutOfMemory}!Air.Inst.Index {
        const sema = block.sema;
        const gpa = sema.gpa;

        try sema.air_instructions.ensureUnusedCapacity(gpa, 1);

        const result_index: Air.Inst.Index = @enumFromInt(sema.air_instructions.len);
        sema.air_instructions.appendAssumeCapacity(inst);

        try block.instructions.insert(gpa, @intFromEnum(index), result_index);
        return result_index;
    }

    pub fn ownerModule(block: Block) *Package.Module {
        const zcu = block.sema.pt.zcu;
        return zcu.namespacePtr(block.namespace).fileScope(zcu).mod.?;
    }

    fn trackZir(block: *Block, inst: Zir.Inst.Index) Allocator.Error!InternPool.TrackedInst.Index {
        const pt = block.sema.pt;
        block.sema.code.assertTrackable(inst);
        return pt.zcu.intern_pool.trackZir(pt.zcu.gpa, pt.tid, .{
            .file = block.getFileScopeIndex(pt.zcu),
            .inst = inst,
        });
    }

    /// Returns the `*Block` that should be passed to `Sema.failWithOwnedErrorMsg`, because all inline
    /// calls below it have already been reported with "called at comptime from here" notes.
    fn explainWhyBlockIsComptime(start_block: *Block, err_msg: *Zcu.ErrorMsg) !*Block {
        const sema = start_block.sema;
        var block = start_block;
        while (true) {
            switch (block.comptime_reason.?) {
                .inlining_parent => {
                    const inlining = block.inlining.?;
                    try sema.errNote(inlining.call_src, err_msg, "called at comptime from here", .{});
                    block = inlining.call_block;
                },
                .reason => |r| {
                    try r.r.explain(sema, r.src, err_msg);
                    return block;
                },
            }
        }
    }
};

/// Represents the reason we are resolving a value or evaluating code at comptime.
/// Most reasons are represented by a `std.zig.SimpleComptimeReason`, which provides a plain message.
const ComptimeReason = union(enum) {
    /// Evaluating at comptime for a reason in the `std.zig.SimpleComptimeReason` enum.
    simple: std.zig.SimpleComptimeReason,

    /// Evaluating at comptime because of a comptime-only type. This field is separate so that
    /// the type in question can be included in the error message. AstGen could never emit this
    /// reason, because it knows nothing of types.
    /// The format string looks like "foo '{}' bar", where "{}" is the comptime-only type.
    /// We will then explain why this type is comptime-only.
    comptime_only: struct {
        ty: Type,
        msg: enum {
            union_init,
            struct_init,
            tuple_init,
        },
    },

    /// Like `comptime_only`, but for a parameter type.
    /// Includes a "parameter type declared here" note.
    comptime_only_param_ty: struct {
        ty: Type,
        param_ty_src: LazySrcLoc,
    },

    /// Like `comptime_only`, but for a return type.
    /// Includes a "return type declared here" note.
    comptime_only_ret_ty: struct {
        ty: Type,
        is_generic_inst: bool,
        ret_ty_src: LazySrcLoc,
    },

    /// Evaluating at comptime because we're evaluating an argument to a parameter marked `comptime`.
    comptime_param: struct {
        comptime_src: LazySrcLoc,
    },

    fn explain(reason: ComptimeReason, sema: *Sema, src: LazySrcLoc, err_msg: *Zcu.ErrorMsg) !void {
        switch (reason) {
            .simple => |simple| {
                try sema.errNote(src, err_msg, "{s}", .{simple.message()});
            },
            .comptime_only => |co| {
                const pre, const post = switch (co.msg) {
                    .union_init => .{ "initializer of comptime-only union", "must be comptime-known" },
                    .struct_init => .{ "initializer of comptime-only struct", "must be comptime-known" },
                    .tuple_init => .{ "initializer of comptime-only tuple", "must be comptime-known" },
                };
                try sema.errNote(src, err_msg, "{s} '{}' {s}", .{ pre, co.ty.fmt(sema.pt), post });
                try sema.explainWhyTypeIsComptime(err_msg, src, co.ty);
            },
            .comptime_only_param_ty => |co| {
                try sema.errNote(src, err_msg, "argument to parameter with comptime-only type '{}' must be comptime-known", .{co.ty.fmt(sema.pt)});
                try sema.errNote(co.param_ty_src, err_msg, "parameter type declared here", .{});
                try sema.explainWhyTypeIsComptime(err_msg, src, co.ty);
            },
            .comptime_only_ret_ty => |co| {
                const function_with: []const u8 = if (co.is_generic_inst) "generic function instantiated with" else "function with";
                try sema.errNote(src, err_msg, "call to {s} comptime-only return type '{}' is evaluated at comptime", .{ function_with, co.ty.fmt(sema.pt) });
                try sema.errNote(co.ret_ty_src, err_msg, "return type declared here", .{});
                try sema.explainWhyTypeIsComptime(err_msg, src, co.ty);
            },
            .comptime_param => |cp| {
                try sema.errNote(src, err_msg, "argument to comptime parameter must be comptime-known", .{});
                try sema.errNote(cp.comptime_src, err_msg, "parameter declared comptime here", .{});
            },
        }
    }
};

/// Represents the reason a `Block` is being evaluated at comptime.
const BlockComptimeReason = union(enum) {
    /// This block inherits being comptime-only from the `inlining` call site.
    inlining_parent,

    /// Comptime evaluation began somewhere in the current function for a given `ComptimeReason`.
    reason: struct {
        /// The source location which this reason originates from. `r` is reported here.
        src: LazySrcLoc,
        r: ComptimeReason,
    },
};

const LabeledBlock = struct {
    block: Block,
    label: Block.Label,

    fn destroy(lb: *LabeledBlock, gpa: Allocator) void {
        lb.block.instructions.deinit(gpa);
        lb.label.merges.deinit(gpa);
        gpa.destroy(lb);
    }
};

/// The value stored in the inferred allocation. This will go into
/// peer type resolution. This is stored in a separate list so that
/// the items are contiguous in memory and thus can be passed to
/// `Zcu.resolvePeerTypes`.
const InferredAlloc = struct {
    /// The placeholder `store` instructions used before the result pointer type
    /// is known. These should be rewritten to perform any required coercions
    /// when the type is resolved.
    /// Allocated from `sema.arena`.
    prongs: std.ArrayListUnmanaged(Air.Inst.Index) = .empty,
};

pub fn deinit(sema: *Sema) void {
    const gpa = sema.gpa;
    sema.air_instructions.deinit(gpa);
    sema.air_extra.deinit(gpa);
    sema.inst_map.deinit(gpa);
    {
        var it = sema.post_hoc_blocks.iterator();
        while (it.next()) |entry| {
            const labeled_block = entry.value_ptr.*;
            labeled_block.destroy(gpa);
        }
        sema.post_hoc_blocks.deinit(gpa);
    }
    sema.unresolved_inferred_allocs.deinit(gpa);
    sema.base_allocs.deinit(gpa);
    sema.maybe_comptime_allocs.deinit(gpa);
    sema.comptime_allocs.deinit(gpa);
    sema.exports.deinit(gpa);
    sema.references.deinit(gpa);
    sema.type_references.deinit(gpa);
    sema.dependencies.deinit(gpa);
    sema.* = undefined;
}

/// Performs semantic analysis of a ZIR body which is behind a runtime condition. If comptime
/// control flow happens here, Sema will convert it to runtime control flow by introducing post-hoc
/// blocks where necessary.
/// Returns the branch hint for this branch.
fn analyzeBodyRuntimeBreak(sema: *Sema, block: *Block, body: []const Zir.Inst.Index) !std.builtin.BranchHint {
    const parent_hint = sema.branch_hint;
    defer sema.branch_hint = parent_hint;
    sema.branch_hint = null;

    sema.analyzeBodyInner(block, body) catch |err| switch (err) {
        error.ComptimeBreak => {
            const zir_datas = sema.code.instructions.items(.data);
            const break_data = zir_datas[@intFromEnum(sema.comptime_break_inst)].@"break";
            const extra = sema.code.extraData(Zir.Inst.Break, break_data.payload_index).data;
            try sema.addRuntimeBreak(block, extra.block_inst, break_data.operand);
        },
        else => |e| return e,
    };

    return sema.branch_hint orelse .none;
}

/// Semantically analyze a ZIR function body. It is guranteed by AstGen that such a body cannot
/// trigger comptime control flow to move above the function body.
pub fn analyzeFnBody(
    sema: *Sema,
    block: *Block,
    body: []const Zir.Inst.Index,
) !void {
    sema.analyzeBodyInner(block, body) catch |err| switch (err) {
        error.ComptimeBreak => unreachable, // unexpected comptime control flow
        else => |e| return e,
    };
}

/// Given a ZIR body which can be exited via a `break_inline` instruction, or a non-inline body which
/// we are evaluating at comptime, semantically analyze the body and return the result from it.
/// Returns `null` if control flow did not break from this block, but instead terminated with some
/// other runtime noreturn instruction. Compile-time breaks to blocks further up the stack still
/// return `error.ComptimeBreak`. If `block.isComptime()`, this function will never return `null`.
fn analyzeInlineBody(
    sema: *Sema,
    block: *Block,
    body: []const Zir.Inst.Index,
    /// The index which a break instruction can target to break from this body.
    break_target: Zir.Inst.Index,
) CompileError!?Air.Inst.Ref {
    if (sema.analyzeBodyInner(block, body)) |_| {
        return null;
    } else |err| switch (err) {
        error.ComptimeBreak => {},
        else => |e| return e,
    }
    const break_inst = sema.code.instructions.get(@intFromEnum(sema.comptime_break_inst));
    switch (break_inst.tag) {
        .switch_continue => {
            // This is handled by separate logic.
            return error.ComptimeBreak;
        },
        .break_inline, .@"break" => {},
        else => unreachable,
    }
    const extra = sema.code.extraData(Zir.Inst.Break, break_inst.data.@"break".payload_index).data;
    if (extra.block_inst != break_target) {
        // This control flow goes further up the stack.
        return error.ComptimeBreak;
    }
    return try sema.resolveInst(break_inst.data.@"break".operand);
}

/// Like `analyzeInlineBody`, but if the body does not break with a value, returns
/// `.unreachable_value` instead of `null`. Notably, use this to evaluate an arbitrary
/// body at comptime to a single result value.
pub fn resolveInlineBody(
    sema: *Sema,
    block: *Block,
    body: []const Zir.Inst.Index,
    /// The index which a break instruction can target to break from this body.
    break_target: Zir.Inst.Index,
) CompileError!Air.Inst.Ref {
    return (try sema.analyzeInlineBody(block, body, break_target)) orelse .unreachable_value;
}

/// This function is the main loop of `Sema`. It analyzes a single body of ZIR instructions.
///
/// If this function returns normally, the merges of `block` were populated with all possible
/// (runtime) results of this block. Peer type resolution should be performed on the result,
/// and relevant runtime instructions written to perform necessary coercions and breaks. See
/// `resolveAnalyzedBlock`. This form of return is impossible if `block.isComptime()`.
///
/// Alternatively, this function may return `error.ComptimeBreak`. This indicates that comptime
/// control flow is happening, and we are breaking at comptime from a block indicated by the
/// break instruction in `sema.comptime_break_inst`. This occurs for any `break_inline`, or for a
/// standard `break` at comptime. This error is pushed up the stack until the target block is
/// reached, at which point the break operand will be fetched.
///
/// It is rare to call this function directly. Usually, you want one of the following wrappers:
/// * If the body is exited via a `break_inline`, or is being evaluated at comptime,
///   use `Sema.analyzeInlineBody` or `Sema.resolveInlineBody`.
/// * If the body is behind a fresh runtime condition, use `Sema.analyzeBodyRuntimeBreak`.
/// * If the body is an entire function body, use `Sema.analyzeFnBody`.
/// * If the body is to be generated into an AIR `block`, use `Sema.resolveBlockBody`.
/// * Otherwise, direct usage of `Sema.analyzeBodyInner` may be necessary.
fn analyzeBodyInner(
    sema: *Sema,
    block: *Block,
    body: []const Zir.Inst.Index,
) CompileError!void {
    // No tracy calls here, to avoid interfering with the tail call mechanism.

    try sema.inst_map.ensureSpaceForInstructions(sema.gpa, body);

    const pt = sema.pt;
    const zcu = pt.zcu;
    const map = &sema.inst_map;
    const tags = sema.code.instructions.items(.tag);
    const datas = sema.code.instructions.items(.data);

    var crash_info = crash_report.prepAnalyzeBody(sema, block, body);
    crash_info.push();
    defer crash_info.pop();

    // We use a while (true) loop here to avoid a redundant way of breaking out of
    // the loop. The only way to break out of the loop is with a `noreturn`
    // instruction.
    var i: u32 = 0;
    while (true) {
        crash_info.setBodyIndex(i);
        const inst = body[i];

        // The hashmap lookup in here is a little expensive, and LLVM fails to optimize it away.
        if (build_options.enable_logging) {
            std.log.scoped(.sema_zir).debug("sema ZIR {} %{d}", .{ path: {
                const file_index = block.src_base_inst.resolveFile(&zcu.intern_pool);
                const file = zcu.fileByIndex(file_index);
                break :path file.path.fmt(zcu.comp);
            }, inst });
        }

        const air_inst: Air.Inst.Ref = inst: switch (tags[@intFromEnum(inst)]) {
            // zig fmt: off
            .alloc                        => try sema.zirAlloc(block, inst),
            .alloc_inferred               => try sema.zirAllocInferred(block, true),
            .alloc_inferred_mut           => try sema.zirAllocInferred(block, false),
            .alloc_inferred_comptime      => try sema.zirAllocInferredComptime(true),
            .alloc_inferred_comptime_mut  => try sema.zirAllocInferredComptime(false),
            .resolve_inferred_alloc       => try sema.zirResolveInferredAlloc(block, inst),
            .alloc_mut                    => try sema.zirAllocMut(block, inst),
            .alloc_comptime_mut           => try sema.zirAllocComptime(block, inst),
            .make_ptr_const               => try sema.zirMakePtrConst(block, inst),
            .anyframe_type                => try sema.zirAnyframeType(block, inst),
            .array_cat                    => try sema.zirArrayCat(block, inst),
            .array_mul                    => try sema.zirArrayMul(block, inst),
            .array_type                   => try sema.zirArrayType(block, inst),
            .array_type_sentinel          => try sema.zirArrayTypeSentinel(block, inst),
            .vector_type                  => try sema.zirVectorType(block, inst),
            .as_node                      => try sema.zirAsNode(block, inst),
            .as_shift_operand             => try sema.zirAsShiftOperand(block, inst),
            .bit_and                      => try sema.zirBitwise(block, inst, .bit_and),
            .bit_not                      => try sema.zirBitNot(block, inst),
            .bit_or                       => try sema.zirBitwise(block, inst, .bit_or),
            .bitcast                      => try sema.zirBitcast(block, inst),
            .suspend_block                => try sema.zirSuspendBlock(block, inst),
            .bool_not                     => try sema.zirBoolNot(block, inst),
            .bool_br_and                  => try sema.zirBoolBr(block, inst, false),
            .bool_br_or                   => try sema.zirBoolBr(block, inst, true),
            .c_import                     => try sema.zirCImport(block, inst),
            .call                         => try sema.zirCall(block, inst, .direct),
            .field_call                   => try sema.zirCall(block, inst, .field),
            .cmp_lt                       => try sema.zirCmp(block, inst, .lt),
            .cmp_lte                      => try sema.zirCmp(block, inst, .lte),
            .cmp_eq                       => try sema.zirCmpEq(block, inst, .eq, Air.Inst.Tag.fromCmpOp(.eq, block.float_mode == .optimized)),
            .cmp_gte                      => try sema.zirCmp(block, inst, .gte),
            .cmp_gt                       => try sema.zirCmp(block, inst, .gt),
            .cmp_neq                      => try sema.zirCmpEq(block, inst, .neq, Air.Inst.Tag.fromCmpOp(.neq, block.float_mode == .optimized)),
            .decl_ref                     => try sema.zirDeclRef(block, inst),
            .decl_val                     => try sema.zirDeclVal(block, inst),
            .load                         => try sema.zirLoad(block, inst),
            .elem_ptr                     => try sema.zirElemPtr(block, inst),
            .elem_ptr_node                => try sema.zirElemPtrNode(block, inst),
            .elem_val                     => try sema.zirElemVal(block, inst),
            .elem_val_node                => try sema.zirElemValNode(block, inst),
            .elem_val_imm                 => try sema.zirElemValImm(block, inst),
            .elem_type                    => try sema.zirElemType(block, inst),
            .indexable_ptr_elem_type      => try sema.zirIndexablePtrElemType(block, inst),
            .vec_arr_elem_type            => try sema.zirVecArrElemType(block, inst),
            .enum_literal                 => try sema.zirEnumLiteral(block, inst),
            .decl_literal                 => try sema.zirDeclLiteral(block, inst, true),
            .decl_literal_no_coerce       => try sema.zirDeclLiteral(block, inst, false),
            .int_from_enum                => try sema.zirIntFromEnum(block, inst),
            .enum_from_int                => try sema.zirEnumFromInt(block, inst),
            .err_union_code               => try sema.zirErrUnionCode(block, inst),
            .err_union_code_ptr           => try sema.zirErrUnionCodePtr(block, inst),
            .err_union_payload_unsafe     => try sema.zirErrUnionPayload(block, inst),
            .err_union_payload_unsafe_ptr => try sema.zirErrUnionPayloadPtr(block, inst),
            .error_union_type             => try sema.zirErrorUnionType(block, inst),
            .error_value                  => try sema.zirErrorValue(block, inst),
            .field_ptr                    => try sema.zirFieldPtr(block, inst),
            .field_ptr_named              => try sema.zirFieldPtrNamed(block, inst),
            .field_val                    => try sema.zirFieldVal(block, inst),
            .field_val_named              => try sema.zirFieldValNamed(block, inst),
            .func                         => try sema.zirFunc(block, inst, false),
            .func_inferred                => try sema.zirFunc(block, inst, true),
            .func_fancy                   => try sema.zirFuncFancy(block, inst),
            .import                       => try sema.zirImport(block, inst),
            .indexable_ptr_len            => try sema.zirIndexablePtrLen(block, inst),
            .int                          => try sema.zirInt(block, inst),
            .int_big                      => try sema.zirIntBig(block, inst),
            .float                        => try sema.zirFloat(block, inst),
            .float128                     => try sema.zirFloat128(block, inst),
            .int_type                     => try sema.zirIntType(inst),
            .is_non_err                   => try sema.zirIsNonErr(block, inst),
            .is_non_err_ptr               => try sema.zirIsNonErrPtr(block, inst),
            .ret_is_non_err               => try sema.zirRetIsNonErr(block, inst),
            .is_non_null                  => try sema.zirIsNonNull(block, inst),
            .is_non_null_ptr              => try sema.zirIsNonNullPtr(block, inst),
            .merge_error_sets             => try sema.zirMergeErrorSets(block, inst),
            .negate                       => try sema.zirNegate(block, inst),
            .negate_wrap                  => try sema.zirNegateWrap(block, inst),
            .optional_payload_safe        => try sema.zirOptionalPayload(block, inst, true),
            .optional_payload_safe_ptr    => try sema.zirOptionalPayloadPtr(block, inst, true),
            .optional_payload_unsafe      => try sema.zirOptionalPayload(block, inst, false),
            .optional_payload_unsafe_ptr  => try sema.zirOptionalPayloadPtr(block, inst, false),
            .optional_type                => try sema.zirOptionalType(block, inst),
            .ptr_type                     => try sema.zirPtrType(block, inst),
            .ref                          => try sema.zirRef(block, inst),
            .ret_err_value_code           => try sema.zirRetErrValueCode(inst),
            .shr                          => try sema.zirShr(block, inst, .shr),
            .shr_exact                    => try sema.zirShr(block, inst, .shr_exact),
            .slice_end                    => try sema.zirSliceEnd(block, inst),
            .slice_sentinel               => try sema.zirSliceSentinel(block, inst),
            .slice_start                  => try sema.zirSliceStart(block, inst),
            .slice_length                 => try sema.zirSliceLength(block, inst),
            .slice_sentinel_ty            => try sema.zirSliceSentinelTy(block, inst),
            .str                          => try sema.zirStr(inst),
            .switch_block                 => try sema.zirSwitchBlock(block, inst, false),
            .switch_block_ref             => try sema.zirSwitchBlock(block, inst, true),
            .switch_block_err_union       => try sema.zirSwitchBlockErrUnion(block, inst),
            .type_info                    => try sema.zirTypeInfo(block, inst),
            .size_of                      => try sema.zirSizeOf(block, inst),
            .bit_size_of                  => try sema.zirBitSizeOf(block, inst),
            .typeof                       => try sema.zirTypeof(block, inst),
            .typeof_builtin               => try sema.zirTypeofBuiltin(block, inst),
            .typeof_log2_int_type         => try sema.zirTypeofLog2IntType(block, inst),
            .xor                          => try sema.zirBitwise(block, inst, .xor),
            .struct_init_empty            => try sema.zirStructInitEmpty(block, inst),
            .struct_init_empty_result     => try sema.zirStructInitEmptyResult(block, inst, false),
            .struct_init_empty_ref_result => try sema.zirStructInitEmptyResult(block, inst, true),
            .struct_init_anon             => try sema.zirStructInitAnon(block, inst),
            .struct_init                  => try sema.zirStructInit(block, inst, false),
            .struct_init_ref              => try sema.zirStructInit(block, inst, true),
            .struct_init_field_type       => try sema.zirStructInitFieldType(block, inst),
            .struct_init_field_ptr        => try sema.zirStructInitFieldPtr(block, inst),
            .array_init_anon              => try sema.zirArrayInitAnon(block, inst),
            .array_init                   => try sema.zirArrayInit(block, inst, false),
            .array_init_ref               => try sema.zirArrayInit(block, inst, true),
            .array_init_elem_type         => try sema.zirArrayInitElemType(block, inst),
            .array_init_elem_ptr          => try sema.zirArrayInitElemPtr(block, inst),
            .union_init                   => try sema.zirUnionInit(block, inst),
            .field_type_ref               => try sema.zirFieldTypeRef(block, inst),
            .int_from_ptr                 => try sema.zirIntFromPtr(block, inst),
            .align_of                     => try sema.zirAlignOf(block, inst),
            .int_from_bool                => try sema.zirIntFromBool(block, inst),
            .embed_file                   => try sema.zirEmbedFile(block, inst),
            .error_name                   => try sema.zirErrorName(block, inst),
            .tag_name                     => try sema.zirTagName(block, inst),
            .type_name                    => try sema.zirTypeName(block, inst),
            .frame_type                   => try sema.zirFrameType(block, inst),
            .frame_size                   => try sema.zirFrameSize(block, inst),
            .int_from_float               => try sema.zirIntFromFloat(block, inst),
            .float_from_int               => try sema.zirFloatFromInt(block, inst),
            .ptr_from_int                 => try sema.zirPtrFromInt(block, inst),
            .float_cast                   => try sema.zirFloatCast(block, inst),
            .int_cast                     => try sema.zirIntCast(block, inst),
            .ptr_cast                     => try sema.zirPtrCast(block, inst),
            .truncate                     => try sema.zirTruncate(block, inst),
            .has_decl                     => try sema.zirHasDecl(block, inst),
            .has_field                    => try sema.zirHasField(block, inst),
            .byte_swap                    => try sema.zirByteSwap(block, inst),
            .bit_reverse                  => try sema.zirBitReverse(block, inst),
            .bit_offset_of                => try sema.zirBitOffsetOf(block, inst),
            .offset_of                    => try sema.zirOffsetOf(block, inst),
            .splat                        => try sema.zirSplat(block, inst),
            .reduce                       => try sema.zirReduce(block, inst),
            .shuffle                      => try sema.zirShuffle(block, inst),
            .atomic_load                  => try sema.zirAtomicLoad(block, inst),
            .atomic_rmw                   => try sema.zirAtomicRmw(block, inst),
            .mul_add                      => try sema.zirMulAdd(block, inst),
            .builtin_call                 => try sema.zirBuiltinCall(block, inst),
            .@"resume"                    => try sema.zirResume(block, inst),
            .@"await"                     => try sema.zirAwait(block, inst),
            .for_len                      => try sema.zirForLen(block, inst),
            .validate_array_init_ref_ty   => try sema.zirValidateArrayInitRefTy(block, inst),
            .opt_eu_base_ptr_init         => try sema.zirOptEuBasePtrInit(block, inst),
            .coerce_ptr_elem_ty           => try sema.zirCoercePtrElemTy(block, inst),

            .clz       => try sema.zirBitCount(block, inst, .clz,      Value.clz),
            .ctz       => try sema.zirBitCount(block, inst, .ctz,      Value.ctz),
            .pop_count => try sema.zirBitCount(block, inst, .popcount, Value.popCount),
            .abs       => try sema.zirAbs(block, inst),

            .sqrt  => try sema.zirUnaryMath(block, inst, .sqrt, Value.sqrt),
            .sin   => try sema.zirUnaryMath(block, inst, .sin, Value.sin),
            .cos   => try sema.zirUnaryMath(block, inst, .cos, Value.cos),
            .tan   => try sema.zirUnaryMath(block, inst, .tan, Value.tan),
            .exp   => try sema.zirUnaryMath(block, inst, .exp, Value.exp),
            .exp2  => try sema.zirUnaryMath(block, inst, .exp2, Value.exp2),
            .log   => try sema.zirUnaryMath(block, inst, .log, Value.log),
            .log2  => try sema.zirUnaryMath(block, inst, .log2, Value.log2),
            .log10 => try sema.zirUnaryMath(block, inst, .log10, Value.log10),
            .floor => try sema.zirUnaryMath(block, inst, .floor, Value.floor),
            .ceil  => try sema.zirUnaryMath(block, inst, .ceil, Value.ceil),
            .round => try sema.zirUnaryMath(block, inst, .round, Value.round),
            .trunc => try sema.zirUnaryMath(block, inst, .trunc_float, Value.trunc),

            .error_set_decl => try sema.zirErrorSetDecl(inst),

            .add        => try sema.zirArithmetic(block, inst, .add,        true),
            .addwrap    => try sema.zirArithmetic(block, inst, .addwrap,    true),
            .add_sat    => try sema.zirArithmetic(block, inst, .add_sat,    true),
            .add_unsafe => try sema.zirArithmetic(block, inst, .add_unsafe, false),
            .mul        => try sema.zirArithmetic(block, inst, .mul,        true),
            .mulwrap    => try sema.zirArithmetic(block, inst, .mulwrap,    true),
            .mul_sat    => try sema.zirArithmetic(block, inst, .mul_sat,    true),
            .sub        => try sema.zirArithmetic(block, inst, .sub,        true),
            .subwrap    => try sema.zirArithmetic(block, inst, .subwrap,    true),
            .sub_sat    => try sema.zirArithmetic(block, inst, .sub_sat,    true),

            .div       => try sema.zirDiv(block, inst),
            .div_exact => try sema.zirDivExact(block, inst),
            .div_floor => try sema.zirDivFloor(block, inst),
            .div_trunc => try sema.zirDivTrunc(block, inst),

            .mod_rem => try sema.zirModRem(block, inst),
            .mod     => try sema.zirMod(block, inst),
            .rem     => try sema.zirRem(block, inst),

            .max => try sema.zirMinMax(block, inst, .max),
            .min => try sema.zirMinMax(block, inst, .min),

            .shl       => try sema.zirShl(block, inst, .shl),
            .shl_exact => try sema.zirShl(block, inst, .shl_exact),
            .shl_sat   => try sema.zirShl(block, inst, .shl_sat),

            .ret_ptr  => try sema.zirRetPtr(block, inst),
            .ret_type => Air.internedToRef(sema.fn_ret_ty.toIntern()),

            // Instructions that we know to *always* be noreturn based solely on their tag.
            // These functions match the return type of analyzeBody so that we can
            // tail call them here.
            .compile_error  => break try sema.zirCompileError(block, inst),
            .ret_implicit   => break try sema.zirRetImplicit(block, inst),
            .ret_node       => break try sema.zirRetNode(block, inst),
            .ret_load       => break try sema.zirRetLoad(block, inst),
            .ret_err_value  => break try sema.zirRetErrValue(block, inst),
            .@"unreachable" => break try sema.zirUnreachable(block, inst),
            .panic          => break try sema.zirPanic(block, inst),
            .trap           => break try sema.zirTrap(block, inst),
            // zig fmt: on

            // This instruction never exists in an analyzed body. It exists only in the declaration
            // list for a container type.
            .declaration => unreachable,

            .extended => ext: {
                const extended = datas[@intFromEnum(inst)].extended;
                break :ext switch (extended.opcode) {
                    // zig fmt: off
                    .struct_decl        => try sema.zirStructDecl(        block, extended, inst),
                    .enum_decl          => try sema.zirEnumDecl(          block, extended, inst),
                    .union_decl         => try sema.zirUnionDecl(         block, extended, inst),
                    .opaque_decl        => try sema.zirOpaqueDecl(        block, extended, inst),
                    .tuple_decl         => try sema.zirTupleDecl(         block, extended),
                    .this               => try sema.zirThis(              block, extended),
                    .ret_addr           => try sema.zirRetAddr(           block, extended),
                    .builtin_src        => try sema.zirBuiltinSrc(        block, extended),
                    .error_return_trace => try sema.zirErrorReturnTrace(  block),
                    .frame              => try sema.zirFrame(             block, extended),
                    .frame_address      => try sema.zirFrameAddress(      block, extended),
                    .alloc              => try sema.zirAllocExtended(     block, extended),
                    .builtin_extern     => try sema.zirBuiltinExtern(     block, extended),
                    .@"asm"             => try sema.zirAsm(               block, extended, false),
                    .asm_expr           => try sema.zirAsm(               block, extended, true),
                    .typeof_peer        => try sema.zirTypeofPeer(        block, extended, inst),
                    .compile_log        => try sema.zirCompileLog(        block, extended),
                    .min_multi          => try sema.zirMinMaxMulti(       block, extended, .min),
                    .max_multi          => try sema.zirMinMaxMulti(       block, extended, .max),
                    .add_with_overflow  => try sema.zirOverflowArithmetic(block, extended, extended.opcode),
                    .sub_with_overflow  => try sema.zirOverflowArithmetic(block, extended, extended.opcode),
                    .mul_with_overflow  => try sema.zirOverflowArithmetic(block, extended, extended.opcode),
                    .shl_with_overflow  => try sema.zirOverflowArithmetic(block, extended, extended.opcode),
                    .c_undef            => try sema.zirCUndef(            block, extended),
                    .c_include          => try sema.zirCInclude(          block, extended),
                    .c_define           => try sema.zirCDefine(           block, extended),
                    .wasm_memory_size   => try sema.zirWasmMemorySize(    block, extended),
                    .wasm_memory_grow   => try sema.zirWasmMemoryGrow(    block, extended),
                    .prefetch           => try sema.zirPrefetch(          block, extended),
                    .error_cast         => try sema.zirErrorCast(         block, extended),
                    .await_nosuspend    => try sema.zirAwaitNosuspend(    block, extended),
                    .select             => try sema.zirSelect(            block, extended),
                    .int_from_error     => try sema.zirIntFromError(      block, extended),
                    .error_from_int     => try sema.zirErrorFromInt(      block, extended),
                    .reify              => try sema.zirReify(             block, extended, inst),
                    .builtin_async_call => try sema.zirBuiltinAsyncCall(  block, extended),
                    .cmpxchg            => try sema.zirCmpxchg(           block, extended),
                    .c_va_arg           => try sema.zirCVaArg(            block, extended),
                    .c_va_copy          => try sema.zirCVaCopy(           block, extended),
                    .c_va_end           => try sema.zirCVaEnd(            block, extended),
                    .c_va_start         => try sema.zirCVaStart(          block, extended),
                    .ptr_cast_full      => try sema.zirPtrCastFull(       block, extended),
                    .ptr_cast_no_dest   => try sema.zirPtrCastNoDest(     block, extended),
                    .work_item_id       => try sema.zirWorkItem(          block, extended, extended.opcode),
                    .work_group_size    => try sema.zirWorkItem(          block, extended, extended.opcode),
                    .work_group_id      => try sema.zirWorkItem(          block, extended, extended.opcode),
                    .in_comptime        => try sema.zirInComptime(        block),
                    .closure_get        => try sema.zirClosureGet(        block, extended),
                    // zig fmt: on

                    .set_float_mode => {
                        try sema.zirSetFloatMode(block, extended);
                        i += 1;
                        continue;
                    },
                    .breakpoint => {
                        if (!block.isComptime()) {
                            _ = try block.addNoOp(.breakpoint);
                        }
                        i += 1;
                        continue;
                    },
                    .disable_instrumentation => {
                        try sema.zirDisableInstrumentation();
                        i += 1;
                        continue;
                    },
                    .disable_intrinsics => {
                        try sema.zirDisableIntrinsics();
                        i += 1;
                        continue;
                    },
                    .restore_err_ret_index => {
                        try sema.zirRestoreErrRetIndex(block, extended);
                        i += 1;
                        continue;
                    },
                    .branch_hint => {
                        try sema.zirBranchHint(block, extended);
                        i += 1;
                        continue;
                    },
                    .value_placeholder => unreachable, // never appears in a body
                    .field_parent_ptr => try sema.zirFieldParentPtr(block, extended),
                    .builtin_value => try sema.zirBuiltinValue(block, extended),
                    .inplace_arith_result_ty => try sema.zirInplaceArithResultTy(extended),
                    .dbg_empty_stmt => {
                        try sema.zirDbgEmptyStmt(block, inst);
                        i += 1;
                        continue;
                    },
                    .astgen_error => return error.AnalysisFail,
                };
            },

            // Instructions that we know can *never* be noreturn based solely on
            // their tag. We avoid needlessly checking if they are noreturn and
            // continue the loop.
            // We also know that they cannot be referenced later, so we avoid
            // putting them into the map.
            .dbg_stmt => {
                try sema.zirDbgStmt(block, inst);
                i += 1;
                continue;
            },
            .dbg_var_ptr => {
                try sema.zirDbgVar(block, inst, .dbg_var_ptr);
                i += 1;
                continue;
            },
            .dbg_var_val => {
                try sema.zirDbgVar(block, inst, .dbg_var_val);
                i += 1;
                continue;
            },
            .ensure_err_union_payload_void => {
                try sema.zirEnsureErrUnionPayloadVoid(block, inst);
                i += 1;
                continue;
            },
            .ensure_result_non_error => {
                try sema.zirEnsureResultNonError(block, inst);
                i += 1;
                continue;
            },
            .ensure_result_used => {
                try sema.zirEnsureResultUsed(block, inst);
                i += 1;
                continue;
            },
            .set_eval_branch_quota => {
                try sema.zirSetEvalBranchQuota(block, inst);
                i += 1;
                continue;
            },
            .atomic_store => {
                try sema.zirAtomicStore(block, inst);
                i += 1;
                continue;
            },
            .store_node => {
                try sema.zirStoreNode(block, inst);
                i += 1;
                continue;
            },
            .store_to_inferred_ptr => {
                try sema.zirStoreToInferredPtr(block, inst);
                i += 1;
                continue;
            },
            .validate_struct_init_ty => {
                try sema.zirValidateStructInitTy(block, inst, false);
                i += 1;
                continue;
            },
            .validate_struct_init_result_ty => {
                try sema.zirValidateStructInitTy(block, inst, true);
                i += 1;
                continue;
            },
            .validate_array_init_ty => {
                try sema.zirValidateArrayInitTy(block, inst, false);
                i += 1;
                continue;
            },
            .validate_array_init_result_ty => {
                try sema.zirValidateArrayInitTy(block, inst, true);
                i += 1;
                continue;
            },
            .validate_ptr_struct_init => {
                try sema.zirValidatePtrStructInit(block, inst);
                i += 1;
                continue;
            },
            .validate_ptr_array_init => {
                try sema.zirValidatePtrArrayInit(block, inst);
                i += 1;
                continue;
            },
            .validate_deref => {
                try sema.zirValidateDeref(block, inst);
                i += 1;
                continue;
            },
            .validate_destructure => {
                try sema.zirValidateDestructure(block, inst);
                i += 1;
                continue;
            },
            .validate_ref_ty => {
                try sema.zirValidateRefTy(block, inst);
                i += 1;
                continue;
            },
            .validate_const => {
                try sema.zirValidateConst(block, inst);
                i += 1;
                continue;
            },
            .@"export" => {
                try sema.zirExport(block, inst);
                i += 1;
                continue;
            },
            .set_runtime_safety => {
                try sema.zirSetRuntimeSafety(block, inst);
                i += 1;
                continue;
            },
            .param => {
                try sema.zirParam(block, inst, false);
                i += 1;
                continue;
            },
            .param_comptime => {
                try sema.zirParam(block, inst, true);
                i += 1;
                continue;
            },
            .param_anytype => {
                try sema.zirParamAnytype(block, inst, false);
                i += 1;
                continue;
            },
            .param_anytype_comptime => {
                try sema.zirParamAnytype(block, inst, true);
                i += 1;
                continue;
            },
            .memcpy => {
                try sema.zirMemcpy(block, inst, .memcpy, true);
                i += 1;
                continue;
            },
            .memmove => {
                try sema.zirMemcpy(block, inst, .memmove, false);
                i += 1;
                continue;
            },
            .memset => {
                try sema.zirMemset(block, inst);
                i += 1;
                continue;
            },
            .check_comptime_control_flow => {
                if (!block.isComptime()) {
                    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].un_node;
                    const src = block.nodeOffset(inst_data.src_node);
                    const inline_block = inst_data.operand.toIndex().?;

                    var check_block = block;
                    const target_runtime_index = while (true) {
                        if (check_block.inline_block == inline_block.toOptional()) {
                            break check_block.runtime_index;
                        }
                        check_block = check_block.parent.?;
                    };

                    if (@intFromEnum(target_runtime_index) < @intFromEnum(block.runtime_index)) {
                        const runtime_src = block.runtime_cond orelse block.runtime_loop.?;
                        const msg = msg: {
                            const msg = try sema.errMsg(src, "comptime control flow inside runtime block", .{});
                            errdefer msg.destroy(sema.gpa);
                            try sema.errNote(runtime_src, msg, "runtime control flow here", .{});
                            break :msg msg;
                        };
                        return sema.failWithOwnedErrorMsg(block, msg);
                    }
                }
                i += 1;
                continue;
            },
            .save_err_ret_index => {
                try sema.zirSaveErrRetIndex(block, inst);
                i += 1;
                continue;
            },
            .restore_err_ret_index_unconditional => {
                const un_node = datas[@intFromEnum(inst)].un_node;
                try sema.restoreErrRetIndex(block, block.nodeOffset(un_node.src_node), un_node.operand, .none);
                i += 1;
                continue;
            },
            .restore_err_ret_index_fn_entry => {
                const un_node = datas[@intFromEnum(inst)].un_node;
                try sema.restoreErrRetIndex(block, block.nodeOffset(un_node.src_node), .none, un_node.operand);
                i += 1;
                continue;
            },

            // Special case instructions to handle comptime control flow.
            .@"break" => {
                if (block.isComptime()) {
                    sema.comptime_break_inst = inst;
                    return error.ComptimeBreak;
                } else {
                    try sema.zirBreak(block, inst);
                    break;
                }
            },
            .break_inline => {
                sema.comptime_break_inst = inst;
                return error.ComptimeBreak;
            },
            .repeat => {
                if (block.isComptime()) {
                    // Send comptime control flow back to the beginning of this block.
                    const src = block.nodeOffset(datas[@intFromEnum(inst)].node);
                    try sema.emitBackwardBranch(block, src);
                    i = 0;
                    continue;
                } else {
                    // We are definitely called by `zirLoop`, which will treat the
                    // fact that this body does not terminate `noreturn` as an
                    // implicit repeat.
                    // TODO: since AIR has `repeat` now, we could change ZIR to generate
                    // more optimal code utilizing `repeat` instructions across blocks!
                    break;
                }
            },
            .repeat_inline => {
                // Send comptime control flow back to the beginning of this block.
                const src = block.nodeOffset(datas[@intFromEnum(inst)].node);
                try sema.emitBackwardBranch(block, src);
                i = 0;
                continue;
            },
            .switch_continue => if (block.isComptime()) {
                sema.comptime_break_inst = inst;
                return error.ComptimeBreak;
            } else {
                try sema.zirSwitchContinue(block, inst);
                break;
            },

            .loop => if (block.isComptime()) {
                continue :inst .block_inline;
            } else try sema.zirLoop(block, inst),

            .block => if (block.isComptime()) {
                continue :inst .block_inline;
            } else try sema.zirBlock(block, inst),

            .block_comptime => {
                const pl_node = datas[@intFromEnum(inst)].pl_node;
                const src = block.nodeOffset(pl_node.src_node);
                const extra = sema.code.extraData(Zir.Inst.BlockComptime, pl_node.payload_index);
                const block_body = sema.code.bodySlice(extra.end, extra.data.body_len);

                var child_block = block.makeSubBlock();
                defer child_block.instructions.deinit(sema.gpa);

                // We won't have any merges, but we must ensure this block is properly labeled for
                // any `.restore_err_ret_index_*` instructions.
                var label: Block.Label = .{
                    .zir_block = inst,
                    .merges = undefined,
                };
                child_block.label = &label;

                child_block.comptime_reason = .{ .reason = .{
                    .src = src,
                    .r = .{ .simple = extra.data.reason },
                } };

                const result = try sema.resolveInlineBody(&child_block, block_body, inst);

                // Only check for the result being comptime-known in the outermost `block_comptime`.
                // That way, AstGen can safely elide redundant `block_comptime` without affecting semantics.
                if (!block.isComptime() and !try sema.isComptimeKnown(result)) {
                    return sema.failWithNeededComptime(&child_block, src, null);
                }

                break :inst result;
            },

            .block_inline => blk: {
                // Directly analyze the block body without introducing a new block.
                // However, in the case of a corresponding break_inline which reaches
                // through a runtime conditional branch, we must retroactively emit
                // a block, so we remember the block index here just in case.
                const block_index = block.instructions.items.len;
                const inst_data = datas[@intFromEnum(inst)].pl_node;
                const extra = sema.code.extraData(Zir.Inst.Block, inst_data.payload_index);
                const inline_body = sema.code.bodySlice(extra.end, extra.data.body_len);
                const gpa = sema.gpa;

                const BreakResult = struct {
                    block_inst: Zir.Inst.Index,
                    operand: Zir.Inst.Ref,
                };

                const opt_break_data: ?BreakResult, const need_debug_scope = b: {
                    // Create a temporary child block so that this inline block is properly
                    // labeled for any .restore_err_ret_index instructions
                    var child_block = block.makeSubBlock();
                    var need_debug_scope = false;
                    child_block.need_debug_scope = &need_debug_scope;

                    // If this block contains a function prototype, we need to reset the
                    // current list of parameters and restore it later.
                    // Note: this probably needs to be resolved in a more general manner.
                    const tag_index = @intFromEnum(inline_body[inline_body.len - 1]);
                    child_block.inline_block = (if (tags[tag_index] == .repeat_inline)
                        inline_body[0]
                    else
                        inst).toOptional();

                    var label: Block.Label = .{
                        .zir_block = inst,
                        .merges = undefined,
                    };
                    child_block.label = &label;

                    // Write these instructions directly into the parent block
                    child_block.instructions = block.instructions;
                    defer block.instructions = child_block.instructions;

                    const break_result: ?BreakResult = if (sema.analyzeBodyInner(&child_block, inline_body)) |_| r: {
                        break :r null;
                    } else |err| switch (err) {
                        error.ComptimeBreak => brk_res: {
                            const break_inst = sema.comptime_break_inst;
                            const break_data = sema.code.instructions.items(.data)[@intFromEnum(break_inst)].@"break";
                            const break_extra = sema.code.extraData(Zir.Inst.Break, break_data.payload_index).data;
                            break :brk_res .{
                                .block_inst = break_extra.block_inst,
                                .operand = break_data.operand,
                            };
                        },
                        else => |e| return e,
                    };

                    if (need_debug_scope) {
                        _ = try sema.ensurePostHoc(block, inst);
                    }

                    break :b .{ break_result, need_debug_scope };
                };

                // A runtime conditional branch that needs a post-hoc block to be
                // emitted communicates this by mapping the block index into the inst map.
                if (map.get(inst)) |new_block_ref| ph: {
                    // Comptime control flow populates the map, so we don't actually know
                    // if this is a post-hoc runtime block until we check the
                    // post_hoc_block map.
                    const new_block_inst = new_block_ref.toIndex() orelse break :ph;
                    const labeled_block = sema.post_hoc_blocks.get(new_block_inst) orelse
                        break :ph;

                    // In this case we need to move all the instructions starting at
                    // block_index from the current block into this new one.

                    if (opt_break_data) |break_data| {
                        // This is a comptime break which we now change to a runtime break
                        // since it crosses a runtime branch.
                        // It may pass through our currently being analyzed block_inline or it
                        // may point directly to it. In the latter case, this modifies the
                        // block that we looked up in the post_hoc_blocks map above.
                        try sema.addRuntimeBreak(block, break_data.block_inst, break_data.operand);
                    }

                    try labeled_block.block.instructions.appendSlice(gpa, block.instructions.items[block_index..]);
                    block.instructions.items.len = block_index;

                    const block_result = try sema.resolveAnalyzedBlock(block, block.nodeOffset(inst_data.src_node), &labeled_block.block, &labeled_block.label.merges, need_debug_scope);
                    {
                        // Destroy the ad-hoc block entry so that it does not interfere with
                        // the next iteration of comptime control flow, if any.
                        labeled_block.destroy(gpa);
                        assert(sema.post_hoc_blocks.remove(new_block_inst));
                    }

                    break :blk block_result;
                }

                const break_data = opt_break_data orelse break;
                if (inst == break_data.block_inst) {
                    break :blk try sema.resolveInst(break_data.operand);
                } else {
                    // `comptime_break_inst` preserved from `analyzeBodyInner` above.
                    return error.ComptimeBreak;
                }
            },
            .condbr => if (block.isComptime()) {
                continue :inst .condbr_inline;
            } else {
                try sema.zirCondbr(block, inst);
                break;
            },
            .condbr_inline => {
                const inst_data = datas[@intFromEnum(inst)].pl_node;
                const cond_src = block.src(.{ .node_offset_if_cond = inst_data.src_node });
                const extra = sema.code.extraData(Zir.Inst.CondBr, inst_data.payload_index);
                const then_body = sema.code.bodySlice(extra.end, extra.data.then_body_len);
                const else_body = sema.code.bodySlice(
                    extra.end + then_body.len,
                    extra.data.else_body_len,
                );
                const uncasted_cond = try sema.resolveInst(extra.data.condition);
                const cond = try sema.coerce(block, .bool, uncasted_cond, cond_src);
                const cond_val = try sema.resolveConstDefinedValue(
                    block,
                    cond_src,
                    cond,
                    // If this block is comptime, it's more helpful to just give the outer message.
                    // This is particularly true if this came from a comptime `condbr` above.
                    if (block.isComptime()) null else .{ .simple = .inline_loop_operand },
                );
                const inline_body = if (cond_val.toBool()) then_body else else_body;

                try sema.maybeErrorUnwrapCondbr(block, inline_body, extra.data.condition, cond_src);
                const old_runtime_index = block.runtime_index;
                defer block.runtime_index = old_runtime_index;

                const result = try sema.analyzeInlineBody(block, inline_body, inst) orelse break;
                break :inst result;
            },
            .@"try" => blk: {
                if (!block.isComptime()) break :blk try sema.zirTry(block, inst);
                const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].pl_node;
                const src = block.nodeOffset(inst_data.src_node);
                const operand_src = block.src(.{ .node_offset_try_operand = inst_data.src_node });
                const extra = sema.code.extraData(Zir.Inst.Try, inst_data.payload_index);
                const inline_body = sema.code.bodySlice(extra.end, extra.data.body_len);
                const err_union = try sema.resolveInst(extra.data.operand);
                const err_union_ty = sema.typeOf(err_union);
                if (err_union_ty.zigTypeTag(zcu) != .error_union) {
                    return sema.fail(block, operand_src, "expected error union type, found '{}'", .{
                        err_union_ty.fmt(pt),
                    });
                }
                const is_non_err = try sema.analyzeIsNonErrComptimeOnly(block, operand_src, err_union);
                assert(is_non_err != .none);
                const is_non_err_val = try sema.resolveConstDefinedValue(block, operand_src, is_non_err, null);
                if (is_non_err_val.toBool()) {
                    break :blk try sema.analyzeErrUnionPayload(block, src, err_union_ty, err_union, operand_src, false);
                }
                const result = try sema.analyzeInlineBody(block, inline_body, inst) orelse break;
                break :blk result;
            },
            .try_ptr => blk: {
                if (!block.isComptime()) break :blk try sema.zirTryPtr(block, inst);
                const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].pl_node;
                const src = block.nodeOffset(inst_data.src_node);
                const operand_src = block.src(.{ .node_offset_try_operand = inst_data.src_node });
                const extra = sema.code.extraData(Zir.Inst.Try, inst_data.payload_index);
                const inline_body = sema.code.bodySlice(extra.end, extra.data.body_len);
                const operand = try sema.resolveInst(extra.data.operand);
                const err_union = try sema.analyzeLoad(block, src, operand, operand_src);
                const is_non_err = try sema.analyzeIsNonErrComptimeOnly(block, operand_src, err_union);
                assert(is_non_err != .none);
                const is_non_err_val = try sema.resolveConstDefinedValue(block, operand_src, is_non_err, null);
                if (is_non_err_val.toBool()) {
                    break :blk try sema.analyzeErrUnionPayloadPtr(block, src, operand, false, false);
                }
                const result = try sema.analyzeInlineBody(block, inline_body, inst) orelse break;
                break :blk result;
            },
            .@"defer" => blk: {
                const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].@"defer";
                const defer_body = sema.code.bodySlice(inst_data.index, inst_data.len);
                if (sema.analyzeBodyInner(block, defer_body)) |_| {
                    // The defer terminated noreturn - no more analysis needed.
                    break;
                } else |err| switch (err) {
                    error.ComptimeBreak => {},
                    else => |e| return e,
                }
                if (sema.comptime_break_inst != defer_body[defer_body.len - 1]) {
                    return error.ComptimeBreak;
                }
                break :blk .void_value;
            },
            .defer_err_code => blk: {
                const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].defer_err_code;
                const extra = sema.code.extraData(Zir.Inst.DeferErrCode, inst_data.payload_index).data;
                const defer_body = sema.code.bodySlice(extra.index, extra.len);
                const err_code = try sema.resolveInst(inst_data.err_code);
                try map.ensureSpaceForInstructions(sema.gpa, defer_body);
                map.putAssumeCapacity(extra.remapped_err_code, err_code);
                if (sema.analyzeBodyInner(block, defer_body)) |_| {
                    // The defer terminated noreturn - no more analysis needed.
                    break;
                } else |err| switch (err) {
                    error.ComptimeBreak => {},
                    else => |e| return e,
                }
                if (sema.comptime_break_inst != defer_body[defer_body.len - 1]) {
                    return error.ComptimeBreak;
                }
                break :blk .void_value;
            },
        };
        if (sema.isNoReturn(air_inst)) {
            // We're going to assume that the body itself is noreturn, so let's ensure that now
            assert(block.instructions.items.len > 0);
            assert(sema.isNoReturn(block.instructions.items[block.instructions.items.len - 1].toRef()));
            break;
        }
        map.putAssumeCapacity(inst, air_inst);
        i += 1;
    }
}

pub fn resolveInstAllowNone(sema: *Sema, zir_ref: Zir.Inst.Ref) !Air.Inst.Ref {
    if (zir_ref == .none) {
        return .none;
    } else {
        return resolveInst(sema, zir_ref);
    }
}

pub fn resolveInst(sema: *Sema, zir_ref: Zir.Inst.Ref) !Air.Inst.Ref {
    assert(zir_ref != .none);
    if (zir_ref.toIndex()) |i| {
        return sema.inst_map.get(i).?;
    }
    // First section of indexes correspond to a set number of constant values.
    // We intentionally map the same indexes to the same values between ZIR and AIR.
    return @enumFromInt(@intFromEnum(zir_ref));
}

fn resolveConstBool(
    sema: *Sema,
    block: *Block,
    src: LazySrcLoc,
    zir_ref: Zir.Inst.Ref,
    reason: ComptimeReason,
) !bool {
    const air_inst = try sema.resolveInst(zir_ref);
    const wanted_type: Type = .bool;
    const coerced_inst = try sema.coerce(block, wanted_type, air_inst, src);
    const val = try sema.resolveConstDefinedValue(block, src, coerced_inst, reason);
    return val.toBool();
}

fn resolveConstString(
    sema: *Sema,
    block: *Block,
    src: LazySrcLoc,
    zir_ref: Zir.Inst.Ref,
    reason: ComptimeReason,
) ![]u8 {
    const air_inst = try sema.resolveInst(zir_ref);
    return sema.toConstString(block, src, air_inst, reason);
}

pub fn toConstString(
    sema: *Sema,
    block: *Block,
    src: LazySrcLoc,
    air_inst: Air.Inst.Ref,
    reason: ComptimeReason,
) ![]u8 {
    const pt = sema.pt;
    const coerced_inst = try sema.coerce(block, .slice_const_u8, air_inst, src);
    const slice_val = try sema.resolveConstDefinedValue(block, src, coerced_inst, reason);
    const arr_val = try sema.derefSliceAsArray(block, src, slice_val, reason);
    return arr_val.toAllocatedBytes(arr_val.typeOf(pt.zcu), sema.arena, pt);
}

pub fn resolveConstStringIntern(
    sema: *Sema,
    block: *Block,
    src: LazySrcLoc,
    zir_ref: Zir.Inst.Ref,
    reason: ComptimeReason,
) !InternPool.NullTerminatedString {
    const air_inst = try sema.resolveInst(zir_ref);
    const wanted_type: Type = .slice_const_u8;
    const coerced_inst = try sema.coerce(block, wanted_type, air_inst, src);
    const val = try sema.resolveConstDefinedValue(block, src, coerced_inst, reason);
    return sema.sliceToIpString(block, src, val, reason);
}

fn resolveTypeOrPoison(sema: *Sema, block: *Block, src: LazySrcLoc, zir_ref: Zir.Inst.Ref) !?Type {
    const air_inst = try sema.resolveInst(zir_ref);
    const ty = try sema.analyzeAsType(block, src, air_inst);
    if (ty.isGenericPoison()) return null;
    return ty;
}

pub fn resolveType(sema: *Sema, block: *Block, src: LazySrcLoc, zir_ref: Zir.Inst.Ref) !Type {
    return (try sema.resolveTypeOrPoison(block, src, zir_ref)).?;
}

fn resolveDestType(
    sema: *Sema,
    block: *Block,
    src: LazySrcLoc,
    zir_ref: Zir.Inst.Ref,
    strat: enum { remove_eu_opt, remove_eu, remove_opt },
    builtin_name: []const u8,
) !Type {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const remove_eu = switch (strat) {
        .remove_eu_opt, .remove_eu => true,
        .remove_opt => false,
    };
    const remove_opt = switch (strat) {
        .remove_eu_opt, .remove_opt => true,
        .remove_eu => false,
    };

    const raw_ty = try sema.resolveTypeOrPoison(block, src, zir_ref) orelse {
        // Cast builtins use their result type as the destination type, but
        // it could be an anytype argument, which we can't catch in AstGen.
        const msg = msg: {
            const msg = try sema.errMsg(src, "{s} must have a known result type", .{builtin_name});
            errdefer msg.destroy(sema.gpa);
            switch (sema.genericPoisonReason(block, zir_ref)) {
                .anytype_param => |call_src| try sema.errNote(call_src, msg, "result type is unknown due to anytype parameter", .{}),
                .anyopaque_ptr => |ptr_src| try sema.errNote(ptr_src, msg, "result type is unknown due to opaque pointer type", .{}),
                .unknown => {},
            }
            try sema.errNote(src, msg, "use @as to provide explicit result type", .{});
            break :msg msg;
        };
        return sema.failWithOwnedErrorMsg(block, msg);
    };

    if (remove_eu and raw_ty.zigTypeTag(zcu) == .error_union) {
        const eu_child = raw_ty.errorUnionPayload(zcu);
        if (remove_opt and eu_child.zigTypeTag(zcu) == .optional) {
            return eu_child.childType(zcu);
        }
        return eu_child;
    }
    if (remove_opt and raw_ty.zigTypeTag(zcu) == .optional) {
        return raw_ty.childType(zcu);
    }
    return raw_ty;
}

const GenericPoisonReason = union(enum) {
    anytype_param: LazySrcLoc,
    anyopaque_ptr: LazySrcLoc,
    unknown,
};

/// Backtracks through ZIR instructions to determine the reason a generic poison
/// type was created. Used for error reporting.
fn genericPoisonReason(sema: *Sema, block: *Block, ref: Zir.Inst.Ref) GenericPoisonReason {
    var cur = ref;
    while (true) {
        const inst = cur.toIndex() orelse return .unknown;
        switch (sema.code.instructions.items(.tag)[@intFromEnum(inst)]) {
            .validate_array_init_ref_ty => {
                const pl_node = sema.code.instructions.items(.data)[@intFromEnum(inst)].pl_node;
                const extra = sema.code.extraData(Zir.Inst.ArrayInitRefTy, pl_node.payload_index).data;
                cur = extra.ptr_ty;
            },
            .array_init_elem_type => {
                const bin = sema.code.instructions.items(.data)[@intFromEnum(inst)].bin;
                cur = bin.lhs;
            },
            .indexable_ptr_elem_type, .vec_arr_elem_type => {
                const un_node = sema.code.instructions.items(.data)[@intFromEnum(inst)].un_node;
                cur = un_node.operand;
            },
            .struct_init_field_type => {
                const pl_node = sema.code.instructions.items(.data)[@intFromEnum(inst)].pl_node;
                const extra = sema.code.extraData(Zir.Inst.FieldType, pl_node.payload_index).data;
                cur = extra.container_type;
            },
            .elem_type => {
                // There are two cases here: the pointer type may already have been
                // generic poison, or it may have been an anyopaque pointer.
                const un_node = sema.code.instructions.items(.data)[@intFromEnum(inst)].un_node;
                const operand_ref = try sema.resolveInst(un_node.operand);
                const operand_val = operand_ref.toInterned() orelse return .unknown;
                if (operand_val == .generic_poison_type) {
                    // The pointer was generic poison - keep looking.
                    cur = un_node.operand;
                } else {
                    // This must be an anyopaque pointer!
                    return .{ .anyopaque_ptr = block.nodeOffset(un_node.src_node) };
                }
            },
            .call, .field_call => {
                // A function call can never return generic poison, so we must be
                // evaluating an `anytype` function parameter.
                // TODO: better source location - function decl rather than call
                const pl_node = sema.code.instructions.items(.data)[@intFromEnum(inst)].pl_node;
                return .{ .anytype_param = block.nodeOffset(pl_node.src_node) };
            },
            else => return .unknown,
        }
    }
}

fn analyzeAsType(
    sema: *Sema,
    block: *Block,
    src: LazySrcLoc,
    air_inst: Air.Inst.Ref,
) !Type {
    const wanted_type: Type = .type;
    const coerced_inst = try sema.coerce(block, wanted_type, air_inst, src);
    const val = try sema.resolveConstDefinedValue(block, src, coerced_inst, .{ .simple = .type });
    return val.toType();
}

pub fn setupErrorReturnTrace(sema: *Sema, block: *Block, last_arg_index: usize) !void {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const comp = zcu.comp;
    const gpa = sema.gpa;
    const ip = &zcu.intern_pool;
    if (!comp.config.any_error_tracing) return;

    assert(!block.isComptime());
    var err_trace_block = block.makeSubBlock();
    defer err_trace_block.instructions.deinit(gpa);

    const src: LazySrcLoc = LazySrcLoc.unneeded;

    // var addrs: [err_return_trace_addr_count]usize = undefined;
    const err_return_trace_addr_count = 32;
    const addr_arr_ty = try pt.arrayType(.{
        .len = err_return_trace_addr_count,
        .child = .usize_type,
    });
    const addrs_ptr = try err_trace_block.addTy(.alloc, try pt.singleMutPtrType(addr_arr_ty));

    // var st: StackTrace = undefined;
    const stack_trace_ty = try sema.getBuiltinType(block.nodeOffset(.zero), .StackTrace);
    try stack_trace_ty.resolveFields(pt);
    const st_ptr = try err_trace_block.addTy(.alloc, try pt.singleMutPtrType(stack_trace_ty));

    // st.instruction_addresses = &addrs;
    const instruction_addresses_field_name = try ip.getOrPutString(gpa, pt.tid, "instruction_addresses", .no_embedded_nulls);
    const addr_field_ptr = try sema.fieldPtr(&err_trace_block, src, st_ptr, instruction_addresses_field_name, src, true);
    try sema.storePtr2(&err_trace_block, src, addr_field_ptr, src, addrs_ptr, src, .store);

    // st.index = 0;
    const index_field_name = try ip.getOrPutString(gpa, pt.tid, "index", .no_embedded_nulls);
    const index_field_ptr = try sema.fieldPtr(&err_trace_block, src, st_ptr, index_field_name, src, true);
    try sema.storePtr2(&err_trace_block, src, index_field_ptr, src, .zero_usize, src, .store);

    // @errorReturnTrace() = &st;
    _ = try err_trace_block.addUnOp(.set_err_return_trace, st_ptr);

    try block.instructions.insertSlice(gpa, last_arg_index, err_trace_block.instructions.items);
}

/// Return the Value corresponding to a given AIR ref, or `null` if it refers to a runtime value.
fn resolveValue(sema: *Sema, inst: Air.Inst.Ref) CompileError!?Value {
    const zcu = sema.pt.zcu;
    assert(inst != .none);

    if (try sema.typeHasOnePossibleValue(sema.typeOf(inst))) |opv| {
        return opv;
    }

    if (inst.toInterned()) |ip_index| {
        const val: Value = .fromInterned(ip_index);
        assert(val.getVariable(zcu) == null);
        return val;
    } else {
        // Runtime-known value.
        const air_tags = sema.air_instructions.items(.tag);
        switch (air_tags[@intFromEnum(inst.toIndex().?)]) {
            .inferred_alloc => unreachable, // assertion failure
            .inferred_alloc_comptime => unreachable, // assertion failure
            else => {},
        }
        return null;
    }
}

/// Like `resolveValue`, but emits an error if the value is not comptime-known.
fn resolveConstValue(
    sema: *Sema,
    block: *Block,
    src: LazySrcLoc,
    inst: Air.Inst.Ref,
    reason: ?ComptimeReason,
) CompileError!Value {
    return try sema.resolveValue(inst) orelse {
        return sema.failWithNeededComptime(block, src, reason);
    };
}

/// Like `resolveValue`, but emits an error if the value is comptime-known to be undefined.
fn resolveDefinedValue(
    sema: *Sema,
    block: *Block,
    src: LazySrcLoc,
    air_ref: Air.Inst.Ref,
) CompileError!?Value {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const val = try sema.resolveValue(air_ref) orelse return null;
    if (val.isUndef(zcu)) {
        return sema.failWithUseOfUndef(block, src);
    }
    return val;
}

/// Like `resolveValue`, but emits an error if the value is not comptime-known or is undefined.
fn resolveConstDefinedValue(
    sema: *Sema,
    block: *Block,
    src: LazySrcLoc,
    air_ref: Air.Inst.Ref,
    reason: ?ComptimeReason,
) CompileError!Value {
    const val = try sema.resolveConstValue(block, src, air_ref, reason);
    if (val.isUndef(sema.pt.zcu)) return sema.failWithUseOfUndef(block, src);
    return val;
}

/// Like `resolveValue`, but recursively resolves lazy values before returning.
fn resolveValueResolveLazy(sema: *Sema, inst: Air.Inst.Ref) CompileError!?Value {
    return try sema.resolveLazyValue((try sema.resolveValue(inst)) orelse return null);
}

/// Value Tag may be `undef` or `variable`.
pub fn resolveFinalDeclValue(
    sema: *Sema,
    block: *Block,
    src: LazySrcLoc,
    air_ref: Air.Inst.Ref,
) CompileError!Value {
    const zcu = sema.pt.zcu;
    const val = try sema.resolveConstValue(block, src, air_ref, .{ .simple = .container_var_init });
    if (val.canMutateComptimeVarState(zcu)) {
        const ip = &zcu.intern_pool;
        const nav = ip.getNav(sema.owner.unwrap().nav_val);
        return sema.failWithContainsReferenceToComptimeVar(block, src, nav.name, "global variable", val);
    }
    return val;
}

fn failWithNeededComptime(sema: *Sema, block: *Block, src: LazySrcLoc, reason: ?ComptimeReason) CompileError {
    const msg, const fail_block = msg: {
        const msg = try sema.errMsg(src, "unable to resolve comptime value", .{});
        errdefer msg.destroy(sema.gpa);
        const fail_block = if (reason) |r| b: {
            try r.explain(sema, src, msg);
            break :b block;
        } else b: {
            break :b try block.explainWhyBlockIsComptime(msg);
        };
        break :msg .{ msg, fail_block };
    };
    return sema.failWithOwnedErrorMsg(fail_block, msg);
}

pub fn failWithUseOfUndef(sema: *Sema, block: *Block, src: LazySrcLoc) CompileError {
    return sema.fail(block, src, "use of undefined value here causes illegal behavior", .{});
}

pub fn failWithDivideByZero(sema: *Sema, block: *Block, src: LazySrcLoc) CompileError {
    return sema.fail(block, src, "division by zero here causes illegal behavior", .{});
}

fn failWithModRemNegative(sema: *Sema, block: *Block, src: LazySrcLoc, lhs_ty: Type, rhs_ty: Type) CompileError {
    const pt = sema.pt;
    return sema.fail(block, src, "remainder division with '{}' and '{}': signed integers and floats must use @rem or @mod", .{
        lhs_ty.fmt(pt), rhs_ty.fmt(pt),
    });
}

fn failWithExpectedOptionalType(sema: *Sema, block: *Block, src: LazySrcLoc, non_optional_ty: Type) CompileError {
    const pt = sema.pt;
    const msg = msg: {
        const msg = try sema.errMsg(src, "expected optional type, found '{}'", .{
            non_optional_ty.fmt(pt),
        });
        errdefer msg.destroy(sema.gpa);
        if (non_optional_ty.zigTypeTag(pt.zcu) == .error_union) {
            try sema.errNote(src, msg, "consider using 'try', 'catch', or 'if'", .{});
        }
        try addDeclaredHereNote(sema, msg, non_optional_ty);
        break :msg msg;
    };
    return sema.failWithOwnedErrorMsg(block, msg);
}

fn failWithArrayInitNotSupported(sema: *Sema, block: *Block, src: LazySrcLoc, ty: Type) CompileError {
    const pt = sema.pt;
    const msg = msg: {
        const msg = try sema.errMsg(src, "type '{}' does not support array initialization syntax", .{
            ty.fmt(pt),
        });
        errdefer msg.destroy(sema.gpa);
        if (ty.isSlice(pt.zcu)) {
            try sema.errNote(src, msg, "inferred array length is specified with an underscore: '[_]{}'", .{ty.elemType2(pt.zcu).fmt(pt)});
        }
        break :msg msg;
    };
    return sema.failWithOwnedErrorMsg(block, msg);
}

fn failWithStructInitNotSupported(sema: *Sema, block: *Block, src: LazySrcLoc, ty: Type) CompileError {
    const pt = sema.pt;
    return sema.fail(block, src, "type '{}' does not support struct initialization syntax", .{
        ty.fmt(pt),
    });
}

fn failWithErrorSetCodeMissing(
    sema: *Sema,
    block: *Block,
    src: LazySrcLoc,
    dest_err_set_ty: Type,
    src_err_set_ty: Type,
) CompileError {
    const pt = sema.pt;
    return sema.fail(block, src, "expected type '{}', found type '{}'", .{
        dest_err_set_ty.fmt(pt), src_err_set_ty.fmt(pt),
    });
}

pub fn failWithIntegerOverflow(sema: *Sema, block: *Block, src: LazySrcLoc, int_ty: Type, val: Value, vector_index: ?usize) CompileError {
    const pt = sema.pt;
    return sema.failWithOwnedErrorMsg(block, msg: {
        const msg = try sema.errMsg(src, "overflow of integer type '{}' with value '{}'", .{
            int_ty.fmt(pt), val.fmtValueSema(pt, sema),
        });
        errdefer msg.destroy(sema.gpa);
        if (vector_index) |i| try sema.errNote(src, msg, "when computing vector element at index '{d}'", .{i});
        break :msg msg;
    });
}

fn failWithInvalidComptimeFieldStore(sema: *Sema, block: *Block, init_src: LazySrcLoc, container_ty: Type, field_index: usize) CompileError {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const msg = msg: {
        const msg = try sema.errMsg(init_src, "value stored in comptime field does not match the default value of the field", .{});
        errdefer msg.destroy(sema.gpa);

        const struct_type = zcu.typeToStruct(container_ty) orelse break :msg msg;
        try sema.errNote(.{
            .base_node_inst = struct_type.zir_index,
            .offset = .{ .container_field_value = @intCast(field_index) },
        }, msg, "default value set here", .{});
        break :msg msg;
    };
    return sema.failWithOwnedErrorMsg(block, msg);
}

fn failWithUseOfAsync(sema: *Sema, block: *Block, src: LazySrcLoc) CompileError {
    const msg = msg: {
        const msg = try sema.errMsg(src, "async has not been implemented in the self-hosted compiler yet", .{});
        errdefer msg.destroy(sema.gpa);
        break :msg msg;
    };
    return sema.failWithOwnedErrorMsg(block, msg);
}

fn failWithInvalidFieldAccess(
    sema: *Sema,
    block: *Block,
    src: LazySrcLoc,
    object_ty: Type,
    field_name: InternPool.NullTerminatedString,
) CompileError {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const inner_ty = if (object_ty.isSinglePointer(zcu)) object_ty.childType(zcu) else object_ty;

    if (inner_ty.zigTypeTag(zcu) == .optional) opt: {
        const child_ty = inner_ty.optionalChild(zcu);
        if (!typeSupportsFieldAccess(zcu, child_ty, field_name)) break :opt;
        const msg = msg: {
            const msg = try sema.errMsg(src, "optional type '{}' does not support field access", .{object_ty.fmt(pt)});
            errdefer msg.destroy(sema.gpa);
            try sema.errNote(src, msg, "consider using '.?', 'orelse', or 'if'", .{});
            break :msg msg;
        };
        return sema.failWithOwnedErrorMsg(block, msg);
    } else if (inner_ty.zigTypeTag(zcu) == .error_union) err: {
        const child_ty = inner_ty.errorUnionPayload(zcu);
        if (!typeSupportsFieldAccess(zcu, child_ty, field_name)) break :err;
        const msg = msg: {
            const msg = try sema.errMsg(src, "error union type '{}' does not support field access", .{object_ty.fmt(pt)});
            errdefer msg.destroy(sema.gpa);
            try sema.errNote(src, msg, "consider using 'try', 'catch', or 'if'", .{});
            break :msg msg;
        };
        return sema.failWithOwnedErrorMsg(block, msg);
    }
    return sema.fail(block, src, "type '{}' does not support field access", .{object_ty.fmt(pt)});
}

fn typeSupportsFieldAccess(zcu: *const Zcu, ty: Type, field_name: InternPool.NullTerminatedString) bool {
    const ip = &zcu.intern_pool;
    switch (ty.zigTypeTag(zcu)) {
        .array => return field_name.eqlSlice("len", ip),
        .pointer => {
            const ptr_info = ty.ptrInfo(zcu);
            if (ptr_info.flags.size == .slice) {
                return field_name.eqlSlice("ptr", ip) or field_name.eqlSlice("len", ip);
            } else if (Type.fromInterned(ptr_info.child).zigTypeTag(zcu) == .array) {
                return field_name.eqlSlice("len", ip);
            } else return false;
        },
        .type, .@"struct", .@"union" => return true,
        else => return false,
    }
}

fn failWithComptimeErrorRetTrace(
    sema: *Sema,
    block: *Block,
    src: LazySrcLoc,
    name: InternPool.NullTerminatedString,
) CompileError {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const msg = msg: {
        const msg = try sema.errMsg(src, "caught unexpected error '{}'", .{name.fmt(&zcu.intern_pool)});
        errdefer msg.destroy(sema.gpa);

        for (sema.comptime_err_ret_trace.items) |src_loc| {
            try sema.errNote(src_loc, msg, "error returned here", .{});
        }
        break :msg msg;
    };
    return sema.failWithOwnedErrorMsg(block, msg);
}

fn failWithInvalidPtrArithmetic(sema: *Sema, block: *Block, src: LazySrcLoc, arithmetic: []const u8, supports: []const u8) CompileError {
    const msg = msg: {
        const msg = try sema.errMsg(src, "invalid {s} arithmetic operator", .{arithmetic});
        errdefer msg.destroy(sema.gpa);
        try sema.errNote(src, msg, "{s} arithmetic only supports {s}", .{ arithmetic, supports });
        break :msg msg;
    };
    return sema.failWithOwnedErrorMsg(block, msg);
}

/// We don't return a pointer to the new error note because the pointer
/// becomes invalid when you add another one.
pub fn errNote(
    sema: *Sema,
    src: LazySrcLoc,
    parent: *Zcu.ErrorMsg,
    comptime format: []const u8,
    args: anytype,
) error{OutOfMemory}!void {
    return sema.pt.zcu.errNote(src, parent, format, args);
}

fn addFieldErrNote(
    sema: *Sema,
    container_ty: Type,
    field_index: usize,
    parent: *Zcu.ErrorMsg,
    comptime format: []const u8,
    args: anytype,
) !void {
    @branchHint(.cold);
    const type_src = container_ty.srcLocOrNull(sema.pt.zcu) orelse return;
    const field_src: LazySrcLoc = .{
        .base_node_inst = type_src.base_node_inst,
        .offset = .{ .container_field_name = @intCast(field_index) },
    };
    try sema.errNote(field_src, parent, format, args);
}

pub fn errMsg(
    sema: *Sema,
    src: LazySrcLoc,
    comptime format: []const u8,
    args: anytype,
) Allocator.Error!*Zcu.ErrorMsg {
    assert(src.offset != .unneeded);
    return Zcu.ErrorMsg.create(sema.gpa, src, format, args);
}

pub fn fail(
    sema: *Sema,
    block: *Block,
    src: LazySrcLoc,
    comptime format: []const u8,
    args: anytype,
) CompileError {
    const err_msg = try sema.errMsg(src, format, args);
    inline for (args) |arg| {
        if (@TypeOf(arg) == Type.Formatter) {
            try addDeclaredHereNote(sema, err_msg, arg.data.ty);
        }
    }
    return sema.failWithOwnedErrorMsg(block, err_msg);
}

pub fn failWithOwnedErrorMsg(sema: *Sema, block: ?*Block, err_msg: *Zcu.ErrorMsg) error{ AnalysisFail, OutOfMemory } {
    @branchHint(.cold);
    const gpa = sema.gpa;
    const zcu = sema.pt.zcu;

    if (build_options.enable_debug_extensions and zcu.comp.debug_compile_errors) {
        var wip_errors: std.zig.ErrorBundle.Wip = undefined;
        wip_errors.init(gpa) catch @panic("out of memory");
        Compilation.addModuleErrorMsg(zcu, &wip_errors, err_msg.*, false) catch @panic("out of memory");
        std.debug.print("compile error during Sema:\n", .{});
        var error_bundle = wip_errors.toOwnedBundle("") catch @panic("out of memory");
        error_bundle.renderToStdErr(.{ .ttyconf = .no_color });
        crash_report.compilerPanic("unexpected compile error occurred", null);
    }

    if (block) |start_block| {
        var block_it = start_block;
        while (block_it.inlining) |inlining| {
            const note_str = note: {
                if (inlining.is_generic_instantiation) break :note "generic function instantiated here";
                if (inlining.call_block.isComptime()) break :note "called at comptime here";
                break :note "called inline here";
            };
            try sema.errNote(inlining.call_src, err_msg, "{s}", .{note_str});
            block_it = inlining.call_block;
        }
    }

    err_msg.reference_trace_root = sema.owner.toOptional();

    const gop = try zcu.failed_analysis.getOrPut(gpa, sema.owner);
    if (gop.found_existing) {
        // If there are multiple errors for the same Decl, prefer the first one added.
        sema.err = null;
        err_msg.destroy(gpa);
    } else {
        sema.err = err_msg;
        gop.value_ptr.* = err_msg;
    }

    return error.AnalysisFail;
}

/// Given an ErrorMsg, modify its message and source location to the given values, turning the
/// original message into a note. Notes on the original message are preserved as further notes.
/// Reference trace is preserved.
fn reparentOwnedErrorMsg(
    sema: *Sema,
    src: LazySrcLoc,
    msg: *Zcu.ErrorMsg,
    comptime format: []const u8,
    args: anytype,
) !void {
    const msg_str = try std.fmt.allocPrint(sema.gpa, format, args);

    const orig_notes = msg.notes.len;
    msg.notes = try sema.gpa.realloc(msg.notes, orig_notes + 1);
    std.mem.copyBackwards(Zcu.ErrorMsg, msg.notes[1..], msg.notes[0..orig_notes]);
    msg.notes[0] = .{
        .src_loc = msg.src_loc,
        .msg = msg.msg,
    };

    msg.src_loc = src;
    msg.msg = msg_str;
}

const align_ty: Type = .u29;

pub fn analyzeAsAlign(
    sema: *Sema,
    block: *Block,
    src: LazySrcLoc,
    air_ref: Air.Inst.Ref,
) !Alignment {
    const alignment_big = try sema.analyzeAsInt(block, src, air_ref, align_ty, .{ .simple = .@"align" });
    return sema.validateAlign(block, src, alignment_big);
}

fn validateAlign(
    sema: *Sema,
    block: *Block,
    src: LazySrcLoc,
    alignment: u64,
) !Alignment {
    if (alignment == 0) return sema.fail(block, src, "alignment must be >= 1", .{});
    if (!std.math.isPowerOfTwo(alignment)) {
        return sema.fail(block, src, "alignment value '{d}' is not a power of two", .{
            alignment,
        });
    }
    return Alignment.fromNonzeroByteUnits(alignment);
}

fn resolveAlign(
    sema: *Sema,
    block: *Block,
    src: LazySrcLoc,
    zir_ref: Zir.Inst.Ref,
) !Alignment {
    const air_ref = try sema.resolveInst(zir_ref);
    return sema.analyzeAsAlign(block, src, air_ref);
}

fn resolveInt(
    sema: *Sema,
    block: *Block,
    src: LazySrcLoc,
    zir_ref: Zir.Inst.Ref,
    dest_ty: Type,
    reason: ComptimeReason,
) !u64 {
    const air_ref = try sema.resolveInst(zir_ref);
    return sema.analyzeAsInt(block, src, air_ref, dest_ty, reason);
}

fn analyzeAsInt(
    sema: *Sema,
    block: *Block,
    src: LazySrcLoc,
    air_ref: Air.Inst.Ref,
    dest_ty: Type,
    reason: ComptimeReason,
) !u64 {
    const coerced = try sema.coerce(block, dest_ty, air_ref, src);
    const val = try sema.resolveConstDefinedValue(block, src, coerced, reason);
    return try val.toUnsignedIntSema(sema.pt);
}

fn analyzeValueAsCallconv(
    sema: *Sema,
    block: *Block,
    src: LazySrcLoc,
    unresolved_val: Value,
) !std.builtin.CallingConvention {
    return interpretBuiltinType(sema, block, src, unresolved_val, std.builtin.CallingConvention);
}

fn interpretBuiltinType(
    sema: *Sema,
    block: *Block,
    src: LazySrcLoc,
    unresolved_val: Value,
    comptime T: type,
) !T {
    const resolved_val = try sema.resolveLazyValue(unresolved_val);
    return resolved_val.interpret(T, sema.pt) catch |err| switch (err) {
        error.OutOfMemory => |e| return e,
        error.UndefinedValue => return sema.failWithUseOfUndef(block, src),
        error.TypeMismatch => @panic("std.builtin is corrupt"),
    };
}

fn zirTupleDecl(
    sema: *Sema,
    block: *Block,
    extended: Zir.Inst.Extended.InstData,
) CompileError!Air.Inst.Ref {
    const gpa = sema.gpa;
    const pt = sema.pt;
    const zcu = pt.zcu;
    const fields_len = extended.small;
    const extra = sema.code.extraData(Zir.Inst.TupleDecl, extended.operand);
    var extra_index = extra.end;

    const types = try sema.arena.alloc(InternPool.Index, fields_len);
    const inits = try sema.arena.alloc(InternPool.Index, fields_len);

    const extra_as_refs: []const Zir.Inst.Ref = @ptrCast(sema.code.extra);

    for (types, inits, 0..) |*field_ty, *field_init, field_index| {
        const zir_field_ty, const zir_field_init = extra_as_refs[extra_index..][0..2].*;
        extra_index += 2;

        const type_src = block.src(.{ .tuple_field_type = .{
            .tuple_decl_node_offset = extra.data.src_node,
            .elem_index = @intCast(field_index),
        } });
        const init_src = block.src(.{ .tuple_field_init = .{
            .tuple_decl_node_offset = extra.data.src_node,
            .elem_index = @intCast(field_index),
        } });

        const field_type = try sema.resolveType(block, type_src, zir_field_ty);
        try sema.validateTupleFieldType(block, field_type, type_src);

        field_ty.* = field_type.toIntern();
        field_init.* = init: {
            if (zir_field_init != .none) {
                const uncoerced_field_init = try sema.resolveInst(zir_field_init);
                const coerced_field_init = try sema.coerce(block, field_type, uncoerced_field_init, init_src);
                const field_init_val = try sema.resolveConstDefinedValue(block, init_src, coerced_field_init, .{ .simple = .tuple_field_default_value });
                if (field_init_val.canMutateComptimeVarState(zcu)) {
                    const field_name = try zcu.intern_pool.getOrPutStringFmt(gpa, pt.tid, "{}", .{field_index}, .no_embedded_nulls);
                    return sema.failWithContainsReferenceToComptimeVar(block, init_src, field_name, "field default value", field_init_val);
                }
                break :init field_init_val.toIntern();
            }
            if (try sema.typeHasOnePossibleValue(field_type)) |opv| {
                break :init opv.toIntern();
            }
            break :init .none;
        };
    }

    return Air.internedToRef(try zcu.intern_pool.getTupleType(gpa, pt.tid, .{
        .types = types,
        .values = inits,
    }));
}

fn validateTupleFieldType(
    sema: *Sema,
    block: *Block,
    field_ty: Type,
    field_ty_src: LazySrcLoc,
) CompileError!void {
    const gpa = sema.gpa;
    const zcu = sema.pt.zcu;
    if (field_ty.zigTypeTag(zcu) == .@"opaque") {
        return sema.failWithOwnedErrorMsg(block, msg: {
            const msg = try sema.errMsg(field_ty_src, "opaque types have unknown size and therefore cannot be directly embedded in tuples", .{});
            errdefer msg.destroy(gpa);

            try sema.addDeclaredHereNote(msg, field_ty);
            break :msg msg;
        });
    }
    if (field_ty.zigTypeTag(zcu) == .noreturn) {
        return sema.failWithOwnedErrorMsg(block, msg: {
            const msg = try sema.errMsg(field_ty_src, "tuple fields cannot be 'noreturn'", .{});
            errdefer msg.destroy(gpa);

            try sema.addDeclaredHereNote(msg, field_ty);
            break :msg msg;
        });
    }
}

/// Given a ZIR extra index which points to a list of `Zir.Inst.Capture`,
/// resolves this into a list of `InternPool.CaptureValue` allocated by `arena`.
fn getCaptures(sema: *Sema, block: *Block, type_src: LazySrcLoc, extra_index: usize, captures_len: u32) ![]InternPool.CaptureValue {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const ip = &zcu.intern_pool;
    const parent_ty: Type = .fromInterned(zcu.namespacePtr(block.namespace).owner_type);
    const parent_captures: InternPool.CaptureValue.Slice = parent_ty.getCaptures(zcu);

    const captures = try sema.arena.alloc(InternPool.CaptureValue, captures_len);

    for (sema.code.extra[extra_index..][0..captures_len], sema.code.extra[extra_index + captures_len ..][0..captures_len], captures) |raw, raw_name, *capture| {
        const zir_capture: Zir.Inst.Capture = @bitCast(raw);
        const zir_name: Zir.NullTerminatedString = @enumFromInt(raw_name);
        const zir_name_slice = sema.code.nullTerminatedString(zir_name);
        capture.* = switch (zir_capture.unwrap()) {
            .nested => |parent_idx| parent_captures.get(ip)[parent_idx],
            .instruction_load => |ptr_inst| InternPool.CaptureValue.wrap(capture: {
                const ptr_ref = try sema.resolveInst(ptr_inst.toRef());
                const ptr_val = try sema.resolveValue(ptr_ref) orelse {
                    break :capture .{ .runtime = sema.typeOf(ptr_ref).childType(zcu).toIntern() };
                };
                // TODO: better source location
                const unresolved_loaded_val = try sema.pointerDeref(block, type_src, ptr_val, sema.typeOf(ptr_ref)) orelse {
                    break :capture .{ .runtime = sema.typeOf(ptr_ref).childType(zcu).toIntern() };
                };
                const loaded_val = try sema.resolveLazyValue(unresolved_loaded_val);
                if (loaded_val.canMutateComptimeVarState(zcu)) {
                    const field_name = try ip.getOrPutString(zcu.gpa, pt.tid, zir_name_slice, .no_embedded_nulls);
                    return sema.failWithContainsReferenceToComptimeVar(block, type_src, field_name, "captured value", loaded_val);
                }
                break :capture .{ .@"comptime" = loaded_val.toIntern() };
            }),
            .instruction => |inst| InternPool.CaptureValue.wrap(capture: {
                const air_ref = try sema.resolveInst(inst.toRef());
                if (try sema.resolveValueResolveLazy(air_ref)) |val| {
                    if (val.canMutateComptimeVarState(zcu)) {
                        const field_name = try ip.getOrPutString(zcu.gpa, pt.tid, zir_name_slice, .no_embedded_nulls);
                        return sema.failWithContainsReferenceToComptimeVar(block, type_src, field_name, "captured value", val);
                    }
                    break :capture .{ .@"comptime" = val.toIntern() };
                }
                break :capture .{ .runtime = sema.typeOf(air_ref).toIntern() };
            }),
            .decl_val => |str| capture: {
                const decl_name = try ip.getOrPutString(
                    sema.gpa,
                    pt.tid,
                    sema.code.nullTerminatedString(str),
                    .no_embedded_nulls,
                );
                const nav = try sema.lookupIdentifier(block, LazySrcLoc.unneeded, decl_name); // TODO: could we need this src loc?
                break :capture InternPool.CaptureValue.wrap(.{ .nav_val = nav });
            },
            .decl_ref => |str| capture: {
                const decl_name = try ip.getOrPutString(
                    sema.gpa,
                    pt.tid,
                    sema.code.nullTerminatedString(str),
                    .no_embedded_nulls,
                );
                const nav = try sema.lookupIdentifier(block, LazySrcLoc.unneeded, decl_name); // TODO: could we need this src loc?
                break :capture InternPool.CaptureValue.wrap(.{ .nav_ref = nav });
            },
        };
    }

    return captures;
}

fn zirStructDecl(
    sema: *Sema,
    block: *Block,
    extended: Zir.Inst.Extended.InstData,
    inst: Zir.Inst.Index,
) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const gpa = sema.gpa;
    const ip = &zcu.intern_pool;
    const small: Zir.Inst.StructDecl.Small = @bitCast(extended.small);
    const extra = sema.code.extraData(Zir.Inst.StructDecl, extended.operand);

    const tracked_inst = try block.trackZir(inst);
    const src: LazySrcLoc = .{
        .base_node_inst = tracked_inst,
        .offset = LazySrcLoc.Offset.nodeOffset(.zero),
    };

    var extra_index = extra.end;

    const captures_len = if (small.has_captures_len) blk: {
        const captures_len = sema.code.extra[extra_index];
        extra_index += 1;
        break :blk captures_len;
    } else 0;
    const fields_len = if (small.has_fields_len) blk: {
        const fields_len = sema.code.extra[extra_index];
        extra_index += 1;
        break :blk fields_len;
    } else 0;
    const decls_len = if (small.has_decls_len) blk: {
        const decls_len = sema.code.extra[extra_index];
        extra_index += 1;
        break :blk decls_len;
    } else 0;

    const captures = try sema.getCaptures(block, src, extra_index, captures_len);
    extra_index += captures_len * 2;

    if (small.has_backing_int) {
        const backing_int_body_len = sema.code.extra[extra_index];
        extra_index += 1; // backing_int_body_len
        if (backing_int_body_len == 0) {
            extra_index += 1; // backing_int_ref
        } else {
            extra_index += backing_int_body_len; // backing_int_body_inst
        }
    }

    const struct_init: InternPool.StructTypeInit = .{
        .layout = small.layout,
        .fields_len = fields_len,
        .known_non_opv = small.known_non_opv,
        .requires_comptime = if (small.known_comptime_only) .yes else .unknown,
        .any_comptime_fields = small.any_comptime_fields,
        .any_default_inits = small.any_default_inits,
        .inits_resolved = false,
        .any_aligned_fields = small.any_aligned_fields,
        .key = .{ .declared = .{
            .zir_index = tracked_inst,
            .captures = captures,
        } },
    };
    const wip_ty = switch (try ip.getStructType(gpa, pt.tid, struct_init, false)) {
        .existing => |ty| {
            const new_ty = try pt.ensureTypeUpToDate(ty);

            // Make sure we update the namespace if the declaration is re-analyzed, to pick
            // up on e.g. changed comptime decls.
            try pt.ensureNamespaceUpToDate(Type.fromInterned(new_ty).getNamespaceIndex(zcu));

            try sema.declareDependency(.{ .interned = new_ty });
            try sema.addTypeReferenceEntry(src, new_ty);
            return Air.internedToRef(new_ty);
        },
        .wip => |wip| wip,
    };
    errdefer wip_ty.cancel(ip, pt.tid);

    wip_ty.setName(ip, try sema.createTypeName(
        block,
        small.name_strategy,
        "struct",
        inst,
        wip_ty.index,
    ));

    const new_namespace_index: InternPool.NamespaceIndex = try pt.createNamespace(.{
        .parent = block.namespace.toOptional(),
        .owner_type = wip_ty.index,
        .file_scope = block.getFileScopeIndex(zcu),
        .generation = zcu.generation,
    });
    errdefer pt.destroyNamespace(new_namespace_index);

    if (pt.zcu.comp.incremental) {
        try pt.addDependency(.wrap(.{ .type = wip_ty.index }), .{ .src_hash = tracked_inst });
    }

    const decls = sema.code.bodySlice(extra_index, decls_len);
    try pt.scanNamespace(new_namespace_index, decls);

    try zcu.comp.queueJob(.{ .resolve_type_fully = wip_ty.index });
    codegen_type: {
        if (zcu.comp.config.use_llvm) break :codegen_type;
        if (block.ownerModule().strip) break :codegen_type;
        // This job depends on any resolve_type_fully jobs queued up before it.
        try zcu.comp.queueJob(.{ .codegen_type = wip_ty.index });
    }
    try sema.declareDependency(.{ .interned = wip_ty.index });
    try sema.addTypeReferenceEntry(src, wip_ty.index);
    if (zcu.comp.debugIncremental()) try zcu.incremental_debug_state.newType(zcu, wip_ty.index);
    return Air.internedToRef(wip_ty.finish(ip, new_namespace_index));
}

pub fn createTypeName(
    sema: *Sema,
    block: *Block,
    name_strategy: Zir.Inst.NameStrategy,
    anon_prefix: []const u8,
    inst: ?Zir.Inst.Index,
    /// This is used purely to give the type a unique name in the `anon` case.
    type_index: InternPool.Index,
) !InternPool.NullTerminatedString {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const gpa = zcu.gpa;
    const ip = &zcu.intern_pool;

    switch (name_strategy) {
        .anon => {}, // handled after switch
        .parent => return block.type_name_ctx,
        .func => func_strat: {
            const fn_info = sema.code.getFnInfo(ip.funcZirBodyInst(sema.func_index).resolve(ip) orelse return error.AnalysisFail);
            const zir_tags = sema.code.instructions.items(.tag);

            var buf: std.ArrayListUnmanaged(u8) = .empty;
            defer buf.deinit(gpa);

            const writer = buf.writer(gpa);
            try writer.print("{}(", .{block.type_name_ctx.fmt(ip)});

            var arg_i: usize = 0;
            for (fn_info.param_body) |zir_inst| switch (zir_tags[@intFromEnum(zir_inst)]) {
                .param, .param_comptime, .param_anytype, .param_anytype_comptime => {
                    const arg = sema.inst_map.get(zir_inst).?;
                    // If this is being called in a generic function then analyzeCall will
                    // have already resolved the args and this will work.
                    // If not then this is a struct type being returned from a non-generic
                    // function and the name doesn't matter since it will later
                    // result in a compile error.
                    const arg_val = try sema.resolveValue(arg) orelse break :func_strat; // fall through to anon strat

                    if (arg_i != 0) try writer.writeByte(',');

                    // Limiting the depth here helps avoid type names getting too long, which
                    // in turn helps to avoid unreasonably long symbol names for namespaced
                    // symbols. Such names should ideally be human-readable, and additionally,
                    // some tooling may not support very long symbol names.
                    try writer.print("{}", .{Value.fmtValueSemaFull(.{
                        .val = arg_val,
                        .pt = pt,
                        .opt_sema = sema,
                        .depth = 1,
                    })});

                    arg_i += 1;
                    continue;
                },
                else => continue,
            };

            try writer.writeByte(')');
            return ip.getOrPutString(gpa, pt.tid, buf.items, .no_embedded_nulls);
        },
        .dbg_var => {
            // TODO: this logic is questionable. We ideally should be traversing the `Block` rather than relying on the order of AstGen instructions.
            const ref = inst.?.toRef();
            const zir_tags = sema.code.instructions.items(.tag);
            const zir_data = sema.code.instructions.items(.data);
            for (@intFromEnum(inst.?)..zir_tags.len) |i| switch (zir_tags[i]) {
                .dbg_var_ptr, .dbg_var_val => if (zir_data[i].str_op.operand == ref) {
                    return ip.getOrPutStringFmt(gpa, pt.tid, "{}.{s}", .{
                        block.type_name_ctx.fmt(ip), zir_data[i].str_op.getStr(sema.code),
                    }, .no_embedded_nulls);
                },
                else => {},
            };
            // fall through to anon strat
        },
    }

    // anon strat handling

    // It would be neat to have "struct:line:column" but this name has
    // to survive incremental updates, where it may have been shifted down
    // or up to a different line, but unchanged, and thus not unnecessarily
    // semantically analyzed.
    // TODO: that would be possible, by detecting line number changes and renaming
    // types appropriately. However, `@typeName` becomes a problem then. If we remove
    // that builtin from the language, we can consider this.

    return ip.getOrPutStringFmt(gpa, pt.tid, "{}__{s}_{d}", .{
        block.type_name_ctx.fmt(ip), anon_prefix, @intFromEnum(type_index),
    }, .no_embedded_nulls);
}

fn zirEnumDecl(
    sema: *Sema,
    block: *Block,
    extended: Zir.Inst.Extended.InstData,
    inst: Zir.Inst.Index,
) CompileError!Air.Inst.Ref {
    const tracy = trace(@src());
    defer tracy.end();

    const pt = sema.pt;
    const zcu = pt.zcu;
    const gpa = sema.gpa;
    const ip = &zcu.intern_pool;
    const small: Zir.Inst.EnumDecl.Small = @bitCast(extended.small);
    const extra = sema.code.extraData(Zir.Inst.EnumDecl, extended.operand);
    var extra_index: usize = extra.end;

    const tracked_inst = try block.trackZir(inst);
    const src: LazySrcLoc = .{ .base_node_inst = tracked_inst, .offset = LazySrcLoc.Offset.nodeOffset(.zero) };

    const tag_type_ref = if (small.has_tag_type) blk: {
        const tag_type_ref: Zir.Inst.Ref = @enumFromInt(sema.code.extra[extra_index]);
        extra_index += 1;
        break :blk tag_type_ref;
    } else .none;

    const captures_len = if (small.has_captures_len) blk: {
        const captures_len = sema.code.extra[extra_index];
        extra_index += 1;
        break :blk captures_len;
    } else 0;

    const body_len = if (small.has_body_len) blk: {
        const body_len = sema.code.extra[extra_index];
        extra_index += 1;
        break :blk body_len;
    } else 0;

    const fields_len = if (small.has_fields_len) blk: {
        const fields_len = sema.code.extra[extra_index];
        extra_index += 1;
        break :blk fields_len;
    } else 0;

    const decls_len = if (small.has_decls_len) blk: {
        const decls_len = sema.code.extra[extra_index];
        extra_index += 1;
        break :blk decls_len;
    } else 0;

    const captures = try sema.getCaptures(block, src, extra_index, captures_len);
    extra_index += captures_len * 2;

    const decls = sema.code.bodySlice(extra_index, decls_len);
    extra_index += decls_len;

    const body = sema.code.bodySlice(extra_index, body_len);
    extra_index += body.len;

    const bit_bags_count = std.math.divCeil(usize, fields_len, 32) catch unreachable;
    const body_end = extra_index;
    extra_index += bit_bags_count;

    const any_values = for (sema.code.extra[body_end..][0..bit_bags_count]) |bag| {
        if (bag != 0) break true;
    } else false;

    const enum_init: InternPool.EnumTypeInit = .{
        .has_values = any_values,
        .tag_mode = if (small.nonexhaustive)
            .nonexhaustive
        else if (tag_type_ref == .none)
            .auto
        else
            .explicit,
        .fields_len = fields_len,
        .key = .{ .declared = .{
            .zir_index = tracked_inst,
            .captures = captures,
        } },
    };
    const wip_ty = switch (try ip.getEnumType(gpa, pt.tid, enum_init, false)) {
        .existing => |ty| {
            const new_ty = try pt.ensureTypeUpToDate(ty);

            // Make sure we update the namespace if the declaration is re-analyzed, to pick
            // up on e.g. changed comptime decls.
            try pt.ensureNamespaceUpToDate(Type.fromInterned(new_ty).getNamespaceIndex(zcu));

            try sema.declareDependency(.{ .interned = new_ty });
            try sema.addTypeReferenceEntry(src, new_ty);

            // Since this is an enum, it has to be resolved immediately.
            // `ensureTypeUpToDate` has resolved the new type if necessary.
            // We just need to check for resolution failures.
            const ty_unit: AnalUnit = .wrap(.{ .type = new_ty });
            if (zcu.failed_analysis.contains(ty_unit) or zcu.transitive_failed_analysis.contains(ty_unit)) {
                return error.AnalysisFail;
            }

            return Air.internedToRef(new_ty);
        },
        .wip => |wip| wip,
    };

    // Once this is `true`, we will not delete the decl or type even upon failure, since we
    // have finished constructing the type and are in the process of analyzing it.
    var done = false;

    errdefer if (!done) wip_ty.cancel(ip, pt.tid);

    const type_name = try sema.createTypeName(
        block,
        small.name_strategy,
        "enum",
        inst,
        wip_ty.index,
    );
    wip_ty.setName(ip, type_name);

    const new_namespace_index: InternPool.NamespaceIndex = try pt.createNamespace(.{
        .parent = block.namespace.toOptional(),
        .owner_type = wip_ty.index,
        .file_scope = block.getFileScopeIndex(zcu),
        .generation = zcu.generation,
    });
    errdefer if (!done) pt.destroyNamespace(new_namespace_index);

    try pt.scanNamespace(new_namespace_index, decls);

    try sema.declareDependency(.{ .interned = wip_ty.index });
    try sema.addTypeReferenceEntry(src, wip_ty.index);

    // We've finished the initial construction of this type, and are about to perform analysis.
    // Set the namespace appropriately, and don't destroy anything on failure.
    if (zcu.comp.debugIncremental()) try zcu.incremental_debug_state.newType(zcu, wip_ty.index);
    wip_ty.prepare(ip, new_namespace_index);
    done = true;

    try Sema.resolveDeclaredEnum(
        pt,
        wip_ty,
        inst,
        tracked_inst,
        new_namespace_index,
        type_name,
        small,
        body,
        tag_type_ref,
        any_values,
        fields_len,
        sema.code,
        body_end,
    );

    codegen_type: {
        if (zcu.comp.config.use_llvm) break :codegen_type;
        if (block.ownerModule().strip) break :codegen_type;
        // This job depends on any resolve_type_fully jobs queued up before it.
        try zcu.comp.queueJob(.{ .codegen_type = wip_ty.index });
    }
    return Air.internedToRef(wip_ty.index);
}

fn zirUnionDecl(
    sema: *Sema,
    block: *Block,
    extended: Zir.Inst.Extended.InstData,
    inst: Zir.Inst.Index,
) CompileError!Air.Inst.Ref {
    const tracy = trace(@src());
    defer tracy.end();

    const pt = sema.pt;
    const zcu = pt.zcu;
    const gpa = sema.gpa;
    const ip = &zcu.intern_pool;
    const small: Zir.Inst.UnionDecl.Small = @bitCast(extended.small);
    const extra = sema.code.extraData(Zir.Inst.UnionDecl, extended.operand);
    var extra_index: usize = extra.end;

    const tracked_inst = try block.trackZir(inst);
    const src: LazySrcLoc = .{ .base_node_inst = tracked_inst, .offset = LazySrcLoc.Offset.nodeOffset(.zero) };

    extra_index += @intFromBool(small.has_tag_type);
    const captures_len = if (small.has_captures_len) blk: {
        const captures_len = sema.code.extra[extra_index];
        extra_index += 1;
        break :blk captures_len;
    } else 0;
    extra_index += @intFromBool(small.has_body_len);
    const fields_len = if (small.has_fields_len) blk: {
        const fields_len = sema.code.extra[extra_index];
        extra_index += 1;
        break :blk fields_len;
    } else 0;

    const decls_len = if (small.has_decls_len) blk: {
        const decls_len = sema.code.extra[extra_index];
        extra_index += 1;
        break :blk decls_len;
    } else 0;

    const captures = try sema.getCaptures(block, src, extra_index, captures_len);
    extra_index += captures_len * 2;

    const union_init: InternPool.UnionTypeInit = .{
        .flags = .{
            .layout = small.layout,
            .status = .none,
            .runtime_tag = if (small.has_tag_type or small.auto_enum_tag)
                .tagged
            else if (small.layout != .auto)
                .none
            else switch (block.wantSafeTypes()) {
                true => .safety,
                false => .none,
            },
            .any_aligned_fields = small.any_aligned_fields,
            .requires_comptime = .unknown,
            .assumed_runtime_bits = false,
            .assumed_pointer_aligned = false,
            .alignment = .none,
        },
        .fields_len = fields_len,
        .enum_tag_ty = .none, // set later
        .field_types = &.{}, // set later
        .field_aligns = &.{}, // set later
        .key = .{ .declared = .{
            .zir_index = tracked_inst,
            .captures = captures,
        } },
    };
    const wip_ty = switch (try ip.getUnionType(gpa, pt.tid, union_init, false)) {
        .existing => |ty| {
            const new_ty = try pt.ensureTypeUpToDate(ty);

            // Make sure we update the namespace if the declaration is re-analyzed, to pick
            // up on e.g. changed comptime decls.
            try pt.ensureNamespaceUpToDate(Type.fromInterned(new_ty).getNamespaceIndex(zcu));

            try sema.declareDependency(.{ .interned = new_ty });
            try sema.addTypeReferenceEntry(src, new_ty);
            return Air.internedToRef(new_ty);
        },
        .wip => |wip| wip,
    };
    errdefer wip_ty.cancel(ip, pt.tid);

    wip_ty.setName(ip, try sema.createTypeName(
        block,
        small.name_strategy,
        "union",
        inst,
        wip_ty.index,
    ));

    const new_namespace_index: InternPool.NamespaceIndex = try pt.createNamespace(.{
        .parent = block.namespace.toOptional(),
        .owner_type = wip_ty.index,
        .file_scope = block.getFileScopeIndex(zcu),
        .generation = zcu.generation,
    });
    errdefer pt.destroyNamespace(new_namespace_index);

    if (pt.zcu.comp.incremental) {
        try pt.addDependency(.wrap(.{ .type = wip_ty.index }), .{ .src_hash = tracked_inst });
    }

    const decls = sema.code.bodySlice(extra_index, decls_len);
    try pt.scanNamespace(new_namespace_index, decls);

    try zcu.comp.queueJob(.{ .resolve_type_fully = wip_ty.index });
    codegen_type: {
        if (zcu.comp.config.use_llvm) break :codegen_type;
        if (block.ownerModule().strip) break :codegen_type;
        // This job depends on any resolve_type_fully jobs queued up before it.
        try zcu.comp.queueJob(.{ .codegen_type = wip_ty.index });
    }
    try sema.declareDependency(.{ .interned = wip_ty.index });
    try sema.addTypeReferenceEntry(src, wip_ty.index);
    if (zcu.comp.debugIncremental()) try zcu.incremental_debug_state.newType(zcu, wip_ty.index);
    return Air.internedToRef(wip_ty.finish(ip, new_namespace_index));
}

fn zirOpaqueDecl(
    sema: *Sema,
    block: *Block,
    extended: Zir.Inst.Extended.InstData,
    inst: Zir.Inst.Index,
) CompileError!Air.Inst.Ref {
    const tracy = trace(@src());
    defer tracy.end();

    const pt = sema.pt;
    const zcu = pt.zcu;
    const gpa = sema.gpa;
    const ip = &zcu.intern_pool;

    const small: Zir.Inst.OpaqueDecl.Small = @bitCast(extended.small);
    const extra = sema.code.extraData(Zir.Inst.OpaqueDecl, extended.operand);
    var extra_index: usize = extra.end;

    const tracked_inst = try block.trackZir(inst);
    const src: LazySrcLoc = .{ .base_node_inst = tracked_inst, .offset = LazySrcLoc.Offset.nodeOffset(.zero) };

    const captures_len = if (small.has_captures_len) blk: {
        const captures_len = sema.code.extra[extra_index];
        extra_index += 1;
        break :blk captures_len;
    } else 0;

    const decls_len = if (small.has_decls_len) blk: {
        const decls_len = sema.code.extra[extra_index];
        extra_index += 1;
        break :blk decls_len;
    } else 0;

    const captures = try sema.getCaptures(block, src, extra_index, captures_len);
    extra_index += captures_len * 2;

    const opaque_init: InternPool.OpaqueTypeInit = .{
        .key = .{ .declared = .{
            .zir_index = tracked_inst,
            .captures = captures,
        } },
    };
    const wip_ty = switch (try ip.getOpaqueType(gpa, pt.tid, opaque_init)) {
        .existing => |ty| {
            // Make sure we update the namespace if the declaration is re-analyzed, to pick
            // up on e.g. changed comptime decls.
            try pt.ensureNamespaceUpToDate(Type.fromInterned(ty).getNamespaceIndex(zcu));

            try sema.declareDependency(.{ .interned = ty });
            try sema.addTypeReferenceEntry(src, ty);
            return Air.internedToRef(ty);
        },
        .wip => |wip| wip,
    };
    errdefer wip_ty.cancel(ip, pt.tid);

    wip_ty.setName(ip, try sema.createTypeName(
        block,
        small.name_strategy,
        "opaque",
        inst,
        wip_ty.index,
    ));

    const new_namespace_index: InternPool.NamespaceIndex = try pt.createNamespace(.{
        .parent = block.namespace.toOptional(),
        .owner_type = wip_ty.index,
        .file_scope = block.getFileScopeIndex(zcu),
        .generation = zcu.generation,
    });
    errdefer pt.destroyNamespace(new_namespace_index);

    const decls = sema.code.bodySlice(extra_index, decls_len);
    try pt.scanNamespace(new_namespace_index, decls);

    codegen_type: {
        if (zcu.comp.config.use_llvm) break :codegen_type;
        if (block.ownerModule().strip) break :codegen_type;
        // This job depends on any resolve_type_fully jobs queued up before it.
        try zcu.comp.queueJob(.{ .codegen_type = wip_ty.index });
    }
    try sema.addTypeReferenceEntry(src, wip_ty.index);
    if (zcu.comp.debugIncremental()) try zcu.incremental_debug_state.newType(zcu, wip_ty.index);
    return Air.internedToRef(wip_ty.finish(ip, new_namespace_index));
}

fn zirErrorSetDecl(
    sema: *Sema,
    inst: Zir.Inst.Index,
) CompileError!Air.Inst.Ref {
    const tracy = trace(@src());
    defer tracy.end();

    const pt = sema.pt;
    const zcu = pt.zcu;
    const gpa = sema.gpa;
    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].pl_node;
    const extra = sema.code.extraData(Zir.Inst.ErrorSetDecl, inst_data.payload_index);

    var names: InferredErrorSet.NameMap = .{};
    try names.ensureUnusedCapacity(sema.arena, extra.data.fields_len);

    var extra_index: u32 = @intCast(extra.end);
    const extra_index_end = extra_index + extra.data.fields_len;
    while (extra_index < extra_index_end) : (extra_index += 1) {
        const name_index: Zir.NullTerminatedString = @enumFromInt(sema.code.extra[extra_index]);
        const name = sema.code.nullTerminatedString(name_index);
        const name_ip = try zcu.intern_pool.getOrPutString(gpa, pt.tid, name, .no_embedded_nulls);
        _ = try pt.getErrorValue(name_ip);
        const result = names.getOrPutAssumeCapacity(name_ip);
        assert(!result.found_existing); // verified in AstGen
    }

    return Air.internedToRef((try pt.errorSetFromUnsortedNames(names.keys())).toIntern());
}

fn zirRetPtr(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    const tracy = trace(@src());
    defer tracy.end();

    const pt = sema.pt;

    const src = block.nodeOffset(sema.code.instructions.items(.data)[@intFromEnum(inst)].node);

    if (block.isComptime() or try sema.fn_ret_ty.comptimeOnlySema(pt)) {
        try sema.fn_ret_ty.resolveFields(pt);
        return sema.analyzeComptimeAlloc(block, src, sema.fn_ret_ty, .none);
    }

    const target = pt.zcu.getTarget();
    const ptr_type = try pt.ptrTypeSema(.{
        .child = sema.fn_ret_ty.toIntern(),
        .flags = .{ .address_space = target_util.defaultAddressSpace(target, .local) },
    });

    if (block.inlining != null) {
        // We are inlining a function call; this should be emitted as an alloc, not a ret_ptr.
        // TODO when functions gain result location support, the inlining struct in
        // Block should contain the return pointer, and we would pass that through here.
        return block.addTy(.alloc, ptr_type);
    }

    return block.addTy(.ret_ptr, ptr_type);
}

fn zirRef(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    const tracy = trace(@src());
    defer tracy.end();

    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].un_tok;
    const operand = try sema.resolveInst(inst_data.operand);
    return sema.analyzeRef(block, block.tokenOffset(inst_data.src_tok), operand);
}

fn zirEnsureResultUsed(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!void {
    const tracy = trace(@src());
    defer tracy.end();

    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].un_node;
    const operand = try sema.resolveInst(inst_data.operand);
    const src = block.nodeOffset(inst_data.src_node);

    return sema.ensureResultUsed(block, sema.typeOf(operand), src);
}

fn ensureResultUsed(
    sema: *Sema,
    block: *Block,
    ty: Type,
    src: LazySrcLoc,
) CompileError!void {
    const pt = sema.pt;
    const zcu = pt.zcu;
    switch (ty.zigTypeTag(zcu)) {
        .void, .noreturn => return,
        .error_set => return sema.fail(block, src, "error set is ignored", .{}),
        .error_union => {
            const msg = msg: {
                const msg = try sema.errMsg(src, "error union is ignored", .{});
                errdefer msg.destroy(sema.gpa);
                try sema.errNote(src, msg, "consider using 'try', 'catch', or 'if'", .{});
                break :msg msg;
            };
            return sema.failWithOwnedErrorMsg(block, msg);
        },
        else => {
            const msg = msg: {
                const msg = try sema.errMsg(src, "value of type '{}' ignored", .{ty.fmt(pt)});
                errdefer msg.destroy(sema.gpa);
                try sema.errNote(src, msg, "all non-void values must be used", .{});
                try sema.errNote(src, msg, "to discard the value, assign it to '_'", .{});
                break :msg msg;
            };
            return sema.failWithOwnedErrorMsg(block, msg);
        },
    }
}

fn zirEnsureResultNonError(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!void {
    const tracy = trace(@src());
    defer tracy.end();

    const pt = sema.pt;
    const zcu = pt.zcu;
    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].un_node;
    const operand = try sema.resolveInst(inst_data.operand);
    const src = block.nodeOffset(inst_data.src_node);
    const operand_ty = sema.typeOf(operand);
    switch (operand_ty.zigTypeTag(zcu)) {
        .error_set => return sema.fail(block, src, "error set is discarded", .{}),
        .error_union => {
            const msg = msg: {
                const msg = try sema.errMsg(src, "error union is discarded", .{});
                errdefer msg.destroy(sema.gpa);
                try sema.errNote(src, msg, "consider using 'try', 'catch', or 'if'", .{});
                break :msg msg;
            };
            return sema.failWithOwnedErrorMsg(block, msg);
        },
        else => return,
    }
}

fn zirEnsureErrUnionPayloadVoid(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!void {
    const tracy = trace(@src());
    defer tracy.end();

    const pt = sema.pt;
    const zcu = pt.zcu;
    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].un_node;
    const src = block.nodeOffset(inst_data.src_node);
    const operand = try sema.resolveInst(inst_data.operand);
    const operand_ty = sema.typeOf(operand);
    const err_union_ty = if (operand_ty.zigTypeTag(zcu) == .pointer)
        operand_ty.childType(zcu)
    else
        operand_ty;
    if (err_union_ty.zigTypeTag(zcu) != .error_union) return;
    const payload_ty = err_union_ty.errorUnionPayload(zcu).zigTypeTag(zcu);
    if (payload_ty != .void and payload_ty != .noreturn) {
        const msg = msg: {
            const msg = try sema.errMsg(src, "error union payload is ignored", .{});
            errdefer msg.destroy(sema.gpa);
            try sema.errNote(src, msg, "payload value can be explicitly ignored with '|_|'", .{});
            break :msg msg;
        };
        return sema.failWithOwnedErrorMsg(block, msg);
    }
}

fn zirIndexablePtrLen(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    const tracy = trace(@src());
    defer tracy.end();

    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].un_node;
    const src = block.nodeOffset(inst_data.src_node);
    const object = try sema.resolveInst(inst_data.operand);

    return indexablePtrLen(sema, block, src, object);
}

fn indexablePtrLen(
    sema: *Sema,
    block: *Block,
    src: LazySrcLoc,
    object: Air.Inst.Ref,
) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const object_ty = sema.typeOf(object);
    const is_pointer_to = object_ty.isSinglePointer(zcu);
    const indexable_ty = if (is_pointer_to) object_ty.childType(zcu) else object_ty;
    try sema.checkIndexable(block, src, indexable_ty);
    const field_name = try zcu.intern_pool.getOrPutString(sema.gpa, pt.tid, "len", .no_embedded_nulls);
    return sema.fieldVal(block, src, object, field_name, src);
}

fn indexablePtrLenOrNone(
    sema: *Sema,
    block: *Block,
    src: LazySrcLoc,
    operand: Air.Inst.Ref,
) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const operand_ty = sema.typeOf(operand);
    try checkMemOperand(sema, block, src, operand_ty);
    switch (operand_ty.ptrSize(zcu)) {
        .many, .c => return .none,
        .one, .slice => {},
    }
    const field_name = try zcu.intern_pool.getOrPutString(sema.gpa, pt.tid, "len", .no_embedded_nulls);
    return sema.fieldVal(block, src, operand, field_name, src);
}

fn zirAllocExtended(
    sema: *Sema,
    block: *Block,
    extended: Zir.Inst.Extended.InstData,
) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const gpa = sema.gpa;
    const extra = sema.code.extraData(Zir.Inst.AllocExtended, extended.operand);
    const ty_src = block.src(.{ .node_offset_var_decl_ty = extra.data.src_node });
    const align_src = block.src(.{ .node_offset_var_decl_align = extra.data.src_node });
    const init_src = block.src(.{ .node_offset_var_decl_init = extra.data.src_node });
    const small: Zir.Inst.AllocExtended.Small = @bitCast(extended.small);

    var extra_index: usize = extra.end;

    const var_ty: Type = if (small.has_type) blk: {
        const type_ref: Zir.Inst.Ref = @enumFromInt(sema.code.extra[extra_index]);
        extra_index += 1;
        break :blk try sema.resolveType(block, ty_src, type_ref);
    } else undefined;

    const alignment = if (small.has_align) blk: {
        const align_ref: Zir.Inst.Ref = @enumFromInt(sema.code.extra[extra_index]);
        extra_index += 1;
        break :blk try sema.resolveAlign(block, align_src, align_ref);
    } else .none;

    if (block.isComptime() or small.is_comptime) {
        if (small.has_type) {
            return sema.analyzeComptimeAlloc(block, init_src, var_ty, alignment);
        } else {
            try sema.air_instructions.append(gpa, .{
                .tag = .inferred_alloc_comptime,
                .data = .{ .inferred_alloc_comptime = .{
                    .alignment = alignment,
                    .is_const = small.is_const,
                    .ptr = undefined,
                } },
            });
            return @as(Air.Inst.Index, @enumFromInt(sema.air_instructions.len - 1)).toRef();
        }
    }

    if (small.has_type and try var_ty.comptimeOnlySema(pt)) {
        return sema.analyzeComptimeAlloc(block, init_src, var_ty, alignment);
    }

    if (small.has_type) {
        if (!small.is_const) {
            try sema.validateVarType(block, ty_src, var_ty, false);
        }
        const target = pt.zcu.getTarget();
        try var_ty.resolveLayout(pt);
        if (sema.func_is_naked and try var_ty.hasRuntimeBitsSema(pt)) {
            const var_src = block.src(.{ .node_offset_store_ptr = extra.data.src_node });
            return sema.fail(block, var_src, "local variable in naked function", .{});
        }
        const ptr_type = try sema.pt.ptrTypeSema(.{
            .child = var_ty.toIntern(),
            .flags = .{
                .alignment = alignment,
                .address_space = target_util.defaultAddressSpace(target, .local),
            },
        });
        const ptr = try block.addTy(.alloc, ptr_type);
        if (small.is_const) {
            const ptr_inst = ptr.toIndex().?;
            try sema.maybe_comptime_allocs.put(gpa, ptr_inst, .{ .runtime_index = block.runtime_index });
            try sema.base_allocs.put(gpa, ptr_inst, ptr_inst);
        }
        return ptr;
    }

    const result_index = try block.addInstAsIndex(.{
        .tag = .inferred_alloc,
        .data = .{ .inferred_alloc = .{
            .alignment = alignment,
            .is_const = small.is_const,
        } },
    });
    try sema.unresolved_inferred_allocs.putNoClobber(gpa, result_index, .{});
    if (small.is_const) {
        try sema.maybe_comptime_allocs.put(gpa, result_index, .{ .runtime_index = block.runtime_index });
        try sema.base_allocs.put(gpa, result_index, result_index);
    }
    return result_index.toRef();
}

fn zirAllocComptime(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    const tracy = trace(@src());
    defer tracy.end();

    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].un_node;
    const ty_src = block.src(.{ .node_offset_var_decl_ty = inst_data.src_node });
    const init_src = block.src(.{ .node_offset_var_decl_init = inst_data.src_node });
    const var_ty = try sema.resolveType(block, ty_src, inst_data.operand);
    return sema.analyzeComptimeAlloc(block, init_src, var_ty, .none);
}

fn zirMakePtrConst(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].un_node;
    const alloc = try sema.resolveInst(inst_data.operand);
    const alloc_ty = sema.typeOf(alloc);
    const ptr_info = alloc_ty.ptrInfo(zcu);
    const elem_ty: Type = .fromInterned(ptr_info.child);

    // If the alloc was created in a comptime scope, we already created a comptime alloc for it.
    // However, if the final constructed value does not reference comptime-mutable memory, we wish
    // to promote it to an anon decl.
    already_ct: {
        const ptr_val = try sema.resolveValue(alloc) orelse break :already_ct;

        // If this was a comptime inferred alloc, then `storeToInferredAllocComptime`
        // might have already done our job and created an anon decl ref.
        switch (zcu.intern_pool.indexToKey(ptr_val.toIntern())) {
            .ptr => |ptr| switch (ptr.base_addr) {
                .uav => {
                    // The comptime-ification was already done for us.
                    // Just make sure the pointer is const.
                    return sema.makePtrConst(block, alloc);
                },
                else => {},
            },
            else => {},
        }

        if (!sema.isComptimeMutablePtr(ptr_val)) break :already_ct;
        const ptr = zcu.intern_pool.indexToKey(ptr_val.toIntern()).ptr;
        assert(ptr.byte_offset == 0);
        const alloc_index = ptr.base_addr.comptime_alloc;
        const ct_alloc = sema.getComptimeAlloc(alloc_index);
        const interned = try ct_alloc.val.intern(pt, sema.arena);
        if (interned.canMutateComptimeVarState(zcu)) {
            // Preserve the comptime alloc, just make the pointer const.
            ct_alloc.val = .{ .interned = interned.toIntern() };
            ct_alloc.is_const = true;
            return sema.makePtrConst(block, alloc);
        } else {
            // Promote the constant to an anon decl.
            const new_mut_ptr = Air.internedToRef(try pt.intern(.{ .ptr = .{
                .ty = alloc_ty.toIntern(),
                .base_addr = .{ .uav = .{
                    .val = interned.toIntern(),
                    .orig_ty = alloc_ty.toIntern(),
                } },
                .byte_offset = 0,
            } }));
            return sema.makePtrConst(block, new_mut_ptr);
        }
    }

    // Otherwise, check if the alloc is comptime-known despite being in a runtime scope.
    if (try sema.resolveComptimeKnownAllocPtr(block, alloc, null)) |ptr_val| {
        return sema.makePtrConst(block, Air.internedToRef(ptr_val));
    }

    if (try elem_ty.comptimeOnlySema(pt)) {
        // The value was initialized through RLS, so we didn't detect the runtime condition earlier.
        // TODO: source location of runtime control flow
        const init_src = block.src(.{ .node_offset_var_decl_init = inst_data.src_node });
        return sema.fail(block, init_src, "value with comptime-only type '{}' depends on runtime control flow", .{elem_ty.fmt(pt)});
    }

    // This is a runtime value.
    return sema.makePtrConst(block, alloc);
}

/// If `alloc` is an inferred allocation, `resolved_inferred_ty` is taken to be its resolved
/// type. Otherwise, it may be `null`, and the type will be inferred from `alloc`.
fn resolveComptimeKnownAllocPtr(sema: *Sema, block: *Block, alloc: Air.Inst.Ref, resolved_alloc_ty: ?Type) CompileError!?InternPool.Index {
    const pt = sema.pt;
    const zcu = pt.zcu;

    const alloc_ty = resolved_alloc_ty orelse sema.typeOf(alloc);
    const ptr_info = alloc_ty.ptrInfo(zcu);
    const elem_ty: Type = .fromInterned(ptr_info.child);

    const alloc_inst = alloc.toIndex() orelse return null;
    const comptime_info = sema.maybe_comptime_allocs.fetchRemove(alloc_inst) orelse return null;
    const stores = comptime_info.value.stores.items(.inst);

    // Since the entry existed in `maybe_comptime_allocs`, the allocation is comptime-known.
    // We will resolve and return its value.

    // We expect to have emitted at least one store, unless the elem type is OPV.
    if (stores.len == 0) {
        const val = (try sema.typeHasOnePossibleValue(elem_ty)).?.toIntern();
        return sema.finishResolveComptimeKnownAllocPtr(block, alloc_ty, val, null, alloc_inst, comptime_info.value);
    }

    // In general, we want to create a comptime alloc of the correct type and
    // apply the stores to that alloc in order. However, before going to all
    // that effort, let's optimize for the common case of a single store.

    simple: {
        if (stores.len != 1) break :simple;
        const store_inst = sema.air_instructions.get(@intFromEnum(stores[0]));
        switch (store_inst.tag) {
            .store, .store_safe => {},
            .set_union_tag, .optional_payload_ptr_set, .errunion_payload_ptr_set => break :simple, // there's OPV stuff going on!
            else => unreachable,
        }
        if (store_inst.data.bin_op.lhs != alloc) break :simple;

        const val = store_inst.data.bin_op.rhs.toInterned().?;
        assert(zcu.intern_pool.typeOf(val) == elem_ty.toIntern());
        return sema.finishResolveComptimeKnownAllocPtr(block, alloc_ty, val, null, alloc_inst, comptime_info.value);
    }

    // The simple strategy failed: we must create a mutable comptime alloc and
    // perform all of the runtime store operations at comptime.

    const ct_alloc = try sema.newComptimeAlloc(block, .unneeded, elem_ty, ptr_info.flags.alignment);

    const alloc_ptr = try pt.intern(.{ .ptr = .{
        .ty = alloc_ty.toIntern(),
        .base_addr = .{ .comptime_alloc = ct_alloc },
        .byte_offset = 0,
    } });

    // Maps from pointers into the runtime allocs, to comptime-mutable pointers into the comptime alloc
    var ptr_mapping = std.AutoHashMap(Air.Inst.Index, InternPool.Index).init(sema.arena);
    try ptr_mapping.ensureTotalCapacity(@intCast(stores.len));
    ptr_mapping.putAssumeCapacity(alloc_inst, alloc_ptr);

    // Whilst constructing our mapping, we will also initialize optional and error union payloads when
    // we encounter the corresponding pointers. For this reason, the ordering of `to_map` matters.
    var to_map = try std.ArrayList(Air.Inst.Index).initCapacity(sema.arena, stores.len);
    for (stores) |store_inst_idx| {
        const store_inst = sema.air_instructions.get(@intFromEnum(store_inst_idx));
        const ptr_to_map = switch (store_inst.tag) {
            .store, .store_safe => store_inst.data.bin_op.lhs.toIndex().?, // Map the pointer being stored to.
            .set_union_tag => continue, // We can completely ignore these: we'll do it implicitly when we get the field pointer.
            .optional_payload_ptr_set, .errunion_payload_ptr_set => store_inst_idx, // Map the generated pointer itself.
            else => unreachable,
        };
        to_map.appendAssumeCapacity(ptr_to_map);
    }

    const tmp_air = sema.getTmpAir();

    while (to_map.pop()) |air_ptr| {
        if (ptr_mapping.contains(air_ptr)) continue;
        const PointerMethod = union(enum) {
            same_addr,
            opt_payload,
            eu_payload,
            field: u32,
            elem: u64,
        };
        const inst_tag = tmp_air.instructions.items(.tag)[@intFromEnum(air_ptr)];
        const air_parent_ptr: Air.Inst.Ref, const method: PointerMethod = switch (inst_tag) {
            .struct_field_ptr => blk: {
                const data = tmp_air.extraData(
                    Air.StructField,
                    tmp_air.instructions.items(.data)[@intFromEnum(air_ptr)].ty_pl.payload,
                ).data;
                break :blk .{
                    data.struct_operand,
                    .{ .field = data.field_index },
                };
            },
            .struct_field_ptr_index_0,
            .struct_field_ptr_index_1,
            .struct_field_ptr_index_2,
            .struct_field_ptr_index_3,
            => .{
                tmp_air.instructions.items(.data)[@intFromEnum(air_ptr)].ty_op.operand,
                .{ .field = switch (inst_tag) {
                    .struct_field_ptr_index_0 => 0,
                    .struct_field_ptr_index_1 => 1,
                    .struct_field_ptr_index_2 => 2,
                    .struct_field_ptr_index_3 => 3,
                    else => unreachable,
                } },
            },
            .ptr_slice_ptr_ptr => .{
                tmp_air.instructions.items(.data)[@intFromEnum(air_ptr)].ty_op.operand,
                .{ .field = Value.slice_ptr_index },
            },
            .ptr_slice_len_ptr => .{
                tmp_air.instructions.items(.data)[@intFromEnum(air_ptr)].ty_op.operand,
                .{ .field = Value.slice_len_index },
            },
            .ptr_elem_ptr => blk: {
                const data = tmp_air.extraData(
                    Air.Bin,
                    tmp_air.instructions.items(.data)[@intFromEnum(air_ptr)].ty_pl.payload,
                ).data;
                const idx_val = (try sema.resolveValue(data.rhs)).?;
                break :blk .{
                    data.lhs,
                    .{ .elem = try idx_val.toUnsignedIntSema(pt) },
                };
            },
            .bitcast => .{
                tmp_air.instructions.items(.data)[@intFromEnum(air_ptr)].ty_op.operand,
                .same_addr,
            },
            .optional_payload_ptr_set => .{
                tmp_air.instructions.items(.data)[@intFromEnum(air_ptr)].ty_op.operand,
                .opt_payload,
            },
            .errunion_payload_ptr_set => .{
                tmp_air.instructions.items(.data)[@intFromEnum(air_ptr)].ty_op.operand,
                .eu_payload,
            },
            else => unreachable,
        };

        const decl_parent_ptr = ptr_mapping.get(air_parent_ptr.toIndex().?) orelse {
            // Resolve the parent pointer first.
            // Note that we add in what seems like the wrong order, because we're popping from the end of this array.
            try to_map.appendSlice(&.{ air_ptr, air_parent_ptr.toIndex().? });
            continue;
        };
        const new_ptr_ty = tmp_air.typeOfIndex(air_ptr, &zcu.intern_pool).toIntern();
        const new_ptr = switch (method) {
            .same_addr => try zcu.intern_pool.getCoerced(sema.gpa, pt.tid, decl_parent_ptr, new_ptr_ty),
            .opt_payload => ptr: {
                // Set the optional to non-null at comptime.
                // If the payload is OPV, we must use that value instead of undef.
                const opt_ty = Value.fromInterned(decl_parent_ptr).typeOf(zcu).childType(zcu);
                const payload_ty = opt_ty.optionalChild(zcu);
                const payload_val = try sema.typeHasOnePossibleValue(payload_ty) orelse try pt.undefValue(payload_ty);
                const opt_val = try pt.intern(.{ .opt = .{
                    .ty = opt_ty.toIntern(),
                    .val = payload_val.toIntern(),
                } });
                try sema.storePtrVal(block, LazySrcLoc.unneeded, Value.fromInterned(decl_parent_ptr), Value.fromInterned(opt_val), opt_ty);
                break :ptr (try Value.fromInterned(decl_parent_ptr).ptrOptPayload(pt)).toIntern();
            },
            .eu_payload => ptr: {
                // Set the error union to non-error at comptime.
                // If the payload is OPV, we must use that value instead of undef.
                const eu_ty = Value.fromInterned(decl_parent_ptr).typeOf(zcu).childType(zcu);
                const payload_ty = eu_ty.errorUnionPayload(zcu);
                const payload_val = try sema.typeHasOnePossibleValue(payload_ty) orelse try pt.undefValue(payload_ty);
                const eu_val = try pt.intern(.{ .error_union = .{
                    .ty = eu_ty.toIntern(),
                    .val = .{ .payload = payload_val.toIntern() },
                } });
                try sema.storePtrVal(block, LazySrcLoc.unneeded, Value.fromInterned(decl_parent_ptr), Value.fromInterned(eu_val), eu_ty);
                break :ptr (try Value.fromInterned(decl_parent_ptr).ptrEuPayload(pt)).toIntern();
            },
            .field => |idx| ptr: {
                const maybe_union_ty = Value.fromInterned(decl_parent_ptr).typeOf(zcu).childType(zcu);
                if (zcu.typeToUnion(maybe_union_ty)) |union_obj| {
                    // As this is a union field, we must store to the pointer now to set the tag.
                    // If the payload is OPV, there will not be a payload store, so we store that value.
                    // Otherwise, there will be a payload store to process later, so undef will suffice.
                    const payload_ty: Type = .fromInterned(union_obj.field_types.get(&zcu.intern_pool)[idx]);
                    const payload_val = try sema.typeHasOnePossibleValue(payload_ty) orelse try pt.undefValue(payload_ty);
                    const tag_val = try pt.enumValueFieldIndex(.fromInterned(union_obj.enum_tag_ty), idx);
                    const store_val = try pt.unionValue(maybe_union_ty, tag_val, payload_val);
                    try sema.storePtrVal(block, LazySrcLoc.unneeded, Value.fromInterned(decl_parent_ptr), store_val, maybe_union_ty);
                }
                break :ptr (try Value.fromInterned(decl_parent_ptr).ptrField(idx, pt)).toIntern();
            },
            .elem => |idx| (try Value.fromInterned(decl_parent_ptr).ptrElem(idx, pt)).toIntern(),
        };
        try ptr_mapping.put(air_ptr, new_ptr);
    }

    // We have a correlation between AIR pointers and decl pointers. Perform all stores at comptime.
    // Any implicit stores performed by `optional_payload_ptr_set`, `errunion_payload_ptr_set`, or
    // `set_union_tag` instructions were already done above.

    for (stores) |store_inst_idx| {
        const store_inst = sema.air_instructions.get(@intFromEnum(store_inst_idx));
        switch (store_inst.tag) {
            .set_union_tag => {}, // Handled implicitly by field pointers above
            .optional_payload_ptr_set, .errunion_payload_ptr_set => {}, // Handled explicitly above
            .store, .store_safe => {
                const air_ptr_inst = store_inst.data.bin_op.lhs.toIndex().?;
                const store_val = (try sema.resolveValue(store_inst.data.bin_op.rhs)).?;
                const new_ptr = ptr_mapping.get(air_ptr_inst).?;
                try sema.storePtrVal(block, LazySrcLoc.unneeded, Value.fromInterned(new_ptr), store_val, .fromInterned(zcu.intern_pool.typeOf(store_val.toIntern())));
            },
            else => unreachable,
        }
    }

    // The value is finalized - load it!
    const val = (try sema.pointerDeref(block, LazySrcLoc.unneeded, Value.fromInterned(alloc_ptr), alloc_ty)).?.toIntern();
    return sema.finishResolveComptimeKnownAllocPtr(block, alloc_ty, val, ct_alloc, alloc_inst, comptime_info.value);
}

/// Given the resolved comptime-known value, rewrites the dead AIR to not
/// create a runtime stack allocation. Also places the resulting value into
/// either an anon decl ref or a comptime alloc depending on whether it
/// references comptime-mutable memory. If `existing_comptime_alloc` is
/// passed, it is a scratch allocation which already contains `result_val`.
/// Same return type as `resolveComptimeKnownAllocPtr` so we can tail call.
fn finishResolveComptimeKnownAllocPtr(
    sema: *Sema,
    block: *Block,
    alloc_ty: Type,
    result_val: InternPool.Index,
    existing_comptime_alloc: ?ComptimeAllocIndex,
    alloc_inst: Air.Inst.Index,
    comptime_info: MaybeComptimeAlloc,
) CompileError!?InternPool.Index {
    const pt = sema.pt;
    const zcu = pt.zcu;

    // We're almost done - we have the resolved comptime value. We just need to
    // eliminate the now-dead runtime instructions.

    // We will rewrite the AIR to eliminate the alloc and all stores to it.
    // This will cause instructions deriving field pointers etc of the alloc to
    // become invalid, however, since we are removing all stores to those pointers,
    // they will be eliminated by Liveness before they reach codegen.

    // The specifics of this instruction aren't really important: we just want
    // Liveness to elide it.
    const nop_inst: Air.Inst = .{ .tag = .bitcast, .data = .{ .ty_op = .{ .ty = .u8_type, .operand = .zero_u8 } } };

    sema.air_instructions.set(@intFromEnum(alloc_inst), nop_inst);
    for (comptime_info.stores.items(.inst)) |store_inst| {
        sema.air_instructions.set(@intFromEnum(store_inst), nop_inst);
    }

    if (Value.fromInterned(result_val).canMutateComptimeVarState(zcu)) {
        const alloc_index = existing_comptime_alloc orelse a: {
            const idx = try sema.newComptimeAlloc(block, .unneeded, alloc_ty.childType(zcu), alloc_ty.ptrAlignment(zcu));
            const alloc = sema.getComptimeAlloc(idx);
            alloc.val = .{ .interned = result_val };
            break :a idx;
        };
        sema.getComptimeAlloc(alloc_index).is_const = true;
        return try pt.intern(.{ .ptr = .{
            .ty = alloc_ty.toIntern(),
            .base_addr = .{ .comptime_alloc = alloc_index },
            .byte_offset = 0,
        } });
    } else {
        return try pt.intern(.{ .ptr = .{
            .ty = alloc_ty.toIntern(),
            .base_addr = .{ .uav = .{
                .orig_ty = alloc_ty.toIntern(),
                .val = result_val,
            } },
            .byte_offset = 0,
        } });
    }
}

fn makePtrTyConst(sema: *Sema, ptr_ty: Type) CompileError!Type {
    var ptr_info = ptr_ty.ptrInfo(sema.pt.zcu);
    ptr_info.flags.is_const = true;
    return sema.pt.ptrTypeSema(ptr_info);
}

fn makePtrConst(sema: *Sema, block: *Block, alloc: Air.Inst.Ref) CompileError!Air.Inst.Ref {
    const alloc_ty = sema.typeOf(alloc);
    const const_ptr_ty = try sema.makePtrTyConst(alloc_ty);

    // Detect if a comptime value simply needs to have its type changed.
    if (try sema.resolveValue(alloc)) |val| {
        return Air.internedToRef((try sema.pt.getCoerced(val, const_ptr_ty)).toIntern());
    }

    return block.addBitCast(const_ptr_ty, alloc);
}

fn zirAllocInferredComptime(
    sema: *Sema,
    is_const: bool,
) CompileError!Air.Inst.Ref {
    const gpa = sema.gpa;

    try sema.air_instructions.append(gpa, .{
        .tag = .inferred_alloc_comptime,
        .data = .{ .inferred_alloc_comptime = .{
            .alignment = .none,
            .is_const = is_const,
            .ptr = undefined,
        } },
    });
    return @as(Air.Inst.Index, @enumFromInt(sema.air_instructions.len - 1)).toRef();
}

fn zirAlloc(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    const tracy = trace(@src());
    defer tracy.end();

    const pt = sema.pt;

    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].un_node;
    const ty_src = block.src(.{ .node_offset_var_decl_ty = inst_data.src_node });
    const init_src = block.src(.{ .node_offset_var_decl_init = inst_data.src_node });

    const var_ty = try sema.resolveType(block, ty_src, inst_data.operand);
    if (block.isComptime() or try var_ty.comptimeOnlySema(pt)) {
        return sema.analyzeComptimeAlloc(block, init_src, var_ty, .none);
    }
    if (sema.func_is_naked and try var_ty.hasRuntimeBitsSema(pt)) {
        const mut_src = block.src(.{ .node_offset_store_ptr = inst_data.src_node });
        return sema.fail(block, mut_src, "local variable in naked function", .{});
    }
    const target = pt.zcu.getTarget();
    const ptr_type = try pt.ptrTypeSema(.{
        .child = var_ty.toIntern(),
        .flags = .{ .address_space = target_util.defaultAddressSpace(target, .local) },
    });
    const ptr = try block.addTy(.alloc, ptr_type);
    const ptr_inst = ptr.toIndex().?;
    try sema.maybe_comptime_allocs.put(sema.gpa, ptr_inst, .{ .runtime_index = block.runtime_index });
    try sema.base_allocs.put(sema.gpa, ptr_inst, ptr_inst);
    return ptr;
}

fn zirAllocMut(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    const tracy = trace(@src());
    defer tracy.end();

    const pt = sema.pt;

    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].un_node;
    const ty_src = block.src(.{ .node_offset_var_decl_ty = inst_data.src_node });
    const init_src = block.src(.{ .node_offset_var_decl_init = inst_data.src_node });
    const var_ty = try sema.resolveType(block, ty_src, inst_data.operand);
    if (block.isComptime()) {
        return sema.analyzeComptimeAlloc(block, init_src, var_ty, .none);
    }
    if (sema.func_is_naked and try var_ty.hasRuntimeBitsSema(pt)) {
        const var_src = block.src(.{ .node_offset_store_ptr = inst_data.src_node });
        return sema.fail(block, var_src, "local variable in naked function", .{});
    }
    try sema.validateVarType(block, ty_src, var_ty, false);
    const target = pt.zcu.getTarget();
    const ptr_type = try pt.ptrTypeSema(.{
        .child = var_ty.toIntern(),
        .flags = .{ .address_space = target_util.defaultAddressSpace(target, .local) },
    });
    return block.addTy(.alloc, ptr_type);
}

fn zirAllocInferred(
    sema: *Sema,
    block: *Block,
    is_const: bool,
) CompileError!Air.Inst.Ref {
    const tracy = trace(@src());
    defer tracy.end();

    const gpa = sema.gpa;

    if (block.isComptime()) {
        try sema.air_instructions.append(gpa, .{
            .tag = .inferred_alloc_comptime,
            .data = .{ .inferred_alloc_comptime = .{
                .alignment = .none,
                .is_const = is_const,
                .ptr = undefined,
            } },
        });
        return @as(Air.Inst.Index, @enumFromInt(sema.air_instructions.len - 1)).toRef();
    }

    const result_index = try block.addInstAsIndex(.{
        .tag = .inferred_alloc,
        .data = .{ .inferred_alloc = .{
            .alignment = .none,
            .is_const = is_const,
        } },
    });
    try sema.unresolved_inferred_allocs.putNoClobber(gpa, result_index, .{});
    if (is_const) {
        try sema.maybe_comptime_allocs.put(gpa, result_index, .{ .runtime_index = block.runtime_index });
        try sema.base_allocs.put(sema.gpa, result_index, result_index);
    }
    return result_index.toRef();
}

fn zirResolveInferredAlloc(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    const tracy = trace(@src());
    defer tracy.end();

    const pt = sema.pt;
    const zcu = pt.zcu;
    const gpa = sema.gpa;
    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].un_node;
    const src = block.nodeOffset(inst_data.src_node);
    const ty_src = block.src(.{ .node_offset_var_decl_ty = inst_data.src_node });
    const ptr = try sema.resolveInst(inst_data.operand);
    const ptr_inst = ptr.toIndex().?;
    const target = zcu.getTarget();

    switch (sema.air_instructions.items(.tag)[@intFromEnum(ptr_inst)]) {
        .inferred_alloc_comptime => {
            // The work was already done for us by `Sema.storeToInferredAllocComptime`.
            // All we need to do is return the pointer.
            const iac = sema.air_instructions.items(.data)[@intFromEnum(ptr_inst)].inferred_alloc_comptime;
            const resolved_ptr = iac.ptr;

            if (std.debug.runtime_safety) {
                // The inferred_alloc_comptime should never be referenced again
                sema.air_instructions.set(@intFromEnum(ptr_inst), .{ .tag = undefined, .data = undefined });
            }

            const val = switch (zcu.intern_pool.indexToKey(resolved_ptr).ptr.base_addr) {
                .uav => |a| a.val,
                .comptime_alloc => |i| val: {
                    const alloc = sema.getComptimeAlloc(i);
                    break :val (try alloc.val.intern(pt, sema.arena)).toIntern();
                },
                else => unreachable,
            };
            if (zcu.intern_pool.isFuncBody(val)) {
                const ty: Type = .fromInterned(zcu.intern_pool.typeOf(val));
                if (try ty.fnHasRuntimeBitsSema(pt)) {
                    try sema.addReferenceEntry(block, src, AnalUnit.wrap(.{ .func = val }));
                    try zcu.ensureFuncBodyAnalysisQueued(val);
                }
            }

            return Air.internedToRef(resolved_ptr);
        },
        .inferred_alloc => {
            const ia1 = sema.air_instructions.items(.data)[@intFromEnum(ptr_inst)].inferred_alloc;
            const ia2 = sema.unresolved_inferred_allocs.fetchSwapRemove(ptr_inst).?.value;
            const peer_vals = try sema.arena.alloc(Air.Inst.Ref, ia2.prongs.items.len);
            for (peer_vals, ia2.prongs.items) |*peer_val, store_inst| {
                assert(sema.air_instructions.items(.tag)[@intFromEnum(store_inst)] == .store);
                const bin_op = sema.air_instructions.items(.data)[@intFromEnum(store_inst)].bin_op;
                peer_val.* = bin_op.rhs;
            }
            const final_elem_ty = try sema.resolvePeerTypes(block, ty_src, peer_vals, .none);

            const final_ptr_ty = try pt.ptrTypeSema(.{
                .child = final_elem_ty.toIntern(),
                .flags = .{
                    .alignment = ia1.alignment,
                    .address_space = target_util.defaultAddressSpace(target, .local),
                },
            });

            if (!ia1.is_const) {
                try sema.validateVarType(block, ty_src, final_elem_ty, false);
            } else if (try sema.resolveComptimeKnownAllocPtr(block, ptr, final_ptr_ty)) |ptr_val| {
                const const_ptr_ty = try sema.makePtrTyConst(final_ptr_ty);
                const new_const_ptr = try pt.getCoerced(Value.fromInterned(ptr_val), const_ptr_ty);

                // Unless the block is comptime, `alloc_inferred` always produces
                // a runtime constant. The final inferred type needs to be
                // fully resolved so it can be lowered in codegen.
                try final_elem_ty.resolveFully(pt);

                return Air.internedToRef(new_const_ptr.toIntern());
            }

            if (try final_elem_ty.comptimeOnlySema(pt)) {
                // The alloc wasn't comptime-known per the above logic, so the
                // type cannot be comptime-only.
                // TODO: source location of runtime control flow
                return sema.fail(block, src, "value with comptime-only type '{}' depends on runtime control flow", .{final_elem_ty.fmt(pt)});
            }
            if (sema.func_is_naked and try final_elem_ty.hasRuntimeBitsSema(pt)) {
                const mut_src = block.src(.{ .node_offset_store_ptr = inst_data.src_node });
                return sema.fail(block, mut_src, "local variable in naked function", .{});
            }
            // Change it to a normal alloc.
            sema.air_instructions.set(@intFromEnum(ptr_inst), .{
                .tag = .alloc,
                .data = .{ .ty = final_ptr_ty },
            });

            // Now we need to go back over all the store instructions, and do the logic as if
            // the new result ptr type was available.

            for (ia2.prongs.items) |placeholder_inst| {
                var replacement_block = block.makeSubBlock();
                defer replacement_block.instructions.deinit(gpa);

                assert(sema.air_instructions.items(.tag)[@intFromEnum(placeholder_inst)] == .store);
                const bin_op = sema.air_instructions.items(.data)[@intFromEnum(placeholder_inst)].bin_op;
                try sema.storePtr2(&replacement_block, src, bin_op.lhs, src, bin_op.rhs, src, .store);

                // If only one instruction is produced then we can replace the store
                // placeholder instruction with this instruction; no need for an entire block.
                if (replacement_block.instructions.items.len == 1) {
                    const only_inst = replacement_block.instructions.items[0];
                    sema.air_instructions.set(@intFromEnum(placeholder_inst), sema.air_instructions.get(@intFromEnum(only_inst)));
                    continue;
                }

                // Here we replace the placeholder store instruction with a block
                // that does the actual store logic.
                _ = try replacement_block.addBr(placeholder_inst, .void_value);
                try sema.air_extra.ensureUnusedCapacity(
                    gpa,
                    @typeInfo(Air.Block).@"struct".fields.len + replacement_block.instructions.items.len,
                );
                sema.air_instructions.set(@intFromEnum(placeholder_inst), .{
                    .tag = .block,
                    .data = .{ .ty_pl = .{
                        .ty = .void_type,
                        .payload = sema.addExtraAssumeCapacity(Air.Block{
                            .body_len = @intCast(replacement_block.instructions.items.len),
                        }),
                    } },
                });
                sema.air_extra.appendSliceAssumeCapacity(@ptrCast(replacement_block.instructions.items));
            }

            if (ia1.is_const) {
                return sema.makePtrConst(block, ptr);
            } else {
                return ptr;
            }
        },
        else => unreachable,
    }
}

fn zirForLen(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const gpa = sema.gpa;
    const ip = &zcu.intern_pool;
    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].pl_node;
    const extra = sema.code.extraData(Zir.Inst.MultiOp, inst_data.payload_index);
    const all_args = sema.code.refSlice(extra.end, extra.data.operands_len);
    const arg_pairs: []const [2]Zir.Inst.Ref = @as([*]const [2]Zir.Inst.Ref, @ptrCast(all_args))[0..@divExact(all_args.len, 2)];
    const src = block.nodeOffset(inst_data.src_node);

    var len: Air.Inst.Ref = .none;
    var len_val: ?Value = null;
    var len_idx: u32 = undefined;
    var any_runtime = false;

    const runtime_arg_lens = try gpa.alloc(Air.Inst.Ref, arg_pairs.len);
    defer gpa.free(runtime_arg_lens);

    // First pass to look for comptime values.
    for (arg_pairs, 0..) |zir_arg_pair, i_usize| {
        const i: u32 = @intCast(i_usize);
        runtime_arg_lens[i] = .none;
        if (zir_arg_pair[0] == .none) continue;

        const arg_src = block.src(.{ .for_input = .{
            .for_node_offset = inst_data.src_node,
            .input_index = i,
        } });

        const arg_len_uncoerced = if (zir_arg_pair[1] == .none) l: {
            // This argument is an indexable.
            const object = try sema.resolveInst(zir_arg_pair[0]);
            const object_ty = sema.typeOf(object);
            if (!object_ty.isIndexable(zcu)) {
                // Instead of using checkIndexable we customize this error.
                const msg = msg: {
                    const msg = try sema.errMsg(arg_src, "type '{}' is not indexable and not a range", .{object_ty.fmt(pt)});
                    errdefer msg.destroy(sema.gpa);
                    try sema.errNote(arg_src, msg, "for loop operand must be a range, array, slice, tuple, or vector", .{});

                    if (object_ty.zigTypeTag(zcu) == .error_union) {
                        try sema.errNote(arg_src, msg, "consider using 'try', 'catch', or 'if'", .{});
                    }

                    break :msg msg;
                };
                return sema.failWithOwnedErrorMsg(block, msg);
            }
            if (!object_ty.indexableHasLen(zcu)) continue;
            break :l try sema.fieldVal(block, arg_src, object, try ip.getOrPutString(gpa, pt.tid, "len", .no_embedded_nulls), arg_src);
        } else l: {
            // This argument is a range.
            const range_start = try sema.resolveInst(zir_arg_pair[0]);
            const range_end = try sema.resolveInst(zir_arg_pair[1]);
            break :l try sema.analyzeArithmetic(block, .sub, range_end, range_start, arg_src, arg_src, arg_src, true);
        };
        const arg_len = try sema.coerce(block, .usize, arg_len_uncoerced, arg_src);
        if (len == .none) {
            len = arg_len;
            len_idx = i;
        }
        if (try sema.resolveDefinedValue(block, src, arg_len)) |arg_val| {
            if (len_val) |v| {
                if (!(try sema.valuesEqual(arg_val, v, .usize))) {
                    const msg = msg: {
                        const msg = try sema.errMsg(src, "non-matching for loop lengths", .{});
                        errdefer msg.destroy(gpa);
                        const a_src = block.src(.{ .for_input = .{
                            .for_node_offset = inst_data.src_node,
                            .input_index = len_idx,
                        } });
                        try sema.errNote(a_src, msg, "length {} here", .{
                            v.fmtValueSema(pt, sema),
                        });
                        try sema.errNote(arg_src, msg, "length {} here", .{
                            arg_val.fmtValueSema(pt, sema),
                        });
                        break :msg msg;
                    };
                    return sema.failWithOwnedErrorMsg(block, msg);
                }
            } else {
                len = arg_len;
                len_val = arg_val;
                len_idx = i;
            }
            continue;
        }
        runtime_arg_lens[i] = arg_len;
        any_runtime = true;
    }

    if (len == .none) {
        const msg = msg: {
            const msg = try sema.errMsg(src, "unbounded for loop", .{});
            errdefer msg.destroy(gpa);
            for (arg_pairs, 0..) |zir_arg_pair, i_usize| {
                const i: u32 = @intCast(i_usize);
                if (zir_arg_pair[0] == .none) continue;
                if (zir_arg_pair[1] != .none) continue;
                const object = try sema.resolveInst(zir_arg_pair[0]);
                const object_ty = sema.typeOf(object);
                const arg_src = block.src(.{ .for_input = .{
                    .for_node_offset = inst_data.src_node,
                    .input_index = i,
                } });
                try sema.errNote(arg_src, msg, "type '{}' has no upper bound", .{
                    object_ty.fmt(pt),
                });
            }
            break :msg msg;
        };
        return sema.failWithOwnedErrorMsg(block, msg);
    }

    // Now for the runtime checks.
    if (any_runtime and block.wantSafety()) {
        for (runtime_arg_lens, 0..) |arg_len, i| {
            if (arg_len == .none) continue;
            if (i == len_idx) continue;
            const ok = try block.addBinOp(.cmp_eq, len, arg_len);
            try sema.addSafetyCheck(block, src, ok, .for_len_mismatch);
        }
    }

    return len;
}

/// Given any single pointer, retrieve a pointer to the payload of any optional
/// or error union pointed to, initializing these pointers along the way.
/// Given a `*E!?T`, returns a (valid) `*T`.
/// May invalidate already-stored payload data.
fn optEuBasePtrInit(sema: *Sema, block: *Block, ptr: Air.Inst.Ref, src: LazySrcLoc) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    var base_ptr = ptr;
    while (true) switch (sema.typeOf(base_ptr).childType(zcu).zigTypeTag(zcu)) {
        .error_union => base_ptr = try sema.analyzeErrUnionPayloadPtr(block, src, base_ptr, false, true),
        .optional => base_ptr = try sema.analyzeOptionalPayloadPtr(block, src, base_ptr, false, true),
        else => break,
    };
    try sema.checkKnownAllocPtr(block, ptr, base_ptr);
    return base_ptr;
}

fn zirOptEuBasePtrInit(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    const un_node = sema.code.instructions.items(.data)[@intFromEnum(inst)].un_node;
    const ptr = try sema.resolveInst(un_node.operand);
    return sema.optEuBasePtrInit(block, ptr, block.nodeOffset(un_node.src_node));
}

fn zirCoercePtrElemTy(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const pl_node = sema.code.instructions.items(.data)[@intFromEnum(inst)].pl_node;
    const src = block.nodeOffset(pl_node.src_node);
    const extra = sema.code.extraData(Zir.Inst.Bin, pl_node.payload_index).data;
    const uncoerced_val = try sema.resolveInst(extra.rhs);
    const maybe_wrapped_ptr_ty = try sema.resolveTypeOrPoison(block, LazySrcLoc.unneeded, extra.lhs) orelse return uncoerced_val;
    const ptr_ty = maybe_wrapped_ptr_ty.optEuBaseType(zcu);
    assert(ptr_ty.zigTypeTag(zcu) == .pointer); // validated by a previous instruction
    const elem_ty = ptr_ty.childType(zcu);
    switch (ptr_ty.ptrSize(zcu)) {
        .one => {
            const uncoerced_ty = sema.typeOf(uncoerced_val);
            if (elem_ty.zigTypeTag(zcu) == .array and elem_ty.childType(zcu).toIntern() == uncoerced_ty.toIntern()) {
                // We're trying to initialize a *[1]T with a reference to a T - don't perform any coercion.
                return uncoerced_val;
            }
            // If the destination type is anyopaque, don't coerce - the pointer will coerce instead.
            if (elem_ty.toIntern() == .anyopaque_type) {
                return uncoerced_val;
            } else {
                return sema.coerce(block, elem_ty, uncoerced_val, src);
            }
        },
        .slice, .many => {
            // Our goal is to coerce `uncoerced_val` to an array of `elem_ty`.
            const val_ty = sema.typeOf(uncoerced_val);
            switch (val_ty.zigTypeTag(zcu)) {
                .array, .vector => {},
                else => if (!val_ty.isTuple(zcu)) {
                    return sema.fail(block, src, "expected array of '{}', found '{}'", .{ elem_ty.fmt(pt), val_ty.fmt(pt) });
                },
            }
            const want_ty = try pt.arrayType(.{
                .len = val_ty.arrayLen(zcu),
                .child = elem_ty.toIntern(),
                .sentinel = if (ptr_ty.sentinel(zcu)) |s| s.toIntern() else .none,
            });
            return sema.coerce(block, want_ty, uncoerced_val, src);
        },
        .c => {
            // There's nothing meaningful to do here, because we don't know if this is meant to be a
            // single-pointer or a many-pointer.
            return uncoerced_val;
        },
    }
}

fn zirTryOperandTy(sema: *Sema, block: *Block, inst: Zir.Inst.Index, is_ref: bool) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const un_node = sema.code.instructions.items(.data)[@intFromEnum(inst)].un_node;
    const src = block.nodeOffset(un_node.src_node);

    const operand_ty = try sema.resolveTypeOrPoison(block, src, un_node.operand) orelse return .generic_poison_type;

    const payload_ty = if (is_ref) ty: {
        if (!operand_ty.isSinglePointer(zcu)) {
            return .generic_poison_type; // we can't get a meaningful result type here, since it will be `*E![n]T`, and we don't know `n`.
        }
        break :ty operand_ty.childType(zcu);
    } else operand_ty;

    const err_set_ty: Type = err_set: {
        // There are awkward cases, like `?E`. Our strategy is to repeatedly unwrap optionals
        // until we hit an error union or set.
        var cur_ty = sema.fn_ret_ty;
        while (true) {
            switch (cur_ty.zigTypeTag(zcu)) {
                .error_set => break :err_set cur_ty,
                .error_union => break :err_set cur_ty.errorUnionSet(zcu),
                .optional => cur_ty = cur_ty.optionalChild(zcu),
                else => {
                    // This function cannot return an error.
                    // `try` is still valid if the error case is impossible, i.e. no error is returned.
                    // So, the result type has an error set of `error{}`.
                    break :err_set .fromInterned(try zcu.intern_pool.getErrorSetType(zcu.gpa, pt.tid, &.{}));
                },
            }
        }
    };

    const eu_ty = try pt.errorUnionType(err_set_ty, payload_ty);

    if (is_ref) {
        var ptr_info = operand_ty.ptrInfo(zcu);
        ptr_info.child = eu_ty.toIntern();
        const eu_ptr_ty = try pt.ptrTypeSema(ptr_info);
        return Air.internedToRef(eu_ptr_ty.toIntern());
    } else {
        return Air.internedToRef(eu_ty.toIntern());
    }
}

fn zirValidateRefTy(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!void {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const un_tok = sema.code.instructions.items(.data)[@intFromEnum(inst)].un_tok;
    const src = block.tokenOffset(un_tok.src_tok);
    // In case of GenericPoison, we don't actually have a type, so this will be
    // treated as an untyped address-of operator.
    const ty_operand = try sema.resolveTypeOrPoison(block, src, un_tok.operand) orelse return;
    if (ty_operand.optEuBaseType(zcu).zigTypeTag(zcu) != .pointer) {
        return sema.failWithOwnedErrorMsg(block, msg: {
            const msg = try sema.errMsg(src, "expected type '{}', found pointer", .{ty_operand.fmt(pt)});
            errdefer msg.destroy(sema.gpa);
            try sema.errNote(src, msg, "address-of operator always returns a pointer", .{});
            break :msg msg;
        });
    }
}

fn zirValidateConst(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!void {
    if (!block.isComptime()) return;

    const un_node = sema.code.instructions.items(.data)[@intFromEnum(inst)].un_node;
    const src = block.nodeOffset(un_node.src_node);
    const init_ref = try sema.resolveInst(un_node.operand);
    if (!try sema.isComptimeKnown(init_ref)) {
        return sema.failWithNeededComptime(block, src, null);
    }
}

fn zirValidateArrayInitRefTy(
    sema: *Sema,
    block: *Block,
    inst: Zir.Inst.Index,
) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const pl_node = sema.code.instructions.items(.data)[@intFromEnum(inst)].pl_node;
    const src = block.nodeOffset(pl_node.src_node);
    const extra = sema.code.extraData(Zir.Inst.ArrayInitRefTy, pl_node.payload_index).data;
    const maybe_wrapped_ptr_ty = try sema.resolveTypeOrPoison(block, LazySrcLoc.unneeded, extra.ptr_ty) orelse return .generic_poison_type;
    const ptr_ty = maybe_wrapped_ptr_ty.optEuBaseType(zcu);
    assert(ptr_ty.zigTypeTag(zcu) == .pointer); // validated by a previous instruction
    switch (zcu.intern_pool.indexToKey(ptr_ty.toIntern())) {
        .ptr_type => |ptr_type| switch (ptr_type.flags.size) {
            .slice, .many => {
                // Use array of correct length
                const arr_ty = try pt.arrayType(.{
                    .len = extra.elem_count,
                    .child = ptr_ty.childType(zcu).toIntern(),
                    .sentinel = if (ptr_ty.sentinel(zcu)) |s| s.toIntern() else .none,
                });
                return Air.internedToRef(arr_ty.toIntern());
            },
            else => {},
        },
        else => {},
    }
    // Otherwise, we just want the pointer child type
    const ret_ty = ptr_ty.childType(zcu);
    if (ret_ty.toIntern() == .anyopaque_type) {
        // The actual array type is unknown, which we represent with a generic poison.
        return .generic_poison_type;
    }
    const arr_ty = ret_ty.optEuBaseType(zcu);
    try sema.validateArrayInitTy(block, src, src, extra.elem_count, arr_ty);
    return Air.internedToRef(ret_ty.toIntern());
}

fn zirValidateArrayInitTy(
    sema: *Sema,
    block: *Block,
    inst: Zir.Inst.Index,
    is_result_ty: bool,
) CompileError!void {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].pl_node;
    const src = block.nodeOffset(inst_data.src_node);
    const ty_src: LazySrcLoc = if (is_result_ty) src else block.src(.{ .node_offset_init_ty = inst_data.src_node });
    const extra = sema.code.extraData(Zir.Inst.ArrayInit, inst_data.payload_index).data;
    // It's okay for the type to be poison: this will result in an anonymous array init.
    const ty = try sema.resolveTypeOrPoison(block, ty_src, extra.ty) orelse return;
    const arr_ty = if (is_result_ty) ty.optEuBaseType(zcu) else ty;
    return sema.validateArrayInitTy(block, src, ty_src, extra.init_count, arr_ty);
}

fn validateArrayInitTy(
    sema: *Sema,
    block: *Block,
    src: LazySrcLoc,
    ty_src: LazySrcLoc,
    init_count: u32,
    ty: Type,
) CompileError!void {
    const pt = sema.pt;
    const zcu = pt.zcu;
    switch (ty.zigTypeTag(zcu)) {
        .array => {
            const array_len = ty.arrayLen(zcu);
            if (init_count != array_len) {
                return sema.fail(block, src, "expected {d} array elements; found {d}", .{
                    array_len, init_count,
                });
            }
            return;
        },
        .vector => {
            const array_len = ty.arrayLen(zcu);
            if (init_count != array_len) {
                return sema.fail(block, src, "expected {d} vector elements; found {d}", .{
                    array_len, init_count,
                });
            }
            return;
        },
        .@"struct" => if (ty.isTuple(zcu)) {
            try ty.resolveFields(pt);
            const array_len = ty.arrayLen(zcu);
            if (init_count > array_len) {
                return sema.fail(block, src, "expected at most {d} tuple fields; found {d}", .{
                    array_len, init_count,
                });
            }
            return;
        },
        else => {},
    }
    return sema.failWithArrayInitNotSupported(block, ty_src, ty);
}

fn zirValidateStructInitTy(
    sema: *Sema,
    block: *Block,
    inst: Zir.Inst.Index,
    is_result_ty: bool,
) CompileError!void {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].un_node;
    const src = block.nodeOffset(inst_data.src_node);
    // It's okay for the type to be poison: this will result in an anonymous struct init.
    const ty = try sema.resolveTypeOrPoison(block, src, inst_data.operand) orelse return;
    const struct_ty = if (is_result_ty) ty.optEuBaseType(zcu) else ty;

    switch (struct_ty.zigTypeTag(zcu)) {
        .@"struct", .@"union" => return,
        else => {},
    }
    return sema.failWithStructInitNotSupported(block, src, struct_ty);
}

fn zirValidatePtrStructInit(
    sema: *Sema,
    block: *Block,
    inst: Zir.Inst.Index,
) CompileError!void {
    const tracy = trace(@src());
    defer tracy.end();

    const pt = sema.pt;
    const zcu = pt.zcu;
    const validate_inst = sema.code.instructions.items(.data)[@intFromEnum(inst)].pl_node;
    const init_src = block.nodeOffset(validate_inst.src_node);
    const validate_extra = sema.code.extraData(Zir.Inst.Block, validate_inst.payload_index);
    const instrs = sema.code.bodySlice(validate_extra.end, validate_extra.data.body_len);
    const field_ptr_data = sema.code.instructions.items(.data)[@intFromEnum(instrs[0])].pl_node;
    const field_ptr_extra = sema.code.extraData(Zir.Inst.Field, field_ptr_data.payload_index).data;
    const object_ptr = try sema.resolveInst(field_ptr_extra.lhs);
    const agg_ty = sema.typeOf(object_ptr).childType(zcu).optEuBaseType(zcu);
    switch (agg_ty.zigTypeTag(zcu)) {
        .@"struct" => return sema.validateStructInit(
            block,
            agg_ty,
            init_src,
            instrs,
        ),
        .@"union" => return sema.validateUnionInit(
            block,
            agg_ty,
            init_src,
            instrs,
            object_ptr,
        ),
        else => unreachable,
    }
}

fn validateUnionInit(
    sema: *Sema,
    block: *Block,
    union_ty: Type,
    init_src: LazySrcLoc,
    instrs: []const Zir.Inst.Index,
    union_ptr: Air.Inst.Ref,
) CompileError!void {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const gpa = sema.gpa;

    if (instrs.len != 1) {
        const msg = msg: {
            const msg = try sema.errMsg(
                init_src,
                "cannot initialize multiple union fields at once; unions can only have one active field",
                .{},
            );
            errdefer msg.destroy(gpa);

            for (instrs[1..]) |inst| {
                const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].pl_node;
                const inst_src = block.src(.{ .node_offset_initializer = inst_data.src_node });
                try sema.errNote(inst_src, msg, "additional initializer here", .{});
            }
            try sema.addDeclaredHereNote(msg, union_ty);
            break :msg msg;
        };
        return sema.failWithOwnedErrorMsg(block, msg);
    }

    if (block.isComptime() and
        (try sema.resolveDefinedValue(block, init_src, union_ptr)) != null)
    {
        // In this case, comptime machinery already did everything. No work to do here.
        return;
    }

    const field_ptr = instrs[0];
    const field_ptr_data = sema.code.instructions.items(.data)[@intFromEnum(field_ptr)].pl_node;
    const field_src = block.src(.{ .node_offset_initializer = field_ptr_data.src_node });
    const field_ptr_extra = sema.code.extraData(Zir.Inst.Field, field_ptr_data.payload_index).data;
    const field_name = try zcu.intern_pool.getOrPutString(
        gpa,
        pt.tid,
        sema.code.nullTerminatedString(field_ptr_extra.field_name_start),
        .no_embedded_nulls,
    );
    const field_index = try sema.unionFieldIndex(block, union_ty, field_name, field_src);
    const air_tags = sema.air_instructions.items(.tag);
    const air_datas = sema.air_instructions.items(.data);
    const field_ptr_ref = sema.inst_map.get(field_ptr).?;

    // Our task here is to determine if the union is comptime-known. In such case,
    // we erase the runtime AIR instructions for initializing the union, and replace
    // the mapping with the comptime value. Either way, we will need to populate the tag.

    // We expect to see something like this in the current block AIR:
    //   %a = alloc(*const U)
    //   %b = bitcast(*U, %a)
    //   %c = field_ptr(..., %b)
    //   %e!= store(%c!, %d!)
    // If %d is a comptime operand, the union is comptime.
    // If the union is comptime, we want `first_block_index`
    // to point at %c so that the bitcast becomes the last instruction in the block.
    //
    // Store instruction may be missing; if field type has only one possible value, this case is handled below.
    //
    // In the case of a comptime-known pointer to a union, the
    // the field_ptr instruction is missing, so we have to pattern-match
    // based only on the store instructions.
    // `first_block_index` needs to point to the `field_ptr` if it exists;
    // the `store` otherwise.
    var first_block_index = block.instructions.items.len;
    var block_index = block.instructions.items.len - 1;
    var init_val: ?Value = null;
    var init_ref: ?Air.Inst.Ref = null;
    while (block_index > 0) : (block_index -= 1) {
        const store_inst = block.instructions.items[block_index];
        if (store_inst.toRef() == field_ptr_ref) {
            first_block_index = block_index;
            break;
        }
        switch (air_tags[@intFromEnum(store_inst)]) {
            .store, .store_safe => {},
            else => continue,
        }
        const bin_op = air_datas[@intFromEnum(store_inst)].bin_op;
        var ptr_ref = bin_op.lhs;
        if (ptr_ref.toIndex()) |ptr_inst| if (air_tags[@intFromEnum(ptr_inst)] == .bitcast) {
            ptr_ref = air_datas[@intFromEnum(ptr_inst)].ty_op.operand;
        };
        if (ptr_ref != field_ptr_ref) continue;
        first_block_index = @min(if (field_ptr_ref.toIndex()) |field_ptr_inst|
            std.mem.lastIndexOfScalar(
                Air.Inst.Index,
                block.instructions.items[0..block_index],
                field_ptr_inst,
            ).?
        else
            block_index, first_block_index);
        init_ref = bin_op.rhs;
        init_val = try sema.resolveValue(bin_op.rhs);
        break;
    }

    const tag_ty = union_ty.unionTagTypeHypothetical(zcu);
    const tag_val = try pt.enumValueFieldIndex(tag_ty, field_index);
    const field_type = union_ty.unionFieldType(tag_val, zcu).?;

    if (try sema.typeHasOnePossibleValue(field_type)) |field_only_value| {
        init_val = field_only_value;
    }

    if (init_val) |val| {
        // Our task is to delete all the `field_ptr` and `store` instructions, and insert
        // instead a single `store` to the result ptr with a comptime union value.
        block_index = first_block_index;
        for (block.instructions.items[first_block_index..]) |cur_inst| {
            switch (air_tags[@intFromEnum(cur_inst)]) {
                .struct_field_ptr,
                .struct_field_ptr_index_0,
                .struct_field_ptr_index_1,
                .struct_field_ptr_index_2,
                .struct_field_ptr_index_3,
                => if (cur_inst.toRef() == field_ptr_ref) continue,
                .bitcast => if (air_datas[@intFromEnum(cur_inst)].ty_op.operand == field_ptr_ref) continue,
                .store, .store_safe => {
                    var ptr_ref = air_datas[@intFromEnum(cur_inst)].bin_op.lhs;
                    if (ptr_ref.toIndex()) |ptr_inst| if (air_tags[@intFromEnum(ptr_inst)] == .bitcast) {
                        ptr_ref = air_datas[@intFromEnum(ptr_inst)].ty_op.operand;
                    };
                    if (ptr_ref == field_ptr_ref) continue;
                },
                else => {},
            }
            block.instructions.items[block_index] = cur_inst;
            block_index += 1;
        }
        block.instructions.shrinkRetainingCapacity(block_index);

        const union_val = try pt.internUnion(.{
            .ty = union_ty.toIntern(),
            .tag = tag_val.toIntern(),
            .val = val.toIntern(),
        });
        const union_init = Air.internedToRef(union_val);
        try sema.storePtr2(block, init_src, union_ptr, init_src, union_init, init_src, .store);
        return;
    } else if (try union_ty.comptimeOnlySema(pt)) {
        const src = block.nodeOffset(field_ptr_data.src_node);
        return sema.failWithNeededComptime(block, src, .{ .comptime_only = .{
            .ty = union_ty,
            .msg = .union_init,
        } });
    }
    if (init_ref) |v| try sema.validateRuntimeValue(block, block.nodeOffset(field_ptr_data.src_node), v);

    const new_tag = Air.internedToRef(tag_val.toIntern());
    const set_tag_inst = try block.addBinOp(.set_union_tag, union_ptr, new_tag);
    try sema.checkComptimeKnownStore(block, set_tag_inst, LazySrcLoc.unneeded); // `unneeded` since this isn't a "proper" store
}

fn validateStructInit(
    sema: *Sema,
    block: *Block,
    struct_ty: Type,
    init_src: LazySrcLoc,
    instrs: []const Zir.Inst.Index,
) CompileError!void {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const gpa = sema.gpa;
    const ip = &zcu.intern_pool;

    const field_indices = try gpa.alloc(u32, instrs.len);
    defer gpa.free(field_indices);

    // Maps field index to field_ptr index of where it was already initialized.
    const found_fields = try gpa.alloc(Zir.Inst.OptionalIndex, struct_ty.structFieldCount(zcu));
    defer gpa.free(found_fields);
    @memset(found_fields, .none);

    var struct_ptr_zir_ref: Zir.Inst.Ref = undefined;

    for (instrs, field_indices) |field_ptr, *field_index| {
        const field_ptr_data = sema.code.instructions.items(.data)[@intFromEnum(field_ptr)].pl_node;
        const field_src = block.src(.{ .node_offset_initializer = field_ptr_data.src_node });
        const field_ptr_extra = sema.code.extraData(Zir.Inst.Field, field_ptr_data.payload_index).data;
        struct_ptr_zir_ref = field_ptr_extra.lhs;
        const field_name = try ip.getOrPutString(
            gpa,
            pt.tid,
            sema.code.nullTerminatedString(field_ptr_extra.field_name_start),
            .no_embedded_nulls,
        );
        field_index.* = if (struct_ty.isTuple(zcu))
            try sema.tupleFieldIndex(block, struct_ty, field_name, field_src)
        else
            try sema.structFieldIndex(block, struct_ty, field_name, field_src);
        assert(found_fields[field_index.*] == .none);
        found_fields[field_index.*] = field_ptr.toOptional();
    }

    var root_msg: ?*Zcu.ErrorMsg = null;
    errdefer if (root_msg) |msg| msg.destroy(sema.gpa);

    const struct_ptr = try sema.resolveInst(struct_ptr_zir_ref);
    if (block.isComptime() and
        (try sema.resolveDefinedValue(block, init_src, struct_ptr)) != null)
    {
        try struct_ty.resolveLayout(pt);
        // In this case the only thing we need to do is evaluate the implicit
        // store instructions for default field values, and report any missing fields.
        // Avoid the cost of the extra machinery for detecting a comptime struct init value.
        for (found_fields, 0..) |field_ptr, i_usize| {
            const i: u32 = @intCast(i_usize);
            if (field_ptr != .none) continue;

            try struct_ty.resolveStructFieldInits(pt);
            const default_val = struct_ty.structFieldDefaultValue(i, zcu);
            if (default_val.toIntern() == .unreachable_value) {
                const field_name = struct_ty.structFieldName(i, zcu).unwrap() orelse {
                    const template = "missing tuple field with index {d}";
                    if (root_msg) |msg| {
                        try sema.errNote(init_src, msg, template, .{i});
                    } else {
                        root_msg = try sema.errMsg(init_src, template, .{i});
                    }
                    continue;
                };
                const template = "missing struct field: {}";
                const args = .{field_name.fmt(ip)};
                if (root_msg) |msg| {
                    try sema.errNote(init_src, msg, template, args);
                } else {
                    root_msg = try sema.errMsg(init_src, template, args);
                }
                continue;
            }

            const field_src = init_src; // TODO better source location
            const default_field_ptr = if (struct_ty.isTuple(zcu))
                try sema.tupleFieldPtr(block, init_src, struct_ptr, field_src, @intCast(i), true)
            else
                try sema.structFieldPtrByIndex(block, init_src, struct_ptr, @intCast(i), struct_ty);
            const init = Air.internedToRef(default_val.toIntern());
            try sema.storePtr2(block, init_src, default_field_ptr, init_src, init, field_src, .store);
        }

        if (root_msg) |msg| {
            try sema.addDeclaredHereNote(msg, struct_ty);
            root_msg = null;
            return sema.failWithOwnedErrorMsg(block, msg);
        }

        return;
    }

    var fields_allow_runtime = true;

    var struct_is_comptime = true;
    var first_block_index = block.instructions.items.len;

    const require_comptime = try struct_ty.comptimeOnlySema(pt);
    const air_tags = sema.air_instructions.items(.tag);
    const air_datas = sema.air_instructions.items(.data);

    try struct_ty.resolveStructFieldInits(pt);

    // We collect the comptime field values in case the struct initialization
    // ends up being comptime-known.
    const field_values = try sema.arena.alloc(InternPool.Index, struct_ty.structFieldCount(zcu));

    field: for (found_fields, 0..) |opt_field_ptr, i_usize| {
        const i: u32 = @intCast(i_usize);
        if (opt_field_ptr.unwrap()) |field_ptr| {
            // Determine whether the value stored to this pointer is comptime-known.
            const field_ty = struct_ty.fieldType(i, zcu);
            if (try sema.typeHasOnePossibleValue(field_ty)) |opv| {
                field_values[i] = opv.toIntern();
                continue;
            }

            const field_ptr_ref = sema.inst_map.get(field_ptr).?;

            //std.debug.print("validateStructInit (field_ptr_ref=%{d}):\n", .{field_ptr_ref});
            //for (block.instructions.items) |item| {
            //    std.debug.print("  %{d} = {s}\n", .{item, @tagName(air_tags[@intFromEnum(item)])});
            //}

            // We expect to see something like this in the current block AIR:
            //   %a = field_ptr(...)
            //   store(%a, %b)
            // With an optional bitcast between the store and the field_ptr.
            // If %b is a comptime operand, this field is comptime.
            //
            // However, in the case of a comptime-known pointer to a struct, the
            // the field_ptr instruction is missing, so we have to pattern-match
            // based only on the store instructions.
            // `first_block_index` needs to point to the `field_ptr` if it exists;
            // the `store` otherwise.

            // Possible performance enhancement: save the `block_index` between iterations
            // of the for loop.
            var block_index = block.instructions.items.len;
            while (block_index > 0) {
                block_index -= 1;
                const store_inst = block.instructions.items[block_index];
                if (store_inst.toRef() == field_ptr_ref) {
                    struct_is_comptime = false;
                    continue :field;
                }
                switch (air_tags[@intFromEnum(store_inst)]) {
                    .store, .store_safe => {},
                    else => continue,
                }
                const bin_op = air_datas[@intFromEnum(store_inst)].bin_op;
                var ptr_ref = bin_op.lhs;
                if (ptr_ref.toIndex()) |ptr_inst| if (air_tags[@intFromEnum(ptr_inst)] == .bitcast) {
                    ptr_ref = air_datas[@intFromEnum(ptr_inst)].ty_op.operand;
                };
                if (ptr_ref != field_ptr_ref) continue;
                first_block_index = @min(if (field_ptr_ref.toIndex()) |field_ptr_inst|
                    std.mem.lastIndexOfScalar(
                        Air.Inst.Index,
                        block.instructions.items[0..block_index],
                        field_ptr_inst,
                    ).?
                else
                    block_index, first_block_index);
                if (!sema.checkRuntimeValue(bin_op.rhs)) fields_allow_runtime = false;
                if (try sema.resolveValue(bin_op.rhs)) |val| {
                    field_values[i] = val.toIntern();
                } else if (require_comptime) {
                    const field_ptr_data = sema.code.instructions.items(.data)[@intFromEnum(field_ptr)].pl_node;
                    const src = block.nodeOffset(field_ptr_data.src_node);
                    return sema.failWithNeededComptime(block, src, .{ .comptime_only = .{
                        .ty = struct_ty,
                        .msg = .struct_init,
                    } });
                } else {
                    struct_is_comptime = false;
                }
                continue :field;
            }
            struct_is_comptime = false;
            continue :field;
        }

        const default_val = struct_ty.structFieldDefaultValue(i, zcu);
        if (default_val.toIntern() == .unreachable_value) {
            const field_name = struct_ty.structFieldName(i, zcu).unwrap() orelse {
                const template = "missing tuple field with index {d}";
                if (root_msg) |msg| {
                    try sema.errNote(init_src, msg, template, .{i});
                } else {
                    root_msg = try sema.errMsg(init_src, template, .{i});
                }
                continue;
            };
            const template = "missing struct field: {}";
            const args = .{field_name.fmt(ip)};
            if (root_msg) |msg| {
                try sema.errNote(init_src, msg, template, args);
            } else {
                root_msg = try sema.errMsg(init_src, template, args);
            }
            continue;
        }
        field_values[i] = default_val.toIntern();
    }

    if (!struct_is_comptime and !fields_allow_runtime and root_msg == null) {
        root_msg = try sema.errMsg(init_src, "runtime value contains reference to comptime var", .{});
        try sema.errNote(init_src, root_msg.?, "comptime var pointers are not available at runtime", .{});
    }

    if (root_msg) |msg| {
        try sema.addDeclaredHereNote(msg, struct_ty);
        root_msg = null;
        return sema.failWithOwnedErrorMsg(block, msg);
    }

    if (struct_is_comptime) {
        // Our task is to delete all the `field_ptr` and `store` instructions, and insert
        // instead a single `store` to the struct_ptr with a comptime struct value.
        var init_index: usize = 0;
        var field_ptr_ref = Air.Inst.Ref.none;
        var block_index = first_block_index;
        for (block.instructions.items[first_block_index..]) |cur_inst| {
            while (field_ptr_ref == .none and init_index < instrs.len) : (init_index += 1) {
                const field_ty = struct_ty.fieldType(field_indices[init_index], zcu);
                if (try field_ty.onePossibleValue(pt)) |_| continue;
                field_ptr_ref = sema.inst_map.get(instrs[init_index]).?;
            }
            switch (air_tags[@intFromEnum(cur_inst)]) {
                .struct_field_ptr,
                .struct_field_ptr_index_0,
                .struct_field_ptr_index_1,
                .struct_field_ptr_index_2,
                .struct_field_ptr_index_3,
                => if (cur_inst.toRef() == field_ptr_ref) continue,
                .bitcast => if (air_datas[@intFromEnum(cur_inst)].ty_op.operand == field_ptr_ref) continue,
                .store, .store_safe => {
                    var ptr_ref = air_datas[@intFromEnum(cur_inst)].bin_op.lhs;
                    if (ptr_ref.toIndex()) |ptr_inst| if (air_tags[@intFromEnum(ptr_inst)] == .bitcast) {
                        ptr_ref = air_datas[@intFromEnum(ptr_inst)].ty_op.operand;
                    };
                    if (ptr_ref == field_ptr_ref) {
                        field_ptr_ref = .none;
                        continue;
                    }
                },
                else => {},
            }
            block.instructions.items[block_index] = cur_inst;
            block_index += 1;
        }
        block.instructions.shrinkRetainingCapacity(block_index);

        const struct_val = try pt.intern(.{ .aggregate = .{
            .ty = struct_ty.toIntern(),
            .storage = .{ .elems = field_values },
        } });
        const struct_init = Air.internedToRef(struct_val);
        try sema.storePtr2(block, init_src, struct_ptr, init_src, struct_init, init_src, .store);
        return;
    }
    try struct_ty.resolveLayout(pt);

    // Our task is to insert `store` instructions for all the default field values.
    for (found_fields, 0..) |field_ptr, i| {
        if (field_ptr != .none) continue;

        const field_src = init_src; // TODO better source location
        const default_field_ptr = if (struct_ty.isTuple(zcu))
            try sema.tupleFieldPtr(block, init_src, struct_ptr, field_src, @intCast(i), true)
        else
            try sema.structFieldPtrByIndex(block, init_src, struct_ptr, @intCast(i), struct_ty);
        try sema.checkKnownAllocPtr(block, struct_ptr, default_field_ptr);
        const init = Air.internedToRef(field_values[i]);
        try sema.storePtr2(block, init_src, default_field_ptr, init_src, init, field_src, .store);
    }
}

fn zirValidatePtrArrayInit(
    sema: *Sema,
    block: *Block,
    inst: Zir.Inst.Index,
) CompileError!void {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const validate_inst = sema.code.instructions.items(.data)[@intFromEnum(inst)].pl_node;
    const init_src = block.nodeOffset(validate_inst.src_node);
    const validate_extra = sema.code.extraData(Zir.Inst.Block, validate_inst.payload_index);
    const instrs = sema.code.bodySlice(validate_extra.end, validate_extra.data.body_len);
    const first_elem_ptr_data = sema.code.instructions.items(.data)[@intFromEnum(instrs[0])].pl_node;
    const elem_ptr_extra = sema.code.extraData(Zir.Inst.ElemPtrImm, first_elem_ptr_data.payload_index).data;
    const array_ptr = try sema.resolveInst(elem_ptr_extra.ptr);
    const array_ty = sema.typeOf(array_ptr).childType(zcu).optEuBaseType(zcu);
    const array_len = array_ty.arrayLen(zcu);

    // Collect the comptime element values in case the array literal ends up
    // being comptime-known.
    const element_vals = try sema.arena.alloc(
        InternPool.Index,
        try sema.usizeCast(block, init_src, array_len),
    );

    if (instrs.len != array_len) switch (array_ty.zigTypeTag(zcu)) {
        .@"struct" => {
            var root_msg: ?*Zcu.ErrorMsg = null;
            errdefer if (root_msg) |msg| msg.destroy(sema.gpa);

            try array_ty.resolveStructFieldInits(pt);
            var i = instrs.len;
            while (i < array_len) : (i += 1) {
                const default_val = array_ty.structFieldDefaultValue(i, zcu).toIntern();
                if (default_val == .unreachable_value) {
                    const template = "missing tuple field with index {d}";
                    if (root_msg) |msg| {
                        try sema.errNote(init_src, msg, template, .{i});
                    } else {
                        root_msg = try sema.errMsg(init_src, template, .{i});
                    }
                    continue;
                }

                element_vals[i] = default_val;
            }

            if (root_msg) |msg| {
                root_msg = null;
                return sema.failWithOwnedErrorMsg(block, msg);
            }
        },
        .array => {
            return sema.fail(block, init_src, "expected {d} array elements; found {d}", .{
                array_len, instrs.len,
            });
        },
        .vector => {
            return sema.fail(block, init_src, "expected {d} vector elements; found {d}", .{
                array_len, instrs.len,
            });
        },
        else => unreachable,
    };

    if (block.isComptime() and
        (try sema.resolveDefinedValue(block, init_src, array_ptr)) != null)
    {
        // In this case the comptime machinery will have evaluated the store instructions
        // at comptime so we have almost nothing to do here. However, in case of a
        // sentinel-terminated array, the sentinel will not have been populated by
        // any ZIR instructions at comptime; we need to do that here.
        if (array_ty.sentinel(zcu)) |sentinel_val| {
            const array_len_ref = try pt.intRef(.usize, array_len);
            const sentinel_ptr = try sema.elemPtrArray(block, init_src, init_src, array_ptr, init_src, array_len_ref, true, true);
            const sentinel = Air.internedToRef(sentinel_val.toIntern());
            try sema.storePtr2(block, init_src, sentinel_ptr, init_src, sentinel, init_src, .store);
        }
        return;
    }

    // If the array has one possible value, the value is always comptime-known.
    if (try sema.typeHasOnePossibleValue(array_ty)) |array_opv| {
        const array_init = Air.internedToRef(array_opv.toIntern());
        try sema.storePtr2(block, init_src, array_ptr, init_src, array_init, init_src, .store);
        return;
    }

    var array_is_comptime = true;
    var first_block_index = block.instructions.items.len;

    const air_tags = sema.air_instructions.items(.tag);
    const air_datas = sema.air_instructions.items(.data);

    outer: for (instrs, 0..) |elem_ptr, i| {
        // Determine whether the value stored to this pointer is comptime-known.

        if (array_ty.isTuple(zcu)) {
            if (array_ty.structFieldIsComptime(i, zcu))
                try array_ty.resolveStructFieldInits(pt);
            if (try array_ty.structFieldValueComptime(pt, i)) |opv| {
                element_vals[i] = opv.toIntern();
                continue;
            }
        }

        const elem_ptr_ref = sema.inst_map.get(elem_ptr).?;

        // We expect to see something like this in the current block AIR:
        //   %a = elem_ptr(...)
        //   store(%a, %b)
        // With an optional bitcast between the store and the elem_ptr.
        // If %b is a comptime operand, this element is comptime.
        //
        // However, in the case of a comptime-known pointer to an array, the
        // the elem_ptr instruction is missing, so we have to pattern-match
        // based only on the store instructions.
        // `first_block_index` needs to point to the `elem_ptr` if it exists;
        // the `store` otherwise.
        //
        // This is nearly identical to similar logic in `validateStructInit`.

        // Possible performance enhancement: save the `block_index` between iterations
        // of the for loop.
        var block_index = block.instructions.items.len;
        while (block_index > 0) {
            block_index -= 1;
            const store_inst = block.instructions.items[block_index];
            if (store_inst.toRef() == elem_ptr_ref) {
                array_is_comptime = false;
                continue :outer;
            }
            switch (air_tags[@intFromEnum(store_inst)]) {
                .store, .store_safe => {},
                else => continue,
            }
            const bin_op = air_datas[@intFromEnum(store_inst)].bin_op;
            var ptr_ref = bin_op.lhs;
            if (ptr_ref.toIndex()) |ptr_inst| if (air_tags[@intFromEnum(ptr_inst)] == .bitcast) {
                ptr_ref = air_datas[@intFromEnum(ptr_inst)].ty_op.operand;
            };
            if (ptr_ref != elem_ptr_ref) continue;
            first_block_index = @min(if (elem_ptr_ref.toIndex()) |elem_ptr_inst|
                std.mem.lastIndexOfScalar(
                    Air.Inst.Index,
                    block.instructions.items[0..block_index],
                    elem_ptr_inst,
                ).?
            else
                block_index, first_block_index);
            if (try sema.resolveValue(bin_op.rhs)) |val| {
                element_vals[i] = val.toIntern();
            } else {
                array_is_comptime = false;
            }
            continue :outer;
        }
        array_is_comptime = false;
        continue :outer;
    }

    if (array_is_comptime) {
        if (try sema.resolveDefinedValue(block, init_src, array_ptr)) |ptr_val| {
            switch (zcu.intern_pool.indexToKey(ptr_val.toIntern())) {
                .ptr => |ptr| switch (ptr.base_addr) {
                    .comptime_field => return, // This store was validated by the individual elem ptrs.
                    else => {},
                },
                else => {},
            }
        }

        // Our task is to delete all the `elem_ptr` and `store` instructions, and insert
        // instead a single `store` to the array_ptr with a comptime struct value.
        var elem_index: usize = 0;
        var elem_ptr_ref = Air.Inst.Ref.none;
        var block_index = first_block_index;
        for (block.instructions.items[first_block_index..]) |cur_inst| {
            while (elem_ptr_ref == .none and elem_index < instrs.len) : (elem_index += 1) {
                if (array_ty.isTuple(zcu) and array_ty.structFieldIsComptime(elem_index, zcu)) continue;
                elem_ptr_ref = sema.inst_map.get(instrs[elem_index]).?;
            }
            switch (air_tags[@intFromEnum(cur_inst)]) {
                .ptr_elem_ptr => if (cur_inst.toRef() == elem_ptr_ref) continue,
                .bitcast => if (air_datas[@intFromEnum(cur_inst)].ty_op.operand == elem_ptr_ref) continue,
                .store, .store_safe => {
                    var ptr_ref = air_datas[@intFromEnum(cur_inst)].bin_op.lhs;
                    if (ptr_ref.toIndex()) |ptr_inst| if (air_tags[@intFromEnum(ptr_inst)] == .bitcast) {
                        ptr_ref = air_datas[@intFromEnum(ptr_inst)].ty_op.operand;
                    };
                    if (ptr_ref == elem_ptr_ref) {
                        elem_ptr_ref = .none;
                        continue;
                    }
                },
                else => {},
            }
            block.instructions.items[block_index] = cur_inst;
            block_index += 1;
        }
        block.instructions.shrinkRetainingCapacity(block_index);

        const array_val = try pt.intern(.{ .aggregate = .{
            .ty = array_ty.toIntern(),
            .storage = .{ .elems = element_vals },
        } });
        const array_init = Air.internedToRef(array_val);
        try sema.storePtr2(block, init_src, array_ptr, init_src, array_init, init_src, .store);
    }
}

fn zirValidateDeref(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!void {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].un_node;
    const src = block.nodeOffset(inst_data.src_node);
    const operand = try sema.resolveInst(inst_data.operand);
    const operand_ty = sema.typeOf(operand);

    if (operand_ty.zigTypeTag(zcu) != .pointer) {
        return sema.fail(block, src, "cannot dereference non-pointer type '{}'", .{operand_ty.fmt(pt)});
    } else switch (operand_ty.ptrSize(zcu)) {
        .one, .c => {},
        .many => return sema.fail(block, src, "index syntax required for unknown-length pointer type '{}'", .{operand_ty.fmt(pt)}),
        .slice => return sema.fail(block, src, "index syntax required for slice type '{}'", .{operand_ty.fmt(pt)}),
    }

    if ((try sema.typeHasOnePossibleValue(operand_ty.childType(zcu))) != null) {
        // No need to validate the actual pointer value, we don't need it!
        return;
    }

    const elem_ty = operand_ty.elemType2(zcu);
    if (try sema.resolveValue(operand)) |val| {
        if (val.isUndef(zcu)) {
            return sema.fail(block, src, "cannot dereference undefined value", .{});
        }
    } else if (try elem_ty.comptimeOnlySema(pt)) {
        const msg = msg: {
            const msg = try sema.errMsg(
                src,
                "values of type '{}' must be comptime-known, but operand value is runtime-known",
                .{elem_ty.fmt(pt)},
            );
            errdefer msg.destroy(sema.gpa);

            try sema.explainWhyTypeIsComptime(msg, src, elem_ty);
            break :msg msg;
        };
        return sema.failWithOwnedErrorMsg(block, msg);
    }
}

fn typeIsDestructurable(ty: Type, zcu: *const Zcu) bool {
    return switch (ty.zigTypeTag(zcu)) {
        .array, .vector => true,
        .@"struct" => ty.isTuple(zcu),
        else => false,
    };
}

fn zirValidateDestructure(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!void {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].pl_node;
    const extra = sema.code.extraData(Zir.Inst.ValidateDestructure, inst_data.payload_index).data;
    const src = block.nodeOffset(inst_data.src_node);
    const destructure_src = block.nodeOffset(extra.destructure_node);
    const operand = try sema.resolveInst(extra.operand);
    const operand_ty = sema.typeOf(operand);

    if (!typeIsDestructurable(operand_ty, zcu)) {
        return sema.failWithOwnedErrorMsg(block, msg: {
            const msg = try sema.errMsg(src, "type '{}' cannot be destructured", .{operand_ty.fmt(pt)});
            errdefer msg.destroy(sema.gpa);
            try sema.errNote(destructure_src, msg, "result destructured here", .{});
            if (operand_ty.zigTypeTag(pt.zcu) == .error_union) {
                const base_op_ty = operand_ty.errorUnionPayload(zcu);
                if (typeIsDestructurable(base_op_ty, zcu))
                    try sema.errNote(src, msg, "consider using 'try', 'catch', or 'if'", .{});
            }
            break :msg msg;
        });
    }

    if (operand_ty.arrayLen(zcu) != extra.expect_len) {
        return sema.failWithOwnedErrorMsg(block, msg: {
            const msg = try sema.errMsg(src, "expected {} elements for destructure, found {}", .{
                extra.expect_len,
                operand_ty.arrayLen(zcu),
            });
            errdefer msg.destroy(sema.gpa);
            try sema.errNote(destructure_src, msg, "result destructured here", .{});
            break :msg msg;
        });
    }
}

fn failWithBadMemberAccess(
    sema: *Sema,
    block: *Block,
    agg_ty: Type,
    field_src: LazySrcLoc,
    field_name: InternPool.NullTerminatedString,
) CompileError {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const ip = &zcu.intern_pool;
    const kw_name = switch (agg_ty.zigTypeTag(zcu)) {
        .@"union" => "union",
        .@"struct" => "struct",
        .@"opaque" => "opaque",
        .@"enum" => "enum",
        else => unreachable,
    };
    if (agg_ty.typeDeclInst(zcu)) |inst| if ((inst.resolve(ip) orelse return error.AnalysisFail) == .main_struct_inst) {
        return sema.fail(block, field_src, "root source file struct '{}' has no member named '{}'", .{
            agg_ty.fmt(pt), field_name.fmt(ip),
        });
    };

    return sema.fail(block, field_src, "{s} '{}' has no member named '{}'", .{
        kw_name, agg_ty.fmt(pt), field_name.fmt(ip),
    });
}

fn failWithBadStructFieldAccess(
    sema: *Sema,
    block: *Block,
    struct_ty: Type,
    struct_type: InternPool.LoadedStructType,
    field_src: LazySrcLoc,
    field_name: InternPool.NullTerminatedString,
) CompileError {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const ip = &zcu.intern_pool;

    const msg = msg: {
        const msg = try sema.errMsg(
            field_src,
            "no field named '{}' in struct '{}'",
            .{ field_name.fmt(ip), struct_type.name.fmt(ip) },
        );
        errdefer msg.destroy(sema.gpa);
        try sema.errNote(struct_ty.srcLoc(zcu), msg, "struct declared here", .{});
        break :msg msg;
    };
    return sema.failWithOwnedErrorMsg(block, msg);
}

fn failWithBadUnionFieldAccess(
    sema: *Sema,
    block: *Block,
    union_ty: Type,
    union_obj: InternPool.LoadedUnionType,
    field_src: LazySrcLoc,
    field_name: InternPool.NullTerminatedString,
) CompileError {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const ip = &zcu.intern_pool;
    const gpa = sema.gpa;

    const msg = msg: {
        const msg = try sema.errMsg(
            field_src,
            "no field named '{}' in union '{}'",
            .{ field_name.fmt(ip), union_obj.name.fmt(ip) },
        );
        errdefer msg.destroy(gpa);
        try sema.errNote(union_ty.srcLoc(zcu), msg, "union declared here", .{});
        break :msg msg;
    };
    return sema.failWithOwnedErrorMsg(block, msg);
}

fn addDeclaredHereNote(sema: *Sema, parent: *Zcu.ErrorMsg, decl_ty: Type) !void {
    const zcu = sema.pt.zcu;
    const src_loc = decl_ty.srcLocOrNull(zcu) orelse return;
    const category = switch (decl_ty.zigTypeTag(zcu)) {
        .@"union" => "union",
        .@"struct" => "struct",
        .@"enum" => "enum",
        .@"opaque" => "opaque",
        else => unreachable,
    };
    try sema.errNote(src_loc, parent, "{s} declared here", .{category});
}

fn zirStoreToInferredPtr(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!void {
    const tracy = trace(@src());
    defer tracy.end();

    const pl_node = sema.code.instructions.items(.data)[@intFromEnum(inst)].pl_node;
    const src = block.nodeOffset(pl_node.src_node);
    const bin = sema.code.extraData(Zir.Inst.Bin, pl_node.payload_index).data;
    const ptr = try sema.resolveInst(bin.lhs);
    const operand = try sema.resolveInst(bin.rhs);
    const ptr_inst = ptr.toIndex().?;
    const air_datas = sema.air_instructions.items(.data);

    switch (sema.air_instructions.items(.tag)[@intFromEnum(ptr_inst)]) {
        .inferred_alloc_comptime => {
            const iac = &air_datas[@intFromEnum(ptr_inst)].inferred_alloc_comptime;
            return sema.storeToInferredAllocComptime(block, src, operand, iac);
        },
        .inferred_alloc => {
            const ia = sema.unresolved_inferred_allocs.getPtr(ptr_inst).?;
            return sema.storeToInferredAlloc(block, src, ptr, operand, ia);
        },
        else => unreachable,
    }
}

fn storeToInferredAlloc(
    sema: *Sema,
    block: *Block,
    src: LazySrcLoc,
    ptr: Air.Inst.Ref,
    operand: Air.Inst.Ref,
    inferred_alloc: *InferredAlloc,
) CompileError!void {
    // Create a store instruction as a placeholder.  This will be replaced by a
    // proper store sequence once we know the stored type.
    const dummy_store = try block.addBinOp(.store, ptr, operand);
    try sema.checkComptimeKnownStore(block, dummy_store, src);
    // Add the stored instruction to the set we will use to resolve peer types
    // for the inferred allocation.
    try inferred_alloc.prongs.append(sema.arena, dummy_store.toIndex().?);
}

fn storeToInferredAllocComptime(
    sema: *Sema,
    block: *Block,
    src: LazySrcLoc,
    operand: Air.Inst.Ref,
    iac: *Air.Inst.Data.InferredAllocComptime,
) CompileError!void {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const operand_ty = sema.typeOf(operand);
    // There will be only one store_to_inferred_ptr because we are running at comptime.
    // The alloc will turn into a Decl or a ComptimeAlloc.
    const operand_val = try sema.resolveValue(operand) orelse {
        return sema.failWithNeededComptime(block, src, .{ .simple = .stored_to_comptime_var });
    };
    const alloc_ty = try pt.ptrTypeSema(.{
        .child = operand_ty.toIntern(),
        .flags = .{
            .alignment = iac.alignment,
            .is_const = iac.is_const,
        },
    });
    if (iac.is_const and !operand_val.canMutateComptimeVarState(zcu)) {
        iac.ptr = try pt.intern(.{ .ptr = .{
            .ty = alloc_ty.toIntern(),
            .base_addr = .{ .uav = .{
                .val = operand_val.toIntern(),
                .orig_ty = alloc_ty.toIntern(),
            } },
            .byte_offset = 0,
        } });
    } else {
        const alloc_index = try sema.newComptimeAlloc(block, src, operand_ty, iac.alignment);
        sema.getComptimeAlloc(alloc_index).val = .{ .interned = operand_val.toIntern() };
        iac.ptr = try pt.intern(.{ .ptr = .{
            .ty = alloc_ty.toIntern(),
            .base_addr = .{ .comptime_alloc = alloc_index },
            .byte_offset = 0,
        } });
    }
}

fn zirSetEvalBranchQuota(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!void {
    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].un_node;
    const src = block.nodeOffset(inst_data.src_node);
    const quota: u32 = @intCast(try sema.resolveInt(block, src, inst_data.operand, .u32, .{ .simple = .operand_setEvalBranchQuota }));
    sema.branch_quota = @max(sema.branch_quota, quota);
    sema.allow_memoize = false;
}

fn zirStoreNode(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!void {
    const tracy = trace(@src());
    defer tracy.end();

    const pt = sema.pt;
    const zcu = pt.zcu;
    const zir_tags = sema.code.instructions.items(.tag);
    const zir_datas = sema.code.instructions.items(.data);
    const inst_data = zir_datas[@intFromEnum(inst)].pl_node;
    const src = block.nodeOffset(inst_data.src_node);
    const extra = sema.code.extraData(Zir.Inst.Bin, inst_data.payload_index).data;
    const ptr = try sema.resolveInst(extra.lhs);
    const operand = try sema.resolveInst(extra.rhs);

    const is_ret = if (extra.lhs.toIndex()) |ptr_index|
        zir_tags[@intFromEnum(ptr_index)] == .ret_ptr
    else
        false;

    // Check for the possibility of this pattern:
    //   %a = ret_ptr
    //   %b = store(%a, %c)
    // Where %c is an error union or error set. In such case we need to add
    // to the current function's inferred error set, if any.
    if (is_ret and sema.fn_ret_ty_ies != null) switch (sema.typeOf(operand).zigTypeTag(zcu)) {
        .error_union, .error_set => try sema.addToInferredErrorSet(operand),
        else => {},
    };

    const ptr_src = block.src(.{ .node_offset_store_ptr = inst_data.src_node });
    const operand_src = block.src(.{ .node_offset_store_operand = inst_data.src_node });
    const air_tag: Air.Inst.Tag = if (is_ret)
        .ret_ptr
    else if (block.wantSafety())
        .store_safe
    else
        .store;
    return sema.storePtr2(block, src, ptr, ptr_src, operand, operand_src, air_tag);
}

fn zirStr(sema: *Sema, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    const bytes = sema.code.instructions.items(.data)[@intFromEnum(inst)].str.get(sema.code);
    return sema.addStrLit(
        try sema.pt.zcu.intern_pool.getOrPutString(sema.gpa, sema.pt.tid, bytes, .maybe_embedded_nulls),
        bytes.len,
    );
}

fn addNullTerminatedStrLit(sema: *Sema, string: InternPool.NullTerminatedString) CompileError!Air.Inst.Ref {
    return sema.addStrLit(string.toString(), string.length(&sema.pt.zcu.intern_pool));
}

pub fn addStrLit(sema: *Sema, string: InternPool.String, len: u64) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const array_ty = try pt.arrayType(.{
        .len = len,
        .sentinel = .zero_u8,
        .child = .u8_type,
    });
    const val = try pt.intern(.{ .aggregate = .{
        .ty = array_ty.toIntern(),
        .storage = .{ .bytes = string },
    } });
    return sema.uavRef(val);
}

fn uavRef(sema: *Sema, val: InternPool.Index) CompileError!Air.Inst.Ref {
    return Air.internedToRef(try sema.pt.refValue(val));
}

fn zirInt(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    _ = block;
    const tracy = trace(@src());
    defer tracy.end();

    const int = sema.code.instructions.items(.data)[@intFromEnum(inst)].int;
    return sema.pt.intRef(.comptime_int, int);
}

fn zirIntBig(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    _ = block;
    const tracy = trace(@src());
    defer tracy.end();

    const int = sema.code.instructions.items(.data)[@intFromEnum(inst)].str;
    const byte_count = int.len * @sizeOf(std.math.big.Limb);
    const limb_bytes = sema.code.string_bytes[@intFromEnum(int.start)..][0..byte_count];

    // TODO: this allocation and copy is only needed because the limbs may be unaligned.
    // If ZIR is adjusted so that big int limbs are guaranteed to be aligned, these
    // two lines can be removed.
    const limbs = try sema.arena.alloc(std.math.big.Limb, int.len);
    @memcpy(mem.sliceAsBytes(limbs), limb_bytes);

    return Air.internedToRef((try sema.pt.intValue_big(.comptime_int, .{
        .limbs = limbs,
        .positive = true,
    })).toIntern());
}

fn zirFloat(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    _ = block;
    const number = sema.code.instructions.items(.data)[@intFromEnum(inst)].float;
    return Air.internedToRef((try sema.pt.floatValue(
        .comptime_float,
        number,
    )).toIntern());
}

fn zirFloat128(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    _ = block;
    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].pl_node;
    const extra = sema.code.extraData(Zir.Inst.Float128, inst_data.payload_index).data;
    const number = extra.get();
    return Air.internedToRef((try sema.pt.floatValue(.comptime_float, number)).toIntern());
}

fn zirCompileError(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!void {
    const tracy = trace(@src());
    defer tracy.end();

    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].un_node;
    const src = block.nodeOffset(inst_data.src_node);
    const operand_src = block.builtinCallArgSrc(inst_data.src_node, 0);
    const msg = try sema.resolveConstString(block, operand_src, inst_data.operand, .{ .simple = .compile_error_string });
    return sema.fail(block, src, "{s}", .{msg});
}

fn zirCompileLog(
    sema: *Sema,
    block: *Block,
    extended: Zir.Inst.Extended.InstData,
) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const gpa = zcu.gpa;

    var buf: std.ArrayListUnmanaged(u8) = .empty;
    defer buf.deinit(gpa);

    const writer = buf.writer(gpa);

    const extra = sema.code.extraData(Zir.Inst.NodeMultiOp, extended.operand);
    const src_node = extra.data.src_node;
    const args = sema.code.refSlice(extra.end, extended.small);

    for (args, 0..) |arg_ref, i| {
        if (i != 0) try writer.print(", ", .{});

        const arg = try sema.resolveInst(arg_ref);
        const arg_ty = sema.typeOf(arg);
        if (try sema.resolveValueResolveLazy(arg)) |val| {
            try writer.print("@as({}, {})", .{
                arg_ty.fmt(pt), val.fmtValueSema(pt, sema),
            });
        } else {
            try writer.print("@as({}, [runtime value])", .{arg_ty.fmt(pt)});
        }
    }

    const line_data = try zcu.intern_pool.getOrPutString(gpa, pt.tid, buf.items, .no_embedded_nulls);

    const line_idx: Zcu.CompileLogLine.Index = if (zcu.free_compile_log_lines.pop()) |idx| idx: {
        zcu.compile_log_lines.items[@intFromEnum(idx)] = .{
            .next = .none,
            .data = line_data,
        };
        break :idx idx;
    } else idx: {
        try zcu.compile_log_lines.append(gpa, .{
            .next = .none,
            .data = line_data,
        });
        break :idx @enumFromInt(zcu.compile_log_lines.items.len - 1);
    };

    const gop = try zcu.compile_logs.getOrPut(gpa, sema.owner);
    if (gop.found_existing) {
        const prev_line = gop.value_ptr.last_line.get(zcu);
        assert(prev_line.next == .none);
        prev_line.next = line_idx.toOptional();
        gop.value_ptr.last_line = line_idx;
    } else {
        gop.value_ptr.* = .{
            .base_node_inst = block.src_base_inst,
            .node_offset = src_node,
            .first_line = line_idx,
            .last_line = line_idx,
        };
    }
    return .void_value;
}

fn zirPanic(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!void {
    const pt = sema.pt;
    const zcu = pt.zcu;

    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].un_node;
    const src = block.nodeOffset(inst_data.src_node);
    const msg_inst = try sema.resolveInst(inst_data.operand);

    const coerced_msg = try sema.coerce(block, .slice_const_u8, msg_inst, block.builtinCallArgSrc(inst_data.src_node, 0));

    if (block.isComptime()) {
        return sema.fail(block, src, "encountered @panic at comptime", .{});
    }

    // We only apply the first hint in a branch.
    // This allows user-provided hints to override implicit cold hints.
    if (sema.branch_hint == null) {
        sema.branch_hint = .cold;
    }

    if (!zcu.backendSupportsFeature(.panic_fn)) {
        _ = try block.addNoOp(.trap);
        return;
    }

    try sema.ensureMemoizedStateResolved(src, .panic);
    try zcu.ensureFuncBodyAnalysisQueued(zcu.builtin_decl_values.get(.@"panic.call"));

    const panic_fn = Air.internedToRef(zcu.builtin_decl_values.get(.@"panic.call"));

    const opt_usize_ty = try pt.optionalType(.usize_type);
    const null_ret_addr = Air.internedToRef((try pt.intern(.{ .opt = .{
        .ty = opt_usize_ty.toIntern(),
        .val = .none,
    } })));
    try sema.callBuiltin(block, src, panic_fn, .auto, &.{ coerced_msg, null_ret_addr }, .@"@panic");
}

fn zirTrap(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!void {
    const src_node = sema.code.instructions.items(.data)[@intFromEnum(inst)].node;
    const src = block.nodeOffset(src_node);
    if (block.isComptime())
        return sema.fail(block, src, "encountered @trap at comptime", .{});
    _ = try block.addNoOp(.trap);
}

fn zirLoop(sema: *Sema, parent_block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    const tracy = trace(@src());
    defer tracy.end();

    const pt = sema.pt;
    const zcu = pt.zcu;
    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].pl_node;
    const src = parent_block.nodeOffset(inst_data.src_node);
    const extra = sema.code.extraData(Zir.Inst.Block, inst_data.payload_index);
    const body = sema.code.bodySlice(extra.end, extra.data.body_len);
    const gpa = sema.gpa;

    // AIR expects a block outside the loop block too.
    // Reserve space for a Loop instruction so that generated Break instructions can
    // point to it, even if it doesn't end up getting used because the code ends up being
    // comptime evaluated.
    const block_inst: Air.Inst.Index = @enumFromInt(sema.air_instructions.len);
    const loop_inst: Air.Inst.Index = @enumFromInt(@intFromEnum(block_inst) + 1);
    try sema.air_instructions.ensureUnusedCapacity(gpa, 2);
    sema.air_instructions.appendAssumeCapacity(.{
        .tag = .block,
        .data = undefined,
    });
    sema.air_instructions.appendAssumeCapacity(.{
        .tag = .loop,
        .data = .{ .ty_pl = .{
            .ty = .noreturn_type,
            .payload = undefined,
        } },
    });
    var label: Block.Label = .{
        .zir_block = inst,
        .merges = .{
            .src_locs = .{},
            .results = .{},
            .br_list = .{},
            .block_inst = block_inst,
        },
    };
    var child_block = parent_block.makeSubBlock();
    child_block.label = &label;
    child_block.runtime_cond = null;
    child_block.runtime_loop = src;
    child_block.runtime_index.increment();
    const merges = &child_block.label.?.merges;

    defer child_block.instructions.deinit(gpa);
    defer merges.deinit(gpa);

    var loop_block = child_block.makeSubBlock();
    defer loop_block.instructions.deinit(gpa);

    // Use `analyzeBodyInner` directly to push any comptime control flow up the stack.
    try sema.analyzeBodyInner(&loop_block, body);

    // TODO: since AIR has `repeat` now, we could change ZIR to generate
    // more optimal code utilizing `repeat` instructions across blocks!
    // For now, if the generated loop body does not terminate `noreturn`,
    // then `analyzeBodyInner` is signalling that it ended with `repeat`.

    const loop_block_len = loop_block.instructions.items.len;
    if (loop_block_len > 0 and sema.typeOf(loop_block.instructions.items[loop_block_len - 1].toRef()).isNoReturn(zcu)) {
        // If the loop ended with a noreturn terminator, then there is no way for it to loop,
        // so we can just use the block instead.
        try child_block.instructions.appendSlice(gpa, loop_block.instructions.items);
    } else {
        _ = try loop_block.addInst(.{
            .tag = .repeat,
            .data = .{ .repeat = .{
                .loop_inst = loop_inst,
            } },
        });
        // Note that `loop_block_len` is now off by one.

        try child_block.instructions.append(gpa, loop_inst);

        try sema.air_extra.ensureUnusedCapacity(gpa, @typeInfo(Air.Block).@"struct".fields.len + loop_block_len + 1);
        sema.air_instructions.items(.data)[@intFromEnum(loop_inst)].ty_pl.payload = sema.addExtraAssumeCapacity(
            Air.Block{ .body_len = @intCast(loop_block_len + 1) },
        );
        sema.air_extra.appendSliceAssumeCapacity(@ptrCast(loop_block.instructions.items));
    }
    return sema.resolveAnalyzedBlock(parent_block, src, &child_block, merges, false);
}

fn zirCImport(sema: *Sema, parent_block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    const tracy = trace(@src());
    defer tracy.end();

    const pt = sema.pt;
    const zcu = pt.zcu;
    const comp = zcu.comp;
    const gpa = sema.gpa;
    const pl_node = sema.code.instructions.items(.data)[@intFromEnum(inst)].pl_node;
    const src = parent_block.nodeOffset(pl_node.src_node);
    const extra = sema.code.extraData(Zir.Inst.Block, pl_node.payload_index);
    const body = sema.code.bodySlice(extra.end, extra.data.body_len);

    // we check this here to avoid undefined symbols
    if (!build_options.have_llvm)
        return sema.fail(parent_block, src, "C import unavailable; Zig compiler built without LLVM extensions", .{});

    var c_import_buf = std.ArrayList(u8).init(gpa);
    defer c_import_buf.deinit();

    var child_block: Block = .{
        .parent = parent_block,
        .sema = sema,
        .namespace = parent_block.namespace,
        .instructions = .{},
        .inlining = parent_block.inlining,
        .comptime_reason = .{ .reason = .{
            .src = src,
            .r = .{ .simple = .operand_cImport },
        } },
        .c_import_buf = &c_import_buf,
        .runtime_cond = parent_block.runtime_cond,
        .runtime_loop = parent_block.runtime_loop,
        .runtime_index = parent_block.runtime_index,
        .src_base_inst = parent_block.src_base_inst,
        .type_name_ctx = parent_block.type_name_ctx,
    };
    defer child_block.instructions.deinit(gpa);

    _ = try sema.analyzeInlineBody(&child_block, body, inst);

    var c_import_res = comp.cImport(c_import_buf.items, parent_block.ownerModule()) catch |err|
        return sema.fail(&child_block, src, "C import failed: {s}", .{@errorName(err)});
    defer c_import_res.deinit(gpa);

    if (c_import_res.errors.errorMessageCount() != 0) {
        const msg = msg: {
            const msg = try sema.errMsg(src, "C import failed", .{});
            errdefer msg.destroy(gpa);

            if (!comp.config.link_libc)
                try sema.errNote(src, msg, "libc headers not available; compilation does not link against libc", .{});

            const gop = try zcu.cimport_errors.getOrPut(gpa, sema.owner);
            if (!gop.found_existing) {
                gop.value_ptr.* = c_import_res.errors;
                c_import_res.errors = std.zig.ErrorBundle.empty;
            }
            break :msg msg;
        };
        return sema.failWithOwnedErrorMsg(&child_block, msg);
    }
    const parent_mod = parent_block.ownerModule();
    const digest = Cache.binToHex(c_import_res.digest);

    const new_file_index = file: {
        const c_import_zig_path = try comp.arena.dupe(u8, "o" ++ std.fs.path.sep_str ++ digest);
        const c_import_mod = Package.Module.create(comp.arena, .{
            .paths = .{
                .root = try .fromRoot(comp.arena, comp.dirs, .local_cache, c_import_zig_path),
                .root_src_path = "cimport.zig",
            },
            .fully_qualified_name = c_import_zig_path,
            .cc_argv = parent_mod.cc_argv,
            .inherited = .{},
            .global = comp.config,
            .parent = parent_mod,
        }) catch |err| switch (err) {
            // None of these are possible because we are creating a package with
            // the exact same configuration as the parent package, which already
            // passed these checks.
            error.ValgrindUnsupportedOnTarget => unreachable,
            error.TargetRequiresSingleThreaded => unreachable,
            error.BackendRequiresSingleThreaded => unreachable,
            error.TargetRequiresPic => unreachable,
            error.PieRequiresPic => unreachable,
            error.DynamicLinkingRequiresPic => unreachable,
            error.TargetHasNoRedZone => unreachable,
            error.StackCheckUnsupportedByTarget => unreachable,
            error.StackProtectorUnsupportedByTarget => unreachable,
            error.StackProtectorUnavailableWithoutLibC => unreachable,

            else => |e| return e,
        };
        const c_import_file_path: Compilation.Path = try c_import_mod.root.join(gpa, comp.dirs, "cimport.zig");
        errdefer c_import_file_path.deinit(gpa);
        const c_import_file = try gpa.create(Zcu.File);
        errdefer gpa.destroy(c_import_file);
        const c_import_file_index = try zcu.intern_pool.createFile(gpa, pt.tid, .{
            .bin_digest = c_import_file_path.digest(),
            .file = c_import_file,
            .root_type = .none,
        });
        c_import_file.* = .{
            .status = .never_loaded,
            .stat = undefined,
            .is_builtin = false,
            .path = c_import_file_path,
            .source = null,
            .tree = null,
            .zir = null,
            .zoir = null,
            .mod = c_import_mod,
            .sub_file_path = "cimport.zig",
            .module_changed = false,
            .prev_zir = null,
            .zoir_invalidated = false,
        };
        break :file c_import_file_index;
    };
    pt.updateFile(new_file_index, zcu.fileByIndex(new_file_index)) catch |err|
        return sema.fail(&child_block, src, "C import failed: {s}", .{@errorName(err)});

    try pt.ensureFileAnalyzed(new_file_index);
    const ty = zcu.fileRootType(new_file_index);
    try sema.declareDependency(.{ .interned = ty });
    try sema.addTypeReferenceEntry(src, ty);
    return Air.internedToRef(ty);
}

fn zirSuspendBlock(sema: *Sema, parent_block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].pl_node;
    const src = parent_block.nodeOffset(inst_data.src_node);
    return sema.failWithUseOfAsync(parent_block, src);
}

fn zirBlock(sema: *Sema, parent_block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    const tracy = trace(@src());
    defer tracy.end();

    const pl_node = sema.code.instructions.items(.data)[@intFromEnum(inst)].pl_node;
    const src = parent_block.nodeOffset(pl_node.src_node);
    const extra = sema.code.extraData(Zir.Inst.Block, pl_node.payload_index);
    const body = sema.code.bodySlice(extra.end, extra.data.body_len);
    const gpa = sema.gpa;

    // Reserve space for a Block instruction so that generated Break instructions can
    // point to it, even if it doesn't end up getting used because the code ends up being
    // comptime evaluated or is an unlabeled block.
    const block_inst: Air.Inst.Index = @enumFromInt(sema.air_instructions.len);
    try sema.air_instructions.append(gpa, .{
        .tag = .block,
        .data = undefined,
    });

    var label: Block.Label = .{
        .zir_block = inst,
        .merges = .{
            .src_locs = .{},
            .results = .{},
            .br_list = .{},
            .block_inst = block_inst,
        },
    };

    var child_block: Block = .{
        .parent = parent_block,
        .sema = sema,
        .namespace = parent_block.namespace,
        .instructions = .{},
        .label = &label,
        .inlining = parent_block.inlining,
        .comptime_reason = parent_block.comptime_reason,
        .is_typeof = parent_block.is_typeof,
        .want_safety = parent_block.want_safety,
        .float_mode = parent_block.float_mode,
        .c_import_buf = parent_block.c_import_buf,
        .runtime_cond = parent_block.runtime_cond,
        .runtime_loop = parent_block.runtime_loop,
        .runtime_index = parent_block.runtime_index,
        .error_return_trace_index = parent_block.error_return_trace_index,
        .src_base_inst = parent_block.src_base_inst,
        .type_name_ctx = parent_block.type_name_ctx,
    };

    defer child_block.instructions.deinit(gpa);
    defer label.merges.deinit(gpa);

    return sema.resolveBlockBody(parent_block, src, &child_block, body, inst, &label.merges);
}

/// Semantically analyze the given ZIR body, emitting any resulting runtime code into the AIR block
/// specified by `child_block` if necessary (and emitting this block into `parent_block`).
/// TODO: `merges` is known from `child_block`, remove this parameter.
fn resolveBlockBody(
    sema: *Sema,
    parent_block: *Block,
    src: LazySrcLoc,
    child_block: *Block,
    body: []const Zir.Inst.Index,
    /// This is the instruction that a break instruction within `body` can
    /// use to return from the body.
    body_inst: Zir.Inst.Index,
    merges: *Block.Merges,
) CompileError!Air.Inst.Ref {
    if (child_block.isComptime()) {
        return sema.resolveInlineBody(child_block, body, body_inst);
    } else {
        assert(sema.air_instructions.items(.tag)[@intFromEnum(merges.block_inst)] == .block);
        var need_debug_scope = false;
        child_block.need_debug_scope = &need_debug_scope;
        if (sema.analyzeBodyInner(child_block, body)) |_| {
            return sema.resolveAnalyzedBlock(parent_block, src, child_block, merges, need_debug_scope);
        } else |err| switch (err) {
            error.ComptimeBreak => {
                // Comptime control flow is happening, however child_block may still contain
                // runtime instructions which need to be copied to the parent block.
                if (need_debug_scope and child_block.instructions.items.len > 0) {
                    // We need a runtime block for scoping reasons.
                    _ = try child_block.addBr(merges.block_inst, .void_value);
                    try parent_block.instructions.append(sema.gpa, merges.block_inst);
                    try sema.air_extra.ensureUnusedCapacity(sema.gpa, @typeInfo(Air.Block).@"struct".fields.len +
                        child_block.instructions.items.len);
                    sema.air_instructions.items(.data)[@intFromEnum(merges.block_inst)] = .{ .ty_pl = .{
                        .ty = .void_type,
                        .payload = sema.addExtraAssumeCapacity(Air.Block{
                            .body_len = @intCast(child_block.instructions.items.len),
                        }),
                    } };
                    sema.air_extra.appendSliceAssumeCapacity(@ptrCast(child_block.instructions.items));
                } else {
                    // We can copy instructions directly to the parent block.
                    try parent_block.instructions.appendSlice(sema.gpa, child_block.instructions.items);
                }

                const break_inst = sema.comptime_break_inst;
                const break_data = sema.code.instructions.items(.data)[@intFromEnum(break_inst)].@"break";
                const extra = sema.code.extraData(Zir.Inst.Break, break_data.payload_index).data;
                if (extra.block_inst == body_inst) {
                    return try sema.resolveInst(break_data.operand);
                } else {
                    return error.ComptimeBreak;
                }
            },
            else => |e| return e,
        }
    }
}

/// After a body corresponding to an AIR `block` has been analyzed, this function places them into
/// the block pointed at by `merges.block_inst` if necessary, or the block may be elided in favor of
/// inlining the instructions directly into the parent block. Either way, it considers all merges of
/// this block, and combines them appropriately using peer type resolution, returning the final
/// value of the block.
fn resolveAnalyzedBlock(
    sema: *Sema,
    parent_block: *Block,
    src: LazySrcLoc,
    child_block: *Block,
    merges: *Block.Merges,
    need_debug_scope: bool,
) CompileError!Air.Inst.Ref {
    const tracy = trace(@src());
    defer tracy.end();

    const gpa = sema.gpa;
    const pt = sema.pt;
    const zcu = pt.zcu;

    // Blocks must terminate with noreturn instruction.
    assert(child_block.instructions.items.len != 0);
    assert(sema.typeOf(child_block.instructions.items[child_block.instructions.items.len - 1].toRef()).isNoReturn(zcu));

    const block_tag = sema.air_instructions.items(.tag)[@intFromEnum(merges.block_inst)];
    switch (block_tag) {
        .block => {},
        .dbg_inline_block => assert(need_debug_scope),
        else => unreachable,
    }
    if (merges.results.items.len == 0) {
        switch (block_tag) {
            .block => {
                // No need for a block instruction. We can put the new instructions
                // directly into the parent block.
                if (need_debug_scope) {
                    // The code following this block is unreachable, as the block has no
                    // merges, so we don't necessarily need to emit this as an AIR block.
                    // However, we need a block *somewhere* to make the scoping correct,
                    // so forward this request to the parent block.
                    if (parent_block.need_debug_scope) |ptr| ptr.* = true;
                }
                try parent_block.instructions.appendSlice(gpa, child_block.instructions.items);
                return child_block.instructions.items[child_block.instructions.items.len - 1].toRef();
            },
            .dbg_inline_block => {
                // Create a block containing all instruction from the body.
                try parent_block.instructions.append(gpa, merges.block_inst);
                try sema.air_extra.ensureUnusedCapacity(gpa, @typeInfo(Air.DbgInlineBlock).@"struct".fields.len +
                    child_block.instructions.items.len);
                sema.air_instructions.items(.data)[@intFromEnum(merges.block_inst)] = .{ .ty_pl = .{
                    .ty = .noreturn_type,
                    .payload = sema.addExtraAssumeCapacity(Air.DbgInlineBlock{
                        .func = child_block.inlining.?.func,
                        .body_len = @intCast(child_block.instructions.items.len),
                    }),
                } };
                sema.air_extra.appendSliceAssumeCapacity(@ptrCast(child_block.instructions.items));
                return merges.block_inst.toRef();
            },
            else => unreachable,
        }
    }
    if (merges.results.items.len == 1) {
        // If the `break` is trailing, we may be able to elide the AIR block here
        // by appending the new instructions directly to the parent block.
        if (!need_debug_scope) {
            const last_inst_index = child_block.instructions.items.len - 1;
            const last_inst = child_block.instructions.items[last_inst_index];
            if (sema.getBreakBlock(last_inst)) |br_block| {
                if (br_block == merges.block_inst) {
                    // Great, the last instruction is the break! Put the instructions
                    // directly into the parent block.
                    try parent_block.instructions.appendSlice(gpa, child_block.instructions.items[0..last_inst_index]);
                    return merges.results.items[0];
                }
            }
        }
        // Okay, we need a runtime block. If the value is comptime-known, the
        // block should just return void, and we return the merge result
        // directly. Otherwise, we can defer to the logic below.
        if (try sema.resolveValue(merges.results.items[0])) |result_val| {
            // Create a block containing all instruction from the body.
            try parent_block.instructions.append(gpa, merges.block_inst);
            switch (block_tag) {
                .block => {
                    try sema.air_extra.ensureUnusedCapacity(gpa, @typeInfo(Air.Block).@"struct".fields.len +
                        child_block.instructions.items.len);
                    sema.air_instructions.items(.data)[@intFromEnum(merges.block_inst)] = .{ .ty_pl = .{
                        .ty = .void_type,
                        .payload = sema.addExtraAssumeCapacity(Air.Block{
                            .body_len = @intCast(child_block.instructions.items.len),
                        }),
                    } };
                },
                .dbg_inline_block => {
                    try sema.air_extra.ensureUnusedCapacity(gpa, @typeInfo(Air.DbgInlineBlock).@"struct".fields.len +
                        child_block.instructions.items.len);
                    sema.air_instructions.items(.data)[@intFromEnum(merges.block_inst)] = .{ .ty_pl = .{
                        .ty = .void_type,
                        .payload = sema.addExtraAssumeCapacity(Air.DbgInlineBlock{
                            .func = child_block.inlining.?.func,
                            .body_len = @intCast(child_block.instructions.items.len),
                        }),
                    } };
                },
                else => unreachable,
            }
            sema.air_extra.appendSliceAssumeCapacity(@ptrCast(child_block.instructions.items));
            // Rewrite the break to just give value {}; the value is
            // comptime-known and will be returned directly.
            sema.air_instructions.items(.data)[@intFromEnum(merges.br_list.items[0])].br.operand = .void_value;
            return Air.internedToRef(result_val.toIntern());
        }
    }
    // It is impossible to have the number of results be > 1 in a comptime scope.
    assert(!child_block.isComptime()); // Should already got a compile error in the condbr condition.

    // Note that we'll always create an AIR block here, so `need_debug_scope` is irrelevant.

    // Need to set the type and emit the Block instruction. This allows machine code generation
    // to emit a jump instruction to after the block when it encounters the break.
    try parent_block.instructions.append(gpa, merges.block_inst);
    const resolved_ty = try sema.resolvePeerTypes(parent_block, src, merges.results.items, .{ .override = merges.src_locs.items });
    // TODO add note "missing else causes void value"

    const type_src = src; // TODO: better source location
    if (try resolved_ty.comptimeOnlySema(pt)) {
        const msg = msg: {
            const msg = try sema.errMsg(type_src, "value with comptime-only type '{}' depends on runtime control flow", .{resolved_ty.fmt(pt)});
            errdefer msg.destroy(sema.gpa);

            const runtime_src = child_block.runtime_cond orelse child_block.runtime_loop.?;
            try sema.errNote(runtime_src, msg, "runtime control flow here", .{});

            try sema.explainWhyTypeIsComptime(msg, type_src, resolved_ty);

            break :msg msg;
        };
        return sema.failWithOwnedErrorMsg(child_block, msg);
    }
    for (merges.results.items, merges.src_locs.items) |merge_inst, merge_src| {
        try sema.validateRuntimeValue(child_block, merge_src orelse src, merge_inst);
    }

    try sema.checkMergeAllowed(child_block, type_src, resolved_ty);

    const ty_inst = Air.internedToRef(resolved_ty.toIntern());
    switch (block_tag) {
        .block => {
            try sema.air_extra.ensureUnusedCapacity(gpa, @typeInfo(Air.Block).@"struct".fields.len +
                child_block.instructions.items.len);
            sema.air_instructions.items(.data)[@intFromEnum(merges.block_inst)] = .{ .ty_pl = .{
                .ty = ty_inst,
                .payload = sema.addExtraAssumeCapacity(Air.Block{
                    .body_len = @intCast(child_block.instructions.items.len),
                }),
            } };
        },
        .dbg_inline_block => {
            try sema.air_extra.ensureUnusedCapacity(gpa, @typeInfo(Air.DbgInlineBlock).@"struct".fields.len +
                child_block.instructions.items.len);
            sema.air_instructions.items(.data)[@intFromEnum(merges.block_inst)] = .{ .ty_pl = .{
                .ty = ty_inst,
                .payload = sema.addExtraAssumeCapacity(Air.DbgInlineBlock{
                    .func = child_block.inlining.?.func,
                    .body_len = @intCast(child_block.instructions.items.len),
                }),
            } };
        },
        else => unreachable,
    }
    sema.air_extra.appendSliceAssumeCapacity(@ptrCast(child_block.instructions.items));
    // Now that the block has its type resolved, we need to go back into all the break
    // instructions, and insert type coercion on the operands.
    for (merges.br_list.items) |br| {
        const br_operand = sema.air_instructions.items(.data)[@intFromEnum(br)].br.operand;
        const br_operand_src = src;
        const br_operand_ty = sema.typeOf(br_operand);
        if (br_operand_ty.eql(resolved_ty, zcu)) {
            // No type coercion needed.
            continue;
        }
        var coerce_block = parent_block.makeSubBlock();
        defer coerce_block.instructions.deinit(gpa);
        const coerced_operand = try sema.coerce(&coerce_block, resolved_ty, br_operand, br_operand_src);
        // If no instructions were produced, such as in the case of a coercion of a
        // constant value to a new type, we can simply point the br operand to it.
        if (coerce_block.instructions.items.len == 0) {
            sema.air_instructions.items(.data)[@intFromEnum(br)].br.operand = coerced_operand;
            continue;
        }
        assert(coerce_block.instructions.items[coerce_block.instructions.items.len - 1].toRef() == coerced_operand);

        // Convert the br instruction to a block instruction that has the coercion
        // and then a new br inside that returns the coerced instruction.
        const sub_block_len: u32 = @intCast(coerce_block.instructions.items.len + 1);
        try sema.air_extra.ensureUnusedCapacity(gpa, @typeInfo(Air.Block).@"struct".fields.len +
            sub_block_len);
        try sema.air_instructions.ensureUnusedCapacity(gpa, 1);
        const sub_br_inst: Air.Inst.Index = @enumFromInt(sema.air_instructions.len);

        sema.air_instructions.items(.tag)[@intFromEnum(br)] = .block;
        sema.air_instructions.items(.data)[@intFromEnum(br)] = .{ .ty_pl = .{
            .ty = .noreturn_type,
            .payload = sema.addExtraAssumeCapacity(Air.Block{
                .body_len = sub_block_len,
            }),
        } };
        sema.air_extra.appendSliceAssumeCapacity(@ptrCast(coerce_block.instructions.items));
        sema.air_extra.appendAssumeCapacity(@intFromEnum(sub_br_inst));

        sema.air_instructions.appendAssumeCapacity(.{
            .tag = .br,
            .data = .{ .br = .{
                .block_inst = merges.block_inst,
                .operand = coerced_operand,
            } },
        });
    }
    return merges.block_inst.toRef();
}

fn zirExport(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!void {
    const tracy = trace(@src());
    defer tracy.end();

    const pt = sema.pt;
    const zcu = pt.zcu;
    const ip = &zcu.intern_pool;
    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].pl_node;
    const extra = sema.code.extraData(Zir.Inst.Export, inst_data.payload_index).data;

    const src = block.nodeOffset(inst_data.src_node);
    const ptr_src = block.builtinCallArgSrc(inst_data.src_node, 0);
    const options_src = block.builtinCallArgSrc(inst_data.src_node, 1);

    const ptr = try sema.resolveInst(extra.exported);
    const ptr_val = try sema.resolveConstDefinedValue(block, ptr_src, ptr, .{ .simple = .export_target });
    const ptr_ty = ptr_val.typeOf(zcu);

    const options = try sema.resolveExportOptions(block, options_src, extra.options);

    {
        if (ptr_ty.zigTypeTag(zcu) != .pointer) {
            return sema.fail(block, ptr_src, "expected pointer type, found '{}'", .{ptr_ty.fmt(pt)});
        }
        const ptr_ty_info = ptr_ty.ptrInfo(zcu);
        if (ptr_ty_info.flags.size == .slice) {
            return sema.fail(block, ptr_src, "export target cannot be slice", .{});
        }
        if (ptr_ty_info.packed_offset.host_size != 0) {
            return sema.fail(block, ptr_src, "export target cannot be bit-pointer", .{});
        }
    }

    const ptr_info = ip.indexToKey(ptr_val.toIntern()).ptr;
    switch (ptr_info.base_addr) {
        .comptime_alloc, .int, .comptime_field => return sema.fail(block, ptr_src, "export target must be a global variable or a comptime-known constant", .{}),
        .eu_payload, .opt_payload, .field, .arr_elem => return sema.fail(block, ptr_src, "TODO: export pointer in middle of value", .{}),
        .uav => |uav| {
            if (ptr_info.byte_offset != 0) {
                return sema.fail(block, ptr_src, "TODO: export pointer in middle of value", .{});
            }
            if (options.linkage == .internal) return;
            const export_ty = Value.fromInterned(uav.val).typeOf(zcu);
            if (!try sema.validateExternType(export_ty, .other)) {
                return sema.failWithOwnedErrorMsg(block, msg: {
                    const msg = try sema.errMsg(src, "unable to export type '{}'", .{export_ty.fmt(pt)});
                    errdefer msg.destroy(sema.gpa);
                    try sema.explainWhyTypeIsNotExtern(msg, src, export_ty, .other);
                    try sema.addDeclaredHereNote(msg, export_ty);
                    break :msg msg;
                });
            }
            try sema.exports.append(zcu.gpa, .{
                .opts = options,
                .src = src,
                .exported = .{ .uav = uav.val },
                .status = .in_progress,
            });
        },
        .nav => |nav| {
            if (ptr_info.byte_offset != 0) {
                return sema.fail(block, ptr_src, "TODO: export pointer in middle of value", .{});
            }
            try sema.analyzeExport(block, src, options, nav);
        },
    }
}

pub fn analyzeExport(
    sema: *Sema,
    block: *Block,
    src: LazySrcLoc,
    options: Zcu.Export.Options,
    orig_nav_index: InternPool.Nav.Index,
) !void {
    const gpa = sema.gpa;
    const pt = sema.pt;
    const zcu = pt.zcu;
    const ip = &zcu.intern_pool;

    if (options.linkage == .internal)
        return;

    try sema.ensureNavResolved(block, src, orig_nav_index, .fully);

    const exported_nav_index = switch (ip.indexToKey(ip.getNav(orig_nav_index).status.fully_resolved.val)) {
        .variable => |v| v.owner_nav,
        .@"extern" => |e| e.owner_nav,
        .func => |f| f.owner_nav,
        else => orig_nav_index,
    };

    const exported_nav = ip.getNav(exported_nav_index);
    const export_ty: Type = .fromInterned(exported_nav.typeOf(ip));

    if (!try sema.validateExternType(export_ty, .other)) {
        return sema.failWithOwnedErrorMsg(block, msg: {
            const msg = try sema.errMsg(src, "unable to export type '{}'", .{export_ty.fmt(pt)});
            errdefer msg.destroy(gpa);

            try sema.explainWhyTypeIsNotExtern(msg, src, export_ty, .other);

            try sema.addDeclaredHereNote(msg, export_ty);
            break :msg msg;
        });
    }

    // TODO: some backends might support re-exporting extern decls
    if (exported_nav.getExtern(ip) != null) {
        return sema.fail(block, src, "export target cannot be extern", .{});
    }

    try sema.maybeQueueFuncBodyAnalysis(block, src, exported_nav_index);

    try sema.exports.append(gpa, .{
        .opts = options,
        .src = src,
        .exported = .{ .nav = exported_nav_index },
        .status = .in_progress,
    });
}

fn zirDisableInstrumentation(sema: *Sema) CompileError!void {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const ip = &zcu.intern_pool;
    const func = switch (sema.owner.unwrap()) {
        .func => |func| func,
        .@"comptime",
        .nav_val,
        .nav_ty,
        .type,
        .memoized_state,
        => return, // does nothing outside a function
    };
    ip.funcSetDisableInstrumentation(func);
    sema.allow_memoize = false;
}

fn zirDisableIntrinsics(sema: *Sema) CompileError!void {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const ip = &zcu.intern_pool;
    const func = switch (sema.owner.unwrap()) {
        .func => |func| func,
        .@"comptime",
        .nav_val,
        .nav_ty,
        .type,
        .memoized_state,
        => return, // does nothing outside a function
    };
    ip.funcSetDisableIntrinsics(func);
    sema.allow_memoize = false;
}

fn zirSetFloatMode(sema: *Sema, block: *Block, extended: Zir.Inst.Extended.InstData) CompileError!void {
    const extra = sema.code.extraData(Zir.Inst.UnNode, extended.operand).data;
    const src = block.builtinCallArgSrc(extra.node, 0);
    block.float_mode = try sema.resolveBuiltinEnum(block, src, extra.operand, .FloatMode, .{ .simple = .operand_setFloatMode });
}

fn zirSetRuntimeSafety(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!void {
    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].un_node;
    const operand_src = block.builtinCallArgSrc(inst_data.src_node, 0);
    block.want_safety = try sema.resolveConstBool(block, operand_src, inst_data.operand, .{ .simple = .operand_setRuntimeSafety });
}

fn zirBreak(sema: *Sema, start_block: *Block, inst: Zir.Inst.Index) CompileError!void {
    const tracy = trace(@src());
    defer tracy.end();

    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].@"break";
    const extra = sema.code.extraData(Zir.Inst.Break, inst_data.payload_index).data;
    const operand = try sema.resolveInst(inst_data.operand);
    const zir_block = extra.block_inst;

    var block = start_block;
    while (true) {
        if (block.label) |label| {
            if (label.zir_block == zir_block) {
                const br_ref = try start_block.addBr(label.merges.block_inst, operand);
                const src_loc = if (extra.operand_src_node.unwrap()) |operand_src_node|
                    start_block.nodeOffset(operand_src_node)
                else
                    null;
                try label.merges.src_locs.append(sema.gpa, src_loc);
                try label.merges.results.append(sema.gpa, operand);
                try label.merges.br_list.append(sema.gpa, br_ref.toIndex().?);
                block.runtime_index.increment();
                if (block.runtime_cond == null and block.runtime_loop == null) {
                    block.runtime_cond = start_block.runtime_cond orelse start_block.runtime_loop;
                    block.runtime_loop = start_block.runtime_loop;
                }
                return;
            }
        }
        block = block.parent.?;
    }
}

fn zirSwitchContinue(sema: *Sema, start_block: *Block, inst: Zir.Inst.Index) CompileError!void {
    const tracy = trace(@src());
    defer tracy.end();

    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].@"break";
    const extra = sema.code.extraData(Zir.Inst.Break, inst_data.payload_index).data;
    const operand_src = start_block.nodeOffset(extra.operand_src_node.unwrap().?);
    const uncoerced_operand = try sema.resolveInst(inst_data.operand);
    const switch_inst = extra.block_inst;

    switch (sema.code.instructions.items(.tag)[@intFromEnum(switch_inst)]) {
        .switch_block, .switch_block_ref => {},
        else => unreachable, // assertion failure
    }

    const switch_payload_index = sema.code.instructions.items(.data)[@intFromEnum(switch_inst)].pl_node.payload_index;
    const switch_operand_ref = sema.code.extraData(Zir.Inst.SwitchBlock, switch_payload_index).data.operand;
    const switch_operand_ty = sema.typeOf(try sema.resolveInst(switch_operand_ref));

    const operand = try sema.coerce(start_block, switch_operand_ty, uncoerced_operand, operand_src);

    try sema.validateRuntimeValue(start_block, operand_src, operand);

    // We want to generate a `switch_dispatch` instruction with the switch condition,
    // possibly preceded by a store to the stack alloc containing the raw operand.
    // However, to avoid too much special-case state in Sema, this is handled by the
    // `switch` lowering logic. As such, we will find the `Block` corresponding to the
    // parent `switch_block[_ref]` instruction, create a dummy `br`, and add a merge
    // to signal to the switch logic to rewrite this into an appropriate dispatch.

    var block = start_block;
    while (true) {
        if (block.label) |label| {
            if (label.zir_block == switch_inst) {
                const br_ref = try start_block.addBr(label.merges.block_inst, operand);
                try label.merges.extra_insts.append(sema.gpa, br_ref.toIndex().?);
                try label.merges.extra_src_locs.append(sema.gpa, operand_src);
                block.runtime_index.increment();
                if (block.runtime_cond == null and block.runtime_loop == null) {
                    block.runtime_cond = start_block.runtime_cond orelse start_block.runtime_loop;
                    block.runtime_loop = start_block.runtime_loop;
                }
                return;
            }
        }
        block = block.parent.?;
    }
}

fn zirDbgStmt(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!void {
    if (block.isComptime() or block.ownerModule().strip) return;

    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].dbg_stmt;

    if (block.instructions.items.len != 0) {
        const idx = block.instructions.items[block.instructions.items.len - 1];
        if (sema.air_instructions.items(.tag)[@intFromEnum(idx)] == .dbg_stmt) {
            // The previous dbg_stmt didn't correspond to any actual code, so replace it.
            sema.air_instructions.items(.data)[@intFromEnum(idx)].dbg_stmt = .{
                .line = inst_data.line,
                .column = inst_data.column,
            };
            return;
        }
    }

    _ = try block.addInst(.{
        .tag = .dbg_stmt,
        .data = .{ .dbg_stmt = .{
            .line = inst_data.line,
            .column = inst_data.column,
        } },
    });
}

fn zirDbgEmptyStmt(_: *Sema, block: *Block, _: Zir.Inst.Index) CompileError!void {
    if (block.isComptime() or block.ownerModule().strip) return;
    _ = try block.addNoOp(.dbg_empty_stmt);
}

fn zirDbgVar(
    sema: *Sema,
    block: *Block,
    inst: Zir.Inst.Index,
    air_tag: Air.Inst.Tag,
) CompileError!void {
    const str_op = sema.code.instructions.items(.data)[@intFromEnum(inst)].str_op;
    const operand = try sema.resolveInst(str_op.operand);
    const name = str_op.getStr(sema.code);
    try sema.addDbgVar(block, operand, air_tag, name);
}

fn addDbgVar(
    sema: *Sema,
    block: *Block,
    operand: Air.Inst.Ref,
    air_tag: Air.Inst.Tag,
    name: []const u8,
) CompileError!void {
    if (block.isComptime() or block.ownerModule().strip) return;

    const pt = sema.pt;
    const zcu = pt.zcu;
    const operand_ty = sema.typeOf(operand);
    const val_ty = switch (air_tag) {
        .dbg_var_ptr => operand_ty.childType(zcu),
        .dbg_var_val, .dbg_arg_inline => operand_ty,
        else => unreachable,
    };
    if (try val_ty.comptimeOnlySema(pt)) return;
    if (!(try val_ty.hasRuntimeBitsSema(pt))) return;
    if (try sema.resolveValue(operand)) |operand_val| {
        if (operand_val.canMutateComptimeVarState(zcu)) return;
    }

    // To ensure the lexical scoping is known to backends, this alloc must be
    // within a real runtime block. We set a flag which communicates information
    // to the closest lexically enclosing block:
    // * If it is a `block_inline`, communicates to logic in `analyzeBodyInner`
    //   to create a post-hoc block.
    // * Otherwise, communicates to logic in `resolveBlockBody` to create a
    //   real `block` instruction.
    if (block.need_debug_scope) |ptr| ptr.* = true;

    // Add the name to the AIR.
    const name_nts = try sema.appendAirString(name);

    _ = try block.addInst(.{
        .tag = air_tag,
        .data = .{ .pl_op = .{
            .payload = @intFromEnum(name_nts),
            .operand = operand,
        } },
    });
}

pub fn appendAirString(sema: *Sema, str: []const u8) Allocator.Error!Air.NullTerminatedString {
    if (str.len == 0) return .none;
    const nts: Air.NullTerminatedString = @enumFromInt(sema.air_extra.items.len);
    const elements_used = str.len / 4 + 1;
    const elements = try sema.air_extra.addManyAsSlice(sema.gpa, elements_used);
    const buffer = mem.sliceAsBytes(elements);
    @memcpy(buffer[0..str.len], str);
    buffer[str.len] = 0;
    return nts;
}

fn zirDeclRef(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].str_tok;
    const src = block.tokenOffset(inst_data.src_tok);
    const decl_name = try zcu.intern_pool.getOrPutString(
        sema.gpa,
        pt.tid,
        inst_data.get(sema.code),
        .no_embedded_nulls,
    );
    const nav_index = try sema.lookupIdentifier(block, src, decl_name);
    return sema.analyzeNavRef(block, src, nav_index);
}

fn zirDeclVal(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].str_tok;
    const src = block.tokenOffset(inst_data.src_tok);
    const decl_name = try zcu.intern_pool.getOrPutString(
        sema.gpa,
        pt.tid,
        inst_data.get(sema.code),
        .no_embedded_nulls,
    );
    const nav = try sema.lookupIdentifier(block, src, decl_name);
    return sema.analyzeNavVal(block, src, nav);
}

fn lookupIdentifier(sema: *Sema, block: *Block, src: LazySrcLoc, name: InternPool.NullTerminatedString) !InternPool.Nav.Index {
    const pt = sema.pt;
    const zcu = pt.zcu;
    var namespace = block.namespace;
    while (true) {
        if (try sema.lookupInNamespace(block, src, namespace, name, false)) |lookup| {
            assert(lookup.accessible);
            return lookup.nav;
        }
        namespace = zcu.namespacePtr(namespace).parent.unwrap() orelse break;
    }
    unreachable; // AstGen detects use of undeclared identifiers.
}

/// This looks up a member of a specific namespace. It is affected by `usingnamespace` but
/// only for ones in the specified namespace.
fn lookupInNamespace(
    sema: *Sema,
    block: *Block,
    src: LazySrcLoc,
    namespace_index: InternPool.NamespaceIndex,
    ident_name: InternPool.NullTerminatedString,
    observe_usingnamespace: bool,
) CompileError!?struct {
    nav: InternPool.Nav.Index,
    /// If `false`, the declaration is in a different file and is not `pub`.
    /// We still return the declaration for better error reporting.
    accessible: bool,
} {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const ip = &zcu.intern_pool;

    try pt.ensureNamespaceUpToDate(namespace_index);

    const namespace = zcu.namespacePtr(namespace_index);

    const adapter: Zcu.Namespace.NameAdapter = .{ .zcu = zcu };

    const src_file = zcu.namespacePtr(block.namespace).file_scope;

    if (Type.fromInterned(namespace.owner_type).typeDeclInst(zcu)) |type_decl_inst| {
        try sema.declareDependency(.{ .namespace_name = .{
            .namespace = type_decl_inst,
            .name = ident_name,
        } });
    }

    if (observe_usingnamespace and (namespace.pub_usingnamespace.items.len != 0 or namespace.priv_usingnamespace.items.len != 0)) {
        const gpa = sema.gpa;
        var checked_namespaces: std.AutoArrayHashMapUnmanaged(*Namespace, void) = .empty;
        defer checked_namespaces.deinit(gpa);

        // Keep track of name conflicts for error notes.
        var candidates: std.ArrayListUnmanaged(InternPool.Nav.Index) = .empty;
        defer candidates.deinit(gpa);

        try checked_namespaces.put(gpa, namespace, {});
        var check_i: usize = 0;

        while (check_i < checked_namespaces.count()) : (check_i += 1) {
            const check_ns = checked_namespaces.keys()[check_i];
            const Pass = enum { @"pub", priv };
            for ([2]Pass{ .@"pub", .priv }) |pass| {
                if (pass == .priv and src_file != check_ns.file_scope) {
                    continue;
                }

                const decls, const usingnamespaces = switch (pass) {
                    .@"pub" => .{ &check_ns.pub_decls, &check_ns.pub_usingnamespace },
                    .priv => .{ &check_ns.priv_decls, &check_ns.priv_usingnamespace },
                };

                if (decls.getKeyAdapted(ident_name, adapter)) |nav_index| {
                    try candidates.append(gpa, nav_index);
                }

                for (usingnamespaces.items) |sub_ns_nav| {
                    try sema.ensureNavResolved(block, src, sub_ns_nav, .fully);
                    const sub_ns_ty: Type = .fromInterned(ip.getNav(sub_ns_nav).status.fully_resolved.val);
                    const sub_ns = zcu.namespacePtr(sub_ns_ty.getNamespaceIndex(zcu));
                    try checked_namespaces.put(gpa, sub_ns, {});
                }
            }
        }

        ignore_self: {
            const skip_nav = switch (sema.owner.unwrap()) {
                .@"comptime", .type, .func, .memoized_state => break :ignore_self,
                .nav_ty, .nav_val => |nav| nav,
            };
            var i: usize = 0;
            while (i < candidates.items.len) {
                if (candidates.items[i] == skip_nav) {
                    _ = candidates.orderedRemove(i);
                } else {
                    i += 1;
                }
            }
        }

        switch (candidates.items.len) {
            0 => {},
            1 => return .{
                .nav = candidates.items[0],
                .accessible = true,
            },
            else => return sema.failWithOwnedErrorMsg(block, msg: {
                const msg = try sema.errMsg(src, "ambiguous reference", .{});
                errdefer msg.destroy(gpa);
                for (candidates.items) |candidate| {
                    try sema.errNote(zcu.navSrcLoc(candidate), msg, "declared here", .{});
                }
                break :msg msg;
            }),
        }
    } else if (namespace.pub_decls.getKeyAdapted(ident_name, adapter)) |nav_index| {
        return .{
            .nav = nav_index,
            .accessible = true,
        };
    } else if (namespace.priv_decls.getKeyAdapted(ident_name, adapter)) |nav_index| {
        return .{
            .nav = nav_index,
            .accessible = src_file == namespace.file_scope,
        };
    }

    return null;
}

fn funcDeclSrcInst(sema: *Sema, func_inst: Air.Inst.Ref) !?InternPool.TrackedInst.Index {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const ip = &zcu.intern_pool;
    const func_val = try sema.resolveValue(func_inst) orelse return null;
    if (func_val.isUndef(zcu)) return null;
    const nav = switch (ip.indexToKey(func_val.toIntern())) {
        .@"extern" => |e| e.owner_nav,
        .func => |f| f.owner_nav,
        .ptr => |ptr| switch (ptr.base_addr) {
            .nav => |nav| if (ptr.byte_offset == 0) nav else return null,
            else => return null,
        },
        else => return null,
    };
    return ip.getNav(nav).srcInst(ip);
}

pub fn analyzeSaveErrRetIndex(sema: *Sema, block: *Block) SemaError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const gpa = sema.gpa;

    if (block.isComptime() or block.is_typeof) {
        const index_val = try pt.intValue_u64(.usize, sema.comptime_err_ret_trace.items.len);
        return Air.internedToRef(index_val.toIntern());
    }

    if (!block.ownerModule().error_tracing) return .none;

    const stack_trace_ty = try sema.getBuiltinType(block.nodeOffset(.zero), .StackTrace);
    try stack_trace_ty.resolveFields(pt);
    const field_name = try zcu.intern_pool.getOrPutString(gpa, pt.tid, "index", .no_embedded_nulls);
    const field_index = sema.structFieldIndex(block, stack_trace_ty, field_name, LazySrcLoc.unneeded) catch |err| switch (err) {
        error.AnalysisFail => @panic("std.builtin.StackTrace is corrupt"),
        error.ComptimeReturn, error.ComptimeBreak => unreachable,
        error.OutOfMemory => |e| return e,
    };

    return try block.addInst(.{
        .tag = .save_err_return_trace_index,
        .data = .{ .ty_pl = .{
            .ty = Air.internedToRef(stack_trace_ty.toIntern()),
            .payload = @intCast(field_index),
        } },
    });
}

/// Add instructions to block to "pop" the error return trace.
/// If `operand` is provided, only pops if operand is non-error.
fn popErrorReturnTrace(
    sema: *Sema,
    block: *Block,
    src: LazySrcLoc,
    operand: Air.Inst.Ref,
    saved_error_trace_index: Air.Inst.Ref,
) CompileError!void {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const gpa = sema.gpa;
    var is_non_error: ?bool = null;
    var is_non_error_inst: Air.Inst.Ref = undefined;
    if (operand != .none) {
        is_non_error_inst = try sema.analyzeIsNonErr(block, src, operand);
        if (try sema.resolveDefinedValue(block, src, is_non_error_inst)) |cond_val|
            is_non_error = cond_val.toBool();
    } else is_non_error = true; // no operand means pop unconditionally

    if (is_non_error == true) {
        // AstGen determined this result does not go to an error-handling expr (try/catch/return etc.), or
        // the result is comptime-known to be a non-error. Either way, pop unconditionally.

        const stack_trace_ty = try sema.getBuiltinType(src, .StackTrace);
        try stack_trace_ty.resolveFields(pt);
        const ptr_stack_trace_ty = try pt.singleMutPtrType(stack_trace_ty);
        const err_return_trace = try block.addTy(.err_return_trace, ptr_stack_trace_ty);
        const field_name = try zcu.intern_pool.getOrPutString(gpa, pt.tid, "index", .no_embedded_nulls);
        const field_ptr = try sema.structFieldPtr(block, src, err_return_trace, field_name, src, stack_trace_ty, true);
        try sema.storePtr2(block, src, field_ptr, src, saved_error_trace_index, src, .store);
    } else if (is_non_error == null) {
        // The result might be an error. If it is, we leave the error trace alone. If it isn't, we need
        // to pop any error trace that may have been propagated from our arguments.

        try sema.air_extra.ensureUnusedCapacity(gpa, @typeInfo(Air.Block).@"struct".fields.len);
        const cond_block_inst = try block.addInstAsIndex(.{
            .tag = .block,
            .data = .{
                .ty_pl = .{
                    .ty = .void_type,
                    .payload = undefined, // updated below
                },
            },
        });

        var then_block = block.makeSubBlock();
        defer then_block.instructions.deinit(gpa);

        // If non-error, then pop the error return trace by restoring the index.
        const stack_trace_ty = try sema.getBuiltinType(src, .StackTrace);
        try stack_trace_ty.resolveFields(pt);
        const ptr_stack_trace_ty = try pt.singleMutPtrType(stack_trace_ty);
        const err_return_trace = try then_block.addTy(.err_return_trace, ptr_stack_trace_ty);
        const field_name = try zcu.intern_pool.getOrPutString(gpa, pt.tid, "index", .no_embedded_nulls);
        const field_ptr = try sema.structFieldPtr(&then_block, src, err_return_trace, field_name, src, stack_trace_ty, true);
        try sema.storePtr2(&then_block, src, field_ptr, src, saved_error_trace_index, src, .store);
        _ = try then_block.addBr(cond_block_inst, .void_value);

        // Otherwise, do nothing
        var else_block = block.makeSubBlock();
        defer else_block.instructions.deinit(gpa);
        _ = try else_block.addBr(cond_block_inst, .void_value);

        try sema.air_extra.ensureUnusedCapacity(gpa, @typeInfo(Air.CondBr).@"struct".fields.len +
            then_block.instructions.items.len + else_block.instructions.items.len +
            @typeInfo(Air.Block).@"struct".fields.len + 1); // +1 for the sole .cond_br instruction in the .block

        const cond_br_inst: Air.Inst.Index = @enumFromInt(sema.air_instructions.len);
        try sema.air_instructions.append(gpa, .{
            .tag = .cond_br,
            .data = .{
                .pl_op = .{
                    .operand = is_non_error_inst,
                    .payload = sema.addExtraAssumeCapacity(Air.CondBr{
                        .then_body_len = @intCast(then_block.instructions.items.len),
                        .else_body_len = @intCast(else_block.instructions.items.len),
                        .branch_hints = .{
                            // Weight against error branch.
                            .true = .likely,
                            .false = .unlikely,
                            // Code coverage is not valuable on either branch.
                            .then_cov = .none,
                            .else_cov = .none,
                        },
                    }),
                },
            },
        });
        sema.air_extra.appendSliceAssumeCapacity(@ptrCast(then_block.instructions.items));
        sema.air_extra.appendSliceAssumeCapacity(@ptrCast(else_block.instructions.items));

        sema.air_instructions.items(.data)[@intFromEnum(cond_block_inst)].ty_pl.payload = sema.addExtraAssumeCapacity(Air.Block{ .body_len = 1 });
        sema.air_extra.appendAssumeCapacity(@intFromEnum(cond_br_inst));
    }
}

fn zirCall(
    sema: *Sema,
    block: *Block,
    inst: Zir.Inst.Index,
    comptime kind: enum { direct, field },
) CompileError!Air.Inst.Ref {
    const tracy = trace(@src());
    defer tracy.end();

    const pt = sema.pt;
    const zcu = pt.zcu;
    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].pl_node;
    const callee_src = block.src(.{ .node_offset_call_func = inst_data.src_node });
    const call_src = block.nodeOffset(inst_data.src_node);
    const ExtraType = switch (kind) {
        .direct => Zir.Inst.Call,
        .field => Zir.Inst.FieldCall,
    };
    const extra = sema.code.extraData(ExtraType, inst_data.payload_index);
    const args_len = extra.data.flags.args_len;

    const modifier: std.builtin.CallModifier = @enumFromInt(extra.data.flags.packed_modifier);
    const ensure_result_used = extra.data.flags.ensure_result_used;
    const pop_error_return_trace = extra.data.flags.pop_error_return_trace;

    const callee: ResolvedFieldCallee = switch (kind) {
        .direct => .{ .direct = try sema.resolveInst(extra.data.callee) },
        .field => blk: {
            const object_ptr = try sema.resolveInst(extra.data.obj_ptr);
            const field_name = try zcu.intern_pool.getOrPutString(
                sema.gpa,
                pt.tid,
                sema.code.nullTerminatedString(extra.data.field_name_start),
                .no_embedded_nulls,
            );
            const field_name_src = block.src(.{ .node_offset_field_name = inst_data.src_node });
            break :blk try sema.fieldCallBind(block, callee_src, object_ptr, field_name, field_name_src);
        },
    };
    const func: Air.Inst.Ref = switch (callee) {
        .direct => |func_inst| func_inst,
        .method => |method| method.func_inst,
    };

    const callee_ty = sema.typeOf(func);
    const total_args = args_len + @intFromBool(callee == .method);
    const func_ty = try sema.checkCallArgumentCount(block, func, callee_src, callee_ty, total_args, callee == .method);

    // The block index before the call, so we can potentially insert an error trace save here later.
    const block_index: Air.Inst.Index = @enumFromInt(block.instructions.items.len);

    // This will be set by `analyzeCall` to indicate whether any parameter was an error (making the
    // error trace potentially dirty).
    var input_is_error = false;

    const args_info: CallArgsInfo = .{ .zir_call = .{
        .bound_arg = switch (callee) {
            .direct => .none,
            .method => |method| method.arg0_inst,
        },
        .bound_arg_src = callee_src,
        .call_inst = inst,
        .call_node_offset = inst_data.src_node,
        .num_args = args_len,
        .args_body = @ptrCast(sema.code.extra[extra.end..]),
        .any_arg_is_error = &input_is_error,
    } };

    // AstGen ensures that a call instruction is always preceded by a dbg_stmt instruction.
    const call_dbg_node: Zir.Inst.Index = @enumFromInt(@intFromEnum(inst) - 1);
    const call_inst = try sema.analyzeCall(block, func, func_ty, callee_src, call_src, modifier, ensure_result_used, args_info, call_dbg_node, .call);

    if (block.ownerModule().error_tracing and
        !block.isComptime() and !block.is_typeof and (input_is_error or pop_error_return_trace))
    {
        const return_ty = sema.typeOf(call_inst);
        if (modifier != .always_tail and return_ty.isNoReturn(zcu))
            return call_inst; // call to "fn (...) noreturn", don't pop

        // TODO: we don't fix up the error trace for always_tail correctly, we should be doing it
        // *before* the recursive call. This will be a bit tricky to do and probably requires
        // moving this logic into analyzeCall. But that's probably a good idea anyway.
        if (modifier == .always_tail)
            return call_inst;

        // If any input is an error-type, we might need to pop any trace it generated. Otherwise, we only
        // need to clean-up our own trace if we were passed to a non-error-handling expression.
        if (input_is_error or (pop_error_return_trace and return_ty.isError(zcu))) {
            const stack_trace_ty = try sema.getBuiltinType(call_src, .StackTrace);
            try stack_trace_ty.resolveFields(pt);
            const field_name = try zcu.intern_pool.getOrPutString(sema.gpa, pt.tid, "index", .no_embedded_nulls);
            const field_index = try sema.structFieldIndex(block, stack_trace_ty, field_name, call_src);

            // Insert a save instruction before the arg resolution + call instructions we just generated
            const save_inst = try block.insertInst(block_index, .{
                .tag = .save_err_return_trace_index,
                .data = .{ .ty_pl = .{
                    .ty = Air.internedToRef(stack_trace_ty.toIntern()),
                    .payload = @intCast(field_index),
                } },
            });

            // Pop the error return trace, testing the result for non-error if necessary
            const operand = if (pop_error_return_trace or modifier == .always_tail) .none else call_inst;
            try sema.popErrorReturnTrace(block, call_src, operand, save_inst);
        }

        return call_inst;
    } else {
        return call_inst;
    }
}

fn checkCallArgumentCount(
    sema: *Sema,
    block: *Block,
    func: Air.Inst.Ref,
    func_src: LazySrcLoc,
    callee_ty: Type,
    total_args: usize,
    member_fn: bool,
) !Type {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const func_ty: Type = func_ty: {
        switch (callee_ty.zigTypeTag(zcu)) {
            .@"fn" => break :func_ty callee_ty,
            .pointer => {
                const ptr_info = callee_ty.ptrInfo(zcu);
                if (ptr_info.flags.size == .one and Type.fromInterned(ptr_info.child).zigTypeTag(zcu) == .@"fn") {
                    break :func_ty .fromInterned(ptr_info.child);
                }
            },
            .optional => {
                const opt_child = callee_ty.optionalChild(zcu);
                if (opt_child.zigTypeTag(zcu) == .@"fn" or (opt_child.isSinglePointer(zcu) and
                    opt_child.childType(zcu).zigTypeTag(zcu) == .@"fn"))
                {
                    const msg = msg: {
                        const msg = try sema.errMsg(func_src, "cannot call optional type '{}'", .{
                            callee_ty.fmt(pt),
                        });
                        errdefer msg.destroy(sema.gpa);
                        try sema.errNote(func_src, msg, "consider using '.?', 'orelse' or 'if'", .{});
                        break :msg msg;
                    };
                    return sema.failWithOwnedErrorMsg(block, msg);
                }
            },
            else => {},
        }
        return sema.fail(block, func_src, "type '{}' not a function", .{callee_ty.fmt(pt)});
    };

    const func_ty_info = zcu.typeToFunc(func_ty).?;
    const fn_params_len = func_ty_info.param_types.len;
    const args_len = total_args - @intFromBool(member_fn);
    if (func_ty_info.is_var_args) {
        assert(callConvSupportsVarArgs(func_ty_info.cc));
        if (total_args >= fn_params_len) return func_ty;
    } else if (fn_params_len == total_args) {
        return func_ty;
    }

    const maybe_func_inst = try sema.funcDeclSrcInst(func);
    const member_str = if (member_fn) "member function " else "";
    const variadic_str = if (func_ty_info.is_var_args) "at least " else "";
    const msg = msg: {
        const msg = try sema.errMsg(
            func_src,
            "{s}expected {s}{d} argument(s), found {d}",
            .{
                member_str,
                variadic_str,
                fn_params_len - @intFromBool(member_fn),
                args_len,
            },
        );
        errdefer msg.destroy(sema.gpa);

        if (maybe_func_inst) |func_inst| {
            try sema.errNote(.{
                .base_node_inst = func_inst,
                .offset = LazySrcLoc.Offset.nodeOffset(.zero),
            }, msg, "function declared here", .{});
        }
        break :msg msg;
    };
    return sema.failWithOwnedErrorMsg(block, msg);
}

fn callBuiltin(
    sema: *Sema,
    block: *Block,
    call_src: LazySrcLoc,
    builtin_fn: Air.Inst.Ref,
    modifier: std.builtin.CallModifier,
    args: []const Air.Inst.Ref,
    operation: CallOperation,
) !void {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const callee_ty = sema.typeOf(builtin_fn);
    const func_ty: Type = func_ty: {
        switch (callee_ty.zigTypeTag(zcu)) {
            .@"fn" => break :func_ty callee_ty,
            .pointer => {
                const ptr_info = callee_ty.ptrInfo(zcu);
                if (ptr_info.flags.size == .one and Type.fromInterned(ptr_info.child).zigTypeTag(zcu) == .@"fn") {
                    break :func_ty .fromInterned(ptr_info.child);
                }
            },
            else => {},
        }
        std.debug.panic("type '{}' is not a function calling builtin fn", .{callee_ty.fmt(pt)});
    };

    const func_ty_info = zcu.typeToFunc(func_ty).?;
    const fn_params_len = func_ty_info.param_types.len;
    if (args.len != fn_params_len or (func_ty_info.is_var_args and args.len < fn_params_len)) {
        std.debug.panic("parameter count mismatch calling builtin fn, expected {d}, found {d}", .{ fn_params_len, args.len });
    }

    _ = try sema.analyzeCall(
        block,
        builtin_fn,
        func_ty,
        call_src,
        call_src,
        modifier,
        false,
        .{ .resolved = .{ .src = call_src, .args = args } },
        null,
        operation,
    );
}

const CallOperation = enum {
    call,
    @"@call",
    @"@panic",
    @"safety check",
    @"error return",
};

const CallArgsInfo = union(enum) {
    /// The full list of resolved (but uncoerced) arguments is known ahead of time.
    resolved: struct {
        src: LazySrcLoc,
        args: []const Air.Inst.Ref,
    },

    /// The list of resolved (but uncoerced) arguments is known ahead of time, but
    /// originated from a usage of the @call builtin at the given node offset.
    call_builtin: struct {
        call_node_offset: std.zig.Ast.Node.Offset,
        args: []const Air.Inst.Ref,
    },

    /// This call corresponds to a ZIR call instruction. The arguments have not yet been
    /// resolved. They must be resolved by `analyzeCall` so that argument resolution and
    /// generic instantiation may be interleaved. This is required for RLS to work on
    /// generic parameters.
    zir_call: struct {
        /// This may be `none`, in which case it is ignored. Otherwise, it is the
        /// already-resolved value of the first argument, from method call syntax.
        bound_arg: Air.Inst.Ref,
        /// The source location of `bound_arg` if it is not `null`. Otherwise `undefined`.
        bound_arg_src: LazySrcLoc,
        /// The ZIR call instruction. The parameter type is placed at this index while
        /// analyzing arguments.
        call_inst: Zir.Inst.Index,
        /// The node offset of `call_inst`.
        call_node_offset: std.zig.Ast.Node.Offset,
        /// The number of arguments to this call, not including `bound_arg`.
        num_args: u32,
        /// The ZIR corresponding to all function arguments (other than `bound_arg`, if it
        /// is not `none`). Format is precisely the same as trailing data of ZIR `call`.
        args_body: []const Zir.Inst.Index,
        /// This bool will be set to true if any argument evaluated turns out to have an error set or error union type.
        /// This is used by the caller to restore the error return trace when necessary.
        any_arg_is_error: *bool,
    },

    fn count(cai: CallArgsInfo) usize {
        return switch (cai) {
            inline .resolved, .call_builtin => |resolved| resolved.args.len,
            .zir_call => |zir_call| zir_call.num_args + @intFromBool(zir_call.bound_arg != .none),
        };
    }

    fn argSrc(cai: CallArgsInfo, block: *Block, arg_index: usize) LazySrcLoc {
        return switch (cai) {
            .resolved => |resolved| resolved.src,
            .call_builtin => |call_builtin| block.src(.{ .call_arg = .{
                .call_node_offset = call_builtin.call_node_offset,
                .arg_index = @intCast(arg_index),
            } }),
            .zir_call => |zir_call| if (arg_index == 0 and zir_call.bound_arg != .none) {
                return zir_call.bound_arg_src;
            } else block.src(.{ .call_arg = .{
                .call_node_offset = zir_call.call_node_offset,
                .arg_index = @intCast(arg_index - @intFromBool(zir_call.bound_arg != .none)),
            } }),
        };
    }

    /// Analyzes the arg at `arg_index` and coerces it to `param_ty`.
    /// `param_ty` may be `generic_poison`. A value of `null` indicates a varargs parameter.
    /// `func_ty_info` may be the type before instantiation, even if a generic instantiation is in progress.
    /// Emits a compile error if the argument is not comptime-known despite either `block.isComptime()` or
    /// the parameter being marked `comptime`.
    fn analyzeArg(
        cai: CallArgsInfo,
        sema: *Sema,
        block: *Block,
        arg_index: usize,
        maybe_param_ty: ?Type,
        func_ty_info: InternPool.Key.FuncType,
        func_inst: Air.Inst.Ref,
        maybe_func_src_inst: ?InternPool.TrackedInst.Index,
    ) CompileError!Air.Inst.Ref {
        const pt = sema.pt;
        const zcu = pt.zcu;
        const param_count = func_ty_info.param_types.len;
        const uncoerced_arg: Air.Inst.Ref = switch (cai) {
            inline .resolved, .call_builtin => |resolved| resolved.args[arg_index],
            .zir_call => |zir_call| arg_val: {
                const has_bound_arg = zir_call.bound_arg != .none;
                if (arg_index == 0 and has_bound_arg) {
                    break :arg_val zir_call.bound_arg;
                }
                const real_arg_idx = arg_index - @intFromBool(has_bound_arg);

                const arg_body = if (real_arg_idx == 0) blk: {
                    const start = zir_call.num_args;
                    const end = @intFromEnum(zir_call.args_body[0]);
                    break :blk zir_call.args_body[start..end];
                } else blk: {
                    const start = @intFromEnum(zir_call.args_body[real_arg_idx - 1]);
                    const end = @intFromEnum(zir_call.args_body[real_arg_idx]);
                    break :blk zir_call.args_body[start..end];
                };

                // Generate args to comptime params in comptime block
                const parent_comptime = block.comptime_reason;
                defer block.comptime_reason = parent_comptime;
                // Note that we are indexing into parameters, not arguments, so use `arg_index` instead of `real_arg_idx`
                if (std.math.cast(u5, arg_index)) |i| {
                    if (i < param_count and func_ty_info.paramIsComptime(i)) {
                        block.comptime_reason = .{
                            .reason = .{
                                .src = cai.argSrc(block, arg_index),
                                .r = .{
                                    .comptime_param = .{
                                        .comptime_src = if (maybe_func_src_inst) |src_inst| .{
                                            .base_node_inst = src_inst,
                                            .offset = .{ .func_decl_param_comptime = @intCast(arg_index) },
                                        } else unreachable, // should be non-null because the function is generic
                                    },
                                },
                            },
                        };
                    }
                }
                // Give the arg its result type
                const provide_param_ty: Type = maybe_param_ty orelse .generic_poison;
                sema.inst_map.putAssumeCapacity(zir_call.call_inst, Air.internedToRef(provide_param_ty.toIntern()));
                // Resolve the arg!
                const uncoerced_arg = try sema.resolveInlineBody(block, arg_body, zir_call.call_inst);

                if (block.isComptime() and !try sema.isComptimeKnown(uncoerced_arg)) {
                    return sema.failWithNeededComptime(block, cai.argSrc(block, arg_index), null);
                }

                if (sema.typeOf(uncoerced_arg).zigTypeTag(zcu) == .noreturn) {
                    // This terminates resolution of arguments. The caller should
                    // propagate this.
                    return uncoerced_arg;
                }

                if (sema.typeOf(uncoerced_arg).isError(zcu)) {
                    zir_call.any_arg_is_error.* = true;
                }

                break :arg_val uncoerced_arg;
            },
        };
        const param_ty = maybe_param_ty orelse {
            return sema.coerceVarArgParam(block, uncoerced_arg, cai.argSrc(block, arg_index));
        };
        switch (param_ty.toIntern()) {
            .generic_poison_type => return uncoerced_arg,
            else => return sema.coerceExtra(
                block,
                param_ty,
                uncoerced_arg,
                cai.argSrc(block, arg_index),
                .{ .param_src = .{
                    .func_inst = func_inst,
                    .param_i = @intCast(arg_index),
                } },
            ) catch |err| switch (err) {
                error.NotCoercible => unreachable,
                else => |e| return e,
            },
        }
    }
};

fn analyzeCall(
    sema: *Sema,
    block: *Block,
    callee: Air.Inst.Ref,
    func_ty: Type,
    func_src: LazySrcLoc,
    call_src: LazySrcLoc,
    modifier: std.builtin.CallModifier,
    ensure_result_used: bool,
    args_info: CallArgsInfo,
    call_dbg_node: ?Zir.Inst.Index,
    operation: CallOperation,
) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const gpa = zcu.gpa;
    const ip = &zcu.intern_pool;
    const arena = sema.arena;

    if (modifier == .async_kw) {
        return sema.failWithUseOfAsync(block, call_src);
    }

    const maybe_func_inst = try sema.funcDeclSrcInst(callee);
    const func_ret_ty_src: LazySrcLoc = if (maybe_func_inst) |fn_decl_inst| .{
        .base_node_inst = fn_decl_inst,
        .offset = .{ .node_offset_fn_type_ret_ty = .zero },
    } else func_src;

    const func_ty_info = zcu.typeToFunc(func_ty).?;
    if (!callConvIsCallable(func_ty_info.cc)) {
        return sema.failWithOwnedErrorMsg(block, msg: {
            const msg = try sema.errMsg(
                func_src,
                "unable to call function with calling convention '{s}'",
                .{@tagName(func_ty_info.cc)},
            );
            errdefer msg.destroy(gpa);
            if (maybe_func_inst) |func_inst| try sema.errNote(.{
                .base_node_inst = func_inst,
                .offset = .nodeOffset(.zero),
            }, msg, "function declared here", .{});
            break :msg msg;
        });
    }

    // We need this value in a few code paths.
    const callee_val = try sema.resolveDefinedValue(block, call_src, callee);
    // If the callee is a comptime-known *non-extern* function, `func_val` is populated.
    // If it is a comptime-known extern function, `func_is_extern` is set instead.
    // If it is not comptime-known, neither is set.
    const func_val: ?Value, const func_is_extern: bool = if (callee_val) |c| switch (ip.indexToKey(c.toIntern())) {
        .func => .{ c, false },
        .ptr => switch (try sema.pointerDerefExtra(block, func_src, c)) {
            .runtime_load, .needed_well_defined, .out_of_bounds => .{ null, false },
            .val => |pointee| switch (ip.indexToKey(pointee.toIntern())) {
                .func => .{ pointee, false },
                .@"extern" => .{ null, true },
                else => unreachable,
            },
        },
        .@"extern" => .{ null, true },
        else => unreachable,
    } else .{ null, false };

    if (func_ty_info.is_generic and func_val == null) {
        return sema.failWithNeededComptime(block, func_src, .{ .simple = .generic_call_target });
    }

    const inline_requested = func_ty_info.cc == .@"inline" or modifier == .always_inline;

    // If the modifier is `.compile_time`, or if the return type is non-generic and comptime-only,
    // then we need to enter a comptime scope *now* to make sure the args are comptime-eval'd.
    const old_block_comptime_reason = block.comptime_reason;
    defer block.comptime_reason = old_block_comptime_reason;
    if (!block.isComptime()) {
        if (modifier == .compile_time) {
            block.comptime_reason = .{ .reason = .{
                .src = call_src,
                .r = .{ .simple = .comptime_call_modifier },
            } };
        } else if (!inline_requested and try Type.fromInterned(func_ty_info.return_type).comptimeOnlySema(pt)) {
            block.comptime_reason = .{
                .reason = .{
                    .src = call_src,
                    .r = .{
                        .comptime_only_ret_ty = .{
                            .ty = .fromInterned(func_ty_info.return_type),
                            .is_generic_inst = false,
                            .ret_ty_src = func_ret_ty_src,
                        },
                    },
                },
            };
        }
    }

    // This is whether we already know this to be an inline call.
    // If so, then comptime-known arguments are propagated when evaluating generic parameter/return types.
    // We might still learn that this call is inline *after* evaluating the generic return type.
    const early_known_inline = inline_requested or block.isComptime();

    // These values are undefined if `func_val == null`.
    const fn_nav: InternPool.Nav, const fn_zir: Zir, const fn_tracked_inst: InternPool.TrackedInst.Index, const fn_zir_inst: Zir.Inst.Index, const fn_zir_info: Zir.FnInfo = if (func_val) |f| b: {
        const info = ip.indexToKey(f.toIntern()).func;
        const nav = ip.getNav(info.owner_nav);
        const resolved_func_inst = info.zir_body_inst.resolveFull(ip) orelse return error.AnalysisFail;
        const file = zcu.fileByIndex(resolved_func_inst.file);
        const zir_info = file.zir.?.getFnInfo(resolved_func_inst.inst);
        break :b .{ nav, file.zir.?, info.zir_body_inst, resolved_func_inst.inst, zir_info };
    } else .{ undefined, undefined, undefined, undefined, undefined };

    // This is the `inst_map` used when evaluating generic parameters and return types.
    var generic_inst_map: InstMap = .{};
    defer generic_inst_map.deinit(gpa);
    if (func_ty_info.is_generic) {
        try generic_inst_map.ensureSpaceForInstructions(gpa, fn_zir_info.param_body);
    }

    // This exists so that `generic_block` below can include a "called from here" note back to this
    // call site when analyzing generic parameter/return types.
    var generic_inlining: Block.Inlining = if (func_ty_info.is_generic) .{
        .call_block = block,
        .call_src = call_src,
        .func = func_val.?.toIntern(),
        .is_generic_instantiation = true, // this allows the following fields to be `undefined`
        .has_comptime_args = undefined,
        .comptime_result = undefined,
        .merges = undefined,
    } else undefined;

    // This is the block in which we evaluate generic function components: that is, generic parameter
    // types and the generic return type. This must not be used if the function is not generic.
    // `comptime_reason` is set as needed.
    var generic_block: Block = if (func_ty_info.is_generic) .{
        .parent = null,
        .sema = sema,
        .namespace = fn_nav.analysis.?.namespace,
        .instructions = .{},
        .inlining = &generic_inlining,
        .src_base_inst = fn_nav.analysis.?.zir_index,
        .type_name_ctx = fn_nav.fqn,
    } else undefined;
    defer if (func_ty_info.is_generic) generic_block.instructions.deinit(gpa);

    if (func_ty_info.is_generic) {
        // We certainly depend on the generic owner's signature!
        try sema.declareDependency(.{ .src_hash = fn_tracked_inst });
    }

    const args = try arena.alloc(Air.Inst.Ref, args_info.count());
    for (args, 0..) |*arg, arg_idx| {
        const param_ty: ?Type = if (arg_idx < func_ty_info.param_types.len) ty: {
            const raw = func_ty_info.param_types.get(ip)[arg_idx];
            if (raw != .generic_poison_type) break :ty .fromInterned(raw);

            // We must discover the generic parameter type.
            assert(func_ty_info.is_generic);
            const param_inst_idx = fn_zir_info.param_body[arg_idx];
            const param_inst = fn_zir.instructions.get(@intFromEnum(param_inst_idx));
            switch (param_inst.tag) {
                .param_anytype, .param_anytype_comptime => break :ty .generic_poison,
                .param, .param_comptime => {},
                else => unreachable,
            }

            // Evaluate the generic parameter type. We need to switch out `sema.code` and `sema.inst_map`, because
            // the function definition may be in a different file to the call site.
            const old_code = sema.code;
            const old_inst_map = sema.inst_map;
            defer {
                generic_inst_map = sema.inst_map;
                sema.code = old_code;
                sema.inst_map = old_inst_map;
            }
            sema.code = fn_zir;
            sema.inst_map = generic_inst_map;

            const extra = sema.code.extraData(Zir.Inst.Param, param_inst.data.pl_tok.payload_index);
            const param_src = generic_block.tokenOffset(param_inst.data.pl_tok.src_tok);
            const body = sema.code.bodySlice(extra.end, extra.data.type.body_len);

            generic_block.comptime_reason = .{ .reason = .{
                .r = .{ .simple = .function_parameters },
                .src = param_src,
            } };

            const ty_ref = try sema.resolveInlineBody(&generic_block, body, param_inst_idx);
            const param_ty = try sema.analyzeAsType(&generic_block, param_src, ty_ref);

            if (!param_ty.isValidParamType(zcu)) {
                const opaque_str = if (param_ty.zigTypeTag(zcu) == .@"opaque") "opaque " else "";
                return sema.fail(block, param_src, "parameter of {s}type '{}' not allowed", .{
                    opaque_str, param_ty.fmt(pt),
                });
            }

            break :ty param_ty;
        } else null; // vararg

        arg.* = try args_info.analyzeArg(sema, block, arg_idx, param_ty, func_ty_info, callee, maybe_func_inst);
        const arg_ty = sema.typeOf(arg.*);
        if (arg_ty.zigTypeTag(zcu) == .noreturn) {
            return arg.*; // terminate analysis here
        }

        if (func_ty_info.is_generic) {
            // We need to put the argument into `generic_inst_map` so that other parameters can refer to it.
            const param_inst_idx = fn_zir_info.param_body[arg_idx];
            const declared_comptime = if (std.math.cast(u5, arg_idx)) |i| func_ty_info.paramIsComptime(i) else false;
            const param_is_comptime = declared_comptime or try arg_ty.comptimeOnlySema(pt);
            // We allow comptime-known arguments to propagate to generic types not only for comptime
            // parameters, but if the call is known to be inline.
            if (param_is_comptime or early_known_inline) {
                if (param_is_comptime and !try sema.isComptimeKnown(arg.*)) {
                    assert(!declared_comptime); // `analyzeArg` handles this
                    const arg_src = args_info.argSrc(block, arg_idx);
                    const param_ty_src: LazySrcLoc = .{
                        .base_node_inst = maybe_func_inst.?, // the function is generic
                        .offset = .{ .func_decl_param_ty = @intCast(arg_idx) },
                    };
                    return sema.failWithNeededComptime(
                        block,
                        arg_src,
                        .{ .comptime_only_param_ty = .{ .ty = arg_ty, .param_ty_src = param_ty_src } },
                    );
                }
                generic_inst_map.putAssumeCapacityNoClobber(param_inst_idx, arg.*);
            } else {
                // We need a dummy instruction with this type. It doesn't actually need to be in any block,
                // since it will never be referenced at runtime!
                const dummy: Air.Inst.Index = @enumFromInt(sema.air_instructions.len);
                try sema.air_instructions.append(gpa, .{ .tag = .alloc, .data = .{ .ty = arg_ty } });
                generic_inst_map.putAssumeCapacityNoClobber(param_inst_idx, dummy.toRef());
            }
        }
    }

    // This return type is never generic poison.
    // However, if it has an IES, it is always associated with the callee value.
    // This is not correct for inline calls (where it should be an ad-hoc IES), nor for generic
    // calls (where it should be the IES of the instantiation). However, it's how we print this
    // in error messages.
    const resolved_ret_ty: Type = ret_ty: {
        if (!func_ty_info.is_generic) break :ret_ty .fromInterned(func_ty_info.return_type);

        const maybe_poison_bare = if (fn_zir_info.inferred_error_set) maybe_poison: {
            break :maybe_poison ip.errorUnionPayload(func_ty_info.return_type);
        } else func_ty_info.return_type;

        if (maybe_poison_bare != .generic_poison_type) break :ret_ty .fromInterned(func_ty_info.return_type);

        // Evaluate the generic return type. As with generic parameters, we switch out `sema.code` and `sema.inst_map`.

        assert(func_ty_info.is_generic);

        const old_code = sema.code;
        const old_inst_map = sema.inst_map;
        defer {
            generic_inst_map = sema.inst_map;
            sema.code = old_code;
            sema.inst_map = old_inst_map;
        }
        sema.code = fn_zir;
        sema.inst_map = generic_inst_map;

        generic_block.comptime_reason = .{ .reason = .{
            .r = .{ .simple = .function_ret_ty },
            .src = func_ret_ty_src,
        } };

        const bare_ty = if (fn_zir_info.ret_ty_ref != .none) bare: {
            assert(fn_zir_info.ret_ty_body.len == 0);
            break :bare try sema.resolveType(&generic_block, func_ret_ty_src, fn_zir_info.ret_ty_ref);
        } else bare: {
            assert(fn_zir_info.ret_ty_body.len != 0);
            const ty_ref = try sema.resolveInlineBody(&generic_block, fn_zir_info.ret_ty_body, fn_zir_inst);
            break :bare try sema.analyzeAsType(&generic_block, func_ret_ty_src, ty_ref);
        };
        assert(bare_ty.toIntern() != .generic_poison_type);

        const full_ty = if (fn_zir_info.inferred_error_set) full: {
            try sema.validateErrorUnionPayloadType(block, bare_ty, func_ret_ty_src);
            const set = ip.errorUnionSet(func_ty_info.return_type);
            break :full try pt.errorUnionType(.fromInterned(set), bare_ty);
        } else bare_ty;

        if (!full_ty.isValidReturnType(zcu)) {
            const opaque_str = if (full_ty.zigTypeTag(zcu) == .@"opaque") "opaque " else "";
            return sema.fail(block, func_ret_ty_src, "{s}return type '{}' not allowed", .{
                opaque_str, full_ty.fmt(pt),
            });
        }

        break :ret_ty full_ty;
    };

    // If we've discovered after evaluating arguments that a generic function instantiation is
    // comptime-only, then we can mark the block as comptime *now*.
    if (!inline_requested and !block.isComptime() and try resolved_ret_ty.comptimeOnlySema(pt)) {
        block.comptime_reason = .{
            .reason = .{
                .src = call_src,
                .r = .{
                    .comptime_only_ret_ty = .{
                        .ty = resolved_ret_ty,
                        .is_generic_inst = true,
                        .ret_ty_src = func_ret_ty_src,
                    },
                },
            },
        };
    }

    if (call_dbg_node) |some| try sema.zirDbgStmt(block, some);

    const is_inline_call = block.isComptime() or inline_requested;

    if (!is_inline_call) {
        if (sema.func_is_naked) return sema.failWithOwnedErrorMsg(block, msg: {
            const msg = try sema.errMsg(call_src, "runtime {s} not allowed in naked function", .{@tagName(operation)});
            errdefer msg.destroy(gpa);
            switch (operation) {
                .call, .@"@call", .@"@panic", .@"error return" => {},
                .@"safety check" => try sema.errNote(call_src, msg, "use @setRuntimeSafety to disable runtime safety", .{}),
            }
            break :msg msg;
        });
        if (func_ty_info.cc == .auto) {
            switch (sema.owner.unwrap()) {
                .@"comptime", .nav_ty, .nav_val, .type, .memoized_state => {},
                .func => |owner_func| ip.funcSetHasErrorTrace(owner_func, true),
            }
        }
        for (args, 0..) |arg, arg_idx| {
            try sema.validateRuntimeValue(block, args_info.argSrc(block, arg_idx), arg);
        }
        const runtime_func: Air.Inst.Ref, const runtime_args: []const Air.Inst.Ref = func: {
            if (!func_ty_info.is_generic) break :func .{ callee, args };

            // Instantiate the generic function!

            // This may be an overestimate, but it's definitely sufficient.
            const max_runtime_args = args_info.count() - @popCount(func_ty_info.comptime_bits);
            var runtime_args: std.ArrayListUnmanaged(Air.Inst.Ref) = try .initCapacity(arena, max_runtime_args);
            var runtime_param_tys: std.ArrayListUnmanaged(InternPool.Index) = try .initCapacity(arena, max_runtime_args);

            const comptime_args = try arena.alloc(InternPool.Index, args_info.count());

            var noalias_bits: u32 = 0;

            for (args, comptime_args, 0..) |arg, *comptime_arg, arg_idx| {
                const arg_ty = sema.typeOf(arg);

                const is_comptime = c: {
                    if (std.math.cast(u5, arg_idx)) |i| {
                        if (func_ty_info.paramIsComptime(i)) {
                            break :c true;
                        }
                    }
                    break :c try arg_ty.comptimeOnlySema(pt);
                };
                const is_noalias = if (std.math.cast(u5, arg_idx)) |i| func_ty_info.paramIsNoalias(i) else false;

                if (is_comptime) {
                    // We already emitted an error if the argument isn't comptime-known.
                    comptime_arg.* = (try sema.resolveValue(arg)).?.toIntern();
                } else {
                    comptime_arg.* = .none;
                    if (is_noalias) {
                        const runtime_idx = runtime_args.items.len;
                        noalias_bits |= @as(u32, 1) << @intCast(runtime_idx);
                    }
                    runtime_args.appendAssumeCapacity(arg);
                    runtime_param_tys.appendAssumeCapacity(arg_ty.toIntern());
                }
            }

            const bare_ret_ty = if (fn_zir_info.inferred_error_set) t: {
                break :t resolved_ret_ty.errorUnionPayload(zcu);
            } else resolved_ret_ty;

            // We now need to actually create the function instance.
            const func_instance = try ip.getFuncInstance(gpa, pt.tid, .{
                .param_types = runtime_param_tys.items,
                .noalias_bits = noalias_bits,
                .bare_return_type = bare_ret_ty.toIntern(),
                .is_noinline = func_ty_info.is_noinline,
                .inferred_error_set = fn_zir_info.inferred_error_set,
                .generic_owner = func_val.?.toIntern(),
                .comptime_args = comptime_args,
            });
            if (zcu.comp.debugIncremental()) {
                const nav = ip.indexToKey(func_instance).func.owner_nav;
                const gop = try zcu.incremental_debug_state.navs.getOrPut(gpa, nav);
                if (!gop.found_existing) gop.value_ptr.* = zcu.generation;
            }

            // This call is problematic as it breaks guarantees about order-independency of semantic analysis.
            // These guarantees are necessary for incremental compilation and parallel semantic analysis.
            // See: #22410
            zcu.funcInfo(func_instance).maxBranchQuota(ip, sema.branch_quota);

            break :func .{ Air.internedToRef(func_instance), runtime_args.items };
        };

        ref_func: {
            const runtime_func_val = try sema.resolveValue(runtime_func) orelse break :ref_func;
            if (!ip.isFuncBody(runtime_func_val.toIntern())) break :ref_func;
            try sema.addReferenceEntry(block, call_src, .wrap(.{ .func = runtime_func_val.toIntern() }));
            try zcu.ensureFuncBodyAnalysisQueued(runtime_func_val.toIntern());
        }

        const call_tag: Air.Inst.Tag = switch (modifier) {
            .auto, .no_async => .call,
            .never_tail => .call_never_tail,
            .never_inline => .call_never_inline,
            .always_tail => .call_always_tail,

            .always_inline,
            .compile_time,
            .async_kw,
            => unreachable,
        };

        try sema.air_extra.ensureUnusedCapacity(gpa, @typeInfo(Air.Call).@"struct".fields.len + runtime_args.len);
        const result = try block.addInst(.{
            .tag = call_tag,
            .data = .{ .pl_op = .{
                .operand = runtime_func,
                .payload = sema.addExtraAssumeCapacity(Air.Call{
                    .args_len = @intCast(runtime_args.len),
                }),
            } },
        });
        sema.appendRefsAssumeCapacity(runtime_args);

        if (ensure_result_used) {
            try sema.ensureResultUsed(block, sema.typeOf(result), call_src);
        }

        if (call_tag == .call_always_tail) {
            const func_or_ptr_ty = sema.typeOf(runtime_func);
            const runtime_func_ty = switch (func_or_ptr_ty.zigTypeTag(zcu)) {
                .@"fn" => func_or_ptr_ty,
                .pointer => func_or_ptr_ty.childType(zcu),
                else => unreachable,
            };
            return sema.handleTailCall(block, call_src, runtime_func_ty, result);
        }

        if (resolved_ret_ty.toIntern() == .noreturn_type) {
            const want_check = c: {
                if (!block.wantSafety()) break :c false;
                if (func_val != null) break :c false;
                break :c true;
            };
            if (want_check) {
                try sema.safetyPanic(block, call_src, .noreturn_returned);
            } else {
                _ = try block.addNoOp(.unreach);
            }
            return .unreachable_value;
        }

        return result;
    }

    // This is an inline call. The function must be comptime-known. We will analyze its body directly using this `Sema`.

    if (func_ty_info.is_noinline and !block.isComptime()) {
        return sema.fail(block, call_src, "inline call of noinline function", .{});
    }

    const call_type: []const u8 = if (block.isComptime()) "comptime" else "inline";
    if (modifier == .never_inline) {
        const msg, const fail_block = msg: {
            const msg = try sema.errMsg(call_src, "cannot perform {s} call with 'never_inline' modifier", .{call_type});
            errdefer msg.destroy(gpa);
            const fail_block = if (block.isComptime()) b: {
                break :b try block.explainWhyBlockIsComptime(msg);
            } else block;
            break :msg .{ msg, fail_block };
        };
        return sema.failWithOwnedErrorMsg(fail_block, msg);
    }
    if (func_ty_info.is_var_args) {
        const msg, const fail_block = msg: {
            const msg = try sema.errMsg(call_src, "{s} call of variadic function", .{call_type});
            errdefer msg.destroy(gpa);
            const fail_block = if (block.isComptime()) b: {
                break :b try block.explainWhyBlockIsComptime(msg);
            } else block;
            break :msg .{ msg, fail_block };
        };
        return sema.failWithOwnedErrorMsg(fail_block, msg);
    }
    if (func_val == null) {
        if (func_is_extern) {
            const msg, const fail_block = msg: {
                const msg = try sema.errMsg(call_src, "{s} call of extern function", .{call_type});
                errdefer msg.destroy(gpa);
                const fail_block = if (block.isComptime()) b: {
                    break :b try block.explainWhyBlockIsComptime(msg);
                } else block;
                break :msg .{ msg, fail_block };
            };
            return sema.failWithOwnedErrorMsg(fail_block, msg);
        }
        return sema.failWithNeededComptime(
            block,
            func_src,
            if (block.isComptime()) null else .{ .simple = .inline_call_target },
        );
    }

    if (block.isComptime()) {
        for (args, 0..) |arg, arg_idx| {
            if (!try sema.isComptimeKnown(arg)) {
                const arg_src = args_info.argSrc(block, arg_idx);
                return sema.failWithNeededComptime(block, arg_src, null);
            }
        }
    }

    // For an inline call, we depend on the source code of the whole function definition.
    try sema.declareDependency(.{ .src_hash = fn_nav.analysis.?.zir_index });

    try sema.emitBackwardBranch(block, call_src);

    const want_memoize = m: {
        // TODO: comptime call memoization is currently not supported under incremental compilation
        // since dependencies are not marked on callers. If we want to keep this around (we should
        // check that it's worthwhile first!), each memoized call needs an `AnalUnit`.
        if (zcu.comp.incremental) break :m false;
        if (!block.isComptime()) break :m false;
        for (args) |a| {
            const val = (try sema.resolveValue(a)).?;
            if (val.canMutateComptimeVarState(zcu)) break :m false;
        }
        break :m true;
    };
    const memoized_arg_values: []const InternPool.Index = if (want_memoize) arg_vals: {
        const vals = try sema.arena.alloc(InternPool.Index, args.len);
        for (vals, args) |*v, a| v.* = (try sema.resolveValue(a)).?.toIntern();
        break :arg_vals vals;
    } else undefined;
    if (want_memoize) memoize: {
        const memoized_call_index = ip.getIfExists(.{
            .memoized_call = .{
                .func = func_val.?.toIntern(),
                .arg_values = memoized_arg_values,
                .result = undefined, // ignored by hash+eql
                .branch_count = undefined, // ignored by hash+eql
            },
        }) orelse break :memoize;
        const memoized_call = ip.indexToKey(memoized_call_index).memoized_call;
        if (sema.branch_count + memoized_call.branch_count > sema.branch_quota) {
            // Let the call play out se we get the correct source location for the
            // "evaluation exceeded X backwards branches" error.
            break :memoize;
        }
        sema.branch_count += memoized_call.branch_count;
        const result = Air.internedToRef(memoized_call.result);
        if (ensure_result_used) {
            try sema.ensureResultUsed(block, sema.typeOf(result), call_src);
        }
        return result;
    }

    var new_ies: InferredErrorSet = .{ .func = .none };

    const old_inst_map = sema.inst_map;
    const old_code = sema.code;
    const old_func_index = sema.func_index;
    const old_fn_ret_ty = sema.fn_ret_ty;
    const old_fn_ret_ty_ies = sema.fn_ret_ty_ies;
    const old_error_return_trace_index_on_fn_entry = sema.error_return_trace_index_on_fn_entry;
    defer {
        sema.inst_map.deinit(gpa);
        sema.inst_map = old_inst_map;
        sema.code = old_code;
        sema.func_index = old_func_index;
        sema.fn_ret_ty = old_fn_ret_ty;
        sema.fn_ret_ty_ies = old_fn_ret_ty_ies;
        sema.error_return_trace_index_on_fn_entry = old_error_return_trace_index_on_fn_entry;
    }
    sema.inst_map = .{};
    sema.code = fn_zir;
    sema.func_index = func_val.?.toIntern();
    sema.fn_ret_ty = if (fn_zir_info.inferred_error_set) try pt.errorUnionType(
        .fromInterned(.adhoc_inferred_error_set_type),
        resolved_ret_ty.errorUnionPayload(zcu),
    ) else resolved_ret_ty;
    sema.fn_ret_ty_ies = if (fn_zir_info.inferred_error_set) &new_ies else null;

    try sema.inst_map.ensureSpaceForInstructions(gpa, fn_zir_info.param_body);
    for (args, 0..) |arg, arg_idx| {
        sema.inst_map.putAssumeCapacityNoClobber(fn_zir_info.param_body[arg_idx], arg);
    }

    const need_debug_scope = !block.isComptime() and !block.is_typeof and !block.ownerModule().strip;
    const block_inst: Air.Inst.Index = @enumFromInt(sema.air_instructions.len);
    try sema.air_instructions.append(gpa, .{
        .tag = if (need_debug_scope) .dbg_inline_block else .block,
        .data = undefined,
    });

    var inlining: Block.Inlining = .{
        .call_block = block,
        .call_src = call_src,
        .func = func_val.?.toIntern(),
        .is_generic_instantiation = false,
        .has_comptime_args = for (args) |a| {
            if (try sema.isComptimeKnown(a)) break true;
        } else false,
        .comptime_result = undefined,
        .merges = .{
            .block_inst = block_inst,
            .results = .empty,
            .br_list = .empty,
            .src_locs = .empty,
        },
    };
    var child_block: Block = .{
        .parent = null,
        .sema = sema,
        .namespace = fn_nav.analysis.?.namespace,
        .instructions = .{},
        .inlining = &inlining,
        .is_typeof = block.is_typeof,
        .comptime_reason = if (block.isComptime()) .inlining_parent else null,
        .error_return_trace_index = block.error_return_trace_index,
        .runtime_cond = block.runtime_cond,
        .runtime_loop = block.runtime_loop,
        .runtime_index = block.runtime_index,
        .src_base_inst = fn_nav.analysis.?.zir_index,
        .type_name_ctx = fn_nav.fqn,
    };

    defer child_block.instructions.deinit(gpa);
    defer inlining.merges.deinit(gpa);

    if (!inlining.has_comptime_args) {
        var block_it = block;
        while (block_it.inlining) |parent_inlining| {
            if (!parent_inlining.is_generic_instantiation and
                !parent_inlining.has_comptime_args and
                parent_inlining.func == func_val.?.toIntern())
            {
                return sema.fail(block, call_src, "inline call is recursive", .{});
            }
            block_it = parent_inlining.call_block;
        }
    }

    if (!block.isComptime() and !block.is_typeof) {
        const zir_tags = sema.code.instructions.items(.tag);
        const zir_datas = sema.code.instructions.items(.data);
        for (fn_zir_info.param_body) |inst| switch (zir_tags[@intFromEnum(inst)]) {
            .param, .param_comptime => {
                const extra = sema.code.extraData(Zir.Inst.Param, zir_datas[@intFromEnum(inst)].pl_tok.payload_index);
                const param_name = sema.code.nullTerminatedString(extra.data.name);
                const air_inst = sema.inst_map.get(inst).?;
                try sema.addDbgVar(&child_block, air_inst, .dbg_arg_inline, param_name);
            },
            .param_anytype, .param_anytype_comptime => {
                const param_name = zir_datas[@intFromEnum(inst)].str_tok.get(sema.code);
                const air_inst = sema.inst_map.get(inst).?;
                try sema.addDbgVar(&child_block, air_inst, .dbg_arg_inline, param_name);
            },
            else => {},
        };
    }

    child_block.error_return_trace_index = try sema.analyzeSaveErrRetIndex(&child_block);
    // Save the error trace as our first action in the function
    // to match the behavior of runtime function calls.
    const error_return_trace_index_on_parent_fn_entry = sema.error_return_trace_index_on_fn_entry;
    sema.error_return_trace_index_on_fn_entry = child_block.error_return_trace_index;
    defer sema.error_return_trace_index_on_fn_entry = error_return_trace_index_on_parent_fn_entry;

    // We temporarily set `allow_memoize` to `true` to track this comptime call.
    // It is restored after the call finishes analysis, so that a caller may
    // know whether an in-progress call (containing this call) may be memoized.
    const old_allow_memoize = sema.allow_memoize;
    defer sema.allow_memoize = old_allow_memoize and sema.allow_memoize;
    sema.allow_memoize = true;

    // Store the current eval branch count so we can find out how many eval branches
    // the comptime call caused.
    const old_branch_count = sema.branch_count;

    const result_raw: Air.Inst.Ref = result: {
        sema.analyzeFnBody(&child_block, fn_zir_info.body) catch |err| switch (err) {
            error.ComptimeReturn => break :result inlining.comptime_result,
            else => |e| return e,
        };
        break :result try sema.resolveAnalyzedBlock(block, call_src, &child_block, &inlining.merges, need_debug_scope);
    };

    const result: Air.Inst.Ref = if (try sema.resolveValue(result_raw)) |result_val| r: {
        const val_resolved = try sema.resolveAdHocInferredErrorSet(block, call_src, result_val.toIntern());
        break :r Air.internedToRef(val_resolved);
    } else r: {
        const resolved_ty = try sema.resolveAdHocInferredErrorSetTy(block, call_src, sema.typeOf(result_raw).toIntern());
        if (resolved_ty == .none) break :r result_raw;
        // TODO: mutate in place the previous instruction if possible
        // rather than adding a bitcast instruction.
        break :r try block.addBitCast(.fromInterned(resolved_ty), result_raw);
    };

    if (block.isComptime()) {
        const result_val = (try sema.resolveValue(result)).?;
        if (want_memoize and sema.allow_memoize and !result_val.canMutateComptimeVarState(zcu)) {
            _ = try pt.intern(.{ .memoized_call = .{
                .func = func_val.?.toIntern(),
                .arg_values = memoized_arg_values,
                .result = result_val.toIntern(),
                .branch_count = sema.branch_count - old_branch_count,
            } });
        }
    }

    if (ensure_result_used) {
        try sema.ensureResultUsed(block, sema.typeOf(result), call_src);
    }

    return result;
}

fn handleTailCall(sema: *Sema, block: *Block, call_src: LazySrcLoc, func_ty: Type, result: Air.Inst.Ref) !Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const target = zcu.getTarget();
    const backend = zcu.comp.getZigBackend();
    if (!target_util.supportsTailCall(target, backend)) {
        return sema.fail(block, call_src, "unable to perform tail call: compiler backend '{s}' does not support tail calls on target architecture '{s}' with the selected CPU feature flags", .{
            @tagName(backend), @tagName(target.cpu.arch),
        });
    }
    const owner_func_ty: Type = .fromInterned(zcu.funcInfo(sema.owner.unwrap().func).ty);
    if (owner_func_ty.toIntern() != func_ty.toIntern()) {
        return sema.fail(block, call_src, "unable to perform tail call: type of function being called '{}' does not match type of calling function '{}'", .{
            func_ty.fmt(pt), owner_func_ty.fmt(pt),
        });
    }
    _ = try block.addUnOp(.ret, result);
    return .unreachable_value;
}

fn zirIntType(sema: *Sema, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    const int_type = sema.code.instructions.items(.data)[@intFromEnum(inst)].int_type;
    const ty = try sema.pt.intType(int_type.signedness, int_type.bit_count);
    return Air.internedToRef(ty.toIntern());
}

fn zirOptionalType(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    const tracy = trace(@src());
    defer tracy.end();

    const pt = sema.pt;
    const zcu = pt.zcu;
    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].un_node;
    const operand_src = block.src(.{ .node_offset_un_op = inst_data.src_node });
    const child_type = try sema.resolveType(block, operand_src, inst_data.operand);
    if (child_type.zigTypeTag(zcu) == .@"opaque") {
        return sema.fail(block, operand_src, "opaque type '{}' cannot be optional", .{child_type.fmt(pt)});
    } else if (child_type.zigTypeTag(zcu) == .null) {
        return sema.fail(block, operand_src, "type '{}' cannot be optional", .{child_type.fmt(pt)});
    }
    const opt_type = try pt.optionalType(child_type.toIntern());

    return Air.internedToRef(opt_type.toIntern());
}

fn zirArrayInitElemType(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const bin = sema.code.instructions.items(.data)[@intFromEnum(inst)].bin;
    const maybe_wrapped_indexable_ty = try sema.resolveTypeOrPoison(block, LazySrcLoc.unneeded, bin.lhs) orelse return .generic_poison_type;
    const indexable_ty = maybe_wrapped_indexable_ty.optEuBaseType(zcu);
    try indexable_ty.resolveFields(pt);
    assert(indexable_ty.isIndexable(zcu)); // validated by a previous instruction
    if (indexable_ty.zigTypeTag(zcu) == .@"struct") {
        const elem_type = indexable_ty.fieldType(@intFromEnum(bin.rhs), zcu);
        return Air.internedToRef(elem_type.toIntern());
    } else {
        const elem_type = indexable_ty.elemType2(zcu);
        return Air.internedToRef(elem_type.toIntern());
    }
}

fn zirElemType(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const un_node = sema.code.instructions.items(.data)[@intFromEnum(inst)].un_node;
    const maybe_wrapped_ptr_ty = try sema.resolveTypeOrPoison(block, LazySrcLoc.unneeded, un_node.operand) orelse return .generic_poison_type;
    const ptr_ty = maybe_wrapped_ptr_ty.optEuBaseType(zcu);
    assert(ptr_ty.zigTypeTag(zcu) == .pointer); // validated by a previous instruction
    const elem_ty = ptr_ty.childType(zcu);
    if (elem_ty.toIntern() == .anyopaque_type) {
        // The pointer's actual child type is effectively unknown, so it makes
        // sense to represent it with a generic poison.
        return .generic_poison_type;
    }
    return Air.internedToRef(ptr_ty.childType(zcu).toIntern());
}

fn zirIndexablePtrElemType(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const un_node = sema.code.instructions.items(.data)[@intFromEnum(inst)].un_node;
    const src = block.nodeOffset(un_node.src_node);
    const ptr_ty = try sema.resolveTypeOrPoison(block, src, un_node.operand) orelse return .generic_poison_type;
    try sema.checkMemOperand(block, src, ptr_ty);
    const elem_ty = switch (ptr_ty.ptrSize(zcu)) {
        .slice, .many, .c => ptr_ty.childType(zcu),
        .one => ptr_ty.childType(zcu).childType(zcu),
    };
    return Air.internedToRef(elem_ty.toIntern());
}

fn zirVecArrElemType(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const un_node = sema.code.instructions.items(.data)[@intFromEnum(inst)].un_node;
    const vec_ty = try sema.resolveTypeOrPoison(block, LazySrcLoc.unneeded, un_node.operand) orelse return .generic_poison_type;
    switch (vec_ty.zigTypeTag(zcu)) {
        .array, .vector => {},
        else => return sema.fail(block, block.nodeOffset(un_node.src_node), "expected array or vector type, found '{}'", .{vec_ty.fmt(pt)}),
    }
    return Air.internedToRef(vec_ty.childType(zcu).toIntern());
}

fn zirVectorType(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].pl_node;
    const len_src = block.builtinCallArgSrc(inst_data.src_node, 0);
    const elem_type_src = block.builtinCallArgSrc(inst_data.src_node, 1);
    const extra = sema.code.extraData(Zir.Inst.Bin, inst_data.payload_index).data;
    const len: u32 = @intCast(try sema.resolveInt(block, len_src, extra.lhs, .u32, .{ .simple = .vector_length }));
    const elem_type = try sema.resolveType(block, elem_type_src, extra.rhs);
    try sema.checkVectorElemType(block, elem_type_src, elem_type);
    const vector_type = try sema.pt.vectorType(.{
        .len = len,
        .child = elem_type.toIntern(),
    });
    return Air.internedToRef(vector_type.toIntern());
}

fn zirArrayType(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    const tracy = trace(@src());
    defer tracy.end();

    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].pl_node;
    const extra = sema.code.extraData(Zir.Inst.Bin, inst_data.payload_index).data;
    const len_src = block.src(.{ .node_offset_array_type_len = inst_data.src_node });
    const elem_src = block.src(.{ .node_offset_array_type_elem = inst_data.src_node });
    const len = try sema.resolveInt(block, len_src, extra.lhs, .usize, .{ .simple = .array_length });
    const elem_type = try sema.resolveType(block, elem_src, extra.rhs);
    try sema.validateArrayElemType(block, elem_type, elem_src);
    const array_ty = try sema.pt.arrayType(.{
        .len = len,
        .child = elem_type.toIntern(),
    });

    return Air.internedToRef(array_ty.toIntern());
}

fn zirArrayTypeSentinel(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    const tracy = trace(@src());
    defer tracy.end();

    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].pl_node;
    const extra = sema.code.extraData(Zir.Inst.ArrayTypeSentinel, inst_data.payload_index).data;
    const len_src = block.src(.{ .node_offset_array_type_len = inst_data.src_node });
    const sentinel_src = block.src(.{ .node_offset_array_type_sentinel = inst_data.src_node });
    const elem_src = block.src(.{ .node_offset_array_type_elem = inst_data.src_node });
    const len = try sema.resolveInt(block, len_src, extra.len, .usize, .{ .simple = .array_length });
    const elem_type = try sema.resolveType(block, elem_src, extra.elem_type);
    try sema.validateArrayElemType(block, elem_type, elem_src);
    const uncasted_sentinel = try sema.resolveInst(extra.sentinel);
    const sentinel = try sema.coerce(block, elem_type, uncasted_sentinel, sentinel_src);
    const sentinel_val = try sema.resolveConstDefinedValue(block, sentinel_src, sentinel, .{ .simple = .array_sentinel });
    const array_ty = try sema.pt.arrayType(.{
        .len = len,
        .sentinel = sentinel_val.toIntern(),
        .child = elem_type.toIntern(),
    });
    try sema.checkSentinelType(block, sentinel_src, elem_type);

    return Air.internedToRef(array_ty.toIntern());
}

fn validateArrayElemType(sema: *Sema, block: *Block, elem_type: Type, elem_src: LazySrcLoc) !void {
    const pt = sema.pt;
    const zcu = pt.zcu;
    if (elem_type.zigTypeTag(zcu) == .@"opaque") {
        return sema.fail(block, elem_src, "array of opaque type '{}' not allowed", .{elem_type.fmt(pt)});
    } else if (elem_type.zigTypeTag(zcu) == .noreturn) {
        return sema.fail(block, elem_src, "array of 'noreturn' not allowed", .{});
    }
}

fn zirAnyframeType(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    const tracy = trace(@src());
    defer tracy.end();

    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].un_node;
    if (true) {
        return sema.failWithUseOfAsync(block, block.nodeOffset(inst_data.src_node));
    }
    const zcu = sema.zcu;
    const operand_src = block.src(.{ .node_offset_anyframe_type = inst_data.src_node });
    const return_type = try sema.resolveType(block, operand_src, inst_data.operand);
    const anyframe_type = try zcu.anyframeType(return_type);

    return Air.internedToRef(anyframe_type.toIntern());
}

fn zirErrorUnionType(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    const tracy = trace(@src());
    defer tracy.end();

    const pt = sema.pt;
    const zcu = pt.zcu;
    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].pl_node;
    const extra = sema.code.extraData(Zir.Inst.Bin, inst_data.payload_index).data;
    const lhs_src = block.src(.{ .node_offset_bin_lhs = inst_data.src_node });
    const rhs_src = block.src(.{ .node_offset_bin_rhs = inst_data.src_node });
    const error_set = try sema.resolveType(block, lhs_src, extra.lhs);
    const payload = try sema.resolveType(block, rhs_src, extra.rhs);

    if (error_set.zigTypeTag(zcu) != .error_set) {
        return sema.fail(block, lhs_src, "expected error set type, found '{}'", .{
            error_set.fmt(pt),
        });
    }
    try sema.validateErrorUnionPayloadType(block, payload, rhs_src);
    const err_union_ty = try pt.errorUnionType(error_set, payload);
    return Air.internedToRef(err_union_ty.toIntern());
}

fn validateErrorUnionPayloadType(sema: *Sema, block: *Block, payload_ty: Type, payload_src: LazySrcLoc) !void {
    const pt = sema.pt;
    const zcu = pt.zcu;
    if (payload_ty.zigTypeTag(zcu) == .@"opaque") {
        return sema.fail(block, payload_src, "error union with payload of opaque type '{}' not allowed", .{
            payload_ty.fmt(pt),
        });
    } else if (payload_ty.zigTypeTag(zcu) == .error_set) {
        return sema.fail(block, payload_src, "error union with payload of error set type '{}' not allowed", .{
            payload_ty.fmt(pt),
        });
    }
}

fn zirErrorValue(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    _ = block;
    const pt = sema.pt;
    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].str_tok;
    const name = try pt.zcu.intern_pool.getOrPutString(
        sema.gpa,
        pt.tid,
        inst_data.get(sema.code),
        .no_embedded_nulls,
    );
    _ = try pt.getErrorValue(name);
    // Create an error set type with only this error value, and return the value.
    const error_set_type = try pt.singleErrorSetType(name);
    return Air.internedToRef((try pt.intern(.{ .err = .{
        .ty = error_set_type.toIntern(),
        .name = name,
    } })));
}

fn zirIntFromError(sema: *Sema, block: *Block, extended: Zir.Inst.Extended.InstData) CompileError!Air.Inst.Ref {
    const tracy = trace(@src());
    defer tracy.end();

    const pt = sema.pt;
    const zcu = pt.zcu;
    const ip = &zcu.intern_pool;
    const extra = sema.code.extraData(Zir.Inst.UnNode, extended.operand).data;
    const src = block.nodeOffset(extra.node);
    const operand_src = block.builtinCallArgSrc(extra.node, 0);
    const uncasted_operand = try sema.resolveInst(extra.operand);
    const operand = try sema.coerce(block, .anyerror, uncasted_operand, operand_src);
    const err_int_ty = try pt.errorIntType();

    if (try sema.resolveValue(operand)) |val| {
        if (val.isUndef(zcu)) {
            return pt.undefRef(err_int_ty);
        }
        const err_name = ip.indexToKey(val.toIntern()).err.name;
        return Air.internedToRef((try pt.intValue(
            err_int_ty,
            try pt.getErrorValue(err_name),
        )).toIntern());
    }

    const op_ty = sema.typeOf(uncasted_operand);
    switch (try sema.resolveInferredErrorSetTy(block, src, op_ty.toIntern())) {
        .anyerror_type => {},
        else => |err_set_ty_index| {
            const names = ip.indexToKey(err_set_ty_index).error_set_type.names;
            switch (names.len) {
                0 => return Air.internedToRef((try pt.intValue(err_int_ty, 0)).toIntern()),
                1 => return pt.intRef(err_int_ty, ip.getErrorValueIfExists(names.get(ip)[0]).?),
                else => {},
            }
        },
    }

    try sema.requireRuntimeBlock(block, src, operand_src);
    return block.addBitCast(err_int_ty, operand);
}

fn zirErrorFromInt(sema: *Sema, block: *Block, extended: Zir.Inst.Extended.InstData) CompileError!Air.Inst.Ref {
    const tracy = trace(@src());
    defer tracy.end();

    const pt = sema.pt;
    const zcu = pt.zcu;
    const ip = &zcu.intern_pool;
    const extra = sema.code.extraData(Zir.Inst.UnNode, extended.operand).data;
    const src = block.nodeOffset(extra.node);
    const operand_src = block.builtinCallArgSrc(extra.node, 0);
    const uncasted_operand = try sema.resolveInst(extra.operand);
    const err_int_ty = try pt.errorIntType();
    const operand = try sema.coerce(block, err_int_ty, uncasted_operand, operand_src);

    if (try sema.resolveDefinedValue(block, operand_src, operand)) |value| {
        const int = try sema.usizeCast(block, operand_src, try value.toUnsignedIntSema(pt));
        if (int > len: {
            const mutate = &ip.global_error_set.mutate;
            mutate.map.mutex.lock();
            defer mutate.map.mutex.unlock();
            break :len mutate.names.len;
        } or int == 0)
            return sema.fail(block, operand_src, "integer value '{d}' represents no error", .{int});
        return Air.internedToRef((try pt.intern(.{ .err = .{
            .ty = .anyerror_type,
            .name = ip.global_error_set.shared.names.acquire().view().items(.@"0")[int - 1],
        } })));
    }
    try sema.requireRuntimeBlock(block, src, operand_src);
    if (block.wantSafety()) {
        const is_lt_len = try block.addUnOp(.cmp_lt_errors_len, operand);
        const zero_val = Air.internedToRef((try pt.intValue(err_int_ty, 0)).toIntern());
        const is_non_zero = try block.addBinOp(.cmp_neq, operand, zero_val);
        const ok = try block.addBinOp(.bool_and, is_lt_len, is_non_zero);
        try sema.addSafetyCheck(block, src, ok, .invalid_error_code);
    }
    return block.addInst(.{
        .tag = .bitcast,
        .data = .{ .ty_op = .{
            .ty = .anyerror_type,
            .operand = operand,
        } },
    });
}

fn zirMergeErrorSets(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    const tracy = trace(@src());
    defer tracy.end();

    const pt = sema.pt;
    const zcu = pt.zcu;
    const ip = &zcu.intern_pool;
    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].pl_node;
    const extra = sema.code.extraData(Zir.Inst.Bin, inst_data.payload_index).data;
    const src = block.src(.{ .node_offset_bin_op = inst_data.src_node });
    const lhs_src = block.src(.{ .node_offset_bin_lhs = inst_data.src_node });
    const rhs_src = block.src(.{ .node_offset_bin_rhs = inst_data.src_node });
    const lhs = try sema.resolveInst(extra.lhs);
    const rhs = try sema.resolveInst(extra.rhs);
    if (sema.typeOf(lhs).zigTypeTag(zcu) == .bool and sema.typeOf(rhs).zigTypeTag(zcu) == .bool) {
        const msg = msg: {
            const msg = try sema.errMsg(lhs_src, "expected error set type, found 'bool'", .{});
            errdefer msg.destroy(sema.gpa);
            try sema.errNote(src, msg, "'||' merges error sets; 'or' performs boolean OR", .{});
            break :msg msg;
        };
        return sema.failWithOwnedErrorMsg(block, msg);
    }
    const lhs_ty = try sema.analyzeAsType(block, lhs_src, lhs);
    const rhs_ty = try sema.analyzeAsType(block, rhs_src, rhs);
    if (lhs_ty.zigTypeTag(zcu) != .error_set)
        return sema.fail(block, lhs_src, "expected error set type, found '{}'", .{lhs_ty.fmt(pt)});
    if (rhs_ty.zigTypeTag(zcu) != .error_set)
        return sema.fail(block, rhs_src, "expected error set type, found '{}'", .{rhs_ty.fmt(pt)});

    // Anything merged with anyerror is anyerror.
    if (lhs_ty.toIntern() == .anyerror_type or rhs_ty.toIntern() == .anyerror_type) {
        return .anyerror_type;
    }

    if (ip.isInferredErrorSetType(lhs_ty.toIntern())) {
        switch (try sema.resolveInferredErrorSet(block, src, lhs_ty.toIntern())) {
            // isAnyError might have changed from a false negative to a true
            // positive after resolution.
            .anyerror_type => return .anyerror_type,
            else => {},
        }
    }
    if (ip.isInferredErrorSetType(rhs_ty.toIntern())) {
        switch (try sema.resolveInferredErrorSet(block, src, rhs_ty.toIntern())) {
            // isAnyError might have changed from a false negative to a true
            // positive after resolution.
            .anyerror_type => return .anyerror_type,
            else => {},
        }
    }

    const err_set_ty = try sema.errorSetMerge(lhs_ty, rhs_ty);
    return Air.internedToRef(err_set_ty.toIntern());
}

fn zirEnumLiteral(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    _ = block;
    const tracy = trace(@src());
    defer tracy.end();

    const pt = sema.pt;
    const zcu = pt.zcu;
    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].str_tok;
    const name = inst_data.get(sema.code);
    return Air.internedToRef((try pt.intern(.{
        .enum_literal = try zcu.intern_pool.getOrPutString(sema.gpa, pt.tid, name, .no_embedded_nulls),
    })));
}

fn zirDeclLiteral(sema: *Sema, block: *Block, inst: Zir.Inst.Index, do_coerce: bool) CompileError!Air.Inst.Ref {
    const tracy = trace(@src());
    defer tracy.end();

    const pt = sema.pt;
    const zcu = pt.zcu;
    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].pl_node;
    const src = block.nodeOffset(inst_data.src_node);
    const extra = sema.code.extraData(Zir.Inst.Field, inst_data.payload_index).data;
    const name = try zcu.intern_pool.getOrPutString(
        sema.gpa,
        pt.tid,
        sema.code.nullTerminatedString(extra.field_name_start),
        .no_embedded_nulls,
    );

    const orig_ty: Type = try sema.resolveTypeOrPoison(block, src, extra.lhs) orelse .generic_poison;

    const uncoerced_result = res: {
        if (orig_ty.toIntern() == .generic_poison_type) {
            // Treat this as a normal enum literal.
            break :res Air.internedToRef(try pt.intern(.{ .enum_literal = name }));
        }

        var ty = orig_ty;
        while (true) switch (ty.zigTypeTag(zcu)) {
            .error_union => ty = ty.errorUnionPayload(zcu),
            .optional => ty = ty.optionalChild(zcu),
            .pointer => ty = if (ty.isSinglePointer(zcu)) ty.childType(zcu) else break,
            .enum_literal, .error_set => {
                // Treat this as a normal enum literal.
                break :res Air.internedToRef(try pt.intern(.{ .enum_literal = name }));
            },
            else => break,
        };

        break :res try sema.fieldVal(block, src, Air.internedToRef(ty.toIntern()), name, src);
    };

    // Decl literals cannot lookup runtime `var`s.
    if (!try sema.isComptimeKnown(uncoerced_result)) {
        return sema.fail(block, src, "decl literal must be comptime-known", .{});
    }

    if (do_coerce) {
        return sema.coerce(block, orig_ty, uncoerced_result, src);
    } else {
        return uncoerced_result;
    }
}

fn zirIntFromEnum(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].un_node;
    const src = block.nodeOffset(inst_data.src_node);
    const operand_src = block.builtinCallArgSrc(inst_data.src_node, 0);
    const operand = try sema.resolveInst(inst_data.operand);
    const operand_ty = sema.typeOf(operand);

    const enum_tag: Air.Inst.Ref = switch (operand_ty.zigTypeTag(zcu)) {
        .@"enum" => operand,
        .@"union" => blk: {
            try operand_ty.resolveFields(pt);
            const tag_ty = operand_ty.unionTagType(zcu) orelse {
                return sema.fail(
                    block,
                    operand_src,
                    "untagged union '{}' cannot be converted to integer",
                    .{operand_ty.fmt(pt)},
                );
            };

            break :blk try sema.unionToTag(block, tag_ty, operand, operand_src);
        },
        else => {
            return sema.fail(block, operand_src, "expected enum or tagged union, found '{}'", .{
                operand_ty.fmt(pt),
            });
        },
    };
    const enum_tag_ty = sema.typeOf(enum_tag);
    const int_tag_ty = enum_tag_ty.intTagType(zcu);

    // TODO: use correct solution
    // https://github.com/ziglang/zig/issues/15909
    if (enum_tag_ty.enumFieldCount(zcu) == 0 and !enum_tag_ty.isNonexhaustiveEnum(zcu)) {
        return sema.fail(block, operand_src, "cannot use @intFromEnum on empty enum '{}'", .{
            enum_tag_ty.fmt(pt),
        });
    }

    if (try sema.typeHasOnePossibleValue(enum_tag_ty)) |opv| {
        return Air.internedToRef((try pt.getCoerced(opv, int_tag_ty)).toIntern());
    }

    if (try sema.resolveValue(enum_tag)) |enum_tag_val| {
        if (enum_tag_val.isUndef(zcu)) {
            return pt.undefRef(int_tag_ty);
        }

        const val = try enum_tag_val.intFromEnum(enum_tag_ty, pt);
        return Air.internedToRef(val.toIntern());
    }

    try sema.requireRuntimeBlock(block, src, operand_src);
    return block.addBitCast(int_tag_ty, enum_tag);
}

fn zirEnumFromInt(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].pl_node;
    const extra = sema.code.extraData(Zir.Inst.Bin, inst_data.payload_index).data;
    const src = block.nodeOffset(inst_data.src_node);
    const operand_src = block.builtinCallArgSrc(inst_data.src_node, 0);
    const dest_ty = try sema.resolveDestType(block, src, extra.lhs, .remove_eu_opt, "@enumFromInt");
    const operand = try sema.resolveInst(extra.rhs);
    const operand_ty = sema.typeOf(operand);

    if (dest_ty.zigTypeTag(zcu) != .@"enum") {
        return sema.fail(block, src, "expected enum, found '{}'", .{dest_ty.fmt(pt)});
    }
    _ = try sema.checkIntType(block, operand_src, operand_ty);

    if (try sema.resolveValue(operand)) |int_val| {
        if (dest_ty.isNonexhaustiveEnum(zcu)) {
            const int_tag_ty = dest_ty.intTagType(zcu);
            if (try sema.intFitsInType(int_val, int_tag_ty, null)) {
                return Air.internedToRef((try pt.getCoerced(int_val, dest_ty)).toIntern());
            }
            return sema.fail(block, src, "int value '{}' out of range of non-exhaustive enum '{}'", .{
                int_val.fmtValueSema(pt, sema), dest_ty.fmt(pt),
            });
        }
        if (int_val.isUndef(zcu)) {
            return sema.failWithUseOfUndef(block, operand_src);
        }
        if (!(try sema.enumHasInt(dest_ty, int_val))) {
            return sema.fail(block, src, "enum '{}' has no tag with value '{}'", .{
                dest_ty.fmt(pt), int_val.fmtValueSema(pt, sema),
            });
        }
        return Air.internedToRef((try pt.getCoerced(int_val, dest_ty)).toIntern());
    }

    if (dest_ty.intTagType(zcu).zigTypeTag(zcu) == .comptime_int) {
        return sema.failWithNeededComptime(block, operand_src, .{ .simple = .casted_to_comptime_enum });
    }

    if (try sema.typeHasOnePossibleValue(dest_ty)) |opv| {
        if (block.wantSafety()) {
            // The operand is runtime-known but the result is comptime-known. In
            // this case we still need a safety check.
            const expect_int_val = switch (zcu.intern_pool.indexToKey(opv.toIntern())) {
                .enum_tag => |enum_tag| enum_tag.int,
                else => unreachable,
            };
            const expect_int_coerced = try pt.getCoerced(.fromInterned(expect_int_val), operand_ty);
            const ok = try block.addBinOp(.cmp_eq, operand, Air.internedToRef(expect_int_coerced.toIntern()));
            try sema.addSafetyCheck(block, src, ok, .invalid_enum_value);
        }
        return Air.internedToRef(opv.toIntern());
    }

    try sema.requireRuntimeBlock(block, src, operand_src);
    if (block.wantSafety()) {
        if (zcu.backendSupportsFeature(.panic_fn)) {
            _ = try sema.preparePanicId(src, .invalid_enum_value);
        }
        return block.addTyOp(.intcast_safe, dest_ty, operand);
    }
    return block.addTyOp(.intcast, dest_ty, operand);
}

/// Pointer in, pointer out.
fn zirOptionalPayloadPtr(
    sema: *Sema,
    block: *Block,
    inst: Zir.Inst.Index,
    safety_check: bool,
) CompileError!Air.Inst.Ref {
    const tracy = trace(@src());
    defer tracy.end();

    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].un_node;
    const optional_ptr = try sema.resolveInst(inst_data.operand);
    const src = block.nodeOffset(inst_data.src_node);

    return sema.analyzeOptionalPayloadPtr(block, src, optional_ptr, safety_check, false);
}

fn analyzeOptionalPayloadPtr(
    sema: *Sema,
    block: *Block,
    src: LazySrcLoc,
    optional_ptr: Air.Inst.Ref,
    safety_check: bool,
    initializing: bool,
) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const optional_ptr_ty = sema.typeOf(optional_ptr);
    assert(optional_ptr_ty.zigTypeTag(zcu) == .pointer);

    const opt_type = optional_ptr_ty.childType(zcu);
    if (opt_type.zigTypeTag(zcu) != .optional) {
        return sema.failWithExpectedOptionalType(block, src, opt_type);
    }

    const child_type = opt_type.optionalChild(zcu);
    const child_pointer = try pt.ptrTypeSema(.{
        .child = child_type.toIntern(),
        .flags = .{
            .is_const = optional_ptr_ty.isConstPtr(zcu),
            .address_space = optional_ptr_ty.ptrAddressSpace(zcu),
        },
    });

    if (try sema.resolveDefinedValue(block, src, optional_ptr)) |ptr_val| {
        if (initializing) {
            if (sema.isComptimeMutablePtr(ptr_val)) {
                // Set the optional to non-null at comptime.
                // If the payload is OPV, we must use that value instead of undef.
                const payload_val = try sema.typeHasOnePossibleValue(child_type) orelse try pt.undefValue(child_type);
                const opt_val = try pt.intern(.{ .opt = .{
                    .ty = opt_type.toIntern(),
                    .val = payload_val.toIntern(),
                } });
                try sema.storePtrVal(block, src, ptr_val, Value.fromInterned(opt_val), opt_type);
            } else {
                // Emit runtime instructions to set the optional non-null bit.
                const opt_payload_ptr = try block.addTyOp(.optional_payload_ptr_set, child_pointer, optional_ptr);
                try sema.checkKnownAllocPtr(block, optional_ptr, opt_payload_ptr);
            }
            return Air.internedToRef((try ptr_val.ptrOptPayload(pt)).toIntern());
        }
        if (try sema.pointerDeref(block, src, ptr_val, optional_ptr_ty)) |val| {
            if (val.isNull(zcu)) {
                return sema.fail(block, src, "unable to unwrap null", .{});
            }
            return Air.internedToRef((try ptr_val.ptrOptPayload(pt)).toIntern());
        }
    }

    try sema.requireRuntimeBlock(block, src, null);
    if (safety_check and block.wantSafety()) {
        const is_non_null = try block.addUnOp(.is_non_null_ptr, optional_ptr);
        try sema.addSafetyCheck(block, src, is_non_null, .unwrap_null);
    }

    if (initializing) {
        const opt_payload_ptr = try block.addTyOp(.optional_payload_ptr_set, child_pointer, optional_ptr);
        try sema.checkKnownAllocPtr(block, optional_ptr, opt_payload_ptr);
        return opt_payload_ptr;
    } else {
        return block.addTyOp(.optional_payload_ptr, child_pointer, optional_ptr);
    }
}

/// Value in, value out.
fn zirOptionalPayload(
    sema: *Sema,
    block: *Block,
    inst: Zir.Inst.Index,
    safety_check: bool,
) CompileError!Air.Inst.Ref {
    const tracy = trace(@src());
    defer tracy.end();

    const pt = sema.pt;
    const zcu = pt.zcu;
    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].un_node;
    const src = block.nodeOffset(inst_data.src_node);
    const operand = try sema.resolveInst(inst_data.operand);
    const operand_ty = sema.typeOf(operand);
    const result_ty = switch (operand_ty.zigTypeTag(zcu)) {
        .optional => operand_ty.optionalChild(zcu),
        .pointer => t: {
            if (operand_ty.ptrSize(zcu) != .c) {
                return sema.failWithExpectedOptionalType(block, src, operand_ty);
            }
            // TODO https://github.com/ziglang/zig/issues/6597
            if (true) break :t operand_ty;
            const ptr_info = operand_ty.ptrInfo(zcu);
            break :t try pt.ptrTypeSema(.{
                .child = ptr_info.child,
                .flags = .{
                    .alignment = ptr_info.flags.alignment,
                    .is_const = ptr_info.flags.is_const,
                    .is_volatile = ptr_info.flags.is_volatile,
                    .is_allowzero = ptr_info.flags.is_allowzero,
                    .address_space = ptr_info.flags.address_space,
                },
            });
        },
        else => return sema.failWithExpectedOptionalType(block, src, operand_ty),
    };

    if (try sema.resolveDefinedValue(block, src, operand)) |val| {
        return if (val.optionalValue(zcu)) |payload|
            Air.internedToRef(payload.toIntern())
        else
            sema.fail(block, src, "unable to unwrap null", .{});
    }

    try sema.requireRuntimeBlock(block, src, null);
    if (safety_check and block.wantSafety()) {
        const is_non_null = try block.addUnOp(.is_non_null, operand);
        try sema.addSafetyCheck(block, src, is_non_null, .unwrap_null);
    }
    return block.addTyOp(.optional_payload, result_ty, operand);
}

/// Value in, value out
fn zirErrUnionPayload(
    sema: *Sema,
    block: *Block,
    inst: Zir.Inst.Index,
) CompileError!Air.Inst.Ref {
    const tracy = trace(@src());
    defer tracy.end();

    const pt = sema.pt;
    const zcu = pt.zcu;
    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].un_node;
    const src = block.nodeOffset(inst_data.src_node);
    const operand = try sema.resolveInst(inst_data.operand);
    const operand_src = src;
    const err_union_ty = sema.typeOf(operand);
    if (err_union_ty.zigTypeTag(zcu) != .error_union) {
        return sema.fail(block, operand_src, "expected error union type, found '{}'", .{
            err_union_ty.fmt(pt),
        });
    }
    return sema.analyzeErrUnionPayload(block, src, err_union_ty, operand, operand_src, false);
}

fn analyzeErrUnionPayload(
    sema: *Sema,
    block: *Block,
    src: LazySrcLoc,
    err_union_ty: Type,
    operand: Air.Inst.Ref,
    operand_src: LazySrcLoc,
    safety_check: bool,
) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const payload_ty = err_union_ty.errorUnionPayload(zcu);
    if (try sema.resolveDefinedValue(block, operand_src, operand)) |val| {
        if (val.getErrorName(zcu).unwrap()) |name| {
            return sema.failWithComptimeErrorRetTrace(block, src, name);
        }
        return Air.internedToRef(zcu.intern_pool.indexToKey(val.toIntern()).error_union.val.payload);
    }

    try sema.requireRuntimeBlock(block, src, null);

    // If the error set has no fields then no safety check is needed.
    if (safety_check and block.wantSafety() and
        !err_union_ty.errorUnionSet(zcu).errorSetIsEmpty(zcu))
    {
        try sema.addSafetyCheckUnwrapError(block, src, operand, .unwrap_errunion_err, .is_non_err);
    }

    return block.addTyOp(.unwrap_errunion_payload, payload_ty, operand);
}

/// Pointer in, pointer out.
fn zirErrUnionPayloadPtr(
    sema: *Sema,
    block: *Block,
    inst: Zir.Inst.Index,
) CompileError!Air.Inst.Ref {
    const tracy = trace(@src());
    defer tracy.end();

    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].un_node;
    const operand = try sema.resolveInst(inst_data.operand);
    const src = block.nodeOffset(inst_data.src_node);

    return sema.analyzeErrUnionPayloadPtr(block, src, operand, false, false);
}

fn analyzeErrUnionPayloadPtr(
    sema: *Sema,
    block: *Block,
    src: LazySrcLoc,
    operand: Air.Inst.Ref,
    safety_check: bool,
    initializing: bool,
) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const operand_ty = sema.typeOf(operand);
    assert(operand_ty.zigTypeTag(zcu) == .pointer);

    if (operand_ty.childType(zcu).zigTypeTag(zcu) != .error_union) {
        return sema.fail(block, src, "expected error union type, found '{}'", .{
            operand_ty.childType(zcu).fmt(pt),
        });
    }

    const err_union_ty = operand_ty.childType(zcu);
    const payload_ty = err_union_ty.errorUnionPayload(zcu);
    const operand_pointer_ty = try pt.ptrTypeSema(.{
        .child = payload_ty.toIntern(),
        .flags = .{
            .is_const = operand_ty.isConstPtr(zcu),
            .address_space = operand_ty.ptrAddressSpace(zcu),
        },
    });

    if (try sema.resolveDefinedValue(block, src, operand)) |ptr_val| {
        if (initializing) {
            if (sema.isComptimeMutablePtr(ptr_val)) {
                // Set the error union to non-error at comptime.
                // If the payload is OPV, we must use that value instead of undef.
                const payload_val = try sema.typeHasOnePossibleValue(payload_ty) orelse try pt.undefValue(payload_ty);
                const eu_val = try pt.intern(.{ .error_union = .{
                    .ty = err_union_ty.toIntern(),
                    .val = .{ .payload = payload_val.toIntern() },
                } });
                try sema.storePtrVal(block, src, ptr_val, Value.fromInterned(eu_val), err_union_ty);
            } else {
                // Emit runtime instructions to set the error union error code.
                try sema.requireRuntimeBlock(block, src, null);
                const eu_payload_ptr = try block.addTyOp(.errunion_payload_ptr_set, operand_pointer_ty, operand);
                try sema.checkKnownAllocPtr(block, operand, eu_payload_ptr);
            }
            return Air.internedToRef((try ptr_val.ptrEuPayload(pt)).toIntern());
        }
        if (try sema.pointerDeref(block, src, ptr_val, operand_ty)) |val| {
            if (val.getErrorName(zcu).unwrap()) |name| {
                return sema.failWithComptimeErrorRetTrace(block, src, name);
            }
            return Air.internedToRef((try ptr_val.ptrEuPayload(pt)).toIntern());
        }
    }

    try sema.requireRuntimeBlock(block, src, null);

    // If the error set has no fields then no safety check is needed.
    if (safety_check and block.wantSafety() and
        !err_union_ty.errorUnionSet(zcu).errorSetIsEmpty(zcu))
    {
        try sema.addSafetyCheckUnwrapError(block, src, operand, .unwrap_errunion_err_ptr, .is_non_err_ptr);
    }

    if (initializing) {
        const eu_payload_ptr = try block.addTyOp(.errunion_payload_ptr_set, operand_pointer_ty, operand);
        try sema.checkKnownAllocPtr(block, operand, eu_payload_ptr);
        return eu_payload_ptr;
    } else {
        return block.addTyOp(.unwrap_errunion_payload_ptr, operand_pointer_ty, operand);
    }
}

/// Value in, value out
fn zirErrUnionCode(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    const tracy = trace(@src());
    defer tracy.end();

    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].un_node;
    const src = block.nodeOffset(inst_data.src_node);
    const operand = try sema.resolveInst(inst_data.operand);
    return sema.analyzeErrUnionCode(block, src, operand);
}

/// If `operand` is comptime-known, asserts that it is an error value rather than a payload value.
fn analyzeErrUnionCode(sema: *Sema, block: *Block, src: LazySrcLoc, operand: Air.Inst.Ref) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const operand_ty = sema.typeOf(operand);
    if (operand_ty.zigTypeTag(zcu) != .error_union) {
        return sema.fail(block, src, "expected error union type, found '{}'", .{
            operand_ty.fmt(pt),
        });
    }

    const result_ty = operand_ty.errorUnionSet(zcu);

    if (try sema.resolveDefinedValue(block, src, operand)) |val| {
        return Air.internedToRef((try pt.intern(.{ .err = .{
            .ty = result_ty.toIntern(),
            .name = zcu.intern_pool.indexToKey(val.toIntern()).error_union.val.err_name,
        } })));
    }

    try sema.requireRuntimeBlock(block, src, null);
    return block.addTyOp(.unwrap_errunion_err, result_ty, operand);
}

/// Pointer in, value out
fn zirErrUnionCodePtr(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    const tracy = trace(@src());
    defer tracy.end();

    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].un_node;
    const src = block.nodeOffset(inst_data.src_node);
    const operand = try sema.resolveInst(inst_data.operand);
    return sema.analyzeErrUnionCodePtr(block, src, operand);
}

fn analyzeErrUnionCodePtr(sema: *Sema, block: *Block, src: LazySrcLoc, operand: Air.Inst.Ref) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const operand_ty = sema.typeOf(operand);
    assert(operand_ty.zigTypeTag(zcu) == .pointer);

    if (operand_ty.childType(zcu).zigTypeTag(zcu) != .error_union) {
        return sema.fail(block, src, "expected error union type, found '{}'", .{
            operand_ty.childType(zcu).fmt(pt),
        });
    }

    const result_ty = operand_ty.childType(zcu).errorUnionSet(zcu);

    if (try sema.resolveDefinedValue(block, src, operand)) |pointer_val| {
        if (try sema.pointerDeref(block, src, pointer_val, operand_ty)) |val| {
            assert(val.getErrorName(zcu) != .none);
            return Air.internedToRef((try pt.intern(.{ .err = .{
                .ty = result_ty.toIntern(),
                .name = zcu.intern_pool.indexToKey(val.toIntern()).error_union.val.err_name,
            } })));
        }
    }

    try sema.requireRuntimeBlock(block, src, null);
    return block.addTyOp(.unwrap_errunion_err_ptr, result_ty, operand);
}

fn zirFunc(
    sema: *Sema,
    block: *Block,
    inst: Zir.Inst.Index,
    inferred_error_set: bool,
) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const ip = &zcu.intern_pool;
    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].pl_node;
    const extra = sema.code.extraData(Zir.Inst.Func, inst_data.payload_index);
    const target = zcu.getTarget();
    const ret_ty_src = block.src(.{ .node_offset_fn_type_ret_ty = inst_data.src_node });
    const src = block.nodeOffset(inst_data.src_node);

    var extra_index = extra.end;

    const ret_ty: Type = if (extra.data.ret_ty.is_generic)
        .generic_poison
    else switch (extra.data.ret_ty.body_len) {
        0 => .void,
        1 => blk: {
            const ret_ty_ref: Zir.Inst.Ref = @enumFromInt(sema.code.extra[extra_index]);
            extra_index += 1;
            break :blk try sema.resolveType(block, ret_ty_src, ret_ty_ref);
        },
        else => blk: {
            const ret_ty_body = sema.code.bodySlice(extra_index, extra.data.ret_ty.body_len);
            extra_index += ret_ty_body.len;

            const ret_ty_val = try sema.resolveGenericBody(block, ret_ty_src, ret_ty_body, inst, .type, .{ .simple = .function_ret_ty });
            break :blk ret_ty_val.toType();
        },
    };

    var src_locs: Zir.Inst.Func.SrcLocs = undefined;
    const has_body = extra.data.body_len != 0;
    if (has_body) {
        extra_index += extra.data.body_len;
        src_locs = sema.code.extraData(Zir.Inst.Func.SrcLocs, extra_index).data;
    }

    // If this instruction has a body, then it's a function declaration, and we decide
    // the callconv based on whether it is exported. Otherwise, the callconv defaults
    // to `.auto`.
    const cc: std.builtin.CallingConvention = if (has_body) cc: {
        const func_decl_nav = sema.owner.unwrap().nav_val;
        const fn_is_exported = exported: {
            const decl_inst = ip.getNav(func_decl_nav).analysis.?.zir_index.resolve(ip) orelse return error.AnalysisFail;
            const zir_decl = sema.code.getDeclaration(decl_inst);
            break :exported zir_decl.linkage == .@"export";
        };
        if (fn_is_exported) {
            break :cc target.cCallingConvention() orelse {
                // This target has no default C calling convention. We sometimes trigger a similar
                // error by trying to evaluate `std.builtin.CallingConvention.c`, so for consistency,
                // let's eval that now and just get the transitive error. (It's guaranteed to error
                // because it does the exact `cCallingConvention` call we just did.)
                const cc_type = try sema.getBuiltinType(src, .CallingConvention);
                _ = try sema.namespaceLookupVal(
                    block,
                    LazySrcLoc.unneeded,
                    cc_type.getNamespaceIndex(zcu),
                    try ip.getOrPutString(sema.gpa, pt.tid, "c", .no_embedded_nulls),
                );
                // The above should have errored.
                @panic("std.builtin is corrupt");
            };
        } else {
            break :cc .auto;
        }
    } else .auto;

    return sema.funcCommon(
        block,
        inst_data.src_node,
        inst,
        cc,
        ret_ty,
        false,
        inferred_error_set,
        has_body,
        src_locs,
        0,
        false,
    );
}

fn resolveGenericBody(
    sema: *Sema,
    block: *Block,
    src: LazySrcLoc,
    body: []const Zir.Inst.Index,
    func_inst: Zir.Inst.Index,
    dest_ty: Type,
    reason: ComptimeReason,
) !Value {
    assert(body.len != 0);

    // Make sure any nested param instructions don't clobber our work.
    const prev_params = block.params;
    block.params = .{};
    defer {
        block.params = prev_params;
    }

    const uncasted = try sema.resolveInlineBody(block, body, func_inst);
    const result = try sema.coerce(block, dest_ty, uncasted, src);
    return sema.resolveConstDefinedValue(block, src, result, reason);
}

/// Given a library name, examines if the library name should end up in
/// `link.File.Options.windows_libs` table (for example, libc is always
/// specified via dedicated flag `link_libc` instead),
/// and puts it there if it doesn't exist.
/// It also dupes the library name which can then be saved as part of the
/// respective `Decl` (either `ExternFn` or `Var`).
/// The liveness of the duped library name is tied to liveness of `Zcu`.
/// To deallocate, call `deinit` on the respective `Decl` (`ExternFn` or `Var`).
pub fn handleExternLibName(
    sema: *Sema,
    block: *Block,
    src_loc: LazySrcLoc,
    lib_name: []const u8,
) CompileError!void {
    blk: {
        const pt = sema.pt;
        const zcu = pt.zcu;
        const comp = zcu.comp;
        const target = zcu.getTarget();
        log.debug("extern fn symbol expected in lib '{s}'", .{lib_name});
        if (std.zig.target.isLibCLibName(target, lib_name)) {
            if (!comp.config.link_libc) {
                return sema.fail(
                    block,
                    src_loc,
                    "dependency on libc must be explicitly specified in the build command",
                    .{},
                );
            }
            break :blk;
        }
        if (std.zig.target.isLibCxxLibName(target, lib_name)) {
            if (!comp.config.link_libcpp) return sema.fail(
                block,
                src_loc,
                "dependency on libc++ must be explicitly specified in the build command",
                .{},
            );
            break :blk;
        }
        if (mem.eql(u8, lib_name, "unwind")) {
            if (!comp.config.link_libunwind) return sema.fail(
                block,
                src_loc,
                "dependency on libunwind must be explicitly specified in the build command",
                .{},
            );
            break :blk;
        }
        if (!target.cpu.arch.isWasm() and !block.ownerModule().pic) {
            return sema.fail(
                block,
                src_loc,
                "dependency on dynamic library '{s}' requires enabling Position Independent Code; fixed by '-l{s}' or '-fPIC'",
                .{ lib_name, lib_name },
            );
        }
        comp.addLinkLib(lib_name) catch |err| {
            return sema.fail(block, src_loc, "unable to add link lib '{s}': {s}", .{
                lib_name, @errorName(err),
            });
        };
    }
}

/// These are calling conventions that are confirmed to work with variadic functions.
/// Any calling conventions not included here are either not yet verified to work with variadic
/// functions or there are no more other calling conventions that support variadic functions.
const calling_conventions_supporting_var_args = [_]std.builtin.CallingConvention.Tag{
    .x86_64_sysv,
    .x86_64_win,
    .x86_sysv,
    .x86_win,
    .aarch64_aapcs,
    .aarch64_aapcs_darwin,
    .aarch64_aapcs_win,
    .aarch64_vfabi,
    .aarch64_vfabi_sve,
    .arm_aapcs,
    .arm_aapcs_vfp,
    .mips64_n64,
    .mips64_n32,
    .mips_o32,
    .riscv64_lp64,
    .riscv64_lp64_v,
    .riscv32_ilp32,
    .riscv32_ilp32_v,
    .sparc64_sysv,
    .sparc_sysv,
    .powerpc64_elf,
    .powerpc64_elf_altivec,
    .powerpc64_elf_v2,
    .powerpc_sysv,
    .powerpc_sysv_altivec,
    .powerpc_aix,
    .powerpc_aix_altivec,
    .wasm_mvp,
    .arc_sysv,
    .avr_gnu,
    .bpf_std,
    .csky_sysv,
    .hexagon_sysv,
    .hexagon_sysv_hvx,
    .lanai_sysv,
    .loongarch64_lp64,
    .loongarch32_ilp32,
    .m68k_sysv,
    .m68k_gnu,
    .m68k_rtd,
    .msp430_eabi,
    .s390x_sysv,
    .s390x_sysv_vx,
    .ve_sysv,
    .xcore_xs1,
    .xcore_xs2,
    .xtensa_call0,
    .xtensa_windowed,
};
fn callConvSupportsVarArgs(cc: std.builtin.CallingConvention.Tag) bool {
    return for (calling_conventions_supporting_var_args) |supported_cc| {
        if (cc == supported_cc) return true;
    } else false;
}
fn checkCallConvSupportsVarArgs(sema: *Sema, block: *Block, src: LazySrcLoc, cc: std.builtin.CallingConvention.Tag) CompileError!void {
    const CallingConventionsSupportingVarArgsList = struct {
        arch: std.Target.Cpu.Arch,
        pub fn format(ctx: @This(), comptime fmt: []const u8, options: std.fmt.FormatOptions, writer: anytype) !void {
            _ = fmt;
            _ = options;
            var first = true;
            for (calling_conventions_supporting_var_args) |cc_inner| {
                for (std.Target.Cpu.Arch.fromCallingConvention(cc_inner)) |supported_arch| {
                    if (supported_arch == ctx.arch) break;
                } else continue; // callconv not supported by this arch
                if (!first) {
                    try writer.writeAll(", ");
                }
                first = false;
                try writer.print("'{s}'", .{@tagName(cc_inner)});
            }
        }
    };

    if (!callConvSupportsVarArgs(cc)) {
        return sema.failWithOwnedErrorMsg(block, msg: {
            const msg = try sema.errMsg(src, "variadic function does not support '{s}' calling convention", .{@tagName(cc)});
            errdefer msg.destroy(sema.gpa);
            const target = sema.pt.zcu.getTarget();
            try sema.errNote(src, msg, "supported calling conventions: {}", .{CallingConventionsSupportingVarArgsList{ .arch = target.cpu.arch }});
            break :msg msg;
        });
    }
}

fn callConvIsCallable(cc: std.builtin.CallingConvention.Tag) bool {
    return switch (cc) {
        .naked,

        .arm_interrupt,
        .avr_interrupt,
        .avr_signal,
        .csky_interrupt,
        .m68k_interrupt,
        .mips_interrupt,
        .mips64_interrupt,
        .riscv32_interrupt,
        .riscv64_interrupt,
        .x86_interrupt,
        .x86_64_interrupt,

        .amdgcn_kernel,
        .nvptx_kernel,
        .spirv_kernel,
        .spirv_fragment,
        .spirv_vertex,
        => false,

        else => true,
    };
}

fn checkMergeAllowed(sema: *Sema, block: *Block, src: LazySrcLoc, peer_ty: Type) !void {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const target = zcu.getTarget();

    if (!peer_ty.isPtrAtRuntime(zcu)) {
        return;
    }

    const as = peer_ty.ptrAddressSpace(zcu);
    if (!target_util.arePointersLogical(target, as)) {
        return;
    }

    return sema.failWithOwnedErrorMsg(block, msg: {
        const msg = try sema.errMsg(src, "value with non-mergable pointer type '{}' depends on runtime control flow", .{peer_ty.fmt(pt)});
        errdefer msg.destroy(sema.gpa);

        const runtime_src = block.runtime_cond orelse block.runtime_loop.?;
        try sema.errNote(runtime_src, msg, "runtime control flow here", .{});

        const backend = target_util.zigBackend(target, zcu.comp.config.use_llvm);
        try sema.errNote(src, msg, "pointers with address space '{s}' cannot be returned from a branch on target {s}-{s} by compiler backend {s}", .{
            @tagName(as),
            @tagName(target.cpu.arch.family()),
            @tagName(target.os.tag),
            @tagName(backend),
        });

        break :msg msg;
    });
}

const Section = union(enum) {
    generic,
    default,
    explicit: InternPool.NullTerminatedString,
};

fn funcCommon(
    sema: *Sema,
    block: *Block,
    src_node_offset: std.zig.Ast.Node.Offset,
    func_inst: Zir.Inst.Index,
    cc: std.builtin.CallingConvention,
    /// this might be Type.generic_poison
    bare_return_type: Type,
    var_args: bool,
    inferred_error_set: bool,
    has_body: bool,
    src_locs: Zir.Inst.Func.SrcLocs,
    noalias_bits: u32,
    is_noinline: bool,
) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const gpa = sema.gpa;
    const target = zcu.getTarget();
    const ip = &zcu.intern_pool;
    const ret_ty_src = block.src(.{ .node_offset_fn_type_ret_ty = src_node_offset });
    const cc_src = block.src(.{ .node_offset_fn_type_cc = src_node_offset });
    const func_src = block.nodeOffset(src_node_offset);

    const ret_ty_requires_comptime = try bare_return_type.comptimeOnlySema(pt);
    var is_generic = bare_return_type.isGenericPoison() or ret_ty_requires_comptime;

    var comptime_bits: u32 = 0;
    for (block.params.items(.ty), block.params.items(.is_comptime), 0..) |param_ty_ip, param_is_comptime, i| {
        const param_ty: Type = .fromInterned(param_ty_ip);
        const is_noalias = blk: {
            const index = std.math.cast(u5, i) orelse break :blk false;
            break :blk @as(u1, @truncate(noalias_bits >> index)) != 0;
        };
        const param_src = block.src(.{ .fn_proto_param = .{
            .fn_proto_node_offset = src_node_offset,
            .param_index = @intCast(i),
        } });
        const param_ty_comptime = try param_ty.comptimeOnlySema(pt);
        const param_ty_generic = param_ty.isGenericPoison();
        if (param_is_comptime or param_ty_comptime or param_ty_generic) {
            is_generic = true;
        }
        if (param_is_comptime) {
            comptime_bits |= @as(u32, 1) << @intCast(i); // TODO: handle cast error
        }
        if (param_is_comptime and !target_util.fnCallConvAllowsZigTypes(cc)) {
            return sema.fail(block, param_src, "comptime parameters not allowed in function with calling convention '{s}'", .{@tagName(cc)});
        }
        if (param_ty_generic and !target_util.fnCallConvAllowsZigTypes(cc)) {
            return sema.fail(block, param_src, "generic parameters not allowed in function with calling convention '{s}'", .{@tagName(cc)});
        }
        if (!param_ty.isValidParamType(zcu)) {
            const opaque_str = if (param_ty.zigTypeTag(zcu) == .@"opaque") "opaque " else "";
            return sema.fail(block, param_src, "parameter of {s}type '{}' not allowed", .{
                opaque_str, param_ty.fmt(pt),
            });
        }
        if (!param_ty_generic and !target_util.fnCallConvAllowsZigTypes(cc) and !try sema.validateExternType(param_ty, .param_ty)) {
            const msg = msg: {
                const msg = try sema.errMsg(param_src, "parameter of type '{}' not allowed in function with calling convention '{s}'", .{
                    param_ty.fmt(pt), @tagName(cc),
                });
                errdefer msg.destroy(sema.gpa);

                try sema.explainWhyTypeIsNotExtern(msg, param_src, param_ty, .param_ty);

                try sema.addDeclaredHereNote(msg, param_ty);
                break :msg msg;
            };
            return sema.failWithOwnedErrorMsg(block, msg);
        }
        if (param_ty_comptime and !param_is_comptime and has_body and !block.isComptime()) {
            const msg = msg: {
                const msg = try sema.errMsg(param_src, "parameter of type '{}' must be declared comptime", .{
                    param_ty.fmt(pt),
                });
                errdefer msg.destroy(sema.gpa);

                try sema.explainWhyTypeIsComptime(msg, param_src, param_ty);

                try sema.addDeclaredHereNote(msg, param_ty);
                break :msg msg;
            };
            return sema.failWithOwnedErrorMsg(block, msg);
        }
        if (!param_ty_generic and is_noalias and
            !(param_ty.zigTypeTag(zcu) == .pointer or param_ty.isPtrLikeOptional(zcu)))
        {
            return sema.fail(block, param_src, "non-pointer parameter declared noalias", .{});
        }
        switch (cc) {
            .x86_64_interrupt, .x86_interrupt => {
                const err_code_size = target.ptrBitWidth();
                switch (i) {
                    0 => if (param_ty.zigTypeTag(zcu) != .pointer) return sema.fail(block, param_src, "first parameter of function with '{s}' calling convention must be a pointer type", .{@tagName(cc)}),
                    1 => if (param_ty.bitSize(zcu) != err_code_size) return sema.fail(block, param_src, "second parameter of function with '{s}' calling convention must be a {d}-bit integer", .{ @tagName(cc), err_code_size }),
                    else => return sema.fail(block, param_src, "'{s}' calling convention supports up to 2 parameters, found {d}", .{ @tagName(cc), i + 1 }),
                }
            },
            .arm_interrupt,
            .mips64_interrupt,
            .mips_interrupt,
            .riscv64_interrupt,
            .riscv32_interrupt,
            .avr_interrupt,
            .csky_interrupt,
            .m68k_interrupt,
            .avr_signal,
            => return sema.fail(block, param_src, "parameters are not allowed with '{s}' calling convention", .{@tagName(cc)}),
            else => {},
        }
    }

    if (var_args) {
        if (is_generic) {
            return sema.fail(block, func_src, "generic function cannot be variadic", .{});
        }
        const va_args_src = block.src(.{
            .fn_proto_param = .{
                .fn_proto_node_offset = src_node_offset,
                .param_index = @intCast(block.params.len), // va_arg must be the last parameter
            },
        });
        try sema.checkCallConvSupportsVarArgs(block, va_args_src, cc);
    }

    const ret_poison = bare_return_type.isGenericPoison();

    const param_types = block.params.items(.ty);

    if (inferred_error_set) {
        assert(has_body);
        if (!ret_poison)
            try sema.validateErrorUnionPayloadType(block, bare_return_type, ret_ty_src);
        const func_index = try ip.getFuncDeclIes(gpa, pt.tid, .{
            .owner_nav = sema.owner.unwrap().nav_val,

            .param_types = param_types,
            .noalias_bits = noalias_bits,
            .comptime_bits = comptime_bits,
            .bare_return_type = bare_return_type.toIntern(),
            .cc = cc,
            .is_var_args = var_args,
            .is_generic = is_generic,
            .is_noinline = is_noinline,

            .zir_body_inst = try block.trackZir(func_inst),
            .lbrace_line = src_locs.lbrace_line,
            .rbrace_line = src_locs.rbrace_line,
            .lbrace_column = @as(u16, @truncate(src_locs.columns)),
            .rbrace_column = @as(u16, @truncate(src_locs.columns >> 16)),
        });
        return finishFunc(
            sema,
            block,
            func_index,
            .none,
            ret_poison,
            bare_return_type,
            ret_ty_src,
            cc,
            ret_ty_requires_comptime,
            func_inst,
            cc_src,
            is_noinline,
            is_generic,
        );
    }

    const func_ty = try ip.getFuncType(gpa, pt.tid, .{
        .param_types = param_types,
        .noalias_bits = noalias_bits,
        .comptime_bits = comptime_bits,
        .return_type = bare_return_type.toIntern(),
        .cc = cc,
        .is_var_args = var_args,
        .is_generic = is_generic,
        .is_noinline = is_noinline,
    });

    if (has_body) {
        const func_index = try ip.getFuncDecl(gpa, pt.tid, .{
            .owner_nav = sema.owner.unwrap().nav_val,
            .ty = func_ty,
            .cc = cc,
            .is_noinline = is_noinline,
            .zir_body_inst = try block.trackZir(func_inst),
            .lbrace_line = src_locs.lbrace_line,
            .rbrace_line = src_locs.rbrace_line,
            .lbrace_column = @as(u16, @truncate(src_locs.columns)),
            .rbrace_column = @as(u16, @truncate(src_locs.columns >> 16)),
        });
        return finishFunc(
            sema,
            block,
            func_index,
            func_ty,
            ret_poison,
            bare_return_type,
            ret_ty_src,
            cc,
            ret_ty_requires_comptime,
            func_inst,
            cc_src,
            is_noinline,
            is_generic,
        );
    }

    return finishFunc(
        sema,
        block,
        .none,
        func_ty,
        ret_poison,
        bare_return_type,
        ret_ty_src,
        cc,
        ret_ty_requires_comptime,
        func_inst,
        cc_src,
        is_noinline,
        is_generic,
    );
}

fn finishFunc(
    sema: *Sema,
    block: *Block,
    opt_func_index: InternPool.Index,
    func_ty: InternPool.Index,
    ret_poison: bool,
    bare_return_type: Type,
    ret_ty_src: LazySrcLoc,
    cc_resolved: std.builtin.CallingConvention,
    ret_ty_requires_comptime: bool,
    func_inst: Zir.Inst.Index,
    cc_src: LazySrcLoc,
    is_noinline: bool,
    is_generic: bool,
) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const ip = &zcu.intern_pool;
    const gpa = sema.gpa;

    const return_type: Type = if (opt_func_index == .none or ret_poison)
        bare_return_type
    else
        .fromInterned(ip.funcTypeReturnType(ip.typeOf(opt_func_index)));

    if (!return_type.isValidReturnType(zcu)) {
        const opaque_str = if (return_type.zigTypeTag(zcu) == .@"opaque") "opaque " else "";
        return sema.fail(block, ret_ty_src, "{s}return type '{}' not allowed", .{
            opaque_str, return_type.fmt(pt),
        });
    }
    if (!ret_poison and !target_util.fnCallConvAllowsZigTypes(cc_resolved) and
        !try sema.validateExternType(return_type, .ret_ty))
    {
        const msg = msg: {
            const msg = try sema.errMsg(ret_ty_src, "return type '{}' not allowed in function with calling convention '{s}'", .{
                return_type.fmt(pt), @tagName(cc_resolved),
            });
            errdefer msg.destroy(gpa);

            try sema.explainWhyTypeIsNotExtern(msg, ret_ty_src, return_type, .ret_ty);

            try sema.addDeclaredHereNote(msg, return_type);
            break :msg msg;
        };
        return sema.failWithOwnedErrorMsg(block, msg);
    }

    // If the return type is comptime-only but not dependent on parameters then
    // all parameter types also need to be comptime.
    if (opt_func_index != .none and ret_ty_requires_comptime and !block.isComptime()) comptime_check: {
        for (block.params.items(.is_comptime)) |is_comptime| {
            if (!is_comptime) break;
        } else break :comptime_check;

        const msg = try sema.errMsg(
            ret_ty_src,
            "function with comptime-only return type '{}' requires all parameters to be comptime",
            .{return_type.fmt(pt)},
        );
        errdefer msg.destroy(sema.gpa);
        try sema.explainWhyTypeIsComptime(msg, ret_ty_src, return_type);

        const tags = sema.code.instructions.items(.tag);
        const data = sema.code.instructions.items(.data);
        const param_body = sema.code.getParamBody(func_inst);
        for (
            block.params.items(.is_comptime),
            block.params.items(.name),
            param_body[0..block.params.len],
        ) |is_comptime, name_nts, param_index| {
            if (!is_comptime) {
                const param_src = block.tokenOffset(switch (tags[@intFromEnum(param_index)]) {
                    .param => data[@intFromEnum(param_index)].pl_tok.src_tok,
                    .param_anytype => data[@intFromEnum(param_index)].str_tok.src_tok,
                    else => unreachable,
                });
                const name = sema.code.nullTerminatedString(name_nts);
                if (name.len != 0) {
                    try sema.errNote(param_src, msg, "param '{s}' is required to be comptime", .{name});
                } else {
                    try sema.errNote(param_src, msg, "param is required to be comptime", .{});
                }
            }
        }
        return sema.failWithOwnedErrorMsg(block, msg);
    }

    validate_incoming_stack_align: {
        const a: u64 = switch (cc_resolved) {
            inline else => |payload| if (@TypeOf(payload) != void and @hasField(@TypeOf(payload), "incoming_stack_alignment"))
                payload.incoming_stack_alignment orelse break :validate_incoming_stack_align
            else
                break :validate_incoming_stack_align,
        };
        if (!std.math.isPowerOfTwo(a)) {
            return sema.fail(block, cc_src, "calling convention incoming stack alignment '{d}' is not a power of two", .{a});
        }
    }

    switch (cc_resolved) {
        .x86_64_interrupt,
        .x86_interrupt,
        .arm_interrupt,
        .mips64_interrupt,
        .mips_interrupt,
        .riscv64_interrupt,
        .riscv32_interrupt,
        .avr_interrupt,
        .csky_interrupt,
        .m68k_interrupt,
        .avr_signal,
        => if (return_type.zigTypeTag(zcu) != .void and return_type.zigTypeTag(zcu) != .noreturn) {
            return sema.fail(block, ret_ty_src, "function with calling convention '{s}' must return 'void' or 'noreturn'", .{@tagName(cc_resolved)});
        },
        .@"inline" => if (is_noinline) {
            return sema.fail(block, cc_src, "'noinline' function cannot have calling convention 'inline'", .{});
        },
        else => {},
    }

    switch (zcu.callconvSupported(cc_resolved)) {
        .ok => {},
        .bad_arch => |allowed_archs| {
            const ArchListFormatter = struct {
                archs: []const std.Target.Cpu.Arch,
                pub fn format(formatter: @This(), comptime fmt: []const u8, options: std.fmt.FormatOptions, writer: anytype) !void {
                    _ = fmt;
                    _ = options;
                    for (formatter.archs, 0..) |arch, i| {
                        if (i != 0)
                            try writer.writeAll(", ");
                        try writer.print("'{s}'", .{@tagName(arch)});
                    }
                }
            };
            return sema.fail(block, cc_src, "calling convention '{s}' only available on architectures {}", .{
                @tagName(cc_resolved),
                ArchListFormatter{ .archs = allowed_archs },
            });
        },
        .bad_backend => |bad_backend| return sema.fail(block, cc_src, "calling convention '{s}' not supported by compiler backend '{s}'", .{
            @tagName(cc_resolved),
            @tagName(bad_backend),
        }),
    }

    if (!is_generic and sema.wantErrorReturnTracing(return_type)) {
        // Make sure that StackTrace's fields are resolved so that the backend can
        // lower this fn type.
        const unresolved_stack_trace_ty = try sema.getBuiltinType(block.nodeOffset(.zero), .StackTrace);
        try unresolved_stack_trace_ty.resolveFields(pt);
    }

    return Air.internedToRef(if (opt_func_index != .none) opt_func_index else func_ty);
}

fn zirParam(
    sema: *Sema,
    block: *Block,
    inst: Zir.Inst.Index,
    comptime_syntax: bool,
) CompileError!void {
    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].pl_tok;
    const src = block.tokenOffset(inst_data.src_tok);
    const extra = sema.code.extraData(Zir.Inst.Param, inst_data.payload_index);
    const param_name: Zir.NullTerminatedString = extra.data.name;
    const body = sema.code.bodySlice(extra.end, extra.data.type.body_len);

    const param_ty: Type = if (extra.data.type.is_generic) .generic_poison else ty: {
        // Make sure any nested param instructions don't clobber our work.
        const prev_params = block.params;
        block.params = .{};
        defer {
            block.params = prev_params;
        }

        const param_ty_inst = try sema.resolveInlineBody(block, body, inst);
        break :ty try sema.analyzeAsType(block, src, param_ty_inst);
    };

    try block.params.append(sema.arena, .{
        .ty = param_ty.toIntern(),
        .is_comptime = comptime_syntax,
        .name = param_name,
    });
}

fn zirParamAnytype(
    sema: *Sema,
    block: *Block,
    inst: Zir.Inst.Index,
    comptime_syntax: bool,
) CompileError!void {
    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].str_tok;
    const param_name: Zir.NullTerminatedString = inst_data.start;

    try block.params.append(sema.arena, .{
        .ty = .generic_poison_type,
        .is_comptime = comptime_syntax,
        .name = param_name,
    });
}

fn zirAsNode(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    const tracy = trace(@src());
    defer tracy.end();

    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].pl_node;
    const src = block.nodeOffset(inst_data.src_node);
    const extra = sema.code.extraData(Zir.Inst.As, inst_data.payload_index).data;
    return sema.analyzeAs(block, src, extra.dest_type, extra.operand, false);
}

fn zirAsShiftOperand(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    const tracy = trace(@src());
    defer tracy.end();

    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].pl_node;
    const src = block.nodeOffset(inst_data.src_node);
    const extra = sema.code.extraData(Zir.Inst.As, inst_data.payload_index).data;
    return sema.analyzeAs(block, src, extra.dest_type, extra.operand, true);
}

fn analyzeAs(
    sema: *Sema,
    block: *Block,
    src: LazySrcLoc,
    zir_dest_type: Zir.Inst.Ref,
    zir_operand: Zir.Inst.Ref,
    no_cast_to_comptime_int: bool,
) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const operand = try sema.resolveInst(zir_operand);
    const dest_ty = try sema.resolveTypeOrPoison(block, src, zir_dest_type) orelse return operand;
    switch (dest_ty.zigTypeTag(zcu)) {
        .@"opaque" => return sema.fail(block, src, "cannot cast to opaque type '{}'", .{dest_ty.fmt(pt)}),
        .noreturn => return sema.fail(block, src, "cannot cast to noreturn", .{}),
        else => {},
    }

    const is_ret = if (zir_dest_type.toIndex()) |ptr_index|
        sema.code.instructions.items(.tag)[@intFromEnum(ptr_index)] == .ret_type
    else
        false;
    return sema.coerceExtra(block, dest_ty, operand, src, .{ .is_ret = is_ret, .no_cast_to_comptime_int = no_cast_to_comptime_int }) catch |err| switch (err) {
        error.NotCoercible => unreachable,
        else => |e| return e,
    };
}

fn zirIntFromPtr(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    const tracy = trace(@src());
    defer tracy.end();

    const pt = sema.pt;
    const zcu = pt.zcu;
    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].un_node;
    const ptr_src = block.builtinCallArgSrc(inst_data.src_node, 0);
    const operand = try sema.resolveInst(inst_data.operand);
    const operand_ty = sema.typeOf(operand);
    const ptr_ty = operand_ty.scalarType(zcu);
    const is_vector = operand_ty.zigTypeTag(zcu) == .vector;
    if (!ptr_ty.isPtrAtRuntime(zcu)) {
        return sema.fail(block, ptr_src, "expected pointer, found '{}'", .{ptr_ty.fmt(pt)});
    }
    const pointee_ty = ptr_ty.childType(zcu);
    if (try ptr_ty.comptimeOnlySema(pt)) {
        const msg = msg: {
            const msg = try sema.errMsg(ptr_src, "comptime-only type '{}' has no pointer address", .{pointee_ty.fmt(pt)});
            errdefer msg.destroy(sema.gpa);
            try sema.explainWhyTypeIsComptime(msg, ptr_src, pointee_ty);
            break :msg msg;
        };
        return sema.failWithOwnedErrorMsg(block, msg);
    }
    const len = if (is_vector) operand_ty.vectorLen(zcu) else undefined;
    const dest_ty: Type = if (is_vector) try pt.vectorType(.{ .child = .usize_type, .len = len }) else .usize;

    if (try sema.resolveValue(operand)) |operand_val| ct: {
        if (!is_vector) {
            if (operand_val.isUndef(zcu)) {
                return .undef_usize;
            }
            const addr = try operand_val.getUnsignedIntSema(pt) orelse {
                // Wasn't an integer pointer. This is a runtime operation.
                break :ct;
            };
            return Air.internedToRef((try pt.intValue(
                .usize,
                addr,
            )).toIntern());
        }
        const new_elems = try sema.arena.alloc(InternPool.Index, len);
        for (new_elems, 0..) |*new_elem, i| {
            const ptr_val = try operand_val.elemValue(pt, i);
            if (ptr_val.isUndef(zcu)) {
                new_elem.* = .undef_usize;
                continue;
            }
            const addr = try ptr_val.getUnsignedIntSema(pt) orelse {
                // A vector element wasn't an integer pointer. This is a runtime operation.
                break :ct;
            };
            new_elem.* = (try pt.intValue(
                .usize,
                addr,
            )).toIntern();
        }
        return Air.internedToRef(try pt.intern(.{ .aggregate = .{
            .ty = dest_ty.toIntern(),
            .storage = .{ .elems = new_elems },
        } }));
    }
    try sema.requireRuntimeBlock(block, block.nodeOffset(inst_data.src_node), ptr_src);
    try sema.validateRuntimeValue(block, ptr_src, operand);
    try sema.checkLogicalPtrOperation(block, ptr_src, ptr_ty);
    return block.addBitCast(dest_ty, operand);
}

fn zirFieldVal(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    const tracy = trace(@src());
    defer tracy.end();

    const pt = sema.pt;
    const zcu = pt.zcu;
    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].pl_node;
    const src = block.nodeOffset(inst_data.src_node);
    const field_name_src = block.src(.{ .node_offset_field_name = inst_data.src_node });
    const extra = sema.code.extraData(Zir.Inst.Field, inst_data.payload_index).data;
    const field_name = try zcu.intern_pool.getOrPutString(
        sema.gpa,
        pt.tid,
        sema.code.nullTerminatedString(extra.field_name_start),
        .no_embedded_nulls,
    );
    const object = try sema.resolveInst(extra.lhs);
    return sema.fieldVal(block, src, object, field_name, field_name_src);
}

fn zirFieldPtr(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    const tracy = trace(@src());
    defer tracy.end();

    const pt = sema.pt;
    const zcu = pt.zcu;
    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].pl_node;
    const src = block.nodeOffset(inst_data.src_node);
    const field_name_src = block.src(.{ .node_offset_field_name = inst_data.src_node });
    const extra = sema.code.extraData(Zir.Inst.Field, inst_data.payload_index).data;
    const field_name = try zcu.intern_pool.getOrPutString(
        sema.gpa,
        pt.tid,
        sema.code.nullTerminatedString(extra.field_name_start),
        .no_embedded_nulls,
    );
    const object_ptr = try sema.resolveInst(extra.lhs);
    return sema.fieldPtr(block, src, object_ptr, field_name, field_name_src, false);
}

fn zirStructInitFieldPtr(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    const tracy = trace(@src());
    defer tracy.end();

    const pt = sema.pt;
    const zcu = pt.zcu;
    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].pl_node;
    const src = block.nodeOffset(inst_data.src_node);
    const field_name_src = block.src(.{ .node_offset_field_name_init = inst_data.src_node });
    const extra = sema.code.extraData(Zir.Inst.Field, inst_data.payload_index).data;
    const field_name = try zcu.intern_pool.getOrPutString(
        sema.gpa,
        pt.tid,
        sema.code.nullTerminatedString(extra.field_name_start),
        .no_embedded_nulls,
    );
    const object_ptr = try sema.resolveInst(extra.lhs);
    const struct_ty = sema.typeOf(object_ptr).childType(zcu);
    switch (struct_ty.zigTypeTag(zcu)) {
        .@"struct", .@"union" => {
            return sema.fieldPtr(block, src, object_ptr, field_name, field_name_src, true);
        },
        else => {
            return sema.failWithStructInitNotSupported(block, src, struct_ty);
        },
    }
}

fn zirFieldValNamed(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    const tracy = trace(@src());
    defer tracy.end();

    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].pl_node;
    const src = block.nodeOffset(inst_data.src_node);
    const field_name_src = block.builtinCallArgSrc(inst_data.src_node, 1);
    const extra = sema.code.extraData(Zir.Inst.FieldNamed, inst_data.payload_index).data;
    const object = try sema.resolveInst(extra.lhs);
    const field_name = try sema.resolveConstStringIntern(block, field_name_src, extra.field_name, .{ .simple = .field_name });
    return sema.fieldVal(block, src, object, field_name, field_name_src);
}

fn zirFieldPtrNamed(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    const tracy = trace(@src());
    defer tracy.end();

    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].pl_node;
    const src = block.nodeOffset(inst_data.src_node);
    const field_name_src = block.builtinCallArgSrc(inst_data.src_node, 1);
    const extra = sema.code.extraData(Zir.Inst.FieldNamed, inst_data.payload_index).data;
    const object_ptr = try sema.resolveInst(extra.lhs);
    const field_name = try sema.resolveConstStringIntern(block, field_name_src, extra.field_name, .{ .simple = .field_name });
    return sema.fieldPtr(block, src, object_ptr, field_name, field_name_src, false);
}

fn zirIntCast(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    const tracy = trace(@src());
    defer tracy.end();

    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].pl_node;
    const src = block.nodeOffset(inst_data.src_node);
    const operand_src = block.builtinCallArgSrc(inst_data.src_node, 0);
    const extra = sema.code.extraData(Zir.Inst.Bin, inst_data.payload_index).data;

    const dest_ty = try sema.resolveDestType(block, src, extra.lhs, .remove_eu_opt, "@intCast");
    const operand = try sema.resolveInst(extra.rhs);

    return sema.intCast(block, block.nodeOffset(inst_data.src_node), dest_ty, src, operand, operand_src);
}

fn intCast(
    sema: *Sema,
    block: *Block,
    src: LazySrcLoc,
    dest_ty: Type,
    dest_ty_src: LazySrcLoc,
    operand: Air.Inst.Ref,
    operand_src: LazySrcLoc,
) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const operand_ty = sema.typeOf(operand);
    const dest_scalar_ty = try sema.checkIntOrVectorAllowComptime(block, dest_ty, dest_ty_src);
    const operand_scalar_ty = try sema.checkIntOrVectorAllowComptime(block, operand_ty, operand_src);

    if (try sema.isComptimeKnown(operand)) {
        return sema.coerce(block, dest_ty, operand, operand_src);
    } else if (dest_scalar_ty.zigTypeTag(zcu) == .comptime_int) {
        return sema.fail(block, operand_src, "unable to cast runtime value to 'comptime_int'", .{});
    }

    try sema.checkVectorizableBinaryOperands(block, operand_src, dest_ty, operand_ty, dest_ty_src, operand_src);
    const is_vector = dest_ty.zigTypeTag(zcu) == .vector;

    if ((try sema.typeHasOnePossibleValue(dest_ty))) |opv| {
        // requirement: intCast(u0, input) iff input == 0
        if (block.wantSafety()) {
            try sema.requireRuntimeBlock(block, src, operand_src);
            const wanted_info = dest_scalar_ty.intInfo(zcu);
            const wanted_bits = wanted_info.bits;

            if (wanted_bits == 0) {
                const ok = if (is_vector) ok: {
                    const zeros = try sema.splat(operand_ty, try pt.intValue(operand_scalar_ty, 0));
                    const zero_inst = Air.internedToRef(zeros.toIntern());
                    const is_in_range = try block.addCmpVector(operand, zero_inst, .eq);
                    const all_in_range = try block.addReduce(is_in_range, .And);
                    break :ok all_in_range;
                } else ok: {
                    const zero_inst = Air.internedToRef((try pt.intValue(operand_ty, 0)).toIntern());
                    const is_in_range = try block.addBinOp(.cmp_lte, operand, zero_inst);
                    break :ok is_in_range;
                };
                try sema.addSafetyCheck(block, src, ok, .integer_out_of_bounds);
            }
        }

        return Air.internedToRef(opv.toIntern());
    }

    try sema.requireRuntimeBlock(block, src, operand_src);
    if (block.wantSafety()) {
        if (zcu.backendSupportsFeature(.panic_fn)) {
            _ = try sema.preparePanicId(src, .integer_out_of_bounds);
        }
        return block.addTyOp(.intcast_safe, dest_ty, operand);
    }
    return block.addTyOp(.intcast, dest_ty, operand);
}

fn zirBitcast(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    const tracy = trace(@src());
    defer tracy.end();

    const pt = sema.pt;
    const zcu = pt.zcu;
    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].pl_node;
    const src = block.nodeOffset(inst_data.src_node);
    const operand_src = block.builtinCallArgSrc(inst_data.src_node, 0);
    const extra = sema.code.extraData(Zir.Inst.Bin, inst_data.payload_index).data;

    const dest_ty = try sema.resolveDestType(block, src, extra.lhs, .remove_eu_opt, "@bitCast");
    const operand = try sema.resolveInst(extra.rhs);
    const operand_ty = sema.typeOf(operand);
    switch (dest_ty.zigTypeTag(zcu)) {
        .@"anyframe",
        .comptime_float,
        .comptime_int,
        .enum_literal,
        .error_set,
        .error_union,
        .@"fn",
        .frame,
        .noreturn,
        .null,
        .@"opaque",
        .optional,
        .type,
        .undefined,
        .void,
        => return sema.fail(block, src, "cannot @bitCast to '{}'", .{dest_ty.fmt(pt)}),

        .@"enum" => {
            const msg = msg: {
                const msg = try sema.errMsg(src, "cannot @bitCast to '{}'", .{dest_ty.fmt(pt)});
                errdefer msg.destroy(sema.gpa);
                switch (operand_ty.zigTypeTag(zcu)) {
                    .int, .comptime_int => try sema.errNote(src, msg, "use @enumFromInt to cast from '{}'", .{operand_ty.fmt(pt)}),
                    else => {},
                }

                break :msg msg;
            };
            return sema.failWithOwnedErrorMsg(block, msg);
        },

        .pointer => {
            const msg = msg: {
                const msg = try sema.errMsg(src, "cannot @bitCast to '{}'", .{dest_ty.fmt(pt)});
                errdefer msg.destroy(sema.gpa);
                switch (operand_ty.zigTypeTag(zcu)) {
                    .int, .comptime_int => try sema.errNote(src, msg, "use @ptrFromInt to cast from '{}'", .{operand_ty.fmt(pt)}),
                    .pointer => try sema.errNote(src, msg, "use @ptrCast to cast from '{}'", .{operand_ty.fmt(pt)}),
                    else => {},
                }

                break :msg msg;
            };
            return sema.failWithOwnedErrorMsg(block, msg);
        },
        .@"struct", .@"union" => if (dest_ty.containerLayout(zcu) == .auto) {
            const container = switch (dest_ty.zigTypeTag(zcu)) {
                .@"struct" => "struct",
                .@"union" => "union",
                else => unreachable,
            };
            return sema.fail(block, src, "cannot @bitCast to '{}'; {s} does not have a guaranteed in-memory layout", .{
                dest_ty.fmt(pt), container,
            });
        },

        .array,
        .bool,
        .float,
        .int,
        .vector,
        => {},
    }
    switch (operand_ty.zigTypeTag(zcu)) {
        .@"anyframe",
        .comptime_float,
        .comptime_int,
        .enum_literal,
        .error_set,
        .error_union,
        .@"fn",
        .frame,
        .noreturn,
        .null,
        .@"opaque",
        .optional,
        .type,
        .undefined,
        .void,
        => return sema.fail(block, operand_src, "cannot @bitCast from '{}'", .{operand_ty.fmt(pt)}),

        .@"enum" => {
            const msg = msg: {
                const msg = try sema.errMsg(operand_src, "cannot @bitCast from '{}'", .{operand_ty.fmt(pt)});
                errdefer msg.destroy(sema.gpa);
                switch (dest_ty.zigTypeTag(zcu)) {
                    .int, .comptime_int => try sema.errNote(operand_src, msg, "use @intFromEnum to cast to '{}'", .{dest_ty.fmt(pt)}),
                    else => {},
                }

                break :msg msg;
            };
            return sema.failWithOwnedErrorMsg(block, msg);
        },
        .pointer => {
            const msg = msg: {
                const msg = try sema.errMsg(operand_src, "cannot @bitCast from '{}'", .{operand_ty.fmt(pt)});
                errdefer msg.destroy(sema.gpa);
                switch (dest_ty.zigTypeTag(zcu)) {
                    .int, .comptime_int => try sema.errNote(operand_src, msg, "use @intFromPtr to cast to '{}'", .{dest_ty.fmt(pt)}),
                    .pointer => try sema.errNote(operand_src, msg, "use @ptrCast to cast to '{}'", .{dest_ty.fmt(pt)}),
                    else => {},
                }

                break :msg msg;
            };
            return sema.failWithOwnedErrorMsg(block, msg);
        },
        .@"struct", .@"union" => if (operand_ty.containerLayout(zcu) == .auto) {
            const container = switch (operand_ty.zigTypeTag(zcu)) {
                .@"struct" => "struct",
                .@"union" => "union",
                else => unreachable,
            };
            return sema.fail(block, operand_src, "cannot @bitCast from '{}'; {s} does not have a guaranteed in-memory layout", .{
                operand_ty.fmt(pt), container,
            });
        },

        .array,
        .bool,
        .float,
        .int,
        .vector,
        => {},
    }
    return sema.bitCast(block, dest_ty, operand, block.nodeOffset(inst_data.src_node), operand_src);
}

fn zirFloatCast(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    const tracy = trace(@src());
    defer tracy.end();

    const pt = sema.pt;
    const zcu = pt.zcu;
    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].pl_node;
    const src = block.nodeOffset(inst_data.src_node);
    const operand_src = block.builtinCallArgSrc(inst_data.src_node, 0);
    const extra = sema.code.extraData(Zir.Inst.Bin, inst_data.payload_index).data;

    const dest_ty = try sema.resolveDestType(block, src, extra.lhs, .remove_eu_opt, "@floatCast");
    const dest_scalar_ty = dest_ty.scalarType(zcu);

    const operand = try sema.resolveInst(extra.rhs);
    const operand_ty = sema.typeOf(operand);
    const operand_scalar_ty = operand_ty.scalarType(zcu);

    try sema.checkVectorizableBinaryOperands(block, operand_src, dest_ty, operand_ty, src, operand_src);
    const is_vector = dest_ty.zigTypeTag(zcu) == .vector;

    const target = zcu.getTarget();
    const dest_is_comptime_float = switch (dest_scalar_ty.zigTypeTag(zcu)) {
        .comptime_float => true,
        .float => false,
        else => return sema.fail(
            block,
            src,
            "expected float or vector type, found '{}'",
            .{dest_ty.fmt(pt)},
        ),
    };

    switch (operand_scalar_ty.zigTypeTag(zcu)) {
        .comptime_float, .float, .comptime_int => {},
        else => return sema.fail(
            block,
            operand_src,
            "expected float or vector type, found '{}'",
            .{operand_ty.fmt(pt)},
        ),
    }

    if (try sema.resolveValue(operand)) |operand_val| {
        if (!is_vector) {
            return Air.internedToRef((try operand_val.floatCast(dest_ty, pt)).toIntern());
        }
        const vec_len = operand_ty.vectorLen(zcu);
        const new_elems = try sema.arena.alloc(InternPool.Index, vec_len);
        for (new_elems, 0..) |*new_elem, i| {
            const old_elem = try operand_val.elemValue(pt, i);
            new_elem.* = (try old_elem.floatCast(dest_scalar_ty, pt)).toIntern();
        }
        return Air.internedToRef(try pt.intern(.{ .aggregate = .{
            .ty = dest_ty.toIntern(),
            .storage = .{ .elems = new_elems },
        } }));
    }
    if (dest_is_comptime_float) {
        return sema.fail(block, operand_src, "unable to cast runtime value to 'comptime_float'", .{});
    }
    try sema.requireRuntimeBlock(block, block.nodeOffset(inst_data.src_node), operand_src);

    const src_bits = operand_scalar_ty.floatBits(target);
    const dst_bits = dest_scalar_ty.floatBits(target);
    if (dst_bits >= src_bits) {
        return sema.coerce(block, dest_ty, operand, operand_src);
    }
    return block.addTyOp(.fptrunc, dest_ty, operand);
}

fn zirElemVal(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    const tracy = trace(@src());
    defer tracy.end();

    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].pl_node;
    const src = block.nodeOffset(inst_data.src_node);
    const extra = sema.code.extraData(Zir.Inst.Bin, inst_data.payload_index).data;
    const array = try sema.resolveInst(extra.lhs);
    const elem_index = try sema.resolveInst(extra.rhs);
    return sema.elemVal(block, src, array, elem_index, src, false);
}

fn zirElemValNode(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    const tracy = trace(@src());
    defer tracy.end();

    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].pl_node;
    const src = block.nodeOffset(inst_data.src_node);
    const elem_index_src = block.src(.{ .node_offset_array_access_index = inst_data.src_node });
    const extra = sema.code.extraData(Zir.Inst.Bin, inst_data.payload_index).data;
    const array = try sema.resolveInst(extra.lhs);
    const uncoerced_elem_index = try sema.resolveInst(extra.rhs);
    const elem_index = try sema.coerce(block, .usize, uncoerced_elem_index, elem_index_src);
    return sema.elemVal(block, src, array, elem_index, elem_index_src, true);
}

fn zirElemValImm(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    const tracy = trace(@src());
    defer tracy.end();

    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].elem_val_imm;
    const array = try sema.resolveInst(inst_data.operand);
    const elem_index = try sema.pt.intRef(.usize, inst_data.idx);
    return sema.elemVal(block, LazySrcLoc.unneeded, array, elem_index, LazySrcLoc.unneeded, false);
}

fn zirElemPtr(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    const tracy = trace(@src());
    defer tracy.end();

    const pt = sema.pt;
    const zcu = pt.zcu;
    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].pl_node;
    const src = block.nodeOffset(inst_data.src_node);
    const extra = sema.code.extraData(Zir.Inst.Bin, inst_data.payload_index).data;
    const array_ptr = try sema.resolveInst(extra.lhs);
    const elem_index = try sema.resolveInst(extra.rhs);
    const indexable_ty = sema.typeOf(array_ptr);
    if (indexable_ty.zigTypeTag(zcu) != .pointer) {
        const capture_src = block.src(.{ .for_capture_from_input = inst_data.src_node });
        const msg = msg: {
            const msg = try sema.errMsg(capture_src, "pointer capture of non pointer type '{}'", .{
                indexable_ty.fmt(pt),
            });
            errdefer msg.destroy(sema.gpa);
            if (indexable_ty.isIndexable(zcu)) {
                try sema.errNote(src, msg, "consider using '&' here", .{});
            }
            break :msg msg;
        };
        return sema.failWithOwnedErrorMsg(block, msg);
    }
    return sema.elemPtrOneLayerOnly(block, src, array_ptr, elem_index, src, false, false);
}

fn zirElemPtrNode(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    const tracy = trace(@src());
    defer tracy.end();

    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].pl_node;
    const src = block.nodeOffset(inst_data.src_node);
    const elem_index_src = block.src(.{ .node_offset_array_access_index = inst_data.src_node });
    const extra = sema.code.extraData(Zir.Inst.Bin, inst_data.payload_index).data;
    const array_ptr = try sema.resolveInst(extra.lhs);
    const uncoerced_elem_index = try sema.resolveInst(extra.rhs);
    const elem_index = try sema.coerce(block, .usize, uncoerced_elem_index, elem_index_src);
    return sema.elemPtr(block, src, array_ptr, elem_index, elem_index_src, false, true);
}

fn zirArrayInitElemPtr(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    const tracy = trace(@src());
    defer tracy.end();

    const pt = sema.pt;
    const zcu = pt.zcu;
    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].pl_node;
    const src = block.nodeOffset(inst_data.src_node);
    const extra = sema.code.extraData(Zir.Inst.ElemPtrImm, inst_data.payload_index).data;
    const array_ptr = try sema.resolveInst(extra.ptr);
    const elem_index = try pt.intRef(.usize, extra.index);
    const array_ty = sema.typeOf(array_ptr).childType(zcu);
    switch (array_ty.zigTypeTag(zcu)) {
        .array, .vector => {},
        else => if (!array_ty.isTuple(zcu)) {
            return sema.failWithArrayInitNotSupported(block, src, array_ty);
        },
    }
    return sema.elemPtr(block, src, array_ptr, elem_index, src, true, true);
}

fn zirSliceStart(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    const tracy = trace(@src());
    defer tracy.end();

    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].pl_node;
    const src = block.nodeOffset(inst_data.src_node);
    const extra = sema.code.extraData(Zir.Inst.SliceStart, inst_data.payload_index).data;
    const array_ptr = try sema.resolveInst(extra.lhs);
    const start = try sema.resolveInst(extra.start);
    const ptr_src = block.src(.{ .node_offset_slice_ptr = inst_data.src_node });
    const start_src = block.src(.{ .node_offset_slice_start = inst_data.src_node });
    const end_src = block.src(.{ .node_offset_slice_end = inst_data.src_node });

    return sema.analyzeSlice(block, src, array_ptr, start, .none, .none, LazySrcLoc.unneeded, ptr_src, start_src, end_src, false);
}

fn zirSliceEnd(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    const tracy = trace(@src());
    defer tracy.end();

    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].pl_node;
    const src = block.nodeOffset(inst_data.src_node);
    const extra = sema.code.extraData(Zir.Inst.SliceEnd, inst_data.payload_index).data;
    const array_ptr = try sema.resolveInst(extra.lhs);
    const start = try sema.resolveInst(extra.start);
    const end = try sema.resolveInst(extra.end);
    const ptr_src = block.src(.{ .node_offset_slice_ptr = inst_data.src_node });
    const start_src = block.src(.{ .node_offset_slice_start = inst_data.src_node });
    const end_src = block.src(.{ .node_offset_slice_end = inst_data.src_node });

    return sema.analyzeSlice(block, src, array_ptr, start, end, .none, LazySrcLoc.unneeded, ptr_src, start_src, end_src, false);
}

fn zirSliceSentinel(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    const tracy = trace(@src());
    defer tracy.end();

    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].pl_node;
    const src = block.nodeOffset(inst_data.src_node);
    const sentinel_src = block.src(.{ .node_offset_slice_sentinel = inst_data.src_node });
    const extra = sema.code.extraData(Zir.Inst.SliceSentinel, inst_data.payload_index).data;
    const array_ptr = try sema.resolveInst(extra.lhs);
    const start = try sema.resolveInst(extra.start);
    const end: Air.Inst.Ref = if (extra.end == .none) .none else try sema.resolveInst(extra.end);
    const sentinel = try sema.resolveInst(extra.sentinel);
    const ptr_src = block.src(.{ .node_offset_slice_ptr = inst_data.src_node });
    const start_src = block.src(.{ .node_offset_slice_start = inst_data.src_node });
    const end_src = block.src(.{ .node_offset_slice_end = inst_data.src_node });

    return sema.analyzeSlice(block, src, array_ptr, start, end, sentinel, sentinel_src, ptr_src, start_src, end_src, false);
}

fn zirSliceLength(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    const tracy = trace(@src());
    defer tracy.end();

    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].pl_node;
    const src = block.nodeOffset(inst_data.src_node);
    const extra = sema.code.extraData(Zir.Inst.SliceLength, inst_data.payload_index).data;
    const array_ptr = try sema.resolveInst(extra.lhs);
    const start = try sema.resolveInst(extra.start);
    const len = try sema.resolveInst(extra.len);
    const sentinel = if (extra.sentinel == .none) .none else try sema.resolveInst(extra.sentinel);
    const ptr_src = block.src(.{ .node_offset_slice_ptr = inst_data.src_node });
    const start_src = block.src(.{ .node_offset_slice_start = extra.start_src_node_offset });
    const end_src = block.src(.{ .node_offset_slice_end = inst_data.src_node });
    const sentinel_src: LazySrcLoc = if (sentinel == .none)
        LazySrcLoc.unneeded
    else
        block.src(.{ .node_offset_slice_sentinel = inst_data.src_node });

    return sema.analyzeSlice(block, src, array_ptr, start, len, sentinel, sentinel_src, ptr_src, start_src, end_src, true);
}

fn zirSliceSentinelTy(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    const tracy = trace(@src());
    defer tracy.end();

    const pt = sema.pt;
    const zcu = pt.zcu;

    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].un_node;

    const src = block.nodeOffset(inst_data.src_node);
    const ptr_src = block.src(.{ .node_offset_slice_ptr = inst_data.src_node });
    const sentinel_src = block.src(.{ .node_offset_slice_sentinel = inst_data.src_node });

    // This is like the logic in `analyzeSlice`; since we've evaluated the LHS as an lvalue, we will
    // have a double pointer if it was already a pointer.

    const lhs_ptr_ty = sema.typeOf(try sema.resolveInst(inst_data.operand));
    const lhs_ty = switch (lhs_ptr_ty.zigTypeTag(zcu)) {
        .pointer => lhs_ptr_ty.childType(zcu),
        else => return sema.fail(block, ptr_src, "expected pointer, found '{}'", .{lhs_ptr_ty.fmt(pt)}),
    };

    const sentinel_ty: Type = switch (lhs_ty.zigTypeTag(zcu)) {
        .array => lhs_ty.childType(zcu),
        .pointer => switch (lhs_ty.ptrSize(zcu)) {
            .many, .c, .slice => lhs_ty.childType(zcu),
            .one => s: {
                const lhs_elem_ty = lhs_ty.childType(zcu);
                break :s switch (lhs_elem_ty.zigTypeTag(zcu)) {
                    .array => lhs_elem_ty.childType(zcu), // array element type
                    else => return sema.fail(block, sentinel_src, "slice of single-item pointer cannot have sentinel", .{}),
                };
            },
        },
        else => return sema.fail(block, src, "slice of non-array type '{}'", .{lhs_ty.fmt(pt)}),
    };

    return Air.internedToRef(sentinel_ty.toIntern());
}

/// Holds common data used when analyzing or resolving switch prong bodies,
/// including setting up captures.
const SwitchProngAnalysis = struct {
    sema: *Sema,
    /// The block containing the `switch_block` itself.
    parent_block: *Block,
    operand: Operand,
    /// If this switch is on an error set, this is the type to assign to the
    /// `else` prong. If `null`, the prong should be unreachable.
    else_error_ty: ?Type,
    /// The index of the `switch_block` instruction itself.
    switch_block_inst: Zir.Inst.Index,
    /// The dummy index into which inline tag captures should be placed. May be
    /// undefined if no prong has a tag capture.
    tag_capture_inst: Zir.Inst.Index,

    const Operand = union(enum) {
        /// This switch will be dispatched only once, with the given operand.
        simple: struct {
            /// The raw switch operand value. Always defined.
            by_val: Air.Inst.Ref,
            /// The switch operand *pointer*. Defined only if there is a prong
            /// with a by-ref capture.
            by_ref: Air.Inst.Ref,
            /// The switch condition value. For unions, `operand` is the union
            /// and `cond` is its enum tag value.
            cond: Air.Inst.Ref,
        },
        /// This switch may be dispatched multiple times with `continue` syntax.
        /// As such, the operand is stored in an alloc if needed.
        loop: struct {
            /// The `alloc` containing the `switch` operand for the active dispatch.
            /// Each prong must load from this `alloc` to get captures.
            /// If there are no captures, this may be undefined.
            operand_alloc: Air.Inst.Ref,
            /// Whether `operand_alloc` contains a by-val operand or a by-ref
            /// operand.
            operand_is_ref: bool,
            /// The switch condition value for the *initial* dispatch. For
            /// unions, this is the enum tag value.
            init_cond: Air.Inst.Ref,
        },
    };

    /// Resolve a switch prong which is determined at comptime to have no peers.
    /// Uses `resolveBlockBody`. Sets up captures as needed.
    fn resolveProngComptime(
        spa: SwitchProngAnalysis,
        child_block: *Block,
        prong_type: enum { normal, special },
        prong_body: []const Zir.Inst.Index,
        capture: Zir.Inst.SwitchBlock.ProngInfo.Capture,
        /// Must use the `switch_capture` field in `offset`.
        capture_src: LazySrcLoc,
        /// The set of all values which can reach this prong. May be undefined
        /// if the prong is special or contains ranges.
        case_vals: []const Air.Inst.Ref,
        /// The inline capture of this prong. If this is not an inline prong,
        /// this is `.none`.
        inline_case_capture: Air.Inst.Ref,
        /// Whether this prong has an inline tag capture. If `true`, then
        /// `inline_case_capture` cannot be `.none`.
        has_tag_capture: bool,
        merges: *Block.Merges,
    ) CompileError!Air.Inst.Ref {
        const sema = spa.sema;
        const src = spa.parent_block.nodeOffset(
            sema.code.instructions.items(.data)[@intFromEnum(spa.switch_block_inst)].pl_node.src_node,
        );

        // We can propagate `.cold` hints from this branch since it's comptime-known
        // to be taken from the parent branch.
        const parent_hint = sema.branch_hint;
        defer sema.branch_hint = parent_hint orelse if (sema.branch_hint == .cold) .cold else null;

        if (has_tag_capture) {
            const tag_ref = try spa.analyzeTagCapture(child_block, capture_src, inline_case_capture);
            sema.inst_map.putAssumeCapacity(spa.tag_capture_inst, tag_ref);
        }
        defer if (has_tag_capture) assert(sema.inst_map.remove(spa.tag_capture_inst));

        switch (capture) {
            .none => {
                return sema.resolveBlockBody(spa.parent_block, src, child_block, prong_body, spa.switch_block_inst, merges);
            },

            .by_val, .by_ref => {
                const capture_ref = try spa.analyzeCapture(
                    child_block,
                    capture == .by_ref,
                    prong_type == .special,
                    capture_src,
                    case_vals,
                    inline_case_capture,
                );

                if (sema.typeOf(capture_ref).isNoReturn(sema.pt.zcu)) {
                    // This prong should be unreachable!
                    return .unreachable_value;
                }

                sema.inst_map.putAssumeCapacity(spa.switch_block_inst, capture_ref);
                defer assert(sema.inst_map.remove(spa.switch_block_inst));

                return sema.resolveBlockBody(spa.parent_block, src, child_block, prong_body, spa.switch_block_inst, merges);
            },
        }
    }

    /// Analyze a switch prong which may have peers at runtime.
    /// Uses `analyzeBodyRuntimeBreak`. Sets up captures as needed.
    /// Returns the `BranchHint` for the prong.
    fn analyzeProngRuntime(
        spa: SwitchProngAnalysis,
        case_block: *Block,
        prong_type: enum { normal, special },
        prong_body: []const Zir.Inst.Index,
        capture: Zir.Inst.SwitchBlock.ProngInfo.Capture,
        /// Must use the `switch_capture` field in `offset`.
        capture_src: LazySrcLoc,
        /// The set of all values which can reach this prong. May be undefined
        /// if the prong is special or contains ranges.
        case_vals: []const Air.Inst.Ref,
        /// The inline capture of this prong. If this is not an inline prong,
        /// this is `.none`.
        inline_case_capture: Air.Inst.Ref,
        /// Whether this prong has an inline tag capture. If `true`, then
        /// `inline_case_capture` cannot be `.none`.
        has_tag_capture: bool,
    ) CompileError!std.builtin.BranchHint {
        const sema = spa.sema;

        if (has_tag_capture) {
            const tag_ref = try spa.analyzeTagCapture(case_block, capture_src, inline_case_capture);
            sema.inst_map.putAssumeCapacity(spa.tag_capture_inst, tag_ref);
        }
        defer if (has_tag_capture) assert(sema.inst_map.remove(spa.tag_capture_inst));

        switch (capture) {
            .none => {
                return sema.analyzeBodyRuntimeBreak(case_block, prong_body);
            },

            .by_val, .by_ref => {
                const capture_ref = try spa.analyzeCapture(
                    case_block,
                    capture == .by_ref,
                    prong_type == .special,
                    capture_src,
                    case_vals,
                    inline_case_capture,
                );

                if (sema.typeOf(capture_ref).isNoReturn(sema.pt.zcu)) {
                    // No need to analyze any further, the prong is unreachable
                    return .none;
                }

                sema.inst_map.putAssumeCapacity(spa.switch_block_inst, capture_ref);
                defer assert(sema.inst_map.remove(spa.switch_block_inst));

                return sema.analyzeBodyRuntimeBreak(case_block, prong_body);
            },
        }
    }

    fn analyzeTagCapture(
        spa: SwitchProngAnalysis,
        block: *Block,
        capture_src: LazySrcLoc,
        inline_case_capture: Air.Inst.Ref,
    ) CompileError!Air.Inst.Ref {
        const sema = spa.sema;
        const pt = sema.pt;
        const zcu = pt.zcu;
        const operand_ty = switch (spa.operand) {
            .simple => |s| sema.typeOf(s.by_val),
            .loop => |l| ty: {
                const alloc_ty = sema.typeOf(l.operand_alloc);
                const alloc_child = alloc_ty.childType(zcu);
                if (l.operand_is_ref) break :ty alloc_child.childType(zcu);
                break :ty alloc_child;
            },
        };
        if (operand_ty.zigTypeTag(zcu) != .@"union") {
            const tag_capture_src: LazySrcLoc = .{
                .base_node_inst = capture_src.base_node_inst,
                .offset = .{ .switch_tag_capture = capture_src.offset.switch_capture },
            };
            return sema.fail(block, tag_capture_src, "cannot capture tag of non-union type '{}'", .{
                operand_ty.fmt(pt),
            });
        }
        assert(inline_case_capture != .none);
        return inline_case_capture;
    }

    fn analyzeCapture(
        spa: SwitchProngAnalysis,
        block: *Block,
        capture_byref: bool,
        is_special_prong: bool,
        capture_src: LazySrcLoc,
        case_vals: []const Air.Inst.Ref,
        inline_case_capture: Air.Inst.Ref,
    ) CompileError!Air.Inst.Ref {
        const sema = spa.sema;
        const pt = sema.pt;
        const zcu = pt.zcu;
        const ip = &zcu.intern_pool;

        const zir_datas = sema.code.instructions.items(.data);
        const switch_node_offset = zir_datas[@intFromEnum(spa.switch_block_inst)].pl_node.src_node;

        const operand_src = block.src(.{ .node_offset_switch_operand = switch_node_offset });

        const operand_val, const operand_ptr = switch (spa.operand) {
            .simple => |s| .{ s.by_val, s.by_ref },
            .loop => |l| op: {
                const loaded = try sema.analyzeLoad(block, operand_src, l.operand_alloc, operand_src);
                if (l.operand_is_ref) {
                    const by_val = try sema.analyzeLoad(block, operand_src, loaded, operand_src);
                    break :op .{ by_val, loaded };
                } else {
                    break :op .{ loaded, undefined };
                }
            },
        };

        const operand_ty = sema.typeOf(operand_val);
        const operand_ptr_ty = if (capture_byref) sema.typeOf(operand_ptr) else undefined;

        if (inline_case_capture != .none) {
            const item_val = sema.resolveConstDefinedValue(block, LazySrcLoc.unneeded, inline_case_capture, undefined) catch unreachable;
            if (operand_ty.zigTypeTag(zcu) == .@"union") {
                const field_index: u32 = @intCast(operand_ty.unionTagFieldIndex(item_val, zcu).?);
                const union_obj = zcu.typeToUnion(operand_ty).?;
                const field_ty: Type = .fromInterned(union_obj.field_types.get(ip)[field_index]);
                if (capture_byref) {
                    const ptr_field_ty = try pt.ptrTypeSema(.{
                        .child = field_ty.toIntern(),
                        .flags = .{
                            .is_const = !operand_ptr_ty.ptrIsMutable(zcu),
                            .is_volatile = operand_ptr_ty.isVolatilePtr(zcu),
                            .address_space = operand_ptr_ty.ptrAddressSpace(zcu),
                        },
                    });
                    if (try sema.resolveDefinedValue(block, operand_src, operand_ptr)) |union_ptr| {
                        return Air.internedToRef((try union_ptr.ptrField(field_index, pt)).toIntern());
                    }
                    return block.addStructFieldPtr(operand_ptr, field_index, ptr_field_ty);
                } else {
                    if (try sema.resolveDefinedValue(block, operand_src, operand_val)) |union_val| {
                        const tag_and_val = ip.indexToKey(union_val.toIntern()).un;
                        return Air.internedToRef(tag_and_val.val);
                    }
                    return block.addStructFieldVal(operand_val, field_index, field_ty);
                }
            } else if (capture_byref) {
                return sema.uavRef(item_val.toIntern());
            } else {
                return inline_case_capture;
            }
        }

        if (is_special_prong) {
            if (capture_byref) {
                return operand_ptr;
            }

            switch (operand_ty.zigTypeTag(zcu)) {
                .error_set => if (spa.else_error_ty) |ty| {
                    return sema.bitCast(block, ty, operand_val, operand_src, null);
                } else {
                    try sema.analyzeUnreachable(block, operand_src, false);
                    return .unreachable_value;
                },
                else => return operand_val,
            }
        }

        switch (operand_ty.zigTypeTag(zcu)) {
            .@"union" => {
                const union_obj = zcu.typeToUnion(operand_ty).?;
                const first_item_val = sema.resolveConstDefinedValue(block, LazySrcLoc.unneeded, case_vals[0], undefined) catch unreachable;

                const first_field_index: u32 = zcu.unionTagFieldIndex(union_obj, first_item_val).?;
                const first_field_ty: Type = .fromInterned(union_obj.field_types.get(ip)[first_field_index]);

                const field_indices = try sema.arena.alloc(u32, case_vals.len);
                for (case_vals, field_indices) |item, *field_idx| {
                    const item_val = sema.resolveConstDefinedValue(block, LazySrcLoc.unneeded, item, undefined) catch unreachable;
                    field_idx.* = zcu.unionTagFieldIndex(union_obj, item_val).?;
                }

                // Fast path: if all the operands are the same type already, we don't need to hit
                // PTR! This will also allow us to emit simpler code.
                const same_types = for (field_indices[1..]) |field_idx| {
                    const field_ty: Type = .fromInterned(union_obj.field_types.get(ip)[field_idx]);
                    if (!field_ty.eql(first_field_ty, zcu)) break false;
                } else true;

                const capture_ty = if (same_types) first_field_ty else capture_ty: {
                    // We need values to run PTR on, so make a bunch of undef constants.
                    const dummy_captures = try sema.arena.alloc(Air.Inst.Ref, case_vals.len);
                    for (dummy_captures, field_indices) |*dummy, field_idx| {
                        const field_ty: Type = .fromInterned(union_obj.field_types.get(ip)[field_idx]);
                        dummy.* = try pt.undefRef(field_ty);
                    }

                    const case_srcs = try sema.arena.alloc(?LazySrcLoc, case_vals.len);
                    for (case_srcs, 0..) |*case_src, i| {
                        case_src.* = .{
                            .base_node_inst = capture_src.base_node_inst,
                            .offset = .{ .switch_case_item = .{
                                .switch_node_offset = switch_node_offset,
                                .case_idx = capture_src.offset.switch_capture.case_idx,
                                .item_idx = .{ .kind = .single, .index = @intCast(i) },
                            } },
                        };
                    }

                    break :capture_ty sema.resolvePeerTypes(block, capture_src, dummy_captures, .{ .override = case_srcs }) catch |err| switch (err) {
                        error.AnalysisFail => {
                            const msg = sema.err orelse return error.AnalysisFail;
                            try sema.reparentOwnedErrorMsg(capture_src, msg, "capture group with incompatible types", .{});
                            return error.AnalysisFail;
                        },
                        else => |e| return e,
                    };
                };

                // By-reference captures have some further restrictions which make them easier to emit
                if (capture_byref) {
                    const operand_ptr_info = operand_ptr_ty.ptrInfo(zcu);
                    const capture_ptr_ty = resolve: {
                        // By-ref captures of hetereogeneous types are only allowed if all field
                        // pointer types are peer resolvable to each other.
                        // We need values to run PTR on, so make a bunch of undef constants.
                        const dummy_captures = try sema.arena.alloc(Air.Inst.Ref, case_vals.len);
                        for (field_indices, dummy_captures) |field_idx, *dummy| {
                            const field_ty: Type = .fromInterned(union_obj.field_types.get(ip)[field_idx]);
                            const field_ptr_ty = try pt.ptrTypeSema(.{
                                .child = field_ty.toIntern(),
                                .flags = .{
                                    .is_const = operand_ptr_info.flags.is_const,
                                    .is_volatile = operand_ptr_info.flags.is_volatile,
                                    .address_space = operand_ptr_info.flags.address_space,
                                    .alignment = union_obj.fieldAlign(ip, field_idx),
                                },
                            });
                            dummy.* = try pt.undefRef(field_ptr_ty);
                        }
                        const case_srcs = try sema.arena.alloc(?LazySrcLoc, case_vals.len);
                        for (case_srcs, 0..) |*case_src, i| {
                            case_src.* = .{
                                .base_node_inst = capture_src.base_node_inst,
                                .offset = .{ .switch_case_item = .{
                                    .switch_node_offset = switch_node_offset,
                                    .case_idx = capture_src.offset.switch_capture.case_idx,
                                    .item_idx = .{ .kind = .single, .index = @intCast(i) },
                                } },
                            };
                        }

                        break :resolve sema.resolvePeerTypes(block, capture_src, dummy_captures, .{ .override = case_srcs }) catch |err| switch (err) {
                            error.AnalysisFail => {
                                const msg = sema.err orelse return error.AnalysisFail;
                                try sema.errNote(capture_src, msg, "this coercion is only possible when capturing by value", .{});
                                try sema.reparentOwnedErrorMsg(capture_src, msg, "capture group with incompatible types", .{});
                                return error.AnalysisFail;
                            },
                            else => |e| return e,
                        };
                    };

                    if (try sema.resolveDefinedValue(block, operand_src, operand_ptr)) |op_ptr_val| {
                        if (op_ptr_val.isUndef(zcu)) return pt.undefRef(capture_ptr_ty);
                        const field_ptr_val = try op_ptr_val.ptrField(first_field_index, pt);
                        return Air.internedToRef((try pt.getCoerced(field_ptr_val, capture_ptr_ty)).toIntern());
                    }

                    try sema.requireRuntimeBlock(block, operand_src, null);
                    return block.addStructFieldPtr(operand_ptr, first_field_index, capture_ptr_ty);
                }

                if (try sema.resolveDefinedValue(block, operand_src, operand_val)) |operand_val_val| {
                    if (operand_val_val.isUndef(zcu)) return pt.undefRef(capture_ty);
                    const union_val = ip.indexToKey(operand_val_val.toIntern()).un;
                    if (Value.fromInterned(union_val.tag).isUndef(zcu)) return pt.undefRef(capture_ty);
                    const uncoerced = Air.internedToRef(union_val.val);
                    return sema.coerce(block, capture_ty, uncoerced, operand_src);
                }

                try sema.requireRuntimeBlock(block, operand_src, null);

                if (same_types) {
                    return block.addStructFieldVal(operand_val, first_field_index, capture_ty);
                }

                // We may have to emit a switch block which coerces the operand to the capture type.
                // If we can, try to avoid that using in-memory coercions.
                const first_non_imc = in_mem: {
                    for (field_indices, 0..) |field_idx, i| {
                        const field_ty: Type = .fromInterned(union_obj.field_types.get(ip)[field_idx]);
                        if (.ok != try sema.coerceInMemoryAllowed(block, capture_ty, field_ty, false, zcu.getTarget(), LazySrcLoc.unneeded, LazySrcLoc.unneeded, null)) {
                            break :in_mem i;
                        }
                    }
                    // All fields are in-memory coercible to the resolved type!
                    // Just take the first field and bitcast the result.
                    const uncoerced = try block.addStructFieldVal(operand_val, first_field_index, first_field_ty);
                    return block.addBitCast(capture_ty, uncoerced);
                };

                // By-val capture with heterogeneous types which are not all in-memory coercible to
                // the resolved capture type. We finally have to fall back to the ugly method.

                // However, let's first track which operands are in-memory coercible. There may well
                // be several, and we can squash all of these cases into the same switch prong using
                // a simple bitcast. We'll make this the 'else' prong.

                var in_mem_coercible = try std.DynamicBitSet.initFull(sema.arena, field_indices.len);
                in_mem_coercible.unset(first_non_imc);
                {
                    const next = first_non_imc + 1;
                    for (field_indices[next..], next..) |field_idx, i| {
                        const field_ty: Type = .fromInterned(union_obj.field_types.get(ip)[field_idx]);
                        if (.ok != try sema.coerceInMemoryAllowed(block, capture_ty, field_ty, false, zcu.getTarget(), LazySrcLoc.unneeded, LazySrcLoc.unneeded, null)) {
                            in_mem_coercible.unset(i);
                        }
                    }
                }

                const capture_block_inst = try block.addInstAsIndex(.{
                    .tag = .block,
                    .data = .{
                        .ty_pl = .{
                            .ty = Air.internedToRef(capture_ty.toIntern()),
                            .payload = undefined, // updated below
                        },
                    },
                });

                const prong_count = field_indices.len - in_mem_coercible.count();

                const estimated_extra = prong_count * 6 + (prong_count / 10); // 2 for Case, 1 item, probably 3 insts; plus hints
                var cases_extra = try std.ArrayList(u32).initCapacity(sema.gpa, estimated_extra);
                defer cases_extra.deinit();

                {
                    // All branch hints are `.none`, so just add zero elems.
                    comptime assert(@intFromEnum(std.builtin.BranchHint.none) == 0);
                    const need_elems = std.math.divCeil(usize, prong_count + 1, 10) catch unreachable;
                    try cases_extra.appendNTimes(0, need_elems);
                }

                {
                    // Non-bitcast cases
                    var it = in_mem_coercible.iterator(.{ .kind = .unset });
                    while (it.next()) |idx| {
                        var coerce_block = block.makeSubBlock();
                        defer coerce_block.instructions.deinit(sema.gpa);

                        const case_src: LazySrcLoc = .{
                            .base_node_inst = capture_src.base_node_inst,
                            .offset = .{ .switch_case_item = .{
                                .switch_node_offset = switch_node_offset,
                                .case_idx = capture_src.offset.switch_capture.case_idx,
                                .item_idx = .{ .kind = .single, .index = @intCast(idx) },
                            } },
                        };

                        const field_idx = field_indices[idx];
                        const field_ty: Type = .fromInterned(union_obj.field_types.get(ip)[field_idx]);
                        const uncoerced = try coerce_block.addStructFieldVal(operand_val, field_idx, field_ty);
                        const coerced = try sema.coerce(&coerce_block, capture_ty, uncoerced, case_src);
                        _ = try coerce_block.addBr(capture_block_inst, coerced);

                        try cases_extra.ensureUnusedCapacity(@typeInfo(Air.SwitchBr.Case).@"struct".fields.len +
                            1 + // `item`, no ranges
                            coerce_block.instructions.items.len);
                        cases_extra.appendSliceAssumeCapacity(&payloadToExtraItems(Air.SwitchBr.Case{
                            .items_len = 1,
                            .ranges_len = 0,
                            .body_len = @intCast(coerce_block.instructions.items.len),
                        }));
                        cases_extra.appendAssumeCapacity(@intFromEnum(case_vals[idx])); // item
                        cases_extra.appendSliceAssumeCapacity(@ptrCast(coerce_block.instructions.items)); // body
                    }
                }
                const else_body_len = len: {
                    // 'else' prong uses a bitcast
                    var coerce_block = block.makeSubBlock();
                    defer coerce_block.instructions.deinit(sema.gpa);

                    const first_imc_item_idx = in_mem_coercible.findFirstSet().?;
                    const first_imc_field_idx = field_indices[first_imc_item_idx];
                    const first_imc_field_ty: Type = .fromInterned(union_obj.field_types.get(ip)[first_imc_field_idx]);
                    const uncoerced = try coerce_block.addStructFieldVal(operand_val, first_imc_field_idx, first_imc_field_ty);
                    const coerced = try coerce_block.addBitCast(capture_ty, uncoerced);
                    _ = try coerce_block.addBr(capture_block_inst, coerced);

                    try cases_extra.appendSlice(@ptrCast(coerce_block.instructions.items));
                    break :len coerce_block.instructions.items.len;
                };

                try sema.air_extra.ensureUnusedCapacity(sema.gpa, @typeInfo(Air.SwitchBr).@"struct".fields.len +
                    cases_extra.items.len +
                    @typeInfo(Air.Block).@"struct".fields.len +
                    1);

                const switch_br_inst: u32 = @intCast(sema.air_instructions.len);
                try sema.air_instructions.append(sema.gpa, .{
                    .tag = .switch_br,
                    .data = .{
                        .pl_op = .{
                            .operand = undefined, // set by switch below
                            .payload = sema.addExtraAssumeCapacity(Air.SwitchBr{
                                .cases_len = @intCast(prong_count),
                                .else_body_len = @intCast(else_body_len),
                            }),
                        },
                    },
                });
                sema.air_extra.appendSliceAssumeCapacity(cases_extra.items);

                // Set up block body
                switch (spa.operand) {
                    .simple => |s| {
                        const air_datas = sema.air_instructions.items(.data);
                        air_datas[switch_br_inst].pl_op.operand = s.cond;
                        air_datas[@intFromEnum(capture_block_inst)].ty_pl.payload = sema.addExtraAssumeCapacity(Air.Block{
                            .body_len = 1,
                        });
                        sema.air_extra.appendAssumeCapacity(switch_br_inst);
                    },
                    .loop => {
                        // The block must first extract the tag from the loaded union.
                        const tag_inst: Air.Inst.Index = @enumFromInt(sema.air_instructions.len);
                        try sema.air_instructions.append(sema.gpa, .{
                            .tag = .get_union_tag,
                            .data = .{ .ty_op = .{
                                .ty = Air.internedToRef(union_obj.enum_tag_ty),
                                .operand = operand_val,
                            } },
                        });
                        const air_datas = sema.air_instructions.items(.data);
                        air_datas[switch_br_inst].pl_op.operand = tag_inst.toRef();
                        air_datas[@intFromEnum(capture_block_inst)].ty_pl.payload = sema.addExtraAssumeCapacity(Air.Block{
                            .body_len = 2,
                        });
                        sema.air_extra.appendAssumeCapacity(@intFromEnum(tag_inst));
                        sema.air_extra.appendAssumeCapacity(switch_br_inst);
                    },
                }

                return capture_block_inst.toRef();
            },
            .error_set => {
                if (capture_byref) {
                    return sema.fail(
                        block,
                        capture_src,
                        "error set cannot be captured by reference",
                        .{},
                    );
                }

                if (case_vals.len == 1) {
                    const item_val = sema.resolveConstDefinedValue(block, LazySrcLoc.unneeded, case_vals[0], undefined) catch unreachable;
                    const item_ty = try pt.singleErrorSetType(item_val.getErrorName(zcu).unwrap().?);
                    return sema.bitCast(block, item_ty, operand_val, operand_src, null);
                }

                var names: InferredErrorSet.NameMap = .{};
                try names.ensureUnusedCapacity(sema.arena, case_vals.len);
                for (case_vals) |err| {
                    const err_val = sema.resolveConstDefinedValue(block, LazySrcLoc.unneeded, err, undefined) catch unreachable;
                    names.putAssumeCapacityNoClobber(err_val.getErrorName(zcu).unwrap().?, {});
                }
                const error_ty = try pt.errorSetFromUnsortedNames(names.keys());
                return sema.bitCast(block, error_ty, operand_val, operand_src, null);
            },
            else => {
                // In this case the capture value is just the passed-through value
                // of the switch condition.
                if (capture_byref) {
                    return operand_ptr;
                } else {
                    return operand_val;
                }
            },
        }
    }
};

fn switchCond(
    sema: *Sema,
    block: *Block,
    src: LazySrcLoc,
    operand: Air.Inst.Ref,
) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const operand_ty = sema.typeOf(operand);
    switch (operand_ty.zigTypeTag(zcu)) {
        .type,
        .void,
        .bool,
        .int,
        .float,
        .comptime_float,
        .comptime_int,
        .enum_literal,
        .pointer,
        .@"fn",
        .error_set,
        .@"enum",
        => {
            if (operand_ty.isSlice(zcu)) {
                return sema.fail(block, src, "switch on type '{}'", .{operand_ty.fmt(pt)});
            }
            if ((try sema.typeHasOnePossibleValue(operand_ty))) |opv| {
                return Air.internedToRef(opv.toIntern());
            }
            return operand;
        },

        .@"union" => {
            try operand_ty.resolveFields(pt);
            const enum_ty = operand_ty.unionTagType(zcu) orelse {
                const msg = msg: {
                    const msg = try sema.errMsg(src, "switch on union with no attached enum", .{});
                    errdefer msg.destroy(sema.gpa);
                    if (operand_ty.srcLocOrNull(zcu)) |union_src| {
                        try sema.errNote(union_src, msg, "consider 'union(enum)' here", .{});
                    }
                    break :msg msg;
                };
                return sema.failWithOwnedErrorMsg(block, msg);
            };
            return sema.unionToTag(block, enum_ty, operand, src);
        },

        .error_union,
        .noreturn,
        .array,
        .@"struct",
        .undefined,
        .null,
        .optional,
        .@"opaque",
        .vector,
        .frame,
        .@"anyframe",
        => return sema.fail(block, src, "switch on type '{}'", .{operand_ty.fmt(pt)}),
    }
}

const SwitchErrorSet = std.AutoHashMap(InternPool.NullTerminatedString, LazySrcLoc);

fn zirSwitchBlockErrUnion(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    const tracy = trace(@src());
    defer tracy.end();

    const pt = sema.pt;
    const zcu = pt.zcu;
    const gpa = sema.gpa;
    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].pl_node;
    const switch_src = block.nodeOffset(inst_data.src_node);
    const switch_src_node_offset = inst_data.src_node;
    const switch_operand_src = block.src(.{ .node_offset_switch_operand = switch_src_node_offset });
    const else_prong_src = block.src(.{ .node_offset_switch_special_prong = switch_src_node_offset });
    const extra = sema.code.extraData(Zir.Inst.SwitchBlockErrUnion, inst_data.payload_index);
    const main_operand_src = block.src(.{ .node_offset_if_cond = extra.data.main_src_node_offset });
    const main_src = block.src(.{ .node_offset_main_token = extra.data.main_src_node_offset });

    const raw_operand_val = try sema.resolveInst(extra.data.operand);

    // AstGen guarantees that the instruction immediately preceding
    // switch_block_err_union is a dbg_stmt
    const cond_dbg_node_index: Zir.Inst.Index = @enumFromInt(@intFromEnum(inst) - 1);

    var header_extra_index: usize = extra.end;

    const scalar_cases_len = extra.data.bits.scalar_cases_len;
    const multi_cases_len = if (extra.data.bits.has_multi_cases) blk: {
        const multi_cases_len = sema.code.extra[header_extra_index];
        header_extra_index += 1;
        break :blk multi_cases_len;
    } else 0;

    const err_capture_inst: Zir.Inst.Index = if (extra.data.bits.any_uses_err_capture) blk: {
        const err_capture_inst: Zir.Inst.Index = @enumFromInt(sema.code.extra[header_extra_index]);
        header_extra_index += 1;
        // SwitchProngAnalysis wants inst_map to have space for the tag capture.
        // Note that the normal capture is referred to via the switch block
        // index, which there is already necessarily space for.
        try sema.inst_map.ensureSpaceForInstructions(gpa, &.{err_capture_inst});
        break :blk err_capture_inst;
    } else undefined;

    var case_vals = try std.ArrayListUnmanaged(Air.Inst.Ref).initCapacity(gpa, scalar_cases_len + 2 * multi_cases_len);
    defer case_vals.deinit(gpa);

    const NonError = struct {
        body: []const Zir.Inst.Index,
        end: usize,
        capture: Zir.Inst.SwitchBlock.ProngInfo.Capture,
    };

    const non_error_case: NonError = non_error: {
        const info: Zir.Inst.SwitchBlock.ProngInfo = @bitCast(sema.code.extra[header_extra_index]);
        const extra_body_start = header_extra_index + 1;
        break :non_error .{
            .body = sema.code.bodySlice(extra_body_start, info.body_len),
            .end = extra_body_start + info.body_len,
            .capture = info.capture,
        };
    };

    const Else = struct {
        body: []const Zir.Inst.Index,
        end: usize,
        is_inline: bool,
        has_capture: bool,
    };

    const else_case: Else = if (!extra.data.bits.has_else) .{
        .body = &.{},
        .end = non_error_case.end,
        .is_inline = false,
        .has_capture = false,
    } else special: {
        const info: Zir.Inst.SwitchBlock.ProngInfo = @bitCast(sema.code.extra[non_error_case.end]);
        const extra_body_start = non_error_case.end + 1;
        assert(info.capture != .by_ref);
        assert(!info.has_tag_capture);
        break :special .{
            .body = sema.code.bodySlice(extra_body_start, info.body_len),
            .end = extra_body_start + info.body_len,
            .is_inline = info.is_inline,
            .has_capture = info.capture != .none,
        };
    };

    var seen_errors = SwitchErrorSet.init(gpa);
    defer seen_errors.deinit();

    const operand_ty = sema.typeOf(raw_operand_val);
    const operand_err_set = if (extra.data.bits.payload_is_ref)
        operand_ty.childType(zcu)
    else
        operand_ty;

    if (operand_err_set.zigTypeTag(zcu) != .error_union) {
        return sema.fail(block, switch_src, "expected error union type, found '{}'", .{
            operand_ty.fmt(pt),
        });
    }

    const operand_err_set_ty = operand_err_set.errorUnionSet(zcu);

    const block_inst: Air.Inst.Index = @enumFromInt(sema.air_instructions.len);
    try sema.air_instructions.append(gpa, .{
        .tag = .block,
        .data = undefined,
    });
    var label: Block.Label = .{
        .zir_block = inst,
        .merges = .{
            .src_locs = .{},
            .results = .{},
            .br_list = .{},
            .block_inst = block_inst,
        },
    };

    var child_block: Block = .{
        .parent = block,
        .sema = sema,
        .namespace = block.namespace,
        .instructions = .{},
        .label = &label,
        .inlining = block.inlining,
        .comptime_reason = block.comptime_reason,
        .is_typeof = block.is_typeof,
        .c_import_buf = block.c_import_buf,
        .runtime_cond = block.runtime_cond,
        .runtime_loop = block.runtime_loop,
        .runtime_index = block.runtime_index,
        .error_return_trace_index = block.error_return_trace_index,
        .want_safety = block.want_safety,
        .src_base_inst = block.src_base_inst,
        .type_name_ctx = block.type_name_ctx,
    };
    const merges = &child_block.label.?.merges;
    defer child_block.instructions.deinit(gpa);
    defer merges.deinit(gpa);

    const resolved_err_set = try sema.resolveInferredErrorSetTy(block, main_src, operand_err_set_ty.toIntern());
    if (Type.fromInterned(resolved_err_set).errorSetIsEmpty(zcu)) {
        return sema.resolveBlockBody(block, main_operand_src, &child_block, non_error_case.body, inst, merges);
    }

    const else_error_ty: ?Type = try validateErrSetSwitch(
        sema,
        block,
        &seen_errors,
        &case_vals,
        operand_err_set_ty,
        inst_data,
        scalar_cases_len,
        multi_cases_len,
        .{ .body = else_case.body, .end = else_case.end, .src = else_prong_src },
        extra.data.bits.has_else,
    );

    var spa: SwitchProngAnalysis = .{
        .sema = sema,
        .parent_block = block,
        .operand = .{
            .simple = .{
                .by_val = undefined, // must be set to the unwrapped error code before use
                .by_ref = undefined,
                .cond = raw_operand_val,
            },
        },
        .else_error_ty = else_error_ty,
        .switch_block_inst = inst,
        .tag_capture_inst = undefined,
    };

    if (try sema.resolveDefinedValue(&child_block, main_src, raw_operand_val)) |ov| {
        const operand_val = if (extra.data.bits.payload_is_ref)
            (try sema.pointerDeref(&child_block, main_src, ov, operand_ty)).?
        else
            ov;

        if (operand_val.errorUnionIsPayload(zcu)) {
            return sema.resolveBlockBody(block, main_operand_src, &child_block, non_error_case.body, inst, merges);
        } else {
            const err_val = Value.fromInterned(try pt.intern(.{
                .err = .{
                    .ty = operand_err_set_ty.toIntern(),
                    .name = operand_val.getErrorName(zcu).unwrap().?,
                },
            }));
            spa.operand.simple.by_val = if (extra.data.bits.payload_is_ref)
                try sema.analyzeErrUnionCodePtr(block, switch_operand_src, raw_operand_val)
            else
                try sema.analyzeErrUnionCode(block, switch_operand_src, raw_operand_val);

            if (extra.data.bits.any_uses_err_capture) {
                sema.inst_map.putAssumeCapacity(err_capture_inst, spa.operand.simple.by_val);
            }
            defer if (extra.data.bits.any_uses_err_capture) assert(sema.inst_map.remove(err_capture_inst));

            return resolveSwitchComptime(
                sema,
                spa,
                &child_block,
                try sema.switchCond(block, switch_operand_src, spa.operand.simple.by_val),
                err_val,
                operand_err_set_ty,
                switch_src_node_offset,
                .{
                    .body = else_case.body,
                    .end = else_case.end,
                    .capture = if (else_case.has_capture) .by_val else .none,
                    .is_inline = else_case.is_inline,
                    .has_tag_capture = false,
                },
                case_vals,
                scalar_cases_len,
                multi_cases_len,
                true,
                false,
            );
        }
    }

    if (scalar_cases_len + multi_cases_len == 0) {
        if (else_error_ty) |ty| if (ty.errorSetIsEmpty(zcu)) {
            return sema.resolveBlockBody(block, main_operand_src, &child_block, non_error_case.body, inst, merges);
        };
    }

    if (child_block.isComptime()) {
        _ = try sema.resolveConstDefinedValue(&child_block, main_operand_src, raw_operand_val, null);
        unreachable;
    }

    const cond = if (extra.data.bits.payload_is_ref) blk: {
        try sema.checkErrorType(block, main_src, sema.typeOf(raw_operand_val).elemType2(zcu));
        const loaded = try sema.analyzeLoad(block, main_src, raw_operand_val, main_src);
        break :blk try sema.analyzeIsNonErr(block, main_src, loaded);
    } else blk: {
        try sema.checkErrorType(block, main_src, sema.typeOf(raw_operand_val));
        break :blk try sema.analyzeIsNonErr(block, main_src, raw_operand_val);
    };

    var sub_block = child_block.makeSubBlock();
    sub_block.runtime_loop = null;
    sub_block.runtime_cond = main_operand_src;
    sub_block.runtime_index.increment();
    sub_block.need_debug_scope = null; // this body is emitted regardless
    defer sub_block.instructions.deinit(gpa);

    const non_error_hint = try sema.analyzeBodyRuntimeBreak(&sub_block, non_error_case.body);
    const true_instructions = try sub_block.instructions.toOwnedSlice(gpa);
    defer gpa.free(true_instructions);

    spa.operand.simple.by_val = if (extra.data.bits.payload_is_ref)
        try sema.analyzeErrUnionCodePtr(&sub_block, switch_operand_src, raw_operand_val)
    else
        try sema.analyzeErrUnionCode(&sub_block, switch_operand_src, raw_operand_val);

    if (extra.data.bits.any_uses_err_capture) {
        sema.inst_map.putAssumeCapacity(err_capture_inst, spa.operand.simple.by_val);
    }
    defer if (extra.data.bits.any_uses_err_capture) assert(sema.inst_map.remove(err_capture_inst));
    _ = try sema.analyzeSwitchRuntimeBlock(
        spa,
        &sub_block,
        switch_src,
        try sema.switchCond(block, switch_operand_src, spa.operand.simple.by_val),
        operand_err_set_ty,
        switch_operand_src,
        case_vals,
        .{
            .body = else_case.body,
            .end = else_case.end,
            .capture = if (else_case.has_capture) .by_val else .none,
            .is_inline = else_case.is_inline,
            .has_tag_capture = false,
        },
        scalar_cases_len,
        multi_cases_len,
        false,
        undefined,
        true,
        switch_src_node_offset,
        else_prong_src,
        undefined,
        seen_errors,
        undefined,
        undefined,
        undefined,
        cond_dbg_node_index,
        true,
    );

    try sema.air_extra.ensureUnusedCapacity(gpa, @typeInfo(Air.CondBr).@"struct".fields.len +
        true_instructions.len + sub_block.instructions.items.len);

    _ = try child_block.addInst(.{
        .tag = .cond_br,
        .data = .{
            .pl_op = .{
                .operand = cond,
                .payload = sema.addExtraAssumeCapacity(Air.CondBr{
                    .then_body_len = @intCast(true_instructions.len),
                    .else_body_len = @intCast(sub_block.instructions.items.len),
                    .branch_hints = .{
                        .true = non_error_hint,
                        .false = .none,
                        // Code coverage is desired for error handling.
                        .then_cov = .poi,
                        .else_cov = .poi,
                    },
                }),
            },
        },
    });
    sema.air_extra.appendSliceAssumeCapacity(@ptrCast(true_instructions));
    sema.air_extra.appendSliceAssumeCapacity(@ptrCast(sub_block.instructions.items));

    return sema.resolveAnalyzedBlock(block, main_src, &child_block, merges, false);
}

fn zirSwitchBlock(sema: *Sema, block: *Block, inst: Zir.Inst.Index, operand_is_ref: bool) CompileError!Air.Inst.Ref {
    const tracy = trace(@src());
    defer tracy.end();

    const pt = sema.pt;
    const zcu = pt.zcu;
    const gpa = sema.gpa;
    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].pl_node;
    const src = block.nodeOffset(inst_data.src_node);
    const src_node_offset = inst_data.src_node;
    const operand_src = block.src(.{ .node_offset_switch_operand = src_node_offset });
    const special_prong_src = block.src(.{ .node_offset_switch_special_prong = src_node_offset });
    const extra = sema.code.extraData(Zir.Inst.SwitchBlock, inst_data.payload_index);

    const operand: SwitchProngAnalysis.Operand, const raw_operand_ty: Type = op: {
        const maybe_ptr = try sema.resolveInst(extra.data.operand);
        const val, const ref = if (operand_is_ref)
            .{ try sema.analyzeLoad(block, src, maybe_ptr, operand_src), maybe_ptr }
        else
            .{ maybe_ptr, undefined };

        const init_cond = try sema.switchCond(block, operand_src, val);

        const operand_ty = sema.typeOf(val);

        if (extra.data.bits.has_continue and !block.isComptime()) {
            // Even if the operand is comptime-known, this `switch` is runtime.
            if (try operand_ty.comptimeOnlySema(pt)) {
                return sema.failWithOwnedErrorMsg(block, msg: {
                    const msg = try sema.errMsg(operand_src, "operand of switch loop has comptime-only type '{}'", .{operand_ty.fmt(pt)});
                    errdefer msg.destroy(gpa);
                    try sema.errNote(operand_src, msg, "switch loops are evalauted at runtime outside of comptime scopes", .{});
                    break :msg msg;
                });
            }
            try sema.validateRuntimeValue(block, operand_src, maybe_ptr);
            const operand_alloc = if (extra.data.bits.any_non_inline_capture) a: {
                const operand_ptr_ty = try pt.singleMutPtrType(sema.typeOf(maybe_ptr));
                const operand_alloc = try block.addTy(.alloc, operand_ptr_ty);
                _ = try block.addBinOp(.store, operand_alloc, maybe_ptr);
                break :a operand_alloc;
            } else undefined;
            break :op .{
                .{ .loop = .{
                    .operand_alloc = operand_alloc,
                    .operand_is_ref = operand_is_ref,
                    .init_cond = init_cond,
                } },
                operand_ty,
            };
        }

        // We always use `simple` in the comptime case, because as far as the dispatching logic
        // is concerned, it really is dispatching a single prong. `resolveSwitchComptime` will
        // be resposible for recursively resolving different prongs as needed.
        break :op .{
            .{ .simple = .{
                .by_val = val,
                .by_ref = ref,
                .cond = init_cond,
            } },
            operand_ty,
        };
    };

    const union_originally = raw_operand_ty.zigTypeTag(zcu) == .@"union";
    const err_set = raw_operand_ty.zigTypeTag(zcu) == .error_set;
    const cond_ty = switch (raw_operand_ty.zigTypeTag(zcu)) {
        .@"union" => raw_operand_ty.unionTagType(zcu).?, // validated by `switchCond` above
        else => raw_operand_ty,
    };

    // AstGen guarantees that the instruction immediately preceding
    // switch_block(_ref) is a dbg_stmt
    const cond_dbg_node_index: Zir.Inst.Index = @enumFromInt(@intFromEnum(inst) - 1);

    var header_extra_index: usize = extra.end;

    const scalar_cases_len = extra.data.bits.scalar_cases_len;
    const multi_cases_len = if (extra.data.bits.has_multi_cases) blk: {
        const multi_cases_len = sema.code.extra[header_extra_index];
        header_extra_index += 1;
        break :blk multi_cases_len;
    } else 0;

    const tag_capture_inst: Zir.Inst.Index = if (extra.data.bits.any_has_tag_capture) blk: {
        const tag_capture_inst: Zir.Inst.Index = @enumFromInt(sema.code.extra[header_extra_index]);
        header_extra_index += 1;
        // SwitchProngAnalysis wants inst_map to have space for the tag capture.
        // Note that the normal capture is referred to via the switch block
        // index, which there is already necessarily space for.
        try sema.inst_map.ensureSpaceForInstructions(gpa, &.{tag_capture_inst});
        break :blk tag_capture_inst;
    } else undefined;

    var case_vals = try std.ArrayListUnmanaged(Air.Inst.Ref).initCapacity(gpa, scalar_cases_len + 2 * multi_cases_len);
    defer case_vals.deinit(gpa);

    const special_prong = extra.data.bits.specialProng();
    const special: SpecialProng = switch (special_prong) {
        .none => .{
            .body = &.{},
            .end = header_extra_index,
            .capture = .none,
            .is_inline = false,
            .has_tag_capture = false,
        },
        .under, .@"else" => blk: {
            const info: Zir.Inst.SwitchBlock.ProngInfo = @bitCast(sema.code.extra[header_extra_index]);
            const extra_body_start = header_extra_index + 1;
            break :blk .{
                .body = sema.code.bodySlice(extra_body_start, info.body_len),
                .end = extra_body_start + info.body_len,
                .capture = info.capture,
                .is_inline = info.is_inline,
                .has_tag_capture = info.has_tag_capture,
            };
        },
    };

    // Duplicate checking variables later also used for `inline else`.
    var seen_enum_fields: []?LazySrcLoc = &.{};
    var seen_errors = SwitchErrorSet.init(gpa);
    var range_set = RangeSet.init(gpa, zcu);
    var true_count: u8 = 0;
    var false_count: u8 = 0;

    defer {
        range_set.deinit();
        gpa.free(seen_enum_fields);
        seen_errors.deinit();
    }

    var empty_enum = false;

    var else_error_ty: ?Type = null;

    // Validate usage of '_' prongs.
    if (special_prong == .under and !raw_operand_ty.isNonexhaustiveEnum(zcu)) {
        const msg = msg: {
            const msg = try sema.errMsg(
                src,
                "'_' prong only allowed when switching on non-exhaustive enums",
                .{},
            );
            errdefer msg.destroy(gpa);
            try sema.errNote(
                special_prong_src,
                msg,
                "'_' prong here",
                .{},
            );
            try sema.errNote(
                src,
                msg,
                "consider using 'else'",
                .{},
            );
            break :msg msg;
        };
        return sema.failWithOwnedErrorMsg(block, msg);
    }

    // Validate for duplicate items, missing else prong, and invalid range.
    switch (cond_ty.zigTypeTag(zcu)) {
        .@"union" => unreachable, // handled in `switchCond`
        .@"enum" => {
            seen_enum_fields = try gpa.alloc(?LazySrcLoc, cond_ty.enumFieldCount(zcu));
            empty_enum = seen_enum_fields.len == 0 and !cond_ty.isNonexhaustiveEnum(zcu);
            @memset(seen_enum_fields, null);
            // `range_set` is used for non-exhaustive enum values that do not correspond to any tags.

            var extra_index: usize = special.end;
            {
                var scalar_i: u32 = 0;
                while (scalar_i < scalar_cases_len) : (scalar_i += 1) {
                    const item_ref: Zir.Inst.Ref = @enumFromInt(sema.code.extra[extra_index]);
                    extra_index += 1;
                    const info: Zir.Inst.SwitchBlock.ProngInfo = @bitCast(sema.code.extra[extra_index]);
                    extra_index += 1 + info.body_len;

                    case_vals.appendAssumeCapacity(try sema.validateSwitchItemEnum(
                        block,
                        seen_enum_fields,
                        &range_set,
                        item_ref,
                        cond_ty,
                        block.src(.{ .switch_case_item = .{
                            .switch_node_offset = src_node_offset,
                            .case_idx = .{ .kind = .scalar, .index = @intCast(scalar_i) },
                            .item_idx = .{ .kind = .single, .index = 0 },
                        } }),
                    ));
                }
            }
            {
                var multi_i: u32 = 0;
                while (multi_i < multi_cases_len) : (multi_i += 1) {
                    const items_len = sema.code.extra[extra_index];
                    extra_index += 1;
                    const ranges_len = sema.code.extra[extra_index];
                    extra_index += 1;
                    const info: Zir.Inst.SwitchBlock.ProngInfo = @bitCast(sema.code.extra[extra_index]);
                    extra_index += 1;
                    const items = sema.code.refSlice(extra_index, items_len);
                    extra_index += items_len + info.body_len;

                    try case_vals.ensureUnusedCapacity(gpa, items.len);
                    for (items, 0..) |item_ref, item_i| {
                        case_vals.appendAssumeCapacity(try sema.validateSwitchItemEnum(
                            block,
                            seen_enum_fields,
                            &range_set,
                            item_ref,
                            cond_ty,
                            block.src(.{ .switch_case_item = .{
                                .switch_node_offset = src_node_offset,
                                .case_idx = .{ .kind = .multi, .index = @intCast(multi_i) },
                                .item_idx = .{ .kind = .single, .index = @intCast(item_i) },
                            } }),
                        ));
                    }

                    try sema.validateSwitchNoRange(block, ranges_len, cond_ty, src_node_offset);
                }
            }
            const all_tags_handled = for (seen_enum_fields) |seen_src| {
                if (seen_src == null) break false;
            } else true;

            if (special_prong == .@"else") {
                if (all_tags_handled and !cond_ty.isNonexhaustiveEnum(zcu)) return sema.fail(
                    block,
                    special_prong_src,
                    "unreachable else prong; all cases already handled",
                    .{},
                );
            } else if (!all_tags_handled) {
                const msg = msg: {
                    const msg = try sema.errMsg(
                        src,
                        "switch must handle all possibilities",
                        .{},
                    );
                    errdefer msg.destroy(sema.gpa);
                    for (seen_enum_fields, 0..) |seen_src, i| {
                        if (seen_src != null) continue;

                        const field_name = cond_ty.enumFieldName(i, zcu);
                        try sema.addFieldErrNote(
                            cond_ty,
                            i,
                            msg,
                            "unhandled enumeration value: '{}'",
                            .{field_name.fmt(&zcu.intern_pool)},
                        );
                    }
                    try sema.errNote(
                        cond_ty.srcLoc(zcu),
                        msg,
                        "enum '{}' declared here",
                        .{cond_ty.fmt(pt)},
                    );
                    break :msg msg;
                };
                return sema.failWithOwnedErrorMsg(block, msg);
            } else if (special_prong == .none and cond_ty.isNonexhaustiveEnum(zcu) and !union_originally) {
                return sema.fail(
                    block,
                    src,
                    "switch on non-exhaustive enum must include 'else' or '_' prong",
                    .{},
                );
            }
        },
        .error_set => else_error_ty = try validateErrSetSwitch(
            sema,
            block,
            &seen_errors,
            &case_vals,
            cond_ty,
            inst_data,
            scalar_cases_len,
            multi_cases_len,
            .{ .body = special.body, .end = special.end, .src = special_prong_src },
            special_prong == .@"else",
        ),
        .int, .comptime_int => {
            var extra_index: usize = special.end;
            {
                var scalar_i: u32 = 0;
                while (scalar_i < scalar_cases_len) : (scalar_i += 1) {
                    const item_ref: Zir.Inst.Ref = @enumFromInt(sema.code.extra[extra_index]);
                    extra_index += 1;
                    const info: Zir.Inst.SwitchBlock.ProngInfo = @bitCast(sema.code.extra[extra_index]);
                    extra_index += 1 + info.body_len;

                    case_vals.appendAssumeCapacity(try sema.validateSwitchItemInt(
                        block,
                        &range_set,
                        item_ref,
                        cond_ty,
                        block.src(.{ .switch_case_item = .{
                            .switch_node_offset = src_node_offset,
                            .case_idx = .{ .kind = .scalar, .index = @intCast(scalar_i) },
                            .item_idx = .{ .kind = .single, .index = 0 },
                        } }),
                    ));
                }
            }
            {
                var multi_i: u32 = 0;
                while (multi_i < multi_cases_len) : (multi_i += 1) {
                    const items_len = sema.code.extra[extra_index];
                    extra_index += 1;
                    const ranges_len = sema.code.extra[extra_index];
                    extra_index += 1;
                    const info: Zir.Inst.SwitchBlock.ProngInfo = @bitCast(sema.code.extra[extra_index]);
                    extra_index += 1;
                    const items = sema.code.refSlice(extra_index, items_len);
                    extra_index += items_len;

                    try case_vals.ensureUnusedCapacity(gpa, items.len);
                    for (items, 0..) |item_ref, item_i| {
                        case_vals.appendAssumeCapacity(try sema.validateSwitchItemInt(
                            block,
                            &range_set,
                            item_ref,
                            cond_ty,
                            block.src(.{ .switch_case_item = .{
                                .switch_node_offset = src_node_offset,
                                .case_idx = .{ .kind = .multi, .index = @intCast(multi_i) },
                                .item_idx = .{ .kind = .single, .index = @intCast(item_i) },
                            } }),
                        ));
                    }

                    try case_vals.ensureUnusedCapacity(gpa, 2 * ranges_len);
                    var range_i: u32 = 0;
                    while (range_i < ranges_len) : (range_i += 1) {
                        const item_first: Zir.Inst.Ref = @enumFromInt(sema.code.extra[extra_index]);
                        extra_index += 1;
                        const item_last: Zir.Inst.Ref = @enumFromInt(sema.code.extra[extra_index]);
                        extra_index += 1;

                        const vals = try sema.validateSwitchRange(
                            block,
                            &range_set,
                            item_first,
                            item_last,
                            cond_ty,
                            block.src(.{ .switch_case_item = .{
                                .switch_node_offset = src_node_offset,
                                .case_idx = .{ .kind = .multi, .index = @intCast(multi_i) },
                                .item_idx = .{ .kind = .range, .index = @intCast(range_i) },
                            } }),
                        );
                        case_vals.appendAssumeCapacity(vals[0]);
                        case_vals.appendAssumeCapacity(vals[1]);
                    }

                    extra_index += info.body_len;
                }
            }

            check_range: {
                if (cond_ty.zigTypeTag(zcu) == .int) {
                    const min_int = try cond_ty.minInt(pt, cond_ty);
                    const max_int = try cond_ty.maxInt(pt, cond_ty);
                    if (try range_set.spans(min_int.toIntern(), max_int.toIntern())) {
                        if (special_prong == .@"else") {
                            return sema.fail(
                                block,
                                special_prong_src,
                                "unreachable else prong; all cases already handled",
                                .{},
                            );
                        }
                        break :check_range;
                    }
                }
                if (special_prong != .@"else") {
                    return sema.fail(
                        block,
                        src,
                        "switch must handle all possibilities",
                        .{},
                    );
                }
            }
        },
        .bool => {
            var extra_index: usize = special.end;
            {
                var scalar_i: u32 = 0;
                while (scalar_i < scalar_cases_len) : (scalar_i += 1) {
                    const item_ref: Zir.Inst.Ref = @enumFromInt(sema.code.extra[extra_index]);
                    extra_index += 1;
                    const info: Zir.Inst.SwitchBlock.ProngInfo = @bitCast(sema.code.extra[extra_index]);
                    extra_index += 1 + info.body_len;

                    case_vals.appendAssumeCapacity(try sema.validateSwitchItemBool(
                        block,
                        &true_count,
                        &false_count,
                        item_ref,
                        block.src(.{ .switch_case_item = .{
                            .switch_node_offset = src_node_offset,
                            .case_idx = .{ .kind = .scalar, .index = @intCast(scalar_i) },
                            .item_idx = .{ .kind = .single, .index = 0 },
                        } }),
                    ));
                }
            }
            {
                var multi_i: u32 = 0;
                while (multi_i < multi_cases_len) : (multi_i += 1) {
                    const items_len = sema.code.extra[extra_index];
                    extra_index += 1;
                    const ranges_len = sema.code.extra[extra_index];
                    extra_index += 1;
                    const info: Zir.Inst.SwitchBlock.ProngInfo = @bitCast(sema.code.extra[extra_index]);
                    extra_index += 1;
                    const items = sema.code.refSlice(extra_index, items_len);
                    extra_index += items_len + info.body_len;

                    try case_vals.ensureUnusedCapacity(gpa, items.len);
                    for (items, 0..) |item_ref, item_i| {
                        case_vals.appendAssumeCapacity(try sema.validateSwitchItemBool(
                            block,
                            &true_count,
                            &false_count,
                            item_ref,
                            block.src(.{ .switch_case_item = .{
                                .switch_node_offset = src_node_offset,
                                .case_idx = .{ .kind = .multi, .index = @intCast(multi_i) },
                                .item_idx = .{ .kind = .single, .index = @intCast(item_i) },
                            } }),
                        ));
                    }

                    try sema.validateSwitchNoRange(block, ranges_len, cond_ty, src_node_offset);
                }
            }
            switch (special_prong) {
                .@"else" => {
                    if (true_count + false_count == 2) {
                        return sema.fail(
                            block,
                            special_prong_src,
                            "unreachable else prong; all cases already handled",
                            .{},
                        );
                    }
                },
                .under, .none => {
                    if (true_count + false_count < 2) {
                        return sema.fail(
                            block,
                            src,
                            "switch must handle all possibilities",
                            .{},
                        );
                    }
                },
            }
        },
        .enum_literal, .void, .@"fn", .pointer, .type => {
            if (special_prong != .@"else") {
                return sema.fail(
                    block,
                    src,
                    "else prong required when switching on type '{}'",
                    .{cond_ty.fmt(pt)},
                );
            }

            var seen_values = ValueSrcMap{};
            defer seen_values.deinit(gpa);

            var extra_index: usize = special.end;
            {
                var scalar_i: u32 = 0;
                while (scalar_i < scalar_cases_len) : (scalar_i += 1) {
                    const item_ref: Zir.Inst.Ref = @enumFromInt(sema.code.extra[extra_index]);
                    extra_index += 1;
                    const info: Zir.Inst.SwitchBlock.ProngInfo = @bitCast(sema.code.extra[extra_index]);
                    extra_index += 1;
                    extra_index += info.body_len;

                    case_vals.appendAssumeCapacity(try sema.validateSwitchItemSparse(
                        block,
                        &seen_values,
                        item_ref,
                        cond_ty,
                        block.src(.{ .switch_case_item = .{
                            .switch_node_offset = src_node_offset,
                            .case_idx = .{ .kind = .scalar, .index = @intCast(scalar_i) },
                            .item_idx = .{ .kind = .single, .index = 0 },
                        } }),
                    ));
                }
            }
            {
                var multi_i: u32 = 0;
                while (multi_i < multi_cases_len) : (multi_i += 1) {
                    const items_len = sema.code.extra[extra_index];
                    extra_index += 1;
                    const ranges_len = sema.code.extra[extra_index];
                    extra_index += 1;
                    const info: Zir.Inst.SwitchBlock.ProngInfo = @bitCast(sema.code.extra[extra_index]);
                    extra_index += 1;
                    const items = sema.code.refSlice(extra_index, items_len);
                    extra_index += items_len + info.body_len;

                    try case_vals.ensureUnusedCapacity(gpa, items.len);
                    for (items, 0..) |item_ref, item_i| {
                        case_vals.appendAssumeCapacity(try sema.validateSwitchItemSparse(
                            block,
                            &seen_values,
                            item_ref,
                            cond_ty,
                            block.src(.{ .switch_case_item = .{
                                .switch_node_offset = src_node_offset,
                                .case_idx = .{ .kind = .multi, .index = @intCast(multi_i) },
                                .item_idx = .{ .kind = .single, .index = @intCast(item_i) },
                            } }),
                        ));
                    }

                    try sema.validateSwitchNoRange(block, ranges_len, cond_ty, src_node_offset);
                }
            }
        },

        .error_union,
        .noreturn,
        .array,
        .@"struct",
        .undefined,
        .null,
        .optional,
        .@"opaque",
        .vector,
        .frame,
        .@"anyframe",
        .comptime_float,
        .float,
        => return sema.fail(block, operand_src, "invalid switch operand type '{}'", .{
            raw_operand_ty.fmt(pt),
        }),
    }

    const spa: SwitchProngAnalysis = .{
        .sema = sema,
        .parent_block = block,
        .operand = operand,
        .else_error_ty = else_error_ty,
        .switch_block_inst = inst,
        .tag_capture_inst = tag_capture_inst,
    };

    const block_inst: Air.Inst.Index = @enumFromInt(sema.air_instructions.len);
    try sema.air_instructions.append(gpa, .{
        .tag = .block,
        .data = undefined,
    });
    var label: Block.Label = .{
        .zir_block = inst,
        .merges = .{
            .src_locs = .{},
            .results = .{},
            .br_list = .{},
            .block_inst = block_inst,
        },
    };

    var child_block: Block = .{
        .parent = block,
        .sema = sema,
        .namespace = block.namespace,
        .instructions = .{},
        .label = &label,
        .inlining = block.inlining,
        .comptime_reason = block.comptime_reason,
        .is_typeof = block.is_typeof,
        .c_import_buf = block.c_import_buf,
        .runtime_cond = block.runtime_cond,
        .runtime_loop = block.runtime_loop,
        .runtime_index = block.runtime_index,
        .want_safety = block.want_safety,
        .error_return_trace_index = block.error_return_trace_index,
        .src_base_inst = block.src_base_inst,
        .type_name_ctx = block.type_name_ctx,
    };
    const merges = &child_block.label.?.merges;
    defer child_block.instructions.deinit(gpa);
    defer merges.deinit(gpa);

    if (scalar_cases_len + multi_cases_len == 0 and !special.is_inline) {
        if (empty_enum) {
            return .void_value;
        }
        if (special_prong == .none) {
            return sema.fail(block, src, "switch must handle all possibilities", .{});
        }
        const init_cond = switch (operand) {
            .simple => |s| s.cond,
            .loop => |l| l.init_cond,
        };
        if (zcu.backendSupportsFeature(.is_named_enum_value) and block.wantSafety() and
            raw_operand_ty.zigTypeTag(zcu) == .@"enum" and !raw_operand_ty.isNonexhaustiveEnum(zcu))
        {
            try sema.zirDbgStmt(block, cond_dbg_node_index);
            const ok = try block.addUnOp(.is_named_enum_value, init_cond);
            try sema.addSafetyCheck(block, src, ok, .corrupt_switch);
        }
        if (err_set and try sema.maybeErrorUnwrap(block, special.body, init_cond, operand_src, false)) {
            return .unreachable_value;
        }
    }

    switch (operand) {
        .loop => {}, // always runtime; evaluation in comptime scope uses `simple`
        .simple => |s| {
            if (try sema.resolveDefinedValue(&child_block, src, s.cond)) |cond_val| {
                return resolveSwitchComptimeLoop(
                    sema,
                    spa,
                    &child_block,
                    if (operand_is_ref)
                        sema.typeOf(s.by_ref)
                    else
                        raw_operand_ty,
                    cond_ty,
                    cond_val,
                    src_node_offset,
                    special,
                    case_vals,
                    scalar_cases_len,
                    multi_cases_len,
                    err_set,
                    empty_enum,
                    operand_is_ref,
                );
            }

            if (scalar_cases_len + multi_cases_len == 0 and !special.is_inline and !extra.data.bits.has_continue) {
                return spa.resolveProngComptime(
                    &child_block,
                    .special,
                    special.body,
                    special.capture,
                    block.src(.{ .switch_capture = .{
                        .switch_node_offset = src_node_offset,
                        .case_idx = LazySrcLoc.Offset.SwitchCaseIndex.special,
                    } }),
                    undefined, // case_vals may be undefined for special prongs
                    .none,
                    false,
                    merges,
                );
            }
        },
    }

    if (child_block.isComptime()) {
        _ = try sema.resolveConstDefinedValue(&child_block, operand_src, operand.simple.cond, null);
        unreachable;
    }

    const air_switch_ref = try sema.analyzeSwitchRuntimeBlock(
        spa,
        &child_block,
        src,
        switch (operand) {
            .simple => |s| s.cond,
            .loop => |l| l.init_cond,
        },
        cond_ty,
        operand_src,
        case_vals,
        special,
        scalar_cases_len,
        multi_cases_len,
        union_originally,
        raw_operand_ty,
        err_set,
        src_node_offset,
        special_prong_src,
        seen_enum_fields,
        seen_errors,
        range_set,
        true_count,
        false_count,
        cond_dbg_node_index,
        false,
    );

    for (merges.extra_insts.items, merges.extra_src_locs.items) |placeholder_inst, dispatch_src| {
        var replacement_block = block.makeSubBlock();
        defer replacement_block.instructions.deinit(gpa);

        assert(sema.air_instructions.items(.tag)[@intFromEnum(placeholder_inst)] == .br);
        const new_operand_maybe_ref = sema.air_instructions.items(.data)[@intFromEnum(placeholder_inst)].br.operand;

        if (extra.data.bits.any_non_inline_capture) {
            _ = try replacement_block.addBinOp(.store, operand.loop.operand_alloc, new_operand_maybe_ref);
        }

        const new_operand_val = if (operand_is_ref)
            try sema.analyzeLoad(&replacement_block, dispatch_src, new_operand_maybe_ref, dispatch_src)
        else
            new_operand_maybe_ref;

        const new_cond = try sema.switchCond(&replacement_block, dispatch_src, new_operand_val);

        if (zcu.backendSupportsFeature(.is_named_enum_value) and block.wantSafety() and
            cond_ty.zigTypeTag(zcu) == .@"enum" and !cond_ty.isNonexhaustiveEnum(zcu) and
            !try sema.isComptimeKnown(new_cond))
        {
            const ok = try replacement_block.addUnOp(.is_named_enum_value, new_cond);
            try sema.addSafetyCheck(&replacement_block, src, ok, .corrupt_switch);
        }

        _ = try replacement_block.addInst(.{
            .tag = .switch_dispatch,
            .data = .{ .br = .{
                .block_inst = air_switch_ref.toIndex().?,
                .operand = new_cond,
            } },
        });

        if (replacement_block.instructions.items.len == 1) {
            // Optimization: we don't need a block!
            sema.air_instructions.set(
                @intFromEnum(placeholder_inst),
                sema.air_instructions.get(@intFromEnum(replacement_block.instructions.items[0])),
            );
            continue;
        }

        // Replace placeholder with a block.
        // No `br` is needed as the block is a switch dispatch so necessarily `noreturn`.
        try sema.air_extra.ensureUnusedCapacity(
            gpa,
            @typeInfo(Air.Block).@"struct".fields.len + replacement_block.instructions.items.len,
        );
        sema.air_instructions.set(@intFromEnum(placeholder_inst), .{
            .tag = .block,
            .data = .{ .ty_pl = .{
                .ty = .noreturn_type,
                .payload = sema.addExtraAssumeCapacity(Air.Block{
                    .body_len = @intCast(replacement_block.instructions.items.len),
                }),
            } },
        });
        sema.air_extra.appendSliceAssumeCapacity(@ptrCast(replacement_block.instructions.items));
    }

    return sema.resolveAnalyzedBlock(block, src, &child_block, merges, false);
}

const SpecialProng = struct {
    body: []const Zir.Inst.Index,
    end: usize,
    capture: Zir.Inst.SwitchBlock.ProngInfo.Capture,
    is_inline: bool,
    has_tag_capture: bool,
};

fn analyzeSwitchRuntimeBlock(
    sema: *Sema,
    spa: SwitchProngAnalysis,
    child_block: *Block,
    src: LazySrcLoc,
    operand: Air.Inst.Ref,
    operand_ty: Type,
    operand_src: LazySrcLoc,
    case_vals: std.ArrayListUnmanaged(Air.Inst.Ref),
    special: SpecialProng,
    scalar_cases_len: usize,
    multi_cases_len: usize,
    union_originally: bool,
    maybe_union_ty: Type,
    err_set: bool,
    switch_node_offset: std.zig.Ast.Node.Offset,
    special_prong_src: LazySrcLoc,
    seen_enum_fields: []?LazySrcLoc,
    seen_errors: SwitchErrorSet,
    range_set: RangeSet,
    true_count: u8,
    false_count: u8,
    cond_dbg_node_index: Zir.Inst.Index,
    allow_err_code_unwrap: bool,
) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const gpa = sema.gpa;
    const ip = &zcu.intern_pool;

    const block = child_block.parent.?;

    const estimated_cases_extra = (scalar_cases_len + multi_cases_len) *
        @typeInfo(Air.SwitchBr.Case).@"struct".fields.len + 2;
    var cases_extra = try std.ArrayListUnmanaged(u32).initCapacity(gpa, estimated_cases_extra);
    defer cases_extra.deinit(gpa);

    var branch_hints = try std.ArrayListUnmanaged(std.builtin.BranchHint).initCapacity(gpa, scalar_cases_len);
    defer branch_hints.deinit(gpa);

    var case_block = child_block.makeSubBlock();
    case_block.runtime_loop = null;
    case_block.runtime_cond = operand_src;
    case_block.runtime_index.increment();
    case_block.need_debug_scope = null; // this body is emitted regardless
    defer case_block.instructions.deinit(gpa);

    var extra_index: usize = special.end;

    var scalar_i: usize = 0;
    while (scalar_i < scalar_cases_len) : (scalar_i += 1) {
        extra_index += 1;
        const info: Zir.Inst.SwitchBlock.ProngInfo = @bitCast(sema.code.extra[extra_index]);
        extra_index += 1;
        const body = sema.code.bodySlice(extra_index, info.body_len);
        extra_index += info.body_len;

        case_block.instructions.shrinkRetainingCapacity(0);
        case_block.error_return_trace_index = child_block.error_return_trace_index;

        const item = case_vals.items[scalar_i];
        // `item` is already guaranteed to be constant known.

        const analyze_body = if (union_originally) blk: {
            const unresolved_item_val = sema.resolveConstDefinedValue(block, LazySrcLoc.unneeded, item, undefined) catch unreachable;
            const item_val = sema.resolveLazyValue(unresolved_item_val) catch unreachable;
            const field_ty = maybe_union_ty.unionFieldType(item_val, zcu).?;
            break :blk field_ty.zigTypeTag(zcu) != .noreturn;
        } else true;

        const prong_hint: std.builtin.BranchHint = if (err_set and
            try sema.maybeErrorUnwrap(&case_block, body, operand, operand_src, allow_err_code_unwrap))
        h: {
            // nothing to do here. weight against error branch
            break :h .unlikely;
        } else if (analyze_body) h: {
            break :h try spa.analyzeProngRuntime(
                &case_block,
                .normal,
                body,
                info.capture,
                child_block.src(.{ .switch_capture = .{
                    .switch_node_offset = switch_node_offset,
                    .case_idx = .{ .kind = .scalar, .index = @intCast(scalar_i) },
                } }),
                &.{item},
                if (info.is_inline) item else .none,
                info.has_tag_capture,
            );
        } else h: {
            _ = try case_block.addNoOp(.unreach);
            break :h .none;
        };

        try branch_hints.append(gpa, prong_hint);
        try cases_extra.ensureUnusedCapacity(gpa, @typeInfo(Air.SwitchBr.Case).@"struct".fields.len +
            1 + // `item`, no ranges
            case_block.instructions.items.len);
        cases_extra.appendSliceAssumeCapacity(&payloadToExtraItems(Air.SwitchBr.Case{
            .items_len = 1,
            .ranges_len = 0,
            .body_len = @intCast(case_block.instructions.items.len),
        }));
        cases_extra.appendAssumeCapacity(@intFromEnum(item));
        cases_extra.appendSliceAssumeCapacity(@ptrCast(case_block.instructions.items));
    }

    var cases_len = scalar_cases_len;
    var case_val_idx: usize = scalar_cases_len;
    var multi_i: u32 = 0;
    while (multi_i < multi_cases_len) : (multi_i += 1) {
        const items_len = sema.code.extra[extra_index];
        extra_index += 1;
        const ranges_len = sema.code.extra[extra_index];
        extra_index += 1;
        const info: Zir.Inst.SwitchBlock.ProngInfo = @bitCast(sema.code.extra[extra_index]);
        extra_index += 1 + items_len + 2 * ranges_len;

        const items = case_vals.items[case_val_idx..][0..items_len];
        case_val_idx += items_len;
        // TODO: @ptrCast slice once Sema supports it
        const ranges: []const [2]Air.Inst.Ref = @as([*]const [2]Air.Inst.Ref, @ptrCast(case_vals.items[case_val_idx..]))[0..ranges_len];
        case_val_idx += ranges_len * 2;

        const body = sema.code.bodySlice(extra_index, info.body_len);
        extra_index += info.body_len;

        case_block.instructions.shrinkRetainingCapacity(0);
        case_block.error_return_trace_index = child_block.error_return_trace_index;

        // Generate all possible cases as scalar prongs.
        if (info.is_inline) {
            var emit_bb = false;

            for (ranges, 0..) |range_items, range_i| {
                var item = sema.resolveConstDefinedValue(block, LazySrcLoc.unneeded, range_items[0], undefined) catch unreachable;
                const item_last = sema.resolveConstDefinedValue(block, LazySrcLoc.unneeded, range_items[1], undefined) catch unreachable;

                while (item.compareScalar(.lte, item_last, operand_ty, zcu)) : ({
                    // Previous validation has resolved any possible lazy values.
                    const result = try arith.incrementDefinedInt(sema, operand_ty, item);
                    assert(!result.overflow);
                    item = result.val;
                }) {
                    cases_len += 1;

                    const item_ref = Air.internedToRef(item.toIntern());

                    case_block.instructions.shrinkRetainingCapacity(0);
                    case_block.error_return_trace_index = child_block.error_return_trace_index;

                    if (emit_bb) try sema.emitBackwardBranch(block, block.src(.{ .switch_case_item = .{
                        .switch_node_offset = switch_node_offset,
                        .case_idx = .{ .kind = .multi, .index = @intCast(multi_i) },
                        .item_idx = .{ .kind = .range, .index = @intCast(range_i) },
                    } }));
                    emit_bb = true;

                    const prong_hint = try spa.analyzeProngRuntime(
                        &case_block,
                        .normal,
                        body,
                        info.capture,
                        child_block.src(.{ .switch_capture = .{
                            .switch_node_offset = switch_node_offset,
                            .case_idx = .{ .kind = .multi, .index = @intCast(multi_i) },
                        } }),
                        undefined, // case_vals may be undefined for ranges
                        item_ref,
                        info.has_tag_capture,
                    );
                    try branch_hints.append(gpa, prong_hint);

                    try cases_extra.ensureUnusedCapacity(gpa, @typeInfo(Air.SwitchBr.Case).@"struct".fields.len +
                        1 + // `item`, no ranges
                        case_block.instructions.items.len);
                    cases_extra.appendSliceAssumeCapacity(&payloadToExtraItems(Air.SwitchBr.Case{
                        .items_len = 1,
                        .ranges_len = 0,
                        .body_len = @intCast(case_block.instructions.items.len),
                    }));
                    cases_extra.appendAssumeCapacity(@intFromEnum(item_ref));
                    cases_extra.appendSliceAssumeCapacity(@ptrCast(case_block.instructions.items));

                    if (item.compareScalar(.eq, item_last, operand_ty, zcu)) break;
                }
            }

            for (items, 0..) |item, item_i| {
                cases_len += 1;

                case_block.instructions.shrinkRetainingCapacity(0);
                case_block.error_return_trace_index = child_block.error_return_trace_index;

                const analyze_body = if (union_originally) blk: {
                    const item_val = sema.resolveConstDefinedValue(block, LazySrcLoc.unneeded, item, undefined) catch unreachable;
                    const field_ty = maybe_union_ty.unionFieldType(item_val, zcu).?;
                    break :blk field_ty.zigTypeTag(zcu) != .noreturn;
                } else true;

                if (emit_bb) try sema.emitBackwardBranch(block, block.src(.{ .switch_case_item = .{
                    .switch_node_offset = switch_node_offset,
                    .case_idx = .{ .kind = .multi, .index = @intCast(multi_i) },
                    .item_idx = .{ .kind = .single, .index = @intCast(item_i) },
                } }));
                emit_bb = true;

                const prong_hint: std.builtin.BranchHint = if (analyze_body) h: {
                    break :h try spa.analyzeProngRuntime(
                        &case_block,
                        .normal,
                        body,
                        info.capture,
                        child_block.src(.{ .switch_capture = .{
                            .switch_node_offset = switch_node_offset,
                            .case_idx = .{ .kind = .multi, .index = @intCast(multi_i) },
                        } }),
                        &.{item},
                        item,
                        info.has_tag_capture,
                    );
                } else h: {
                    _ = try case_block.addNoOp(.unreach);
                    break :h .none;
                };
                try branch_hints.append(gpa, prong_hint);

                try cases_extra.ensureUnusedCapacity(gpa, @typeInfo(Air.SwitchBr.Case).@"struct".fields.len +
                    1 + // `item`, no ranges
                    case_block.instructions.items.len);
                cases_extra.appendSliceAssumeCapacity(&payloadToExtraItems(Air.SwitchBr.Case{
                    .items_len = 1,
                    .ranges_len = 0,
                    .body_len = @intCast(case_block.instructions.items.len),
                }));
                cases_extra.appendAssumeCapacity(@intFromEnum(item));
                cases_extra.appendSliceAssumeCapacity(@ptrCast(case_block.instructions.items));
            }

            continue;
        }

        cases_len += 1;

        const analyze_body = if (union_originally)
            for (items) |item| {
                const item_val = sema.resolveConstDefinedValue(block, LazySrcLoc.unneeded, item, undefined) catch unreachable;
                const field_ty = maybe_union_ty.unionFieldType(item_val, zcu).?;
                if (field_ty.zigTypeTag(zcu) != .noreturn) break true;
            } else false
        else
            true;

        const prong_hint: std.builtin.BranchHint = if (err_set and
            try sema.maybeErrorUnwrap(&case_block, body, operand, operand_src, allow_err_code_unwrap))
        h: {
            // nothing to do here. weight against error branch
            break :h .unlikely;
        } else if (analyze_body) h: {
            break :h try spa.analyzeProngRuntime(
                &case_block,
                .normal,
                body,
                info.capture,
                child_block.src(.{ .switch_capture = .{
                    .switch_node_offset = switch_node_offset,
                    .case_idx = .{ .kind = .multi, .index = @intCast(multi_i) },
                } }),
                items,
                .none,
                false,
            );
        } else h: {
            _ = try case_block.addNoOp(.unreach);
            break :h .none;
        };

        try branch_hints.append(gpa, prong_hint);

        try cases_extra.ensureUnusedCapacity(gpa, @typeInfo(Air.SwitchBr.Case).@"struct".fields.len +
            items.len + 2 * ranges_len +
            case_block.instructions.items.len);
        cases_extra.appendSliceAssumeCapacity(&payloadToExtraItems(Air.SwitchBr.Case{
            .items_len = @intCast(items.len),
            .ranges_len = @intCast(ranges_len),
            .body_len = @intCast(case_block.instructions.items.len),
        }));

        for (items) |item| {
            cases_extra.appendAssumeCapacity(@intFromEnum(item));
        }
        for (ranges) |range| {
            cases_extra.appendSliceAssumeCapacity(&.{
                @intFromEnum(range[0]),
                @intFromEnum(range[1]),
            });
        }

        cases_extra.appendSliceAssumeCapacity(@ptrCast(case_block.instructions.items));
    }

    const else_body: []const Air.Inst.Index = if (special.body.len != 0 or case_block.wantSafety()) else_body: {
        var emit_bb = false;
        if (special.is_inline) switch (operand_ty.zigTypeTag(zcu)) {
            .@"enum" => {
                if (operand_ty.isNonexhaustiveEnum(zcu) and !union_originally) {
                    return sema.fail(block, special_prong_src, "cannot enumerate values of type '{}' for 'inline else'", .{
                        operand_ty.fmt(pt),
                    });
                }
                for (seen_enum_fields, 0..) |f, i| {
                    if (f != null) continue;
                    cases_len += 1;

                    const item_val = try pt.enumValueFieldIndex(operand_ty, @intCast(i));
                    const item_ref = Air.internedToRef(item_val.toIntern());

                    case_block.instructions.shrinkRetainingCapacity(0);
                    case_block.error_return_trace_index = child_block.error_return_trace_index;

                    const analyze_body = if (union_originally) blk: {
                        const field_ty = maybe_union_ty.unionFieldType(item_val, zcu).?;
                        break :blk field_ty.zigTypeTag(zcu) != .noreturn;
                    } else true;

                    if (emit_bb) try sema.emitBackwardBranch(block, special_prong_src);
                    emit_bb = true;

                    const prong_hint: std.builtin.BranchHint = if (analyze_body) h: {
                        break :h try spa.analyzeProngRuntime(
                            &case_block,
                            .special,
                            special.body,
                            special.capture,
                            child_block.src(.{ .switch_capture = .{
                                .switch_node_offset = switch_node_offset,
                                .case_idx = LazySrcLoc.Offset.SwitchCaseIndex.special,
                            } }),
                            &.{item_ref},
                            item_ref,
                            special.has_tag_capture,
                        );
                    } else h: {
                        _ = try case_block.addNoOp(.unreach);
                        break :h .none;
                    };
                    try branch_hints.append(gpa, prong_hint);

                    try cases_extra.ensureUnusedCapacity(gpa, @typeInfo(Air.SwitchBr.Case).@"struct".fields.len +
                        1 + // `item`, no ranges
                        case_block.instructions.items.len);
                    cases_extra.appendSliceAssumeCapacity(&payloadToExtraItems(Air.SwitchBr.Case{
                        .items_len = 1,
                        .ranges_len = 0,
                        .body_len = @intCast(case_block.instructions.items.len),
                    }));
                    cases_extra.appendAssumeCapacity(@intFromEnum(item_ref));
                    cases_extra.appendSliceAssumeCapacity(@ptrCast(case_block.instructions.items));
                }
            },
            .error_set => {
                if (operand_ty.isAnyError(zcu)) {
                    return sema.fail(block, special_prong_src, "cannot enumerate values of type '{}' for 'inline else'", .{
                        operand_ty.fmt(pt),
                    });
                }
                const error_names = operand_ty.errorSetNames(zcu);
                for (0..error_names.len) |name_index| {
                    const error_name = error_names.get(ip)[name_index];
                    if (seen_errors.contains(error_name)) continue;
                    cases_len += 1;

                    const item_val = try pt.intern(.{ .err = .{
                        .ty = operand_ty.toIntern(),
                        .name = error_name,
                    } });
                    const item_ref = Air.internedToRef(item_val);

                    case_block.instructions.shrinkRetainingCapacity(0);
                    case_block.error_return_trace_index = child_block.error_return_trace_index;

                    if (emit_bb) try sema.emitBackwardBranch(block, special_prong_src);
                    emit_bb = true;

                    const prong_hint = try spa.analyzeProngRuntime(
                        &case_block,
                        .special,
                        special.body,
                        special.capture,
                        child_block.src(.{ .switch_capture = .{
                            .switch_node_offset = switch_node_offset,
                            .case_idx = LazySrcLoc.Offset.SwitchCaseIndex.special,
                        } }),
                        &.{item_ref},
                        item_ref,
                        special.has_tag_capture,
                    );
                    try branch_hints.append(gpa, prong_hint);

                    try cases_extra.ensureUnusedCapacity(gpa, @typeInfo(Air.SwitchBr.Case).@"struct".fields.len +
                        1 + // `item`, no ranges
                        case_block.instructions.items.len);
                    cases_extra.appendSliceAssumeCapacity(&payloadToExtraItems(Air.SwitchBr.Case{
                        .items_len = 1,
                        .ranges_len = 0,
                        .body_len = @intCast(case_block.instructions.items.len),
                    }));
                    cases_extra.appendAssumeCapacity(@intFromEnum(item_ref));
                    cases_extra.appendSliceAssumeCapacity(@ptrCast(case_block.instructions.items));
                }
            },
            .int => {
                var it = try RangeSetUnhandledIterator.init(sema, operand_ty, range_set);
                while (try it.next()) |cur| {
                    cases_len += 1;

                    const item_ref = Air.internedToRef(cur);

                    case_block.instructions.shrinkRetainingCapacity(0);
                    case_block.error_return_trace_index = child_block.error_return_trace_index;

                    if (emit_bb) try sema.emitBackwardBranch(block, special_prong_src);
                    emit_bb = true;

                    const prong_hint = try spa.analyzeProngRuntime(
                        &case_block,
                        .special,
                        special.body,
                        special.capture,
                        child_block.src(.{ .switch_capture = .{
                            .switch_node_offset = switch_node_offset,
                            .case_idx = LazySrcLoc.Offset.SwitchCaseIndex.special,
                        } }),
                        &.{item_ref},
                        item_ref,
                        special.has_tag_capture,
                    );
                    try branch_hints.append(gpa, prong_hint);

                    try cases_extra.ensureUnusedCapacity(gpa, @typeInfo(Air.SwitchBr.Case).@"struct".fields.len +
                        1 + // `item`, no ranges
                        case_block.instructions.items.len);
                    cases_extra.appendSliceAssumeCapacity(&payloadToExtraItems(Air.SwitchBr.Case{
                        .items_len = 1,
                        .ranges_len = 0,
                        .body_len = @intCast(case_block.instructions.items.len),
                    }));
                    cases_extra.appendAssumeCapacity(@intFromEnum(item_ref));
                    cases_extra.appendSliceAssumeCapacity(@ptrCast(case_block.instructions.items));
                }
            },
            .bool => {
                if (true_count == 0) {
                    cases_len += 1;

                    case_block.instructions.shrinkRetainingCapacity(0);
                    case_block.error_return_trace_index = child_block.error_return_trace_index;

                    if (emit_bb) try sema.emitBackwardBranch(block, special_prong_src);
                    emit_bb = true;

                    const prong_hint = try spa.analyzeProngRuntime(
                        &case_block,
                        .special,
                        special.body,
                        special.capture,
                        child_block.src(.{ .switch_capture = .{
                            .switch_node_offset = switch_node_offset,
                            .case_idx = LazySrcLoc.Offset.SwitchCaseIndex.special,
                        } }),
                        &.{.bool_true},
                        .bool_true,
                        special.has_tag_capture,
                    );
                    try branch_hints.append(gpa, prong_hint);

                    try cases_extra.ensureUnusedCapacity(gpa, @typeInfo(Air.SwitchBr.Case).@"struct".fields.len +
                        1 + // `item`, no ranges
                        case_block.instructions.items.len);
                    cases_extra.appendSliceAssumeCapacity(&payloadToExtraItems(Air.SwitchBr.Case{
                        .items_len = 1,
                        .ranges_len = 0,
                        .body_len = @intCast(case_block.instructions.items.len),
                    }));
                    cases_extra.appendAssumeCapacity(@intFromEnum(Air.Inst.Ref.bool_true));
                    cases_extra.appendSliceAssumeCapacity(@ptrCast(case_block.instructions.items));
                }
                if (false_count == 0) {
                    cases_len += 1;

                    case_block.instructions.shrinkRetainingCapacity(0);
                    case_block.error_return_trace_index = child_block.error_return_trace_index;

                    if (emit_bb) try sema.emitBackwardBranch(block, special_prong_src);
                    emit_bb = true;

                    const prong_hint = try spa.analyzeProngRuntime(
                        &case_block,
                        .special,
                        special.body,
                        special.capture,
                        child_block.src(.{ .switch_capture = .{
                            .switch_node_offset = switch_node_offset,
                            .case_idx = LazySrcLoc.Offset.SwitchCaseIndex.special,
                        } }),
                        &.{.bool_false},
                        .bool_false,
                        special.has_tag_capture,
                    );
                    try branch_hints.append(gpa, prong_hint);

                    try cases_extra.ensureUnusedCapacity(gpa, @typeInfo(Air.SwitchBr.Case).@"struct".fields.len +
                        1 + // `item`, no ranges
                        case_block.instructions.items.len);
                    cases_extra.appendSliceAssumeCapacity(&payloadToExtraItems(Air.SwitchBr.Case{
                        .items_len = 1,
                        .ranges_len = 0,
                        .body_len = @intCast(case_block.instructions.items.len),
                    }));
                    cases_extra.appendAssumeCapacity(@intFromEnum(Air.Inst.Ref.bool_false));
                    cases_extra.appendSliceAssumeCapacity(@ptrCast(case_block.instructions.items));
                }
            },
            else => return sema.fail(block, special_prong_src, "cannot enumerate values of type '{}' for 'inline else'", .{
                operand_ty.fmt(pt),
            }),
        };

        case_block.instructions.shrinkRetainingCapacity(0);
        case_block.error_return_trace_index = child_block.error_return_trace_index;

        if (zcu.backendSupportsFeature(.is_named_enum_value) and
            special.body.len != 0 and block.wantSafety() and
            operand_ty.zigTypeTag(zcu) == .@"enum" and
            (!operand_ty.isNonexhaustiveEnum(zcu) or union_originally))
        {
            try sema.zirDbgStmt(&case_block, cond_dbg_node_index);
            const ok = try case_block.addUnOp(.is_named_enum_value, operand);
            try sema.addSafetyCheck(&case_block, src, ok, .corrupt_switch);
        }

        const analyze_body = if (union_originally and !special.is_inline)
            for (seen_enum_fields, 0..) |seen_field, index| {
                if (seen_field != null) continue;
                const union_obj = zcu.typeToUnion(maybe_union_ty).?;
                const field_ty: Type = .fromInterned(union_obj.field_types.get(ip)[index]);
                if (field_ty.zigTypeTag(zcu) != .noreturn) break true;
            } else false
        else
            true;
        const else_hint: std.builtin.BranchHint = if (special.body.len != 0 and err_set and
            try sema.maybeErrorUnwrap(&case_block, special.body, operand, operand_src, allow_err_code_unwrap))
        h: {
            // nothing to do here. weight against error branch
            break :h .unlikely;
        } else if (special.body.len != 0 and analyze_body and !special.is_inline) h: {
            break :h try spa.analyzeProngRuntime(
                &case_block,
                .special,
                special.body,
                special.capture,
                child_block.src(.{ .switch_capture = .{
                    .switch_node_offset = switch_node_offset,
                    .case_idx = LazySrcLoc.Offset.SwitchCaseIndex.special,
                } }),
                undefined, // case_vals may be undefined for special prongs
                .none,
                false,
            );
        } else h: {
            // We still need a terminator in this block, but we have proven
            // that it is unreachable.
            if (case_block.wantSafety()) {
                try sema.zirDbgStmt(&case_block, cond_dbg_node_index);
                try sema.safetyPanic(&case_block, src, .corrupt_switch);
            } else {
                _ = try case_block.addNoOp(.unreach);
            }
            // Safety check / unreachable branches are cold.
            break :h .cold;
        };

        try branch_hints.append(gpa, else_hint);
        break :else_body case_block.instructions.items;
    } else else_body: {
        try branch_hints.append(gpa, .none);
        break :else_body &.{};
    };

    assert(branch_hints.items.len == cases_len + 1);

    try sema.air_extra.ensureUnusedCapacity(gpa, @typeInfo(Air.SwitchBr).@"struct".fields.len +
        cases_extra.items.len + else_body.len +
        (std.math.divCeil(usize, branch_hints.items.len, 10) catch unreachable)); // branch hints

    const payload_index = sema.addExtraAssumeCapacity(Air.SwitchBr{
        .cases_len = @intCast(cases_len),
        .else_body_len = @intCast(else_body.len),
    });

    {
        // Add branch hints.
        var cur_bag: u32 = 0;
        for (branch_hints.items, 0..) |hint, idx| {
            const idx_in_bag = idx % 10;
            cur_bag |= @as(u32, @intFromEnum(hint)) << @intCast(idx_in_bag * 3);
            if (idx_in_bag == 9) {
                sema.air_extra.appendAssumeCapacity(cur_bag);
                cur_bag = 0;
            }
        }
        if (branch_hints.items.len % 10 != 0) {
            sema.air_extra.appendAssumeCapacity(cur_bag);
        }
    }
    sema.air_extra.appendSliceAssumeCapacity(@ptrCast(cases_extra.items));
    sema.air_extra.appendSliceAssumeCapacity(@ptrCast(else_body));

    return try child_block.addInst(.{
        .tag = if (spa.operand == .loop) .loop_switch_br else .switch_br,
        .data = .{ .pl_op = .{
            .operand = operand,
            .payload = payload_index,
        } },
    });
}

fn resolveSwitchComptimeLoop(
    sema: *Sema,
    init_spa: SwitchProngAnalysis,
    child_block: *Block,
    maybe_ptr_operand_ty: Type,
    cond_ty: Type,
    init_cond_val: Value,
    switch_node_offset: std.zig.Ast.Node.Offset,
    special: SpecialProng,
    case_vals: std.ArrayListUnmanaged(Air.Inst.Ref),
    scalar_cases_len: u32,
    multi_cases_len: u32,
    err_set: bool,
    empty_enum: bool,
    operand_is_ref: bool,
) CompileError!Air.Inst.Ref {
    var spa = init_spa;
    var cond_val = init_cond_val;

    while (true) {
        if (resolveSwitchComptime(
            sema,
            spa,
            child_block,
            spa.operand.simple.cond,
            cond_val,
            cond_ty,
            switch_node_offset,
            special,
            case_vals,
            scalar_cases_len,
            multi_cases_len,
            err_set,
            empty_enum,
        )) |result| {
            return result;
        } else |err| switch (err) {
            error.ComptimeBreak => {
                const break_inst = sema.code.instructions.get(@intFromEnum(sema.comptime_break_inst));
                if (break_inst.tag != .switch_continue) return error.ComptimeBreak;
                const extra = sema.code.extraData(Zir.Inst.Break, break_inst.data.@"break".payload_index).data;
                if (extra.block_inst != spa.switch_block_inst) return error.ComptimeBreak;
                // This is a `switch_continue` targeting this block. Change the operand and start over.
                const src = child_block.nodeOffset(extra.operand_src_node.unwrap().?);
                const new_operand_uncoerced = try sema.resolveInst(break_inst.data.@"break".operand);
                const new_operand = try sema.coerce(child_block, maybe_ptr_operand_ty, new_operand_uncoerced, src);

                try sema.emitBackwardBranch(child_block, src);

                const val, const ref = if (operand_is_ref)
                    .{ try sema.analyzeLoad(child_block, src, new_operand, src), new_operand }
                else
                    .{ new_operand, undefined };

                const cond_ref = try sema.switchCond(child_block, src, val);

                cond_val = try sema.resolveConstDefinedValue(child_block, src, cond_ref, null);
                spa.operand = .{ .simple = .{
                    .by_val = val,
                    .by_ref = ref,
                    .cond = cond_ref,
                } };
            },
            else => |e| return e,
        }
    }
}

fn resolveSwitchComptime(
    sema: *Sema,
    spa: SwitchProngAnalysis,
    child_block: *Block,
    cond_operand: Air.Inst.Ref,
    operand_val: Value,
    operand_ty: Type,
    switch_node_offset: std.zig.Ast.Node.Offset,
    special: SpecialProng,
    case_vals: std.ArrayListUnmanaged(Air.Inst.Ref),
    scalar_cases_len: u32,
    multi_cases_len: u32,
    err_set: bool,
    empty_enum: bool,
) CompileError!Air.Inst.Ref {
    const merges = &child_block.label.?.merges;
    const resolved_operand_val = try sema.resolveLazyValue(operand_val);

    var extra_index: usize = special.end;
    {
        var scalar_i: usize = 0;
        while (scalar_i < scalar_cases_len) : (scalar_i += 1) {
            extra_index += 1;
            const info: Zir.Inst.SwitchBlock.ProngInfo = @bitCast(sema.code.extra[extra_index]);
            extra_index += 1;
            const body = sema.code.bodySlice(extra_index, info.body_len);
            extra_index += info.body_len;

            const item = case_vals.items[scalar_i];
            const item_val = sema.resolveConstDefinedValue(child_block, LazySrcLoc.unneeded, item, undefined) catch unreachable;
            if (operand_val.eql(item_val, operand_ty, sema.pt.zcu)) {
                if (err_set) try sema.maybeErrorUnwrapComptime(child_block, body, cond_operand);
                return spa.resolveProngComptime(
                    child_block,
                    .normal,
                    body,
                    info.capture,
                    child_block.src(.{ .switch_capture = .{
                        .switch_node_offset = switch_node_offset,
                        .case_idx = .{ .kind = .scalar, .index = @intCast(scalar_i) },
                    } }),
                    &.{item},
                    if (info.is_inline) cond_operand else .none,
                    info.has_tag_capture,
                    merges,
                );
            }
        }
    }
    {
        var multi_i: usize = 0;
        var case_val_idx: usize = scalar_cases_len;
        while (multi_i < multi_cases_len) : (multi_i += 1) {
            const items_len = sema.code.extra[extra_index];
            extra_index += 1;
            const ranges_len = sema.code.extra[extra_index];
            extra_index += 1;
            const info: Zir.Inst.SwitchBlock.ProngInfo = @bitCast(sema.code.extra[extra_index]);
            extra_index += 1 + items_len;
            const body = sema.code.bodySlice(extra_index + 2 * ranges_len, info.body_len);

            const items = case_vals.items[case_val_idx..][0..items_len];
            case_val_idx += items_len;

            for (items) |item| {
                // Validation above ensured these will succeed.
                const item_val = sema.resolveConstDefinedValue(child_block, LazySrcLoc.unneeded, item, undefined) catch unreachable;
                if (operand_val.eql(item_val, operand_ty, sema.pt.zcu)) {
                    if (err_set) try sema.maybeErrorUnwrapComptime(child_block, body, cond_operand);
                    return spa.resolveProngComptime(
                        child_block,
                        .normal,
                        body,
                        info.capture,
                        child_block.src(.{ .switch_capture = .{
                            .switch_node_offset = switch_node_offset,
                            .case_idx = .{ .kind = .multi, .index = @intCast(multi_i) },
                        } }),
                        items,
                        if (info.is_inline) cond_operand else .none,
                        info.has_tag_capture,
                        merges,
                    );
                }
            }

            var range_i: usize = 0;
            while (range_i < ranges_len) : (range_i += 1) {
                const range_items = case_vals.items[case_val_idx..][0..2];
                extra_index += 2;
                case_val_idx += 2;

                // Validation above ensured these will succeed.
                const first_val = sema.resolveConstDefinedValue(child_block, LazySrcLoc.unneeded, range_items[0], undefined) catch unreachable;
                const last_val = sema.resolveConstDefinedValue(child_block, LazySrcLoc.unneeded, range_items[1], undefined) catch unreachable;
                if ((try sema.compareAll(resolved_operand_val, .gte, first_val, operand_ty)) and
                    (try sema.compareAll(resolved_operand_val, .lte, last_val, operand_ty)))
                {
                    if (err_set) try sema.maybeErrorUnwrapComptime(child_block, body, cond_operand);
                    return spa.resolveProngComptime(
                        child_block,
                        .normal,
                        body,
                        info.capture,
                        child_block.src(.{ .switch_capture = .{
                            .switch_node_offset = switch_node_offset,
                            .case_idx = .{ .kind = .multi, .index = @intCast(multi_i) },
                        } }),
                        undefined, // case_vals may be undefined for ranges
                        if (info.is_inline) cond_operand else .none,
                        info.has_tag_capture,
                        merges,
                    );
                }
            }

            extra_index += info.body_len;
        }
    }
    if (err_set) try sema.maybeErrorUnwrapComptime(child_block, special.body, cond_operand);
    if (empty_enum) {
        return .void_value;
    }

    return spa.resolveProngComptime(
        child_block,
        .special,
        special.body,
        special.capture,
        child_block.src(.{ .switch_capture = .{
            .switch_node_offset = switch_node_offset,
            .case_idx = LazySrcLoc.Offset.SwitchCaseIndex.special,
        } }),
        undefined, // case_vals may be undefined for special prongs
        if (special.is_inline) cond_operand else .none,
        special.has_tag_capture,
        merges,
    );
}

const RangeSetUnhandledIterator = struct {
    pt: Zcu.PerThread,
    cur: ?InternPool.Index,
    max: InternPool.Index,
    range_i: usize,
    ranges: []const RangeSet.Range,
    limbs: []math.big.Limb,

    const preallocated_limbs = math.big.int.calcTwosCompLimbCount(128);

    fn init(sema: *Sema, ty: Type, range_set: RangeSet) !RangeSetUnhandledIterator {
        const pt = sema.pt;
        const int_type = pt.zcu.intern_pool.indexToKey(ty.toIntern()).int_type;
        const needed_limbs = math.big.int.calcTwosCompLimbCount(int_type.bits);
        return .{
            .pt = pt,
            .cur = (try ty.minInt(pt, ty)).toIntern(),
            .max = (try ty.maxInt(pt, ty)).toIntern(),
            .range_i = 0,
            .ranges = range_set.ranges.items,
            .limbs = if (needed_limbs > preallocated_limbs)
                try sema.arena.alloc(math.big.Limb, needed_limbs)
            else
                &.{},
        };
    }

    fn addOne(it: *const RangeSetUnhandledIterator, val: InternPool.Index) !?InternPool.Index {
        if (val == it.max) return null;
        const int = it.pt.zcu.intern_pool.indexToKey(val).int;

        switch (int.storage) {
            inline .u64, .i64 => |val_int| {
                const next_int = @addWithOverflow(val_int, 1);
                if (next_int[1] == 0)
                    return (try it.pt.intValue(.fromInterned(int.ty), next_int[0])).toIntern();
            },
            .big_int => {},
            .lazy_align, .lazy_size => unreachable,
        }

        var val_space: InternPool.Key.Int.Storage.BigIntSpace = undefined;
        const val_bigint = int.storage.toBigInt(&val_space);

        var result_limbs: [preallocated_limbs]math.big.Limb = undefined;
        var result_bigint = math.big.int.Mutable.init(
            if (it.limbs.len > 0) it.limbs else &result_limbs,
            0,
        );

        result_bigint.addScalar(val_bigint, 1);
        return (try it.pt.intValue_big(.fromInterned(int.ty), result_bigint.toConst())).toIntern();
    }

    fn next(it: *RangeSetUnhandledIterator) !?InternPool.Index {
        var cur = it.cur orelse return null;
        while (it.range_i < it.ranges.len and cur == it.ranges[it.range_i].first) {
            defer it.range_i += 1;
            cur = (try it.addOne(it.ranges[it.range_i].last)) orelse {
                it.cur = null;
                return null;
            };
        }
        it.cur = try it.addOne(cur);
        return cur;
    }
};

const ResolvedSwitchItem = struct {
    ref: Air.Inst.Ref,
    val: InternPool.Index,
};
fn resolveSwitchItemVal(
    sema: *Sema,
    block: *Block,
    item_ref: Zir.Inst.Ref,
    /// Coerce `item_ref` to this type.
    coerce_ty: Type,
    item_src: LazySrcLoc,
) CompileError!ResolvedSwitchItem {
    const uncoerced_item = try sema.resolveInst(item_ref);

    // Constructing a LazySrcLoc is costly because we only have the switch AST node.
    // Only if we know for sure we need to report a compile error do we resolve the
    // full source locations.

    const item = try sema.coerce(block, coerce_ty, uncoerced_item, item_src);

    const maybe_lazy = try sema.resolveConstDefinedValue(block, item_src, item, .{ .simple = .switch_item });

    const val = try sema.resolveLazyValue(maybe_lazy);
    const new_item = if (val.toIntern() != maybe_lazy.toIntern()) blk: {
        break :blk Air.internedToRef(val.toIntern());
    } else item;

    return .{ .ref = new_item, .val = val.toIntern() };
}

fn validateErrSetSwitch(
    sema: *Sema,
    block: *Block,
    seen_errors: *SwitchErrorSet,
    case_vals: *std.ArrayListUnmanaged(Air.Inst.Ref),
    operand_ty: Type,
    inst_data: @FieldType(Zir.Inst.Data, "pl_node"),
    scalar_cases_len: u32,
    multi_cases_len: u32,
    else_case: struct { body: []const Zir.Inst.Index, end: usize, src: LazySrcLoc },
    has_else: bool,
) CompileError!?Type {
    const gpa = sema.gpa;
    const pt = sema.pt;
    const zcu = pt.zcu;
    const ip = &zcu.intern_pool;

    const src_node_offset = inst_data.src_node;
    const src = block.nodeOffset(src_node_offset);

    var extra_index: usize = else_case.end;
    {
        var scalar_i: u32 = 0;
        while (scalar_i < scalar_cases_len) : (scalar_i += 1) {
            const item_ref: Zir.Inst.Ref = @enumFromInt(sema.code.extra[extra_index]);
            extra_index += 1;
            const info: Zir.Inst.SwitchBlock.ProngInfo = @bitCast(sema.code.extra[extra_index]);
            extra_index += 1 + info.body_len;

            case_vals.appendAssumeCapacity(try sema.validateSwitchItemError(
                block,
                seen_errors,
                item_ref,
                operand_ty,
                block.src(.{ .switch_case_item = .{
                    .switch_node_offset = src_node_offset,
                    .case_idx = .{ .kind = .scalar, .index = @intCast(scalar_i) },
                    .item_idx = .{ .kind = .single, .index = 0 },
                } }),
            ));
        }
    }
    {
        var multi_i: u32 = 0;
        while (multi_i < multi_cases_len) : (multi_i += 1) {
            const items_len = sema.code.extra[extra_index];
            extra_index += 1;
            const ranges_len = sema.code.extra[extra_index];
            extra_index += 1;
            const info: Zir.Inst.SwitchBlock.ProngInfo = @bitCast(sema.code.extra[extra_index]);
            extra_index += 1;
            const items = sema.code.refSlice(extra_index, items_len);
            extra_index += items_len + info.body_len;

            try case_vals.ensureUnusedCapacity(gpa, items.len);
            for (items, 0..) |item_ref, item_i| {
                case_vals.appendAssumeCapacity(try sema.validateSwitchItemError(
                    block,
                    seen_errors,
                    item_ref,
                    operand_ty,
                    block.src(.{ .switch_case_item = .{
                        .switch_node_offset = src_node_offset,
                        .case_idx = .{ .kind = .multi, .index = @intCast(multi_i) },
                        .item_idx = .{ .kind = .single, .index = @intCast(item_i) },
                    } }),
                ));
            }

            try sema.validateSwitchNoRange(block, ranges_len, operand_ty, src_node_offset);
        }
    }

    switch (try sema.resolveInferredErrorSetTy(block, src, operand_ty.toIntern())) {
        .anyerror_type => {
            if (!has_else) {
                return sema.fail(
                    block,
                    src,
                    "else prong required when switching on type 'anyerror'",
                    .{},
                );
            }
            return .anyerror;
        },
        else => |err_set_ty_index| else_validation: {
            const error_names = ip.indexToKey(err_set_ty_index).error_set_type.names;
            var maybe_msg: ?*Zcu.ErrorMsg = null;
            errdefer if (maybe_msg) |msg| msg.destroy(sema.gpa);

            for (error_names.get(ip)) |error_name| {
                if (!seen_errors.contains(error_name) and !has_else) {
                    const msg = maybe_msg orelse blk: {
                        maybe_msg = try sema.errMsg(
                            src,
                            "switch must handle all possibilities",
                            .{},
                        );
                        break :blk maybe_msg.?;
                    };

                    try sema.errNote(
                        src,
                        msg,
                        "unhandled error value: 'error.{}'",
                        .{error_name.fmt(ip)},
                    );
                }
            }

            if (maybe_msg) |msg| {
                maybe_msg = null;
                try sema.addDeclaredHereNote(msg, operand_ty);
                return sema.failWithOwnedErrorMsg(block, msg);
            }

            if (has_else and seen_errors.count() == error_names.len) {
                // In order to enable common patterns for generic code allow simple else bodies
                // else => unreachable,
                // else => return,
                // else => |e| return e,
                // even if all the possible errors were already handled.
                const tags = sema.code.instructions.items(.tag);
                const datas = sema.code.instructions.items(.data);
                for (else_case.body) |else_inst| switch (tags[@intFromEnum(else_inst)]) {
                    .dbg_stmt,
                    .dbg_var_val,
                    .ret_type,
                    .as_node,
                    .ret_node,
                    .@"unreachable",
                    .@"defer",
                    .defer_err_code,
                    .err_union_code,
                    .ret_err_value_code,
                    .save_err_ret_index,
                    .restore_err_ret_index_unconditional,
                    .restore_err_ret_index_fn_entry,
                    .is_non_err,
                    .ret_is_non_err,
                    .condbr,
                    => {},
                    .extended => switch (datas[@intFromEnum(else_inst)].extended.opcode) {
                        .restore_err_ret_index => {},
                        else => break,
                    },
                    else => break,
                } else break :else_validation;

                return sema.fail(
                    block,
                    else_case.src,
                    "unreachable else prong; all cases already handled",
                    .{},
                );
            }

            var names: InferredErrorSet.NameMap = .{};
            try names.ensureUnusedCapacity(sema.arena, error_names.len);
            for (error_names.get(ip)) |error_name| {
                if (seen_errors.contains(error_name)) continue;

                names.putAssumeCapacityNoClobber(error_name, {});
            }
            // No need to keep the hash map metadata correct; here we
            // extract the (sorted) keys only.
            return try pt.errorSetFromUnsortedNames(names.keys());
        },
    }
    return null;
}

fn validateSwitchRange(
    sema: *Sema,
    block: *Block,
    range_set: *RangeSet,
    first_ref: Zir.Inst.Ref,
    last_ref: Zir.Inst.Ref,
    operand_ty: Type,
    item_src: LazySrcLoc,
) CompileError![2]Air.Inst.Ref {
    const first_src: LazySrcLoc = .{
        .base_node_inst = item_src.base_node_inst,
        .offset = .{ .switch_case_item_range_first = item_src.offset.switch_case_item },
    };
    const last_src: LazySrcLoc = .{
        .base_node_inst = item_src.base_node_inst,
        .offset = .{ .switch_case_item_range_last = item_src.offset.switch_case_item },
    };
    const first = try sema.resolveSwitchItemVal(block, first_ref, operand_ty, first_src);
    const last = try sema.resolveSwitchItemVal(block, last_ref, operand_ty, last_src);
    if (try Value.fromInterned(first.val).compareAll(.gt, Value.fromInterned(last.val), operand_ty, sema.pt)) {
        return sema.fail(block, item_src, "range start value is greater than the end value", .{});
    }
    const maybe_prev_src = try range_set.add(first.val, last.val, item_src);
    try sema.validateSwitchDupe(block, maybe_prev_src, item_src);
    return .{ first.ref, last.ref };
}

fn validateSwitchItemInt(
    sema: *Sema,
    block: *Block,
    range_set: *RangeSet,
    item_ref: Zir.Inst.Ref,
    operand_ty: Type,
    item_src: LazySrcLoc,
) CompileError!Air.Inst.Ref {
    const item = try sema.resolveSwitchItemVal(block, item_ref, operand_ty, item_src);
    const maybe_prev_src = try range_set.add(item.val, item.val, item_src);
    try sema.validateSwitchDupe(block, maybe_prev_src, item_src);
    return item.ref;
}

fn validateSwitchItemEnum(
    sema: *Sema,
    block: *Block,
    seen_fields: []?LazySrcLoc,
    range_set: *RangeSet,
    item_ref: Zir.Inst.Ref,
    operand_ty: Type,
    item_src: LazySrcLoc,
) CompileError!Air.Inst.Ref {
    const ip = &sema.pt.zcu.intern_pool;
    const item = try sema.resolveSwitchItemVal(block, item_ref, operand_ty, item_src);
    const int = ip.indexToKey(item.val).enum_tag.int;
    const field_index = ip.loadEnumType(ip.typeOf(item.val)).tagValueIndex(ip, int) orelse {
        const maybe_prev_src = try range_set.add(int, int, item_src);
        try sema.validateSwitchDupe(block, maybe_prev_src, item_src);
        return item.ref;
    };
    const maybe_prev_src = seen_fields[field_index];
    seen_fields[field_index] = item_src;
    try sema.validateSwitchDupe(block, maybe_prev_src, item_src);
    return item.ref;
}

fn validateSwitchItemError(
    sema: *Sema,
    block: *Block,
    seen_errors: *SwitchErrorSet,
    item_ref: Zir.Inst.Ref,
    operand_ty: Type,
    item_src: LazySrcLoc,
) CompileError!Air.Inst.Ref {
    const item = try sema.resolveSwitchItemVal(block, item_ref, operand_ty, item_src);
    const error_name = sema.pt.zcu.intern_pool.indexToKey(item.val).err.name;
    const maybe_prev_src = if (try seen_errors.fetchPut(error_name, item_src)) |prev|
        prev.value
    else
        null;
    try sema.validateSwitchDupe(block, maybe_prev_src, item_src);
    return item.ref;
}

fn validateSwitchDupe(
    sema: *Sema,
    block: *Block,
    maybe_prev_src: ?LazySrcLoc,
    item_src: LazySrcLoc,
) CompileError!void {
    const prev_item_src = maybe_prev_src orelse return;
    return sema.failWithOwnedErrorMsg(block, msg: {
        const msg = try sema.errMsg(
            item_src,
            "duplicate switch value",
            .{},
        );
        errdefer msg.destroy(sema.gpa);
        try sema.errNote(
            prev_item_src,
            msg,
            "previous value here",
            .{},
        );
        break :msg msg;
    });
}

fn validateSwitchItemBool(
    sema: *Sema,
    block: *Block,
    true_count: *u8,
    false_count: *u8,
    item_ref: Zir.Inst.Ref,
    item_src: LazySrcLoc,
) CompileError!Air.Inst.Ref {
    const item = try sema.resolveSwitchItemVal(block, item_ref, .bool, item_src);
    if (Value.fromInterned(item.val).toBool()) {
        true_count.* += 1;
    } else {
        false_count.* += 1;
    }
    if (true_count.* > 1 or false_count.* > 1) {
        return sema.fail(block, item_src, "duplicate switch value", .{});
    }
    return item.ref;
}

const ValueSrcMap = std.AutoHashMapUnmanaged(InternPool.Index, LazySrcLoc);

fn validateSwitchItemSparse(
    sema: *Sema,
    block: *Block,
    seen_values: *ValueSrcMap,
    item_ref: Zir.Inst.Ref,
    operand_ty: Type,
    item_src: LazySrcLoc,
) CompileError!Air.Inst.Ref {
    const item = try sema.resolveSwitchItemVal(block, item_ref, operand_ty, item_src);
    const kv = try seen_values.fetchPut(sema.gpa, item.val, item_src) orelse return item.ref;
    try sema.validateSwitchDupe(block, kv.value, item_src);
    unreachable;
}

fn validateSwitchNoRange(
    sema: *Sema,
    block: *Block,
    ranges_len: u32,
    operand_ty: Type,
    src_node_offset: std.zig.Ast.Node.Offset,
) CompileError!void {
    if (ranges_len == 0)
        return;

    const operand_src = block.src(.{ .node_offset_switch_operand = src_node_offset });
    const range_src = block.src(.{ .node_offset_switch_range = src_node_offset });

    const msg = msg: {
        const msg = try sema.errMsg(
            operand_src,
            "ranges not allowed when switching on type '{}'",
            .{operand_ty.fmt(sema.pt)},
        );
        errdefer msg.destroy(sema.gpa);
        try sema.errNote(
            range_src,
            msg,
            "range here",
            .{},
        );
        break :msg msg;
    };
    return sema.failWithOwnedErrorMsg(block, msg);
}

fn maybeErrorUnwrap(
    sema: *Sema,
    block: *Block,
    body: []const Zir.Inst.Index,
    operand: Air.Inst.Ref,
    operand_src: LazySrcLoc,
    allow_err_code_inst: bool,
) !bool {
    const pt = sema.pt;
    const zcu = pt.zcu;

    const tags = sema.code.instructions.items(.tag);
    for (body) |inst| {
        switch (tags[@intFromEnum(inst)]) {
            .@"unreachable" => if (!block.wantSafety()) return false,
            .err_union_code => if (!allow_err_code_inst) return false,
            .save_err_ret_index,
            .dbg_stmt,
            .str,
            .as_node,
            .panic,
            .field_val,
            => {},
            else => return false,
        }
    }

    for (body) |inst| {
        const air_inst = switch (tags[@intFromEnum(inst)]) {
            .err_union_code => continue,
            .dbg_stmt => {
                try sema.zirDbgStmt(block, inst);
                continue;
            },
            .save_err_ret_index => {
                try sema.zirSaveErrRetIndex(block, inst);
                continue;
            },
            .str => try sema.zirStr(inst),
            .as_node => try sema.zirAsNode(block, inst),
            .field_val => try sema.zirFieldVal(block, inst),
            .@"unreachable" => {
                try safetyPanicUnwrapError(sema, block, operand_src, operand);
                return true;
            },
            .panic => {
                const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].un_node;
                const msg_inst = try sema.resolveInst(inst_data.operand);

                const panic_fn = try getBuiltin(sema, operand_src, .@"panic.call");
                const args: [2]Air.Inst.Ref = .{ msg_inst, .null_value };
                try sema.callBuiltin(block, operand_src, Air.internedToRef(panic_fn), .auto, &args, .@"safety check");
                return true;
            },
            else => unreachable,
        };
        if (sema.typeOf(air_inst).isNoReturn(zcu))
            return true;
        sema.inst_map.putAssumeCapacity(inst, air_inst);
    }
    unreachable;
}

fn maybeErrorUnwrapCondbr(sema: *Sema, block: *Block, body: []const Zir.Inst.Index, cond: Zir.Inst.Ref, cond_src: LazySrcLoc) !void {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const index = cond.toIndex() orelse return;
    if (sema.code.instructions.items(.tag)[@intFromEnum(index)] != .is_non_err) return;

    const err_inst_data = sema.code.instructions.items(.data)[@intFromEnum(index)].un_node;
    const err_operand = try sema.resolveInst(err_inst_data.operand);
    const operand_ty = sema.typeOf(err_operand);
    if (operand_ty.zigTypeTag(zcu) == .error_set) {
        try sema.maybeErrorUnwrapComptime(block, body, err_operand);
        return;
    }
    if (try sema.resolveDefinedValue(block, cond_src, err_operand)) |val| {
        if (!operand_ty.isError(zcu)) return;
        if (val.getErrorName(zcu) == .none) return;
        try sema.maybeErrorUnwrapComptime(block, body, err_operand);
    }
}

fn maybeErrorUnwrapComptime(sema: *Sema, block: *Block, body: []const Zir.Inst.Index, operand: Air.Inst.Ref) !void {
    const tags = sema.code.instructions.items(.tag);
    const inst = for (body) |inst| {
        switch (tags[@intFromEnum(inst)]) {
            .dbg_stmt,
            .save_err_ret_index,
            => {},
            .@"unreachable" => break inst,
            else => return,
        }
    } else return;
    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].@"unreachable";
    const src = block.nodeOffset(inst_data.src_node);

    if (try sema.resolveDefinedValue(block, src, operand)) |val| {
        if (val.getErrorName(sema.pt.zcu).unwrap()) |name| {
            return sema.failWithComptimeErrorRetTrace(block, src, name);
        }
    }
}

fn zirHasField(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].pl_node;
    const extra = sema.code.extraData(Zir.Inst.Bin, inst_data.payload_index).data;
    const ty_src = block.builtinCallArgSrc(inst_data.src_node, 0);
    const name_src = block.builtinCallArgSrc(inst_data.src_node, 1);
    const ty = try sema.resolveType(block, ty_src, extra.lhs);
    const field_name = try sema.resolveConstStringIntern(block, name_src, extra.rhs, .{ .simple = .field_name });
    try ty.resolveFields(pt);
    const ip = &zcu.intern_pool;

    const has_field = hf: {
        switch (ip.indexToKey(ty.toIntern())) {
            .ptr_type => |ptr_type| switch (ptr_type.flags.size) {
                .slice => {
                    if (field_name.eqlSlice("ptr", ip)) break :hf true;
                    if (field_name.eqlSlice("len", ip)) break :hf true;
                    break :hf false;
                },
                else => {},
            },
            .tuple_type => |tuple| {
                const field_index = field_name.toUnsigned(ip) orelse break :hf false;
                break :hf field_index < tuple.types.len;
            },
            .struct_type => {
                break :hf ip.loadStructType(ty.toIntern()).nameIndex(ip, field_name) != null;
            },
            .union_type => {
                const union_type = ip.loadUnionType(ty.toIntern());
                break :hf union_type.loadTagType(ip).nameIndex(ip, field_name) != null;
            },
            .enum_type => {
                break :hf ip.loadEnumType(ty.toIntern()).nameIndex(ip, field_name) != null;
            },
            .array_type => break :hf field_name.eqlSlice("len", ip),
            else => {},
        }
        return sema.fail(block, ty_src, "type '{}' does not support '@hasField'", .{
            ty.fmt(pt),
        });
    };
    return if (has_field) .bool_true else .bool_false;
}

fn zirHasDecl(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].pl_node;
    const extra = sema.code.extraData(Zir.Inst.Bin, inst_data.payload_index).data;
    const src = block.nodeOffset(inst_data.src_node);
    const lhs_src = block.builtinCallArgSrc(inst_data.src_node, 0);
    const rhs_src = block.builtinCallArgSrc(inst_data.src_node, 1);
    const container_type = try sema.resolveType(block, lhs_src, extra.lhs);
    const decl_name = try sema.resolveConstStringIntern(block, rhs_src, extra.rhs, .{ .simple = .decl_name });

    try sema.checkNamespaceType(block, lhs_src, container_type);

    const namespace = container_type.getNamespace(zcu).unwrap() orelse return .bool_false;
    if (try sema.lookupInNamespace(block, src, namespace, decl_name, true)) |lookup| {
        if (lookup.accessible) {
            return .bool_true;
        }
    }
    return .bool_false;
}

fn zirImport(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    const tracy = trace(@src());
    defer tracy.end();

    const pt = sema.pt;
    const zcu = pt.zcu;
    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].pl_tok;
    const extra = sema.code.extraData(Zir.Inst.Import, inst_data.payload_index).data;
    const operand_src = block.tokenOffset(inst_data.src_tok);
    const operand = sema.code.nullTerminatedString(extra.path);

    const result = pt.doImport(block.getFileScope(zcu), operand) catch |err| switch (err) {
        error.ModuleNotFound => return sema.fail(block, operand_src, "no module named '{s}' available within module '{s}'", .{
            operand, block.getFileScope(zcu).mod.?.fully_qualified_name,
        }),
        error.IllegalZigImport => unreachable, // caught before semantic analysis
        error.OutOfMemory => |e| return e,
    };
    const file_index = result.file;
    const file = zcu.fileByIndex(file_index);
    switch (file.getMode()) {
        .zig => {
            try pt.ensureFileAnalyzed(file_index);
            const ty = zcu.fileRootType(file_index);
            try sema.declareDependency(.{ .interned = ty });
            try sema.addTypeReferenceEntry(operand_src, ty);
            return Air.internedToRef(ty);
        },
        .zon => {
            const res_ty: InternPool.Index = b: {
                if (extra.res_ty == .none) break :b .none;
                const res_ty_inst = try sema.resolveInst(extra.res_ty);
                const res_ty = try sema.analyzeAsType(block, operand_src, res_ty_inst);
                if (res_ty.isGenericPoison()) break :b .none;
                break :b res_ty.toIntern();
            };

            try sema.declareDependency(.{ .zon_file = file_index });
            const interned = try LowerZon.run(
                sema,
                file,
                file_index,
                res_ty,
                operand_src,
                block,
            );
            return Air.internedToRef(interned);
        },
    }
}

fn zirEmbedFile(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    const tracy = trace(@src());
    defer tracy.end();

    const pt = sema.pt;
    const zcu = pt.zcu;

    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].un_node;
    const operand_src = block.builtinCallArgSrc(inst_data.src_node, 0);
    const name = try sema.resolveConstString(block, operand_src, inst_data.operand, .{ .simple = .operand_embedFile });

    if (name.len == 0) {
        return sema.fail(block, operand_src, "file path name cannot be empty", .{});
    }

    const ef_idx = pt.embedFile(block.getFileScope(zcu), name) catch |err| switch (err) {
        error.ImportOutsideModulePath => {
            return sema.fail(block, operand_src, "embed of file outside package path: '{s}'", .{name});
        },
        error.CurrentWorkingDirectoryUnlinked => {
            // TODO: this should be some kind of retryable failure, in case the cwd is put back
            return sema.fail(block, operand_src, "unable to resolve '{s}': working directory has been unlinked", .{name});
        },
        error.OutOfMemory => |e| return e,
    };
    try sema.declareDependency(.{ .embed_file = ef_idx });

    const result = ef_idx.get(zcu);
    if (result.val == .none) {
        return sema.fail(block, operand_src, "unable to open '{s}': {s}", .{ name, @errorName(result.err.?) });
    }

    return Air.internedToRef(result.val);
}

fn zirRetErrValueCode(sema: *Sema, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].str_tok;
    const name = try zcu.intern_pool.getOrPutString(
        sema.gpa,
        pt.tid,
        inst_data.get(sema.code),
        .no_embedded_nulls,
    );
    _ = try pt.getErrorValue(name);
    const error_set_type = try pt.singleErrorSetType(name);
    return Air.internedToRef((try pt.intern(.{ .err = .{
        .ty = error_set_type.toIntern(),
        .name = name,
    } })));
}

fn zirShl(
    sema: *Sema,
    block: *Block,
    inst: Zir.Inst.Index,
    air_tag: Air.Inst.Tag,
) CompileError!Air.Inst.Ref {
    const tracy = trace(@src());
    defer tracy.end();

    const pt = sema.pt;
    const zcu = pt.zcu;
    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].pl_node;
    const extra = sema.code.extraData(Zir.Inst.Bin, inst_data.payload_index).data;
    const lhs = try sema.resolveInst(extra.lhs);
    const rhs = try sema.resolveInst(extra.rhs);
    const lhs_ty = sema.typeOf(lhs);
    const rhs_ty = sema.typeOf(rhs);

    const src = block.nodeOffset(inst_data.src_node);
    const lhs_src, const rhs_src = switch (air_tag) {
        .shl, .shl_sat => .{
            block.src(.{ .node_offset_bin_lhs = inst_data.src_node }),
            block.src(.{ .node_offset_bin_rhs = inst_data.src_node }),
        },
        .shl_exact => .{
            block.builtinCallArgSrc(inst_data.src_node, 0),
            block.builtinCallArgSrc(inst_data.src_node, 1),
        },
        else => unreachable,
    };

    try sema.checkVectorizableBinaryOperands(block, src, lhs_ty, rhs_ty, lhs_src, rhs_src);

    const scalar_ty = lhs_ty.scalarType(zcu);
    const scalar_rhs_ty = rhs_ty.scalarType(zcu);

    _ = try sema.checkIntType(block, rhs_src, scalar_rhs_ty);

    const maybe_lhs_val = try sema.resolveValueResolveLazy(lhs);
    const maybe_rhs_val = try sema.resolveValueResolveLazy(rhs);

    if (maybe_rhs_val) |rhs_val| {
        if (rhs_val.isUndef(zcu)) {
            return pt.undefRef(sema.typeOf(lhs));
        }
        // If rhs is 0, return lhs without doing any calculations.
        if (try rhs_val.compareAllWithZeroSema(.eq, pt)) {
            return lhs;
        }
        if (air_tag != .shl_sat and scalar_ty.zigTypeTag(zcu) != .comptime_int) {
            const bit_value = try pt.intValue(.comptime_int, scalar_ty.intInfo(zcu).bits);
            if (rhs_ty.zigTypeTag(zcu) == .vector) {
                var i: usize = 0;
                while (i < rhs_ty.vectorLen(zcu)) : (i += 1) {
                    const rhs_elem = try rhs_val.elemValue(pt, i);
                    if (rhs_elem.compareHetero(.gte, bit_value, zcu)) {
                        return sema.fail(block, rhs_src, "shift amount '{}' at index '{d}' is too large for operand type '{}'", .{
                            rhs_elem.fmtValueSema(pt, sema),
                            i,
                            scalar_ty.fmt(pt),
                        });
                    }
                }
            } else if (rhs_val.compareHetero(.gte, bit_value, zcu)) {
                return sema.fail(block, rhs_src, "shift amount '{}' is too large for operand type '{}'", .{
                    rhs_val.fmtValueSema(pt, sema),
                    scalar_ty.fmt(pt),
                });
            }
        }
        if (rhs_ty.zigTypeTag(zcu) == .vector) {
            var i: usize = 0;
            while (i < rhs_ty.vectorLen(zcu)) : (i += 1) {
                const rhs_elem = try rhs_val.elemValue(pt, i);
                if (rhs_elem.compareHetero(.lt, try pt.intValue(scalar_rhs_ty, 0), zcu)) {
                    return sema.fail(block, rhs_src, "shift by negative amount '{}' at index '{d}'", .{
                        rhs_elem.fmtValueSema(pt, sema),
                        i,
                    });
                }
            }
        } else if (rhs_val.compareHetero(.lt, try pt.intValue(rhs_ty, 0), zcu)) {
            return sema.fail(block, rhs_src, "shift by negative amount '{}'", .{
                rhs_val.fmtValueSema(pt, sema),
            });
        }
    } else if (scalar_rhs_ty.isSignedInt(zcu)) {
        return sema.fail(block, rhs_src, "shift by signed type '{}'", .{rhs_ty.fmt(pt)});
    }

    const runtime_src = if (maybe_lhs_val) |lhs_val| rs: {
        if (lhs_val.isUndef(zcu)) return pt.undefRef(lhs_ty);
        const rhs_val = maybe_rhs_val orelse {
            if (scalar_ty.zigTypeTag(zcu) == .comptime_int) {
                return sema.fail(block, src, "LHS of shift must be a fixed-width integer type, or RHS must be comptime-known", .{});
            }
            break :rs rhs_src;
        };
        const val = if (scalar_ty.zigTypeTag(zcu) == .comptime_int)
            try lhs_val.shl(rhs_val, lhs_ty, sema.arena, pt)
        else switch (air_tag) {
            .shl_exact => val: {
                const shifted = try lhs_val.shlWithOverflow(rhs_val, lhs_ty, sema.arena, pt);
                if (shifted.overflow_bit.compareAllWithZero(.eq, zcu)) {
                    break :val shifted.wrapped_result;
                }
                return sema.fail(block, src, "operation caused overflow", .{});
            },
            .shl_sat => try lhs_val.shlSat(rhs_val, lhs_ty, sema.arena, pt),
            .shl => try lhs_val.shlTrunc(rhs_val, lhs_ty, sema.arena, pt),
            else => unreachable,
        };
        return Air.internedToRef(val.toIntern());
    } else lhs_src;

    const rt_rhs = switch (air_tag) {
        else => unreachable,
        .shl, .shl_exact => rhs,
        // The backend can handle a large runtime rhs better than we can, but
        // we can limit a large comptime rhs better here. This also has the
        // necessary side effect of preventing rhs from being a `comptime_int`.
        .shl_sat => if (maybe_rhs_val) |rhs_val| Air.internedToRef(rt_rhs: {
            const bit_count = scalar_ty.intInfo(zcu).bits;
            const rt_rhs_scalar_ty = try pt.smallestUnsignedInt(bit_count);
            if (!rhs_ty.isVector(zcu)) break :rt_rhs (try pt.intValue(
                rt_rhs_scalar_ty,
                @min(try rhs_val.getUnsignedIntSema(pt) orelse bit_count, bit_count),
            )).toIntern();
            const rhs_len = rhs_ty.vectorLen(zcu);
            const rhs_elems = try sema.arena.alloc(InternPool.Index, rhs_len);
            for (rhs_elems, 0..) |*rhs_elem, i| rhs_elem.* = (try pt.intValue(
                rt_rhs_scalar_ty,
                @min(try (try rhs_val.elemValue(pt, i)).getUnsignedIntSema(pt) orelse bit_count, bit_count),
            )).toIntern();
            break :rt_rhs try pt.intern(.{ .aggregate = .{
                .ty = (try pt.vectorType(.{
                    .len = rhs_len,
                    .child = rt_rhs_scalar_ty.toIntern(),
                })).toIntern(),
                .storage = .{ .elems = rhs_elems },
            } });
        }) else rhs,
    };

    try sema.requireRuntimeBlock(block, src, runtime_src);
    if (block.wantSafety()) {
        const bit_count = scalar_ty.intInfo(zcu).bits;
        if (!std.math.isPowerOfTwo(bit_count)) {
            const bit_count_val = try pt.intValue(scalar_rhs_ty, bit_count);
            const ok = if (rhs_ty.zigTypeTag(zcu) == .vector) ok: {
                const bit_count_inst = Air.internedToRef((try sema.splat(rhs_ty, bit_count_val)).toIntern());
                const lt = try block.addCmpVector(rhs, bit_count_inst, .lt);
                break :ok try block.addReduce(lt, .And);
            } else ok: {
                const bit_count_inst = Air.internedToRef(bit_count_val.toIntern());
                break :ok try block.addBinOp(.cmp_lt, rhs, bit_count_inst);
            };
            try sema.addSafetyCheck(block, src, ok, .shift_rhs_too_big);
        }

        if (air_tag == .shl_exact) {
            const op_ov_tuple_ty = try pt.overflowArithmeticTupleType(lhs_ty);
            const op_ov = try block.addInst(.{
                .tag = .shl_with_overflow,
                .data = .{ .ty_pl = .{
                    .ty = Air.internedToRef(op_ov_tuple_ty.toIntern()),
                    .payload = try sema.addExtra(Air.Bin{
                        .lhs = lhs,
                        .rhs = rhs,
                    }),
                } },
            });
            const ov_bit = try sema.tupleFieldValByIndex(block, op_ov, 1, op_ov_tuple_ty);
            const any_ov_bit = if (lhs_ty.zigTypeTag(zcu) == .vector)
                try block.addReduce(ov_bit, .Or)
            else
                ov_bit;
            const no_ov = try block.addBinOp(.cmp_eq, any_ov_bit, .zero_u1);

            try sema.addSafetyCheck(block, src, no_ov, .shl_overflow);
            return sema.tupleFieldValByIndex(block, op_ov, 0, op_ov_tuple_ty);
        }
    }
    return block.addBinOp(air_tag, lhs, rt_rhs);
}

fn zirShr(
    sema: *Sema,
    block: *Block,
    inst: Zir.Inst.Index,
    air_tag: Air.Inst.Tag,
) CompileError!Air.Inst.Ref {
    const tracy = trace(@src());
    defer tracy.end();

    const pt = sema.pt;
    const zcu = pt.zcu;
    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].pl_node;
    const extra = sema.code.extraData(Zir.Inst.Bin, inst_data.payload_index).data;
    const lhs = try sema.resolveInst(extra.lhs);
    const rhs = try sema.resolveInst(extra.rhs);
    const lhs_ty = sema.typeOf(lhs);
    const rhs_ty = sema.typeOf(rhs);

    const src = block.nodeOffset(inst_data.src_node);
    const lhs_src = switch (air_tag) {
        .shr => block.src(.{ .node_offset_bin_lhs = inst_data.src_node }),
        .shr_exact => block.builtinCallArgSrc(inst_data.src_node, 0),
        else => unreachable,
    };
    const rhs_src = switch (air_tag) {
        .shr => block.src(.{ .node_offset_bin_rhs = inst_data.src_node }),
        .shr_exact => block.builtinCallArgSrc(inst_data.src_node, 1),
        else => unreachable,
    };

    try sema.checkVectorizableBinaryOperands(block, src, lhs_ty, rhs_ty, lhs_src, rhs_src);
    const scalar_ty = lhs_ty.scalarType(zcu);

    const maybe_lhs_val = try sema.resolveValueResolveLazy(lhs);
    const maybe_rhs_val = try sema.resolveValueResolveLazy(rhs);

    const runtime_src = if (maybe_rhs_val) |rhs_val| rs: {
        if (rhs_val.isUndef(zcu)) {
            return pt.undefRef(lhs_ty);
        }
        // If rhs is 0, return lhs without doing any calculations.
        if (try rhs_val.compareAllWithZeroSema(.eq, pt)) {
            return lhs;
        }
        if (scalar_ty.zigTypeTag(zcu) != .comptime_int) {
            const bit_value = try pt.intValue(.comptime_int, scalar_ty.intInfo(zcu).bits);
            if (rhs_ty.zigTypeTag(zcu) == .vector) {
                var i: usize = 0;
                while (i < rhs_ty.vectorLen(zcu)) : (i += 1) {
                    const rhs_elem = try rhs_val.elemValue(pt, i);
                    if (rhs_elem.compareHetero(.gte, bit_value, zcu)) {
                        return sema.fail(block, rhs_src, "shift amount '{}' at index '{d}' is too large for operand type '{}'", .{
                            rhs_elem.fmtValueSema(pt, sema),
                            i,
                            scalar_ty.fmt(pt),
                        });
                    }
                }
            } else if (rhs_val.compareHetero(.gte, bit_value, zcu)) {
                return sema.fail(block, rhs_src, "shift amount '{}' is too large for operand type '{}'", .{
                    rhs_val.fmtValueSema(pt, sema),
                    scalar_ty.fmt(pt),
                });
            }
        }
        if (rhs_ty.zigTypeTag(zcu) == .vector) {
            var i: usize = 0;
            while (i < rhs_ty.vectorLen(zcu)) : (i += 1) {
                const rhs_elem = try rhs_val.elemValue(pt, i);
                if (rhs_elem.compareHetero(.lt, try pt.intValue(rhs_ty.childType(zcu), 0), zcu)) {
                    return sema.fail(block, rhs_src, "shift by negative amount '{}' at index '{d}'", .{
                        rhs_elem.fmtValueSema(pt, sema),
                        i,
                    });
                }
            }
        } else if (rhs_val.compareHetero(.lt, try pt.intValue(rhs_ty, 0), zcu)) {
            return sema.fail(block, rhs_src, "shift by negative amount '{}'", .{
                rhs_val.fmtValueSema(pt, sema),
            });
        }
        if (maybe_lhs_val) |lhs_val| {
            if (lhs_val.isUndef(zcu)) {
                return pt.undefRef(lhs_ty);
            }
            if (air_tag == .shr_exact) {
                // Detect if any ones would be shifted out.
                const truncated = try lhs_val.intTruncBitsAsValue(lhs_ty, sema.arena, .unsigned, rhs_val, pt);
                if (!(try truncated.compareAllWithZeroSema(.eq, pt))) {
                    return sema.fail(block, src, "exact shift shifted out 1 bits", .{});
                }
            }
            const val = try lhs_val.shr(rhs_val, lhs_ty, sema.arena, pt);
            return Air.internedToRef(val.toIntern());
        } else {
            break :rs lhs_src;
        }
    } else rhs_src;

    if (maybe_rhs_val == null and scalar_ty.zigTypeTag(zcu) == .comptime_int) {
        return sema.fail(block, src, "LHS of shift must be a fixed-width integer type, or RHS must be comptime-known", .{});
    }

    try sema.requireRuntimeBlock(block, src, runtime_src);
    const result = try block.addBinOp(air_tag, lhs, rhs);
    if (block.wantSafety()) {
        const bit_count = scalar_ty.intInfo(zcu).bits;
        if (!std.math.isPowerOfTwo(bit_count)) {
            const bit_count_val = try pt.intValue(rhs_ty.scalarType(zcu), bit_count);

            const ok = if (rhs_ty.zigTypeTag(zcu) == .vector) ok: {
                const bit_count_inst = Air.internedToRef((try sema.splat(rhs_ty, bit_count_val)).toIntern());
                const lt = try block.addCmpVector(rhs, bit_count_inst, .lt);
                break :ok try block.addReduce(lt, .And);
            } else ok: {
                const bit_count_inst = Air.internedToRef(bit_count_val.toIntern());
                break :ok try block.addBinOp(.cmp_lt, rhs, bit_count_inst);
            };
            try sema.addSafetyCheck(block, src, ok, .shift_rhs_too_big);
        }

        if (air_tag == .shr_exact) {
            const back = try block.addBinOp(.shl, result, rhs);

            const ok = if (rhs_ty.zigTypeTag(zcu) == .vector) ok: {
                const eql = try block.addCmpVector(lhs, back, .eq);
                break :ok try block.addReduce(eql, .And);
            } else try block.addBinOp(.cmp_eq, lhs, back);
            try sema.addSafetyCheck(block, src, ok, .shr_overflow);
        }
    }
    return result;
}

fn zirBitwise(
    sema: *Sema,
    block: *Block,
    inst: Zir.Inst.Index,
    air_tag: Air.Inst.Tag,
) CompileError!Air.Inst.Ref {
    const tracy = trace(@src());
    defer tracy.end();

    const pt = sema.pt;
    const zcu = pt.zcu;
    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].pl_node;
    const src = block.src(.{ .node_offset_bin_op = inst_data.src_node });
    const lhs_src = block.src(.{ .node_offset_bin_lhs = inst_data.src_node });
    const rhs_src = block.src(.{ .node_offset_bin_rhs = inst_data.src_node });
    const extra = sema.code.extraData(Zir.Inst.Bin, inst_data.payload_index).data;
    const lhs = try sema.resolveInst(extra.lhs);
    const rhs = try sema.resolveInst(extra.rhs);
    const lhs_ty = sema.typeOf(lhs);
    const rhs_ty = sema.typeOf(rhs);
    try sema.checkVectorizableBinaryOperands(block, src, lhs_ty, rhs_ty, lhs_src, rhs_src);

    const instructions = &[_]Air.Inst.Ref{ lhs, rhs };
    const resolved_type = try sema.resolvePeerTypes(block, src, instructions, .{ .override = &[_]?LazySrcLoc{ lhs_src, rhs_src } });
    const scalar_type = resolved_type.scalarType(zcu);
    const scalar_tag = scalar_type.zigTypeTag(zcu);

    const casted_lhs = try sema.coerce(block, resolved_type, lhs, lhs_src);
    const casted_rhs = try sema.coerce(block, resolved_type, rhs, rhs_src);

    const is_int = scalar_tag == .int or scalar_tag == .comptime_int;

    if (!is_int) {
        return sema.fail(block, src, "invalid operands to binary bitwise expression: '{s}' and '{s}'", .{ @tagName(lhs_ty.zigTypeTag(zcu)), @tagName(rhs_ty.zigTypeTag(zcu)) });
    }

    const runtime_src = runtime: {
        // TODO: ask the linker what kind of relocations are available, and
        // in some cases emit a Value that means "this decl's address AND'd with this operand".
        if (try sema.resolveValueResolveLazy(casted_lhs)) |lhs_val| {
            if (try sema.resolveValueResolveLazy(casted_rhs)) |rhs_val| {
                const result_val = switch (air_tag) {
                    .bit_and => try lhs_val.bitwiseAnd(rhs_val, resolved_type, sema.arena, pt),
                    .bit_or => try lhs_val.bitwiseOr(rhs_val, resolved_type, sema.arena, pt),
                    .xor => try lhs_val.bitwiseXor(rhs_val, resolved_type, sema.arena, pt),
                    else => unreachable,
                };
                return Air.internedToRef(result_val.toIntern());
            } else {
                break :runtime rhs_src;
            }
        } else {
            break :runtime lhs_src;
        }
    };

    try sema.requireRuntimeBlock(block, src, runtime_src);
    return block.addBinOp(air_tag, casted_lhs, casted_rhs);
}

fn zirBitNot(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    const tracy = trace(@src());
    defer tracy.end();

    const pt = sema.pt;
    const zcu = pt.zcu;
    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].un_node;
    const src = block.nodeOffset(inst_data.src_node);
    const operand_src = block.src(.{ .node_offset_un_op = inst_data.src_node });

    const operand = try sema.resolveInst(inst_data.operand);
    const operand_type = sema.typeOf(operand);
    const scalar_type = operand_type.scalarType(zcu);

    if (scalar_type.zigTypeTag(zcu) != .int) {
        return sema.fail(block, src, "unable to perform binary not operation on type '{}'", .{
            operand_type.fmt(pt),
        });
    }

    if (try sema.resolveValue(operand)) |val| {
        if (val.isUndef(zcu)) {
            return pt.undefRef(operand_type);
        } else if (operand_type.zigTypeTag(zcu) == .vector) {
            const vec_len = try sema.usizeCast(block, operand_src, operand_type.vectorLen(zcu));
            const elems = try sema.arena.alloc(InternPool.Index, vec_len);
            for (elems, 0..) |*elem, i| {
                const elem_val = try val.elemValue(pt, i);
                elem.* = (try elem_val.bitwiseNot(scalar_type, sema.arena, pt)).toIntern();
            }
            return Air.internedToRef((try pt.intern(.{ .aggregate = .{
                .ty = operand_type.toIntern(),
                .storage = .{ .elems = elems },
            } })));
        } else {
            const result_val = try val.bitwiseNot(operand_type, sema.arena, pt);
            return Air.internedToRef(result_val.toIntern());
        }
    }

    try sema.requireRuntimeBlock(block, src, null);
    return block.addTyOp(.not, operand_type, operand);
}

fn analyzeTupleCat(
    sema: *Sema,
    block: *Block,
    src_node: std.zig.Ast.Node.Offset,
    lhs: Air.Inst.Ref,
    rhs: Air.Inst.Ref,
) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const lhs_ty = sema.typeOf(lhs);
    const rhs_ty = sema.typeOf(rhs);
    const src = block.nodeOffset(src_node);

    const lhs_len = lhs_ty.structFieldCount(zcu);
    const rhs_len = rhs_ty.structFieldCount(zcu);
    const dest_fields = lhs_len + rhs_len;

    if (dest_fields == 0) {
        return .empty_tuple;
    }
    if (lhs_len == 0) {
        return rhs;
    }
    if (rhs_len == 0) {
        return lhs;
    }
    const final_len = try sema.usizeCast(block, src, dest_fields);

    const types = try sema.arena.alloc(InternPool.Index, final_len);
    const values = try sema.arena.alloc(InternPool.Index, final_len);

    const opt_runtime_src = rs: {
        var runtime_src: ?LazySrcLoc = null;
        var i: u32 = 0;
        while (i < lhs_len) : (i += 1) {
            types[i] = lhs_ty.fieldType(i, zcu).toIntern();
            const default_val = lhs_ty.structFieldDefaultValue(i, zcu);
            values[i] = default_val.toIntern();
            const operand_src = block.src(.{ .array_cat_lhs = .{
                .array_cat_offset = src_node,
                .elem_index = i,
            } });
            if (default_val.toIntern() == .unreachable_value) {
                runtime_src = operand_src;
                values[i] = .none;
            }
        }
        i = 0;
        while (i < rhs_len) : (i += 1) {
            types[i + lhs_len] = rhs_ty.fieldType(i, zcu).toIntern();
            const default_val = rhs_ty.structFieldDefaultValue(i, zcu);
            values[i + lhs_len] = default_val.toIntern();
            const operand_src = block.src(.{ .array_cat_rhs = .{
                .array_cat_offset = src_node,
                .elem_index = i,
            } });
            if (default_val.toIntern() == .unreachable_value) {
                runtime_src = operand_src;
                values[i + lhs_len] = .none;
            }
        }
        break :rs runtime_src;
    };

    const tuple_ty = try zcu.intern_pool.getTupleType(zcu.gpa, pt.tid, .{
        .types = types,
        .values = values,
    });

    const runtime_src = opt_runtime_src orelse {
        const tuple_val = try pt.intern(.{ .aggregate = .{
            .ty = tuple_ty,
            .storage = .{ .elems = values },
        } });
        return Air.internedToRef(tuple_val);
    };

    try sema.requireRuntimeBlock(block, src, runtime_src);

    const element_refs = try sema.arena.alloc(Air.Inst.Ref, final_len);
    var i: u32 = 0;
    while (i < lhs_len) : (i += 1) {
        element_refs[i] = try sema.tupleFieldValByIndex(block, lhs, i, lhs_ty);
    }
    i = 0;
    while (i < rhs_len) : (i += 1) {
        element_refs[i + lhs_len] =
            try sema.tupleFieldValByIndex(block, rhs, i, rhs_ty);
    }

    return block.addAggregateInit(.fromInterned(tuple_ty), element_refs);
}

fn zirArrayCat(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    const tracy = trace(@src());
    defer tracy.end();

    const pt = sema.pt;
    const zcu = pt.zcu;
    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].pl_node;
    const extra = sema.code.extraData(Zir.Inst.Bin, inst_data.payload_index).data;
    const lhs = try sema.resolveInst(extra.lhs);
    const rhs = try sema.resolveInst(extra.rhs);
    const lhs_ty = sema.typeOf(lhs);
    const rhs_ty = sema.typeOf(rhs);
    const src = block.nodeOffset(inst_data.src_node);

    const lhs_is_tuple = lhs_ty.isTuple(zcu);
    const rhs_is_tuple = rhs_ty.isTuple(zcu);
    if (lhs_is_tuple and rhs_is_tuple) {
        return sema.analyzeTupleCat(block, inst_data.src_node, lhs, rhs);
    }

    const lhs_src = block.src(.{ .node_offset_bin_lhs = inst_data.src_node });
    const rhs_src = block.src(.{ .node_offset_bin_rhs = inst_data.src_node });

    const lhs_info = try sema.getArrayCatInfo(block, lhs_src, lhs, rhs_ty) orelse lhs_info: {
        if (lhs_is_tuple) break :lhs_info undefined;
        return sema.fail(block, lhs_src, "expected indexable; found '{}'", .{lhs_ty.fmt(pt)});
    };
    const rhs_info = try sema.getArrayCatInfo(block, rhs_src, rhs, lhs_ty) orelse {
        assert(!rhs_is_tuple);
        return sema.fail(block, rhs_src, "expected indexable; found '{}'", .{rhs_ty.fmt(pt)});
    };

    const resolved_elem_ty = t: {
        var trash_block = block.makeSubBlock();
        trash_block.comptime_reason = null;
        defer trash_block.instructions.deinit(sema.gpa);

        const instructions = [_]Air.Inst.Ref{
            try trash_block.addBitCast(lhs_info.elem_type, .void_value),
            try trash_block.addBitCast(rhs_info.elem_type, .void_value),
        };
        break :t try sema.resolvePeerTypes(block, src, &instructions, .{
            .override = &[_]?LazySrcLoc{ lhs_src, rhs_src },
        });
    };

    // When there is a sentinel mismatch, no sentinel on the result.
    // Otherwise, use the sentinel value provided by either operand,
    // coercing it to the peer-resolved element type.
    const res_sent_val: ?Value = s: {
        if (lhs_info.sentinel) |lhs_sent_val| {
            const lhs_sent = Air.internedToRef(lhs_sent_val.toIntern());
            if (rhs_info.sentinel) |rhs_sent_val| {
                const rhs_sent = Air.internedToRef(rhs_sent_val.toIntern());
                const lhs_sent_casted = try sema.coerce(block, resolved_elem_ty, lhs_sent, lhs_src);
                const rhs_sent_casted = try sema.coerce(block, resolved_elem_ty, rhs_sent, rhs_src);
                const lhs_sent_casted_val = (try sema.resolveDefinedValue(block, lhs_src, lhs_sent_casted)).?;
                const rhs_sent_casted_val = (try sema.resolveDefinedValue(block, rhs_src, rhs_sent_casted)).?;
                if (try sema.valuesEqual(lhs_sent_casted_val, rhs_sent_casted_val, resolved_elem_ty)) {
                    break :s lhs_sent_casted_val;
                } else {
                    break :s null;
                }
            } else {
                const lhs_sent_casted = try sema.coerce(block, resolved_elem_ty, lhs_sent, lhs_src);
                const lhs_sent_casted_val = (try sema.resolveDefinedValue(block, lhs_src, lhs_sent_casted)).?;
                break :s lhs_sent_casted_val;
            }
        } else {
            if (rhs_info.sentinel) |rhs_sent_val| {
                const rhs_sent = Air.internedToRef(rhs_sent_val.toIntern());
                const rhs_sent_casted = try sema.coerce(block, resolved_elem_ty, rhs_sent, rhs_src);
                const rhs_sent_casted_val = (try sema.resolveDefinedValue(block, rhs_src, rhs_sent_casted)).?;
                break :s rhs_sent_casted_val;
            } else {
                break :s null;
            }
        }
    };

    const lhs_len = try sema.usizeCast(block, lhs_src, lhs_info.len);
    const rhs_len = try sema.usizeCast(block, rhs_src, rhs_info.len);
    const result_len = std.math.add(usize, lhs_len, rhs_len) catch |err| switch (err) {
        error.Overflow => return sema.fail(
            block,
            src,
            "concatenating arrays of length {d} and {d} produces an array too large for this compiler implementation to handle",
            .{ lhs_len, rhs_len },
        ),
    };

    const result_ty = try pt.arrayType(.{
        .len = result_len,
        .sentinel = if (res_sent_val) |v| v.toIntern() else .none,
        .child = resolved_elem_ty.toIntern(),
    });
    const ptr_addrspace = p: {
        if (lhs_ty.zigTypeTag(zcu) == .pointer) break :p lhs_ty.ptrAddressSpace(zcu);
        if (rhs_ty.zigTypeTag(zcu) == .pointer) break :p rhs_ty.ptrAddressSpace(zcu);
        break :p null;
    };

    const runtime_src = if (switch (lhs_ty.zigTypeTag(zcu)) {
        .array, .@"struct" => try sema.resolveValue(lhs),
        .pointer => try sema.resolveDefinedValue(block, lhs_src, lhs),
        else => unreachable,
    }) |lhs_val| rs: {
        if (switch (rhs_ty.zigTypeTag(zcu)) {
            .array, .@"struct" => try sema.resolveValue(rhs),
            .pointer => try sema.resolveDefinedValue(block, rhs_src, rhs),
            else => unreachable,
        }) |rhs_val| {
            const lhs_sub_val = if (lhs_ty.isSinglePointer(zcu))
                try sema.pointerDeref(block, lhs_src, lhs_val, lhs_ty) orelse break :rs lhs_src
            else if (lhs_ty.isSlice(zcu))
                try sema.maybeDerefSliceAsArray(block, lhs_src, lhs_val) orelse break :rs lhs_src
            else
                lhs_val;

            const rhs_sub_val = if (rhs_ty.isSinglePointer(zcu))
                try sema.pointerDeref(block, rhs_src, rhs_val, rhs_ty) orelse break :rs rhs_src
            else if (rhs_ty.isSlice(zcu))
                try sema.maybeDerefSliceAsArray(block, rhs_src, rhs_val) orelse break :rs rhs_src
            else
                rhs_val;

            const element_vals = try sema.arena.alloc(InternPool.Index, result_len);
            var elem_i: u32 = 0;
            while (elem_i < lhs_len) : (elem_i += 1) {
                const lhs_elem_i = elem_i;
                const elem_default_val = if (lhs_is_tuple) lhs_ty.structFieldDefaultValue(lhs_elem_i, zcu) else Value.@"unreachable";
                const elem_val = if (elem_default_val.toIntern() == .unreachable_value) try lhs_sub_val.elemValue(pt, lhs_elem_i) else elem_default_val;
                const elem_val_inst = Air.internedToRef(elem_val.toIntern());
                const operand_src = block.src(.{ .array_cat_lhs = .{
                    .array_cat_offset = inst_data.src_node,
                    .elem_index = elem_i,
                } });
                const coerced_elem_val_inst = try sema.coerce(block, resolved_elem_ty, elem_val_inst, operand_src);
                const coerced_elem_val = try sema.resolveConstValue(block, operand_src, coerced_elem_val_inst, undefined);
                element_vals[elem_i] = coerced_elem_val.toIntern();
            }
            while (elem_i < result_len) : (elem_i += 1) {
                const rhs_elem_i = elem_i - lhs_len;
                const elem_default_val = if (rhs_is_tuple) rhs_ty.structFieldDefaultValue(rhs_elem_i, zcu) else Value.@"unreachable";
                const elem_val = if (elem_default_val.toIntern() == .unreachable_value) try rhs_sub_val.elemValue(pt, rhs_elem_i) else elem_default_val;
                const elem_val_inst = Air.internedToRef(elem_val.toIntern());
                const operand_src = block.src(.{ .array_cat_rhs = .{
                    .array_cat_offset = inst_data.src_node,
                    .elem_index = @intCast(rhs_elem_i),
                } });
                const coerced_elem_val_inst = try sema.coerce(block, resolved_elem_ty, elem_val_inst, operand_src);
                const coerced_elem_val = try sema.resolveConstValue(block, operand_src, coerced_elem_val_inst, undefined);
                element_vals[elem_i] = coerced_elem_val.toIntern();
            }
            return sema.addConstantMaybeRef(try pt.intern(.{ .aggregate = .{
                .ty = result_ty.toIntern(),
                .storage = .{ .elems = element_vals },
            } }), ptr_addrspace != null);
        } else break :rs rhs_src;
    } else lhs_src;

    try sema.requireRuntimeBlock(block, src, runtime_src);

    if (ptr_addrspace) |ptr_as| {
        const constant_alloc_ty = try pt.ptrTypeSema(.{
            .child = result_ty.toIntern(),
            .flags = .{
                .address_space = ptr_as,
                .is_const = true,
            },
        });
        const alloc_ty = try pt.ptrTypeSema(.{
            .child = result_ty.toIntern(),
            .flags = .{ .address_space = ptr_as },
        });
        const elem_ptr_ty = try pt.ptrTypeSema(.{
            .child = resolved_elem_ty.toIntern(),
            .flags = .{ .address_space = ptr_as },
        });

        const mutable_alloc = try block.addTy(.alloc, alloc_ty);

        // if both the source and destination are arrays
        // we can hotpath via a memcpy.
        if (lhs_ty.zigTypeTag(zcu) == .pointer and
            rhs_ty.zigTypeTag(zcu) == .pointer)
        {
            const slice_ty = try pt.ptrTypeSema(.{
                .child = resolved_elem_ty.toIntern(),
                .flags = .{
                    .size = .slice,
                    .address_space = ptr_as,
                },
            });

            const many_ty = slice_ty.slicePtrFieldType(zcu);
            const many_alloc = try block.addBitCast(many_ty, mutable_alloc);

            // lhs_dest_slice = dest[0..lhs.len]
            const slice_ty_ref = Air.internedToRef(slice_ty.toIntern());
            const lhs_len_ref = try pt.intRef(.usize, lhs_len);
            const lhs_dest_slice = try block.addInst(.{
                .tag = .slice,
                .data = .{ .ty_pl = .{
                    .ty = slice_ty_ref,
                    .payload = try sema.addExtra(Air.Bin{
                        .lhs = many_alloc,
                        .rhs = lhs_len_ref,
                    }),
                } },
            });

            _ = try block.addBinOp(.memcpy, lhs_dest_slice, lhs);

            // rhs_dest_slice = dest[lhs.len..][0..rhs.len]
            const rhs_len_ref = try pt.intRef(.usize, rhs_len);
            const rhs_dest_offset = try block.addInst(.{
                .tag = .ptr_add,
                .data = .{ .ty_pl = .{
                    .ty = Air.internedToRef(many_ty.toIntern()),
                    .payload = try sema.addExtra(Air.Bin{
                        .lhs = many_alloc,
                        .rhs = lhs_len_ref,
                    }),
                } },
            });
            const rhs_dest_slice = try block.addInst(.{
                .tag = .slice,
                .data = .{ .ty_pl = .{
                    .ty = slice_ty_ref,
                    .payload = try sema.addExtra(Air.Bin{
                        .lhs = rhs_dest_offset,
                        .rhs = rhs_len_ref,
                    }),
                } },
            });

            _ = try block.addBinOp(.memcpy, rhs_dest_slice, rhs);

            if (res_sent_val) |sent_val| {
                const elem_index = try pt.intRef(.usize, result_len);
                const elem_ptr = try block.addPtrElemPtr(mutable_alloc, elem_index, elem_ptr_ty);
                const init = Air.internedToRef((try pt.getCoerced(sent_val, lhs_info.elem_type)).toIntern());
                try sema.storePtr2(block, src, elem_ptr, src, init, lhs_src, .store);
            }

            return block.addBitCast(constant_alloc_ty, mutable_alloc);
        }

        var elem_i: u32 = 0;
        while (elem_i < lhs_len) : (elem_i += 1) {
            const elem_index = try pt.intRef(.usize, elem_i);
            const elem_ptr = try block.addPtrElemPtr(mutable_alloc, elem_index, elem_ptr_ty);
            const operand_src = block.src(.{ .array_cat_lhs = .{
                .array_cat_offset = inst_data.src_node,
                .elem_index = elem_i,
            } });
            const init = try sema.elemVal(block, operand_src, lhs, elem_index, src, true);
            try sema.storePtr2(block, src, elem_ptr, src, init, operand_src, .store);
        }
        while (elem_i < result_len) : (elem_i += 1) {
            const rhs_elem_i = elem_i - lhs_len;
            const elem_index = try pt.intRef(.usize, elem_i);
            const rhs_index = try pt.intRef(.usize, rhs_elem_i);
            const elem_ptr = try block.addPtrElemPtr(mutable_alloc, elem_index, elem_ptr_ty);
            const operand_src = block.src(.{ .array_cat_rhs = .{
                .array_cat_offset = inst_data.src_node,
                .elem_index = @intCast(rhs_elem_i),
            } });
            const init = try sema.elemVal(block, operand_src, rhs, rhs_index, src, true);
            try sema.storePtr2(block, src, elem_ptr, src, init, operand_src, .store);
        }
        if (res_sent_val) |sent_val| {
            const elem_index = try pt.intRef(.usize, result_len);
            const elem_ptr = try block.addPtrElemPtr(mutable_alloc, elem_index, elem_ptr_ty);
            const init = Air.internedToRef((try pt.getCoerced(sent_val, lhs_info.elem_type)).toIntern());
            try sema.storePtr2(block, src, elem_ptr, src, init, lhs_src, .store);
        }

        return block.addBitCast(constant_alloc_ty, mutable_alloc);
    }

    const element_refs = try sema.arena.alloc(Air.Inst.Ref, result_len);
    {
        var elem_i: u32 = 0;
        while (elem_i < lhs_len) : (elem_i += 1) {
            const index = try pt.intRef(.usize, elem_i);
            const operand_src = block.src(.{ .array_cat_lhs = .{
                .array_cat_offset = inst_data.src_node,
                .elem_index = elem_i,
            } });
            const init = try sema.elemVal(block, operand_src, lhs, index, src, true);
            element_refs[elem_i] = try sema.coerce(block, resolved_elem_ty, init, operand_src);
        }
        while (elem_i < result_len) : (elem_i += 1) {
            const rhs_elem_i = elem_i - lhs_len;
            const index = try pt.intRef(.usize, rhs_elem_i);
            const operand_src = block.src(.{ .array_cat_rhs = .{
                .array_cat_offset = inst_data.src_node,
                .elem_index = @intCast(rhs_elem_i),
            } });
            const init = try sema.elemVal(block, operand_src, rhs, index, src, true);
            element_refs[elem_i] = try sema.coerce(block, resolved_elem_ty, init, operand_src);
        }
    }

    return block.addAggregateInit(result_ty, element_refs);
}

fn getArrayCatInfo(sema: *Sema, block: *Block, src: LazySrcLoc, operand: Air.Inst.Ref, peer_ty: Type) !?Type.ArrayInfo {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const operand_ty = sema.typeOf(operand);
    switch (operand_ty.zigTypeTag(zcu)) {
        .array => return operand_ty.arrayInfo(zcu),
        .pointer => {
            const ptr_info = operand_ty.ptrInfo(zcu);
            switch (ptr_info.flags.size) {
                .slice => {
                    const val = try sema.resolveConstDefinedValue(block, src, operand, .{ .simple = .slice_cat_operand });
                    return .{
                        .elem_type = .fromInterned(ptr_info.child),
                        .sentinel = switch (ptr_info.sentinel) {
                            .none => null,
                            else => Value.fromInterned(ptr_info.sentinel),
                        },
                        .len = try val.sliceLen(pt),
                    };
                },
                .one => {
                    if (Type.fromInterned(ptr_info.child).zigTypeTag(zcu) == .array) {
                        return Type.fromInterned(ptr_info.child).arrayInfo(zcu);
                    }
                },
                .c, .many => {},
            }
        },
        .@"struct" => {
            if (operand_ty.isTuple(zcu) and peer_ty.isIndexable(zcu)) {
                assert(!peer_ty.isTuple(zcu));
                return .{
                    .elem_type = peer_ty.elemType2(zcu),
                    .sentinel = null,
                    .len = operand_ty.arrayLen(zcu),
                };
            }
        },
        else => {},
    }
    return null;
}

fn analyzeTupleMul(
    sema: *Sema,
    block: *Block,
    src_node: std.zig.Ast.Node.Offset,
    operand: Air.Inst.Ref,
    factor: usize,
) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const operand_ty = sema.typeOf(operand);
    const src = block.nodeOffset(src_node);
    const len_src = block.src(.{ .node_offset_bin_rhs = src_node });

    const tuple_len = operand_ty.structFieldCount(zcu);
    const final_len = std.math.mul(usize, tuple_len, factor) catch
        return sema.fail(block, len_src, "operation results in overflow", .{});

    if (final_len == 0) {
        return .empty_tuple;
    }
    const types = try sema.arena.alloc(InternPool.Index, final_len);
    const values = try sema.arena.alloc(InternPool.Index, final_len);

    const opt_runtime_src = rs: {
        var runtime_src: ?LazySrcLoc = null;
        for (0..tuple_len) |i| {
            types[i] = operand_ty.fieldType(i, zcu).toIntern();
            values[i] = operand_ty.structFieldDefaultValue(i, zcu).toIntern();
            const operand_src = block.src(.{ .array_cat_lhs = .{
                .array_cat_offset = src_node,
                .elem_index = @intCast(i),
            } });
            if (values[i] == .unreachable_value) {
                runtime_src = operand_src;
                values[i] = .none; // TODO don't treat unreachable_value as special
            }
        }
        for (0..factor) |i| {
            mem.copyForwards(InternPool.Index, types[tuple_len * i ..], types[0..tuple_len]);
            mem.copyForwards(InternPool.Index, values[tuple_len * i ..], values[0..tuple_len]);
        }
        break :rs runtime_src;
    };

    const tuple_ty = try zcu.intern_pool.getTupleType(zcu.gpa, pt.tid, .{
        .types = types,
        .values = values,
    });

    const runtime_src = opt_runtime_src orelse {
        const tuple_val = try pt.intern(.{ .aggregate = .{
            .ty = tuple_ty,
            .storage = .{ .elems = values },
        } });
        return Air.internedToRef(tuple_val);
    };

    try sema.requireRuntimeBlock(block, src, runtime_src);

    const element_refs = try sema.arena.alloc(Air.Inst.Ref, final_len);
    var i: u32 = 0;
    while (i < tuple_len) : (i += 1) {
        element_refs[i] = try sema.tupleFieldValByIndex(block, operand, @intCast(i), operand_ty);
    }
    i = 1;
    while (i < factor) : (i += 1) {
        @memcpy(element_refs[tuple_len * i ..][0..tuple_len], element_refs[0..tuple_len]);
    }

    return block.addAggregateInit(.fromInterned(tuple_ty), element_refs);
}

fn zirArrayMul(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    const tracy = trace(@src());
    defer tracy.end();

    const pt = sema.pt;
    const zcu = pt.zcu;
    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].pl_node;
    const extra = sema.code.extraData(Zir.Inst.ArrayMul, inst_data.payload_index).data;
    const uncoerced_lhs = try sema.resolveInst(extra.lhs);
    const uncoerced_lhs_ty = sema.typeOf(uncoerced_lhs);
    const src: LazySrcLoc = block.nodeOffset(inst_data.src_node);
    const lhs_src = block.src(.{ .node_offset_bin_lhs = inst_data.src_node });
    const operator_src = block.src(.{ .node_offset_main_token = inst_data.src_node });
    const rhs_src = block.src(.{ .node_offset_bin_rhs = inst_data.src_node });

    const lhs, const lhs_ty = coerced_lhs: {
        // If we have a result type, we might be able to do this more efficiently
        // by coercing the LHS first. Specifically, if we want an array or vector
        // and have a tuple, coerce the tuple immediately.
        no_coerce: {
            if (extra.res_ty == .none) break :no_coerce;
            const res_ty = try sema.resolveTypeOrPoison(block, src, extra.res_ty) orelse break :no_coerce;
            if (!uncoerced_lhs_ty.isTuple(zcu)) break :no_coerce;
            const lhs_len = uncoerced_lhs_ty.structFieldCount(zcu);
            const lhs_dest_ty = switch (res_ty.zigTypeTag(zcu)) {
                else => break :no_coerce,
                .array => try pt.arrayType(.{
                    .child = res_ty.childType(zcu).toIntern(),
                    .len = lhs_len,
                    .sentinel = if (res_ty.sentinel(zcu)) |s| s.toIntern() else .none,
                }),
                .vector => try pt.vectorType(.{
                    .child = res_ty.childType(zcu).toIntern(),
                    .len = lhs_len,
                }),
            };
            // Attempt to coerce to this type, but don't emit an error if it fails. Instead,
            // just exit out of this path and let the usual error happen later, so that error
            // messages are consistent.
            const coerced = sema.coerceExtra(block, lhs_dest_ty, uncoerced_lhs, lhs_src, .{ .report_err = false }) catch |err| switch (err) {
                error.NotCoercible => break :no_coerce,
                else => |e| return e,
            };
            break :coerced_lhs .{ coerced, lhs_dest_ty };
        }
        break :coerced_lhs .{ uncoerced_lhs, uncoerced_lhs_ty };
    };

    if (lhs_ty.isTuple(zcu)) {
        // In `**` rhs must be comptime-known, but lhs can be runtime-known
        const factor = try sema.resolveInt(block, rhs_src, extra.rhs, .usize, .{ .simple = .array_mul_factor });
        const factor_casted = try sema.usizeCast(block, rhs_src, factor);
        return sema.analyzeTupleMul(block, inst_data.src_node, lhs, factor_casted);
    }

    // Analyze the lhs first, to catch the case that someone tried to do exponentiation
    const lhs_info = try sema.getArrayCatInfo(block, lhs_src, lhs, lhs_ty) orelse {
        const msg = msg: {
            const msg = try sema.errMsg(lhs_src, "expected indexable; found '{}'", .{lhs_ty.fmt(pt)});
            errdefer msg.destroy(sema.gpa);
            switch (lhs_ty.zigTypeTag(zcu)) {
                .int, .float, .comptime_float, .comptime_int, .vector => {
                    try sema.errNote(operator_src, msg, "this operator multiplies arrays; use std.math.pow for exponentiation", .{});
                },
                else => {},
            }
            break :msg msg;
        };
        return sema.failWithOwnedErrorMsg(block, msg);
    };

    // In `**` rhs must be comptime-known, but lhs can be runtime-known
    const factor = try sema.resolveInt(block, rhs_src, extra.rhs, .usize, .{ .simple = .array_mul_factor });

    const result_len_u64 = std.math.mul(u64, lhs_info.len, factor) catch
        return sema.fail(block, rhs_src, "operation results in overflow", .{});
    const result_len = try sema.usizeCast(block, src, result_len_u64);

    const result_ty = try pt.arrayType(.{
        .len = result_len,
        .sentinel = if (lhs_info.sentinel) |s| s.toIntern() else .none,
        .child = lhs_info.elem_type.toIntern(),
    });

    const ptr_addrspace = if (lhs_ty.zigTypeTag(zcu) == .pointer) lhs_ty.ptrAddressSpace(zcu) else null;
    const lhs_len = try sema.usizeCast(block, lhs_src, lhs_info.len);

    if (try sema.resolveDefinedValue(block, lhs_src, lhs)) |lhs_val| ct: {
        const lhs_sub_val = if (lhs_ty.isSinglePointer(zcu))
            try sema.pointerDeref(block, lhs_src, lhs_val, lhs_ty) orelse break :ct
        else if (lhs_ty.isSlice(zcu))
            try sema.maybeDerefSliceAsArray(block, lhs_src, lhs_val) orelse break :ct
        else
            lhs_val;

        const val = v: {
            // Optimization for the common pattern of a single element repeated N times, such
            // as zero-filling a byte array.
            if (lhs_len == 1 and lhs_info.sentinel == null) {
                const elem_val = try lhs_sub_val.elemValue(pt, 0);
                break :v try pt.intern(.{ .aggregate = .{
                    .ty = result_ty.toIntern(),
                    .storage = .{ .repeated_elem = elem_val.toIntern() },
                } });
            }

            const element_vals = try sema.arena.alloc(InternPool.Index, result_len);
            var elem_i: usize = 0;
            while (elem_i < result_len) {
                var lhs_i: usize = 0;
                while (lhs_i < lhs_len) : (lhs_i += 1) {
                    const elem_val = try lhs_sub_val.elemValue(pt, lhs_i);
                    element_vals[elem_i] = elem_val.toIntern();
                    elem_i += 1;
                }
            }
            break :v try pt.intern(.{ .aggregate = .{
                .ty = result_ty.toIntern(),
                .storage = .{ .elems = element_vals },
            } });
        };
        return sema.addConstantMaybeRef(val, ptr_addrspace != null);
    }

    try sema.requireRuntimeBlock(block, src, lhs_src);

    // Grab all the LHS values ahead of time, rather than repeatedly emitting instructions
    // to get the same elem values.
    const lhs_vals = try sema.arena.alloc(Air.Inst.Ref, lhs_len);
    for (lhs_vals, 0..) |*lhs_val, idx| {
        const idx_ref = try pt.intRef(.usize, idx);
        lhs_val.* = try sema.elemVal(block, lhs_src, lhs, idx_ref, src, false);
    }

    if (ptr_addrspace) |ptr_as| {
        const alloc_ty = try pt.ptrTypeSema(.{
            .child = result_ty.toIntern(),
            .flags = .{
                .address_space = ptr_as,
                .is_const = true,
            },
        });
        const alloc = try block.addTy(.alloc, alloc_ty);
        const elem_ptr_ty = try pt.ptrTypeSema(.{
            .child = lhs_info.elem_type.toIntern(),
            .flags = .{ .address_space = ptr_as },
        });

        var elem_i: usize = 0;
        while (elem_i < result_len) {
            for (lhs_vals) |lhs_val| {
                const elem_index = try pt.intRef(.usize, elem_i);
                const elem_ptr = try block.addPtrElemPtr(alloc, elem_index, elem_ptr_ty);
                try sema.storePtr2(block, src, elem_ptr, src, lhs_val, lhs_src, .store);
                elem_i += 1;
            }
        }
        if (lhs_info.sentinel) |sent_val| {
            const elem_index = try pt.intRef(.usize, result_len);
            const elem_ptr = try block.addPtrElemPtr(alloc, elem_index, elem_ptr_ty);
            const init = Air.internedToRef(sent_val.toIntern());
            try sema.storePtr2(block, src, elem_ptr, src, init, lhs_src, .store);
        }

        return alloc;
    }

    const element_refs = try sema.arena.alloc(Air.Inst.Ref, result_len);
    for (0..try sema.usizeCast(block, rhs_src, factor)) |i| {
        @memcpy(element_refs[i * lhs_len ..][0..lhs_len], lhs_vals);
    }
    return block.addAggregateInit(result_ty, element_refs);
}

fn zirNegate(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].un_node;
    const src = block.nodeOffset(inst_data.src_node);
    const lhs_src = src;
    const rhs_src = block.src(.{ .node_offset_un_op = inst_data.src_node });

    const rhs = try sema.resolveInst(inst_data.operand);
    const rhs_ty = sema.typeOf(rhs);
    const rhs_scalar_ty = rhs_ty.scalarType(zcu);

    if (rhs_scalar_ty.isUnsignedInt(zcu) or switch (rhs_scalar_ty.zigTypeTag(zcu)) {
        .int, .comptime_int, .float, .comptime_float => false,
        else => true,
    }) {
        return sema.fail(block, src, "negation of type '{}'", .{rhs_ty.fmt(pt)});
    }

    if (rhs_scalar_ty.isAnyFloat()) {
        // We handle float negation here to ensure negative zero is represented in the bits.
        if (try sema.resolveValue(rhs)) |rhs_val| {
            const result = try arith.negateFloat(sema, rhs_ty, rhs_val);
            return Air.internedToRef(result.toIntern());
        }
        try sema.requireRuntimeBlock(block, src, null);
        return block.addUnOp(if (block.float_mode == .optimized) .neg_optimized else .neg, rhs);
    }

    const lhs = Air.internedToRef((try sema.splat(rhs_ty, try pt.intValue(rhs_scalar_ty, 0))).toIntern());
    return sema.analyzeArithmetic(block, .sub, lhs, rhs, src, lhs_src, rhs_src, true);
}

fn zirNegateWrap(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].un_node;
    const src = block.nodeOffset(inst_data.src_node);
    const lhs_src = src;
    const rhs_src = block.src(.{ .node_offset_un_op = inst_data.src_node });

    const rhs = try sema.resolveInst(inst_data.operand);
    const rhs_ty = sema.typeOf(rhs);
    const rhs_scalar_ty = rhs_ty.scalarType(zcu);

    switch (rhs_scalar_ty.zigTypeTag(zcu)) {
        .int, .comptime_int, .float, .comptime_float => {},
        else => return sema.fail(block, src, "negation of type '{}'", .{rhs_ty.fmt(pt)}),
    }

    const lhs = Air.internedToRef((try sema.splat(rhs_ty, try pt.intValue(rhs_scalar_ty, 0))).toIntern());
    return sema.analyzeArithmetic(block, .subwrap, lhs, rhs, src, lhs_src, rhs_src, true);
}

fn zirArithmetic(
    sema: *Sema,
    block: *Block,
    inst: Zir.Inst.Index,
    zir_tag: Zir.Inst.Tag,
    safety: bool,
) CompileError!Air.Inst.Ref {
    const tracy = trace(@src());
    defer tracy.end();

    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].pl_node;
    const src = block.src(.{ .node_offset_bin_op = inst_data.src_node });
    const lhs_src = block.src(.{ .node_offset_bin_lhs = inst_data.src_node });
    const rhs_src = block.src(.{ .node_offset_bin_rhs = inst_data.src_node });
    const extra = sema.code.extraData(Zir.Inst.Bin, inst_data.payload_index).data;
    const lhs = try sema.resolveInst(extra.lhs);
    const rhs = try sema.resolveInst(extra.rhs);

    return sema.analyzeArithmetic(block, zir_tag, lhs, rhs, src, lhs_src, rhs_src, safety);
}

fn zirDiv(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].pl_node;
    const src = block.src(.{ .node_offset_bin_op = inst_data.src_node });
    const lhs_src = block.src(.{ .node_offset_bin_lhs = inst_data.src_node });
    const rhs_src = block.src(.{ .node_offset_bin_rhs = inst_data.src_node });
    const extra = sema.code.extraData(Zir.Inst.Bin, inst_data.payload_index).data;
    const lhs = try sema.resolveInst(extra.lhs);
    const rhs = try sema.resolveInst(extra.rhs);
    const lhs_ty = sema.typeOf(lhs);
    const rhs_ty = sema.typeOf(rhs);
    const lhs_zig_ty_tag = lhs_ty.zigTypeTag(zcu);
    const rhs_zig_ty_tag = rhs_ty.zigTypeTag(zcu);
    try sema.checkVectorizableBinaryOperands(block, src, lhs_ty, rhs_ty, lhs_src, rhs_src);
    try sema.checkInvalidPtrIntArithmetic(block, src, lhs_ty);

    const resolved_type = try sema.resolvePeerTypes(block, src, &.{ lhs, rhs }, .{
        .override = &.{ lhs_src, rhs_src },
    });

    const casted_lhs = try sema.coerce(block, resolved_type, lhs, lhs_src);
    const casted_rhs = try sema.coerce(block, resolved_type, rhs, rhs_src);

    const lhs_scalar_ty = lhs_ty.scalarType(zcu);
    const scalar_tag = resolved_type.scalarType(zcu).zigTypeTag(zcu);

    const is_int = scalar_tag == .int or scalar_tag == .comptime_int;

    try sema.checkArithmeticOp(block, src, scalar_tag, lhs_zig_ty_tag, rhs_zig_ty_tag, .div);

    const maybe_lhs_val = try sema.resolveValueResolveLazy(casted_lhs);
    const maybe_rhs_val = try sema.resolveValueResolveLazy(casted_rhs);

    if ((lhs_ty.zigTypeTag(zcu) == .comptime_float and rhs_ty.zigTypeTag(zcu) == .comptime_int) or
        (lhs_ty.zigTypeTag(zcu) == .comptime_int and rhs_ty.zigTypeTag(zcu) == .comptime_float))
    {
        // If it makes a difference whether we coerce to ints or floats before doing the division, error.
        // If lhs % rhs is 0, it doesn't matter.
        const lhs_val = maybe_lhs_val orelse unreachable;
        const rhs_val = maybe_rhs_val orelse unreachable;
        const rem = lhs_val.floatRem(rhs_val, resolved_type, sema.arena, pt) catch unreachable;
        if (!rem.compareAllWithZero(.eq, zcu)) {
            return sema.fail(
                block,
                src,
                "ambiguous coercion of division operands '{}' and '{}'; non-zero remainder '{}'",
                .{ lhs_ty.fmt(pt), rhs_ty.fmt(pt), rem.fmtValueSema(pt, sema) },
            );
        }
    }

    // TODO: emit compile error when .div is used on integers and there would be an
    // ambiguous result between div_floor and div_trunc.

    // The rules here are like those in `analyzeArithmetic`:
    //
    // * If both operands are comptime-known, call the corresponding function in `arith`.
    //   Inputs which would be IB at runtime are compile errors.
    //
    // * Otherwise, if one operand is comptime-known `undefined`, we either trigger a compile error
    //   or return `undefined`, depending on whether this operator can trigger IB.
    //
    // * No other comptime operand determines a comptime result, so remaining cases are runtime ops.

    const allow_div_zero = !is_int and
        resolved_type.toIntern() != .comptime_float_type and
        block.float_mode == .strict;

    if (maybe_lhs_val) |lhs_val| {
        if (maybe_rhs_val) |rhs_val| {
            const result = try arith.div(sema, block, resolved_type, lhs_val, rhs_val, src, lhs_src, rhs_src, .div);
            return Air.internedToRef(result.toIntern());
        }
        if (allow_div_zero) {
            if (lhs_val.isUndefDeep(zcu)) return pt.undefRef(resolved_type);
        } else {
            if (lhs_val.anyScalarIsUndef(zcu)) return sema.failWithUseOfUndef(block, lhs_src);
        }
    } else if (maybe_rhs_val) |rhs_val| {
        if (allow_div_zero) {
            if (rhs_val.isUndefDeep(zcu)) return pt.undefRef(resolved_type);
        } else {
            if (rhs_val.anyScalarIsUndef(zcu)) return sema.failWithUseOfUndef(block, rhs_src);
            if (rhs_val.anyScalarIsZero(zcu)) return sema.failWithDivideByZero(block, rhs_src);
        }
    }

    if (block.wantSafety()) {
        try sema.addDivIntOverflowSafety(block, src, resolved_type, lhs_scalar_ty, maybe_lhs_val, maybe_rhs_val, casted_lhs, casted_rhs, is_int);
        try sema.addDivByZeroSafety(block, src, resolved_type, maybe_rhs_val, casted_rhs, is_int);
    }

    const air_tag = if (is_int) blk: {
        if (lhs_ty.isSignedInt(zcu) or rhs_ty.isSignedInt(zcu)) {
            return sema.fail(
                block,
                src,
                "division with '{}' and '{}': signed integers must use @divTrunc, @divFloor, or @divExact",
                .{ lhs_ty.fmt(pt), rhs_ty.fmt(pt) },
            );
        }
        break :blk Air.Inst.Tag.div_trunc;
    } else switch (block.float_mode) {
        .optimized => Air.Inst.Tag.div_float_optimized,
        .strict => Air.Inst.Tag.div_float,
    };
    return block.addBinOp(air_tag, casted_lhs, casted_rhs);
}

fn zirDivExact(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].pl_node;
    const src = block.src(.{ .node_offset_bin_op = inst_data.src_node });
    const lhs_src = block.builtinCallArgSrc(inst_data.src_node, 0);
    const rhs_src = block.builtinCallArgSrc(inst_data.src_node, 1);
    const extra = sema.code.extraData(Zir.Inst.Bin, inst_data.payload_index).data;
    const lhs = try sema.resolveInst(extra.lhs);
    const rhs = try sema.resolveInst(extra.rhs);
    const lhs_ty = sema.typeOf(lhs);
    const rhs_ty = sema.typeOf(rhs);
    const lhs_zig_ty_tag = lhs_ty.zigTypeTag(zcu);
    const rhs_zig_ty_tag = rhs_ty.zigTypeTag(zcu);
    try sema.checkVectorizableBinaryOperands(block, src, lhs_ty, rhs_ty, lhs_src, rhs_src);
    try sema.checkInvalidPtrIntArithmetic(block, src, lhs_ty);

    const resolved_type = try sema.resolvePeerTypes(block, src, &.{ lhs, rhs }, .{
        .override = &.{ lhs_src, rhs_src },
    });

    const casted_lhs = try sema.coerce(block, resolved_type, lhs, lhs_src);
    const casted_rhs = try sema.coerce(block, resolved_type, rhs, rhs_src);

    const lhs_scalar_ty = lhs_ty.scalarType(zcu);
    const scalar_tag = resolved_type.scalarType(zcu).zigTypeTag(zcu);

    const is_int = scalar_tag == .int or scalar_tag == .comptime_int;

    try sema.checkArithmeticOp(block, src, scalar_tag, lhs_zig_ty_tag, rhs_zig_ty_tag, .div_exact);

    const maybe_lhs_val = try sema.resolveValueResolveLazy(casted_lhs);
    const maybe_rhs_val = try sema.resolveValueResolveLazy(casted_rhs);

    // Because `@divExact` can trigger Illegal Behavior, undefined operands trigger Illegal Behavior.

    if (maybe_lhs_val) |lhs_val| {
        if (maybe_rhs_val) |rhs_val| {
            const result = try arith.div(sema, block, resolved_type, lhs_val, rhs_val, src, lhs_src, rhs_src, .div_exact);
            return Air.internedToRef(result.toIntern());
        }
        if (lhs_val.anyScalarIsUndef(zcu)) return sema.failWithUseOfUndef(block, lhs_src);
    } else if (maybe_rhs_val) |rhs_val| {
        if (rhs_val.anyScalarIsUndef(zcu)) return sema.failWithUseOfUndef(block, rhs_src);
        if (rhs_val.anyScalarIsZero(zcu)) return sema.failWithDivideByZero(block, rhs_src);
    }

    // Depending on whether safety is enabled, we will have a slightly different strategy
    // here. The `div_exact` AIR instruction causes illegal behavior if a remainder
    // is produced, so in the safety check case, it cannot be used. Instead we do a
    // div_trunc and check for remainder.

    if (block.wantSafety()) {
        try sema.addDivIntOverflowSafety(block, src, resolved_type, lhs_scalar_ty, maybe_lhs_val, maybe_rhs_val, casted_lhs, casted_rhs, is_int);
        try sema.addDivByZeroSafety(block, src, resolved_type, maybe_rhs_val, casted_rhs, is_int);

        const result = try block.addBinOp(.div_trunc, casted_lhs, casted_rhs);
        const ok = if (!is_int) ok: {
            const floored = try block.addUnOp(.floor, result);

            if (resolved_type.zigTypeTag(zcu) == .vector) {
                const eql = try block.addCmpVector(result, floored, .eq);
                break :ok try block.addReduce(eql, .And);
            } else {
                const is_in_range = try block.addBinOp(switch (block.float_mode) {
                    .strict => .cmp_eq,
                    .optimized => .cmp_eq_optimized,
                }, result, floored);
                break :ok is_in_range;
            }
        } else ok: {
            const remainder = try block.addBinOp(.rem, casted_lhs, casted_rhs);

            const scalar_zero = switch (scalar_tag) {
                .comptime_float, .float => try pt.floatValue(resolved_type.scalarType(zcu), 0.0),
                .comptime_int, .int => try pt.intValue(resolved_type.scalarType(zcu), 0),
                else => unreachable,
            };
            if (resolved_type.zigTypeTag(zcu) == .vector) {
                const zero_val = try sema.splat(resolved_type, scalar_zero);
                const zero = Air.internedToRef(zero_val.toIntern());
                const eql = try block.addCmpVector(remainder, zero, .eq);
                break :ok try block.addReduce(eql, .And);
            } else {
                const zero = Air.internedToRef(scalar_zero.toIntern());
                const is_in_range = try block.addBinOp(.cmp_eq, remainder, zero);
                break :ok is_in_range;
            }
        };
        try sema.addSafetyCheck(block, src, ok, .exact_division_remainder);
        return result;
    }

    return block.addBinOp(airTag(block, is_int, .div_exact, .div_exact_optimized), casted_lhs, casted_rhs);
}

fn zirDivFloor(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].pl_node;
    const src = block.src(.{ .node_offset_bin_op = inst_data.src_node });
    const lhs_src = block.builtinCallArgSrc(inst_data.src_node, 0);
    const rhs_src = block.builtinCallArgSrc(inst_data.src_node, 1);
    const extra = sema.code.extraData(Zir.Inst.Bin, inst_data.payload_index).data;
    const lhs = try sema.resolveInst(extra.lhs);
    const rhs = try sema.resolveInst(extra.rhs);
    const lhs_ty = sema.typeOf(lhs);
    const rhs_ty = sema.typeOf(rhs);
    const lhs_zig_ty_tag = lhs_ty.zigTypeTag(zcu);
    const rhs_zig_ty_tag = rhs_ty.zigTypeTag(zcu);
    try sema.checkVectorizableBinaryOperands(block, src, lhs_ty, rhs_ty, lhs_src, rhs_src);
    try sema.checkInvalidPtrIntArithmetic(block, src, lhs_ty);

    const resolved_type = try sema.resolvePeerTypes(block, src, &.{ lhs, rhs }, .{
        .override = &.{ lhs_src, rhs_src },
    });

    const casted_lhs = try sema.coerce(block, resolved_type, lhs, lhs_src);
    const casted_rhs = try sema.coerce(block, resolved_type, rhs, rhs_src);

    const lhs_scalar_ty = lhs_ty.scalarType(zcu);
    const scalar_tag = resolved_type.scalarType(zcu).zigTypeTag(zcu);

    const is_int = scalar_tag == .int or scalar_tag == .comptime_int;

    try sema.checkArithmeticOp(block, src, scalar_tag, lhs_zig_ty_tag, rhs_zig_ty_tag, .div_floor);

    const maybe_lhs_val = try sema.resolveValueResolveLazy(casted_lhs);
    const maybe_rhs_val = try sema.resolveValueResolveLazy(casted_rhs);

    const allow_div_zero = !is_int and
        resolved_type.toIntern() != .comptime_float_type and
        block.float_mode == .strict;

    if (maybe_lhs_val) |lhs_val| {
        if (maybe_rhs_val) |rhs_val| {
            const result = try arith.div(sema, block, resolved_type, lhs_val, rhs_val, src, lhs_src, rhs_src, .div_floor);
            return Air.internedToRef(result.toIntern());
        }
        if (allow_div_zero) {
            if (lhs_val.isUndefDeep(zcu)) return pt.undefRef(resolved_type);
        } else {
            if (lhs_val.anyScalarIsUndef(zcu)) return sema.failWithUseOfUndef(block, lhs_src);
        }
    } else if (maybe_rhs_val) |rhs_val| {
        if (allow_div_zero) {
            if (rhs_val.isUndefDeep(zcu)) return pt.undefRef(resolved_type);
        } else {
            if (rhs_val.anyScalarIsUndef(zcu)) return sema.failWithUseOfUndef(block, rhs_src);
            if (rhs_val.anyScalarIsZero(zcu)) return sema.failWithDivideByZero(block, rhs_src);
        }
    }

    if (block.wantSafety()) {
        try sema.addDivIntOverflowSafety(block, src, resolved_type, lhs_scalar_ty, maybe_lhs_val, maybe_rhs_val, casted_lhs, casted_rhs, is_int);
        try sema.addDivByZeroSafety(block, src, resolved_type, maybe_rhs_val, casted_rhs, is_int);
    }

    return block.addBinOp(airTag(block, is_int, .div_floor, .div_floor_optimized), casted_lhs, casted_rhs);
}

fn zirDivTrunc(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].pl_node;
    const src = block.src(.{ .node_offset_bin_op = inst_data.src_node });
    const lhs_src = block.builtinCallArgSrc(inst_data.src_node, 0);
    const rhs_src = block.builtinCallArgSrc(inst_data.src_node, 1);
    const extra = sema.code.extraData(Zir.Inst.Bin, inst_data.payload_index).data;
    const lhs = try sema.resolveInst(extra.lhs);
    const rhs = try sema.resolveInst(extra.rhs);
    const lhs_ty = sema.typeOf(lhs);
    const rhs_ty = sema.typeOf(rhs);
    const lhs_zig_ty_tag = lhs_ty.zigTypeTag(zcu);
    const rhs_zig_ty_tag = rhs_ty.zigTypeTag(zcu);
    try sema.checkVectorizableBinaryOperands(block, src, lhs_ty, rhs_ty, lhs_src, rhs_src);
    try sema.checkInvalidPtrIntArithmetic(block, src, lhs_ty);

    const resolved_type = try sema.resolvePeerTypes(block, src, &.{ lhs, rhs }, .{
        .override = &.{ lhs_src, rhs_src },
    });

    const casted_lhs = try sema.coerce(block, resolved_type, lhs, lhs_src);
    const casted_rhs = try sema.coerce(block, resolved_type, rhs, rhs_src);

    const lhs_scalar_ty = lhs_ty.scalarType(zcu);
    const scalar_tag = resolved_type.scalarType(zcu).zigTypeTag(zcu);

    const is_int = scalar_tag == .int or scalar_tag == .comptime_int;

    try sema.checkArithmeticOp(block, src, scalar_tag, lhs_zig_ty_tag, rhs_zig_ty_tag, .div_trunc);

    const maybe_lhs_val = try sema.resolveValueResolveLazy(casted_lhs);
    const maybe_rhs_val = try sema.resolveValueResolveLazy(casted_rhs);

    const allow_div_zero = !is_int and
        resolved_type.toIntern() != .comptime_float_type and
        block.float_mode == .strict;

    if (maybe_lhs_val) |lhs_val| {
        if (maybe_rhs_val) |rhs_val| {
            const result = try arith.div(sema, block, resolved_type, lhs_val, rhs_val, src, lhs_src, rhs_src, .div_trunc);
            return Air.internedToRef(result.toIntern());
        }
        if (allow_div_zero) {
            if (lhs_val.isUndefDeep(zcu)) return pt.undefRef(resolved_type);
        } else {
            if (lhs_val.anyScalarIsUndef(zcu)) return sema.failWithUseOfUndef(block, lhs_src);
        }
    } else if (maybe_rhs_val) |rhs_val| {
        if (allow_div_zero) {
            if (rhs_val.isUndefDeep(zcu)) return pt.undefRef(resolved_type);
        } else {
            if (rhs_val.anyScalarIsUndef(zcu)) return sema.failWithUseOfUndef(block, rhs_src);
            if (rhs_val.anyScalarIsZero(zcu)) return sema.failWithDivideByZero(block, rhs_src);
        }
    }

    if (block.wantSafety()) {
        try sema.addDivIntOverflowSafety(block, src, resolved_type, lhs_scalar_ty, maybe_lhs_val, maybe_rhs_val, casted_lhs, casted_rhs, is_int);
        try sema.addDivByZeroSafety(block, src, resolved_type, maybe_rhs_val, casted_rhs, is_int);
    }

    return block.addBinOp(airTag(block, is_int, .div_trunc, .div_trunc_optimized), casted_lhs, casted_rhs);
}

fn addDivIntOverflowSafety(
    sema: *Sema,
    block: *Block,
    src: LazySrcLoc,
    resolved_type: Type,
    lhs_scalar_ty: Type,
    maybe_lhs_val: ?Value,
    maybe_rhs_val: ?Value,
    casted_lhs: Air.Inst.Ref,
    casted_rhs: Air.Inst.Ref,
    is_int: bool,
) CompileError!void {
    const pt = sema.pt;
    const zcu = pt.zcu;
    if (!is_int) return;

    // If the LHS is unsigned, it cannot cause overflow.
    if (!lhs_scalar_ty.isSignedInt(zcu)) return;

    // If the LHS is widened to a larger integer type, no overflow is possible.
    if (lhs_scalar_ty.intInfo(zcu).bits < resolved_type.intInfo(zcu).bits) {
        return;
    }

    const min_int = try resolved_type.minInt(pt, resolved_type);
    const neg_one_scalar = try pt.intValue(lhs_scalar_ty, -1);
    const neg_one = try sema.splat(resolved_type, neg_one_scalar);

    // If the LHS is comptime-known to be not equal to the min int,
    // no overflow is possible.
    if (maybe_lhs_val) |lhs_val| {
        if (try lhs_val.compareAll(.neq, min_int, resolved_type, pt)) return;
    }

    // If the RHS is comptime-known to not be equal to -1, no overflow is possible.
    if (maybe_rhs_val) |rhs_val| {
        if (try rhs_val.compareAll(.neq, neg_one, resolved_type, pt)) return;
    }

    if (resolved_type.zigTypeTag(zcu) == .vector) {
        const vec_len = resolved_type.vectorLen(zcu);

        // This is a bool vector whose elements are true if the LHS element does NOT equal `min_int`.
        const lhs_ok: Air.Inst.Ref = if (maybe_lhs_val) |lhs_val| ok: {
            // The operand is comptime-known; intern a constant bool vector for the potentially unsafe elements.
            const min_int_scalar = try min_int.elemValue(pt, 0);
            const elems_ok = try sema.arena.alloc(InternPool.Index, vec_len);
            for (elems_ok, 0..) |*elem_ok, elem_idx| {
                const elem_val = try lhs_val.elemValue(pt, elem_idx);
                elem_ok.* = if (elem_val.eqlScalarNum(min_int_scalar, zcu)) .bool_false else .bool_true;
            }
            break :ok Air.internedToRef(try pt.intern(.{ .aggregate = .{
                .ty = (try pt.vectorType(.{
                    .len = vec_len,
                    .child = .bool_type,
                })).toIntern(),
                .storage = .{ .elems = elems_ok },
            } }));
        } else ok: {
            // The operand isn't comptime-known; add a runtime comparison.
            const min_int_ref = Air.internedToRef(min_int.toIntern());
            break :ok try block.addCmpVector(casted_lhs, min_int_ref, .neq);
        };

        // This is a bool vector whose elements are true if the RHS element does NOT equal -1.
        const rhs_ok: Air.Inst.Ref = if (maybe_rhs_val) |rhs_val| ok: {
            // The operand is comptime-known; intern a constant bool vector for the potentially unsafe elements.
            const elems_ok = try sema.arena.alloc(InternPool.Index, vec_len);
            for (elems_ok, 0..) |*elem_ok, elem_idx| {
                const elem_val = try rhs_val.elemValue(pt, elem_idx);
                elem_ok.* = if (elem_val.eqlScalarNum(neg_one_scalar, zcu)) .bool_false else .bool_true;
            }
            break :ok Air.internedToRef(try pt.intern(.{ .aggregate = .{
                .ty = (try pt.vectorType(.{
                    .len = vec_len,
                    .child = .bool_type,
                })).toIntern(),
                .storage = .{ .elems = elems_ok },
            } }));
        } else ok: {
            // The operand isn't comptime-known; add a runtime comparison.
            const neg_one_ref = Air.internedToRef(neg_one.toIntern());
            break :ok try block.addCmpVector(casted_rhs, neg_one_ref, .neq);
        };

        const ok = try block.addReduce(try block.addBinOp(.bool_or, lhs_ok, rhs_ok), .And);
        try sema.addSafetyCheck(block, src, ok, .integer_overflow);
    } else {
        const lhs_ok: Air.Inst.Ref = if (maybe_lhs_val == null) ok: {
            const min_int_ref = Air.internedToRef(min_int.toIntern());
            break :ok try block.addBinOp(.cmp_neq, casted_lhs, min_int_ref);
        } else .none; // means false
        const rhs_ok: Air.Inst.Ref = if (maybe_rhs_val == null) ok: {
            const neg_one_ref = Air.internedToRef(neg_one.toIntern());
            break :ok try block.addBinOp(.cmp_neq, casted_rhs, neg_one_ref);
        } else .none; // means false

        const ok = if (lhs_ok != .none and rhs_ok != .none)
            try block.addBinOp(.bool_or, lhs_ok, rhs_ok)
        else if (lhs_ok != .none)
            lhs_ok
        else if (rhs_ok != .none)
            rhs_ok
        else
            unreachable;

        try sema.addSafetyCheck(block, src, ok, .integer_overflow);
    }
}

fn addDivByZeroSafety(
    sema: *Sema,
    block: *Block,
    src: LazySrcLoc,
    resolved_type: Type,
    maybe_rhs_val: ?Value,
    casted_rhs: Air.Inst.Ref,
    is_int: bool,
) CompileError!void {
    // Strict IEEE floats have well-defined division by zero.
    if (!is_int and block.float_mode == .strict) return;

    // If rhs was comptime-known to be zero a compile error would have been
    // emitted above.
    if (maybe_rhs_val != null) return;

    const pt = sema.pt;
    const zcu = pt.zcu;
    const scalar_zero = if (is_int)
        try pt.intValue(resolved_type.scalarType(zcu), 0)
    else
        try pt.floatValue(resolved_type.scalarType(zcu), 0.0);
    const ok = if (resolved_type.zigTypeTag(zcu) == .vector) ok: {
        const zero_val = try sema.splat(resolved_type, scalar_zero);
        const zero = Air.internedToRef(zero_val.toIntern());
        const ok = try block.addCmpVector(casted_rhs, zero, .neq);
        break :ok try block.addReduce(ok, .And);
    } else ok: {
        const zero = Air.internedToRef(scalar_zero.toIntern());
        break :ok try block.addBinOp(if (is_int) .cmp_neq else .cmp_neq_optimized, casted_rhs, zero);
    };
    try sema.addSafetyCheck(block, src, ok, .divide_by_zero);
}

fn airTag(block: *Block, is_int: bool, normal: Air.Inst.Tag, optimized: Air.Inst.Tag) Air.Inst.Tag {
    if (is_int) return normal;
    return switch (block.float_mode) {
        .strict => normal,
        .optimized => optimized,
    };
}

fn zirModRem(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].pl_node;
    const src = block.src(.{ .node_offset_bin_op = inst_data.src_node });
    const lhs_src = block.src(.{ .node_offset_bin_lhs = inst_data.src_node });
    const rhs_src = block.src(.{ .node_offset_bin_rhs = inst_data.src_node });
    const extra = sema.code.extraData(Zir.Inst.Bin, inst_data.payload_index).data;
    const lhs = try sema.resolveInst(extra.lhs);
    const rhs = try sema.resolveInst(extra.rhs);
    const lhs_ty = sema.typeOf(lhs);
    const rhs_ty = sema.typeOf(rhs);
    const lhs_zig_ty_tag = lhs_ty.zigTypeTag(zcu);
    const rhs_zig_ty_tag = rhs_ty.zigTypeTag(zcu);
    try sema.checkVectorizableBinaryOperands(block, src, lhs_ty, rhs_ty, lhs_src, rhs_src);
    try sema.checkInvalidPtrIntArithmetic(block, src, lhs_ty);

    const resolved_type = try sema.resolvePeerTypes(block, src, &.{ lhs, rhs }, .{
        .override = &.{ lhs_src, rhs_src },
    });

    const casted_lhs = try sema.coerce(block, resolved_type, lhs, lhs_src);
    const casted_rhs = try sema.coerce(block, resolved_type, rhs, rhs_src);

    const lhs_scalar_ty = lhs_ty.scalarType(zcu);
    const rhs_scalar_ty = rhs_ty.scalarType(zcu);
    const scalar_tag = resolved_type.scalarType(zcu).zigTypeTag(zcu);

    const is_int = scalar_tag == .int or scalar_tag == .comptime_int;

    try sema.checkArithmeticOp(block, src, scalar_tag, lhs_zig_ty_tag, rhs_zig_ty_tag, .mod_rem);

    const maybe_lhs_val = try sema.resolveValueResolveLazy(casted_lhs);
    const maybe_rhs_val = try sema.resolveValueResolveLazy(casted_rhs);

    const lhs_maybe_negative = a: {
        if (lhs_scalar_ty.isUnsignedInt(zcu)) break :a false;
        const lhs_val = maybe_lhs_val orelse break :a true;
        if (lhs_val.compareAllWithZero(.gte, zcu)) break :a false;
        break :a true;
    };
    const rhs_maybe_negative = a: {
        if (rhs_scalar_ty.isUnsignedInt(zcu)) break :a false;
        const rhs_val = maybe_rhs_val orelse break :a true;
        if (rhs_val.compareAllWithZero(.gte, zcu)) break :a false;
        break :a true;
    };

    if (maybe_lhs_val) |lhs_val| {
        if (maybe_rhs_val) |rhs_val| {
            const result = try arith.modRem(sema, block, resolved_type, lhs_val, rhs_val, lhs_src, rhs_src, .rem);
            if (lhs_maybe_negative or rhs_maybe_negative) {
                if (!result.compareAllWithZero(.eq, zcu)) {
                    // Non-zero result means ambiguity between mod and rem
                    return sema.failWithModRemNegative(block, src: {
                        if (lhs_maybe_negative) break :src lhs_src;
                        if (rhs_maybe_negative) break :src rhs_src;
                        unreachable;
                    }, lhs_ty, rhs_ty);
                }
            }
            return Air.internedToRef(result.toIntern());
        }
    }

    // Result not comptime-known, so floats and signed integers are illegal due to mod/rem ambiguity
    if (lhs_maybe_negative or rhs_maybe_negative) {
        return sema.failWithModRemNegative(block, src: {
            if (lhs_maybe_negative) break :src lhs_src;
            if (rhs_maybe_negative) break :src rhs_src;
            unreachable;
        }, lhs_ty, rhs_ty);
    }

    const allow_div_zero = !is_int and
        resolved_type.toIntern() != .comptime_float_type and
        block.float_mode == .strict;

    if (maybe_lhs_val) |lhs_val| {
        if (allow_div_zero) {
            if (lhs_val.isUndefDeep(zcu)) return pt.undefRef(resolved_type);
        } else {
            if (lhs_val.anyScalarIsUndef(zcu)) return sema.failWithUseOfUndef(block, lhs_src);
        }
    } else if (maybe_rhs_val) |rhs_val| {
        if (allow_div_zero) {
            if (rhs_val.isUndefDeep(zcu)) return pt.undefRef(resolved_type);
        } else {
            if (rhs_val.anyScalarIsUndef(zcu)) return sema.failWithUseOfUndef(block, rhs_src);
            if (rhs_val.anyScalarIsZero(zcu)) return sema.failWithDivideByZero(block, rhs_src);
        }
    }

    if (block.wantSafety()) {
        try sema.addDivByZeroSafety(block, src, resolved_type, maybe_rhs_val, casted_rhs, is_int);
    }

    const air_tag = airTag(block, is_int, .rem, .rem_optimized);
    return block.addBinOp(air_tag, casted_lhs, casted_rhs);
}

fn zirMod(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].pl_node;
    const src = block.src(.{ .node_offset_bin_op = inst_data.src_node });
    const lhs_src = block.builtinCallArgSrc(inst_data.src_node, 0);
    const rhs_src = block.builtinCallArgSrc(inst_data.src_node, 1);
    const extra = sema.code.extraData(Zir.Inst.Bin, inst_data.payload_index).data;
    const lhs = try sema.resolveInst(extra.lhs);
    const rhs = try sema.resolveInst(extra.rhs);
    const lhs_ty = sema.typeOf(lhs);
    const rhs_ty = sema.typeOf(rhs);
    const lhs_zig_ty_tag = lhs_ty.zigTypeTag(zcu);
    const rhs_zig_ty_tag = rhs_ty.zigTypeTag(zcu);
    try sema.checkVectorizableBinaryOperands(block, src, lhs_ty, rhs_ty, lhs_src, rhs_src);
    try sema.checkInvalidPtrIntArithmetic(block, src, lhs_ty);

    const resolved_type = try sema.resolvePeerTypes(block, src, &.{ lhs, rhs }, .{
        .override = &.{ lhs_src, rhs_src },
    });

    const casted_lhs = try sema.coerce(block, resolved_type, lhs, lhs_src);
    const casted_rhs = try sema.coerce(block, resolved_type, rhs, rhs_src);

    const scalar_tag = resolved_type.scalarType(zcu).zigTypeTag(zcu);

    const is_int = scalar_tag == .int or scalar_tag == .comptime_int;

    try sema.checkArithmeticOp(block, src, scalar_tag, lhs_zig_ty_tag, rhs_zig_ty_tag, .mod);

    const maybe_lhs_val = try sema.resolveValueResolveLazy(casted_lhs);
    const maybe_rhs_val = try sema.resolveValueResolveLazy(casted_rhs);

    const allow_div_zero = !is_int and
        resolved_type.toIntern() != .comptime_float_type and
        block.float_mode == .strict;

    if (maybe_lhs_val) |lhs_val| {
        if (maybe_rhs_val) |rhs_val| {
            const result = try arith.modRem(sema, block, resolved_type, lhs_val, rhs_val, lhs_src, rhs_src, .mod);
            return Air.internedToRef(result.toIntern());
        }
        if (allow_div_zero) {
            if (lhs_val.isUndefDeep(zcu)) return pt.undefRef(resolved_type);
        } else {
            if (lhs_val.anyScalarIsUndef(zcu)) return sema.failWithUseOfUndef(block, lhs_src);
        }
    } else if (maybe_rhs_val) |rhs_val| {
        if (allow_div_zero) {
            if (rhs_val.isUndefDeep(zcu)) return pt.undefRef(resolved_type);
        } else {
            if (rhs_val.anyScalarIsUndef(zcu)) return sema.failWithUseOfUndef(block, rhs_src);
            if (rhs_val.anyScalarIsZero(zcu)) return sema.failWithDivideByZero(block, rhs_src);
        }
    }

    if (block.wantSafety()) {
        try sema.addDivByZeroSafety(block, src, resolved_type, maybe_rhs_val, casted_rhs, is_int);
    }

    const air_tag = airTag(block, is_int, .mod, .mod_optimized);
    return block.addBinOp(air_tag, casted_lhs, casted_rhs);
}

fn zirRem(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].pl_node;
    const src = block.src(.{ .node_offset_bin_op = inst_data.src_node });
    const lhs_src = block.builtinCallArgSrc(inst_data.src_node, 0);
    const rhs_src = block.builtinCallArgSrc(inst_data.src_node, 1);
    const extra = sema.code.extraData(Zir.Inst.Bin, inst_data.payload_index).data;
    const lhs = try sema.resolveInst(extra.lhs);
    const rhs = try sema.resolveInst(extra.rhs);
    const lhs_ty = sema.typeOf(lhs);
    const rhs_ty = sema.typeOf(rhs);
    const lhs_zig_ty_tag = lhs_ty.zigTypeTag(zcu);
    const rhs_zig_ty_tag = rhs_ty.zigTypeTag(zcu);
    try sema.checkVectorizableBinaryOperands(block, src, lhs_ty, rhs_ty, lhs_src, rhs_src);
    try sema.checkInvalidPtrIntArithmetic(block, src, lhs_ty);

    const resolved_type = try sema.resolvePeerTypes(block, src, &.{ lhs, rhs }, .{
        .override = &.{ lhs_src, rhs_src },
    });

    const casted_lhs = try sema.coerce(block, resolved_type, lhs, lhs_src);
    const casted_rhs = try sema.coerce(block, resolved_type, rhs, rhs_src);

    const scalar_tag = resolved_type.scalarType(zcu).zigTypeTag(zcu);

    const is_int = scalar_tag == .int or scalar_tag == .comptime_int;

    try sema.checkArithmeticOp(block, src, scalar_tag, lhs_zig_ty_tag, rhs_zig_ty_tag, .rem);

    const maybe_lhs_val = try sema.resolveValueResolveLazy(casted_lhs);
    const maybe_rhs_val = try sema.resolveValueResolveLazy(casted_rhs);

    const allow_div_zero = !is_int and
        resolved_type.toIntern() != .comptime_float_type and
        block.float_mode == .strict;

    if (maybe_lhs_val) |lhs_val| {
        if (maybe_rhs_val) |rhs_val| {
            const result = try arith.modRem(sema, block, resolved_type, lhs_val, rhs_val, lhs_src, rhs_src, .rem);
            return Air.internedToRef(result.toIntern());
        }
        if (allow_div_zero) {
            if (lhs_val.isUndefDeep(zcu)) return pt.undefRef(resolved_type);
        } else {
            if (lhs_val.anyScalarIsUndef(zcu)) return sema.failWithUseOfUndef(block, lhs_src);
        }
    } else if (maybe_rhs_val) |rhs_val| {
        if (allow_div_zero) {
            if (rhs_val.isUndefDeep(zcu)) return pt.undefRef(resolved_type);
        } else {
            if (rhs_val.anyScalarIsUndef(zcu)) return sema.failWithUseOfUndef(block, rhs_src);
            if (rhs_val.anyScalarIsZero(zcu)) return sema.failWithDivideByZero(block, rhs_src);
        }
    }

    if (block.wantSafety()) {
        try sema.addDivByZeroSafety(block, src, resolved_type, maybe_rhs_val, casted_rhs, is_int);
    }

    const air_tag = airTag(block, is_int, .rem, .rem_optimized);
    return block.addBinOp(air_tag, casted_lhs, casted_rhs);
}

fn zirOverflowArithmetic(
    sema: *Sema,
    block: *Block,
    extended: Zir.Inst.Extended.InstData,
    zir_tag: Zir.Inst.Extended,
) CompileError!Air.Inst.Ref {
    const tracy = trace(@src());
    defer tracy.end();

    const extra = sema.code.extraData(Zir.Inst.BinNode, extended.operand).data;
    const src = block.nodeOffset(extra.node);

    const lhs_src = block.builtinCallArgSrc(extra.node, 0);
    const rhs_src = block.builtinCallArgSrc(extra.node, 1);

    const uncasted_lhs = try sema.resolveInst(extra.lhs);
    const uncasted_rhs = try sema.resolveInst(extra.rhs);

    const lhs_ty = sema.typeOf(uncasted_lhs);
    const rhs_ty = sema.typeOf(uncasted_rhs);
    const pt = sema.pt;
    const zcu = pt.zcu;
    const ip = &zcu.intern_pool;

    try sema.checkVectorizableBinaryOperands(block, src, lhs_ty, rhs_ty, lhs_src, rhs_src);

    const instructions = &[_]Air.Inst.Ref{ uncasted_lhs, uncasted_rhs };
    const dest_ty = if (zir_tag == .shl_with_overflow)
        lhs_ty
    else
        try sema.resolvePeerTypes(block, src, instructions, .{
            .override = &[_]?LazySrcLoc{ lhs_src, rhs_src },
        });

    const rhs_dest_ty = if (zir_tag == .shl_with_overflow)
        try sema.log2IntType(block, lhs_ty, src)
    else
        dest_ty;

    const lhs = try sema.coerce(block, dest_ty, uncasted_lhs, lhs_src);
    const rhs = try sema.coerce(block, rhs_dest_ty, uncasted_rhs, rhs_src);

    if (dest_ty.scalarType(zcu).zigTypeTag(zcu) != .int) {
        return sema.fail(block, src, "expected vector of integers or integer tag type, found '{}'", .{dest_ty.fmt(pt)});
    }

    const maybe_lhs_val = try sema.resolveValue(lhs);
    const maybe_rhs_val = try sema.resolveValue(rhs);

    const tuple_ty = try pt.overflowArithmeticTupleType(dest_ty);
    const overflow_ty: Type = .fromInterned(ip.indexToKey(tuple_ty.toIntern()).tuple_type.types.get(ip)[1]);

    var result: struct {
        inst: Air.Inst.Ref = .none,
        wrapped: Value = Value.@"unreachable",
        overflow_bit: Value,
    } = result: {
        switch (zir_tag) {
            .add_with_overflow => {
                // If either of the arguments is zero, `false` is returned and the other is stored
                // to the result, even if it is undefined..
                // Otherwise, if either of the argument is undefined, undefined is returned.
                if (maybe_lhs_val) |lhs_val| {
                    if (!lhs_val.isUndef(zcu) and (try lhs_val.compareAllWithZeroSema(.eq, pt))) {
                        break :result .{ .overflow_bit = try sema.splat(overflow_ty, .zero_u1), .inst = rhs };
                    }
                }
                if (maybe_rhs_val) |rhs_val| {
                    if (!rhs_val.isUndef(zcu) and (try rhs_val.compareAllWithZeroSema(.eq, pt))) {
                        break :result .{ .overflow_bit = try sema.splat(overflow_ty, .zero_u1), .inst = lhs };
                    }
                }
                if (maybe_lhs_val) |lhs_val| {
                    if (maybe_rhs_val) |rhs_val| {
                        if (lhs_val.isUndef(zcu) or rhs_val.isUndef(zcu)) {
                            break :result .{ .overflow_bit = Value.undef, .wrapped = Value.undef };
                        }

                        const result = try arith.addWithOverflow(sema, dest_ty, lhs_val, rhs_val);
                        break :result .{ .overflow_bit = result.overflow_bit, .wrapped = result.wrapped_result };
                    }
                }
            },
            .sub_with_overflow => {
                // If the rhs is zero, then the result is lhs and no overflow occured.
                // Otherwise, if either result is undefined, both results are undefined.
                if (maybe_rhs_val) |rhs_val| {
                    if (rhs_val.isUndef(zcu)) {
                        break :result .{ .overflow_bit = Value.undef, .wrapped = Value.undef };
                    } else if (try rhs_val.compareAllWithZeroSema(.eq, pt)) {
                        break :result .{ .overflow_bit = try sema.splat(overflow_ty, .zero_u1), .inst = lhs };
                    } else if (maybe_lhs_val) |lhs_val| {
                        if (lhs_val.isUndef(zcu)) {
                            break :result .{ .overflow_bit = Value.undef, .wrapped = Value.undef };
                        }

                        const result = try arith.subWithOverflow(sema, dest_ty, lhs_val, rhs_val);
                        break :result .{ .overflow_bit = result.overflow_bit, .wrapped = result.wrapped_result };
                    }
                }
            },
            .mul_with_overflow => {
                // If either of the arguments is zero, the result is zero and no overflow occured.
                // If either of the arguments is one, the result is the other and no overflow occured.
                // Otherwise, if either of the arguments is undefined, both results are undefined.
                const scalar_one = try pt.intValue(dest_ty.scalarType(zcu), 1);
                if (maybe_lhs_val) |lhs_val| {
                    if (!lhs_val.isUndef(zcu)) {
                        if (try lhs_val.compareAllWithZeroSema(.eq, pt)) {
                            break :result .{ .overflow_bit = try sema.splat(overflow_ty, .zero_u1), .inst = lhs };
                        } else if (try sema.compareAll(lhs_val, .eq, try sema.splat(dest_ty, scalar_one), dest_ty)) {
                            break :result .{ .overflow_bit = try sema.splat(overflow_ty, .zero_u1), .inst = rhs };
                        }
                    }
                }

                if (maybe_rhs_val) |rhs_val| {
                    if (!rhs_val.isUndef(zcu)) {
                        if (try rhs_val.compareAllWithZeroSema(.eq, pt)) {
                            break :result .{ .overflow_bit = try sema.splat(overflow_ty, .zero_u1), .inst = rhs };
                        } else if (try sema.compareAll(rhs_val, .eq, try sema.splat(dest_ty, scalar_one), dest_ty)) {
                            break :result .{ .overflow_bit = try sema.splat(overflow_ty, .zero_u1), .inst = lhs };
                        }
                    }
                }

                if (maybe_lhs_val) |lhs_val| {
                    if (maybe_rhs_val) |rhs_val| {
                        if (lhs_val.isUndef(zcu) or rhs_val.isUndef(zcu)) {
                            break :result .{ .overflow_bit = Value.undef, .wrapped = Value.undef };
                        }

                        const result = try arith.mulWithOverflow(sema, dest_ty, lhs_val, rhs_val);
                        break :result .{ .overflow_bit = result.overflow_bit, .wrapped = result.wrapped_result };
                    }
                }
            },
            .shl_with_overflow => {
                // If lhs is zero, the result is zero and no overflow occurred.
                // If rhs is zero, the result is lhs (even if undefined) and no overflow occurred.
                // Oterhwise if either of the arguments is undefined, both results are undefined.
                if (maybe_lhs_val) |lhs_val| {
                    if (!lhs_val.isUndef(zcu) and (try lhs_val.compareAllWithZeroSema(.eq, pt))) {
                        break :result .{ .overflow_bit = try sema.splat(overflow_ty, .zero_u1), .inst = lhs };
                    }
                }
                if (maybe_rhs_val) |rhs_val| {
                    if (!rhs_val.isUndef(zcu) and (try rhs_val.compareAllWithZeroSema(.eq, pt))) {
                        break :result .{ .overflow_bit = try sema.splat(overflow_ty, .zero_u1), .inst = lhs };
                    }
                }
                if (maybe_lhs_val) |lhs_val| {
                    if (maybe_rhs_val) |rhs_val| {
                        if (lhs_val.isUndef(zcu) or rhs_val.isUndef(zcu)) {
                            break :result .{ .overflow_bit = Value.undef, .wrapped = Value.undef };
                        }

                        const result = try lhs_val.shlWithOverflow(rhs_val, dest_ty, sema.arena, pt);
                        break :result .{ .overflow_bit = result.overflow_bit, .wrapped = result.wrapped_result };
                    }
                }
            },
            else => unreachable,
        }

        const air_tag: Air.Inst.Tag = switch (zir_tag) {
            .add_with_overflow => .add_with_overflow,
            .mul_with_overflow => .mul_with_overflow,
            .sub_with_overflow => .sub_with_overflow,
            .shl_with_overflow => .shl_with_overflow,
            else => unreachable,
        };

        const runtime_src = if (maybe_lhs_val == null) lhs_src else rhs_src;
        try sema.requireRuntimeBlock(block, src, runtime_src);

        return block.addInst(.{
            .tag = air_tag,
            .data = .{ .ty_pl = .{
                .ty = Air.internedToRef(tuple_ty.toIntern()),
                .payload = try block.sema.addExtra(Air.Bin{
                    .lhs = lhs,
                    .rhs = rhs,
                }),
            } },
        });
    };

    if (result.inst != .none) {
        if (try sema.resolveValue(result.inst)) |some| {
            result.wrapped = some;
            result.inst = .none;
        }
    }

    if (result.inst == .none) {
        return Air.internedToRef((try pt.intern(.{ .aggregate = .{
            .ty = tuple_ty.toIntern(),
            .storage = .{ .elems = &.{
                result.wrapped.toIntern(),
                result.overflow_bit.toIntern(),
            } },
        } })));
    }

    const element_refs = try sema.arena.alloc(Air.Inst.Ref, 2);
    element_refs[0] = result.inst;
    element_refs[1] = Air.internedToRef(result.overflow_bit.toIntern());
    return block.addAggregateInit(tuple_ty, element_refs);
}

fn splat(sema: *Sema, ty: Type, val: Value) !Value {
    const pt = sema.pt;
    const zcu = pt.zcu;
    if (ty.zigTypeTag(zcu) != .vector) return val;
    const repeated = try pt.intern(.{ .aggregate = .{
        .ty = ty.toIntern(),
        .storage = .{ .repeated_elem = val.toIntern() },
    } });
    return Value.fromInterned(repeated);
}

fn analyzeArithmetic(
    sema: *Sema,
    block: *Block,
    /// TODO performance investigation: make this comptime?
    zir_tag: Zir.Inst.Tag,
    lhs: Air.Inst.Ref,
    rhs: Air.Inst.Ref,
    src: LazySrcLoc,
    lhs_src: LazySrcLoc,
    rhs_src: LazySrcLoc,
    want_safety: bool,
) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const lhs_ty = sema.typeOf(lhs);
    const rhs_ty = sema.typeOf(rhs);
    const lhs_zig_ty_tag = lhs_ty.zigTypeTag(zcu);
    const rhs_zig_ty_tag = rhs_ty.zigTypeTag(zcu);
    try sema.checkVectorizableBinaryOperands(block, src, lhs_ty, rhs_ty, lhs_src, rhs_src);

    if (lhs_zig_ty_tag == .pointer) {
        if (rhs_zig_ty_tag == .pointer) {
            if (lhs_ty.ptrSize(zcu) != .slice and rhs_ty.ptrSize(zcu) != .slice) {
                if (zir_tag != .sub) {
                    return sema.failWithInvalidPtrArithmetic(block, src, "pointer-pointer", "subtraction");
                }
                if (!lhs_ty.elemType2(zcu).eql(rhs_ty.elemType2(zcu), zcu)) {
                    return sema.fail(block, src, "incompatible pointer arithmetic operands '{}' and '{}'", .{
                        lhs_ty.fmt(pt), rhs_ty.fmt(pt),
                    });
                }

                const elem_size = lhs_ty.elemType2(zcu).abiSize(zcu);
                if (elem_size == 0) {
                    return sema.fail(block, src, "pointer arithmetic requires element type '{}' to have runtime bits", .{
                        lhs_ty.elemType2(zcu).fmt(pt),
                    });
                }

                const runtime_src = runtime_src: {
                    if (try sema.resolveValue(lhs)) |lhs_value| {
                        if (try sema.resolveValue(rhs)) |rhs_value| {
                            const lhs_ptr = switch (zcu.intern_pool.indexToKey(lhs_value.toIntern())) {
                                .undef => return sema.failWithUseOfUndef(block, lhs_src),
                                .ptr => |ptr| ptr,
                                else => unreachable,
                            };
                            const rhs_ptr = switch (zcu.intern_pool.indexToKey(rhs_value.toIntern())) {
                                .undef => return sema.failWithUseOfUndef(block, rhs_src),
                                .ptr => |ptr| ptr,
                                else => unreachable,
                            };
                            // Make sure the pointers point to the same data.
                            if (!lhs_ptr.base_addr.eql(rhs_ptr.base_addr)) break :runtime_src src;
                            const address = std.math.sub(u64, lhs_ptr.byte_offset, rhs_ptr.byte_offset) catch
                                return sema.fail(block, src, "operation results in overflow", .{});
                            const result = address / elem_size;
                            return try pt.intRef(.usize, result);
                        } else {
                            break :runtime_src lhs_src;
                        }
                    } else {
                        break :runtime_src rhs_src;
                    }
                };

                try sema.requireRuntimeBlock(block, src, runtime_src);
                try sema.checkLogicalPtrOperation(block, src, lhs_ty);
                try sema.checkLogicalPtrOperation(block, src, rhs_ty);
                const lhs_int = try block.addBitCast(.usize, lhs);
                const rhs_int = try block.addBitCast(.usize, rhs);
                const address = try block.addBinOp(.sub_wrap, lhs_int, rhs_int);
                return try block.addBinOp(.div_exact, address, try pt.intRef(.usize, elem_size));
            }
        } else {
            switch (lhs_ty.ptrSize(zcu)) {
                .one, .slice => {},
                .many, .c => {
                    const air_tag: Air.Inst.Tag = switch (zir_tag) {
                        .add => .ptr_add,
                        .sub => .ptr_sub,
                        else => return sema.failWithInvalidPtrArithmetic(block, src, "pointer-integer", "addition and subtraction"),
                    };

                    if (!try lhs_ty.elemType2(zcu).hasRuntimeBitsSema(pt)) {
                        return sema.fail(block, src, "pointer arithmetic requires element type '{}' to have runtime bits", .{
                            lhs_ty.elemType2(zcu).fmt(pt),
                        });
                    }
                    return sema.analyzePtrArithmetic(block, src, lhs, rhs, air_tag, lhs_src, rhs_src);
                },
            }
        }
    }

    const resolved_type = try sema.resolvePeerTypes(block, src, &.{ lhs, rhs }, .{
        .override = &.{ lhs_src, rhs_src },
    });

    const casted_lhs = try sema.coerce(block, resolved_type, lhs, lhs_src);
    const casted_rhs = try sema.coerce(block, resolved_type, rhs, rhs_src);

    const scalar_type = resolved_type.scalarType(zcu);
    const scalar_tag = scalar_type.zigTypeTag(zcu);

    try sema.checkArithmeticOp(block, src, scalar_tag, lhs_zig_ty_tag, rhs_zig_ty_tag, zir_tag);

    // The rules we'll implement below are as follows:
    //
    // * If both operands are comptime-known, we call the corresponding function in `arith` to get
    //   the comptime-known result. Inputs which would be IB at runtime are compile errors.
    //
    // * Otherwise, if one operand is comptime-known `undefined`, we trigger a compile error if this
    //   operator can ever possibly trigger IB, or otherwise return comptime-known `undefined`.
    //
    // * No other comptime operand detemines a comptime result; e.g. `0 * x` isn't always `0` because
    //   of `undefined`. Therefore, the remaining cases all become runtime operations.

    const is_int = switch (scalar_tag) {
        .int, .comptime_int => true,
        .float, .comptime_float => false,
        else => unreachable,
    };

    const maybe_lhs_val = try sema.resolveValueResolveLazy(casted_lhs);
    const maybe_rhs_val = try sema.resolveValueResolveLazy(casted_rhs);

    if (maybe_lhs_val) |lhs_val| {
        if (maybe_rhs_val) |rhs_val| {
            const result_val = switch (zir_tag) {
                .add, .add_unsafe => try arith.add(sema, block, resolved_type, lhs_val, rhs_val, src, lhs_src, rhs_src),
                .addwrap => try arith.addWrap(sema, resolved_type, lhs_val, rhs_val),
                .add_sat => try arith.addSat(sema, resolved_type, lhs_val, rhs_val),
                .sub => try arith.sub(sema, block, resolved_type, lhs_val, rhs_val, src, lhs_src, rhs_src),
                .subwrap => try arith.subWrap(sema, resolved_type, lhs_val, rhs_val),
                .sub_sat => try arith.subSat(sema, resolved_type, lhs_val, rhs_val),
                .mul => try arith.mul(sema, block, resolved_type, lhs_val, rhs_val, src, lhs_src, rhs_src),
                .mulwrap => try arith.mulWrap(sema, resolved_type, lhs_val, rhs_val),
                .mul_sat => try arith.mulSat(sema, resolved_type, lhs_val, rhs_val),
                else => unreachable,
            };
            return Air.internedToRef(result_val.toIntern());
        }
    }

    const air_tag: Air.Inst.Tag, const air_tag_safe: Air.Inst.Tag, const allow_undef: bool = switch (zir_tag) {
        .add, .add_unsafe => .{ if (block.float_mode == .optimized) .add_optimized else .add, .add_safe, !is_int },
        .addwrap => .{ .add_wrap, .add_wrap, true },
        .add_sat => .{ .add_sat, .add_sat, true },
        .sub => .{ if (block.float_mode == .optimized) .sub_optimized else .sub, .sub_safe, !is_int },
        .subwrap => .{ .sub_wrap, .sub_wrap, true },
        .sub_sat => .{ .sub_sat, .sub_sat, true },
        .mul => .{ if (block.float_mode == .optimized) .mul_optimized else .mul, .mul_safe, !is_int },
        .mulwrap => .{ .mul_wrap, .mul_wrap, true },
        .mul_sat => .{ .mul_sat, .mul_sat, true },
        else => unreachable,
    };

    if (allow_undef) {
        if (maybe_lhs_val) |lhs_val| {
            if (lhs_val.isUndefDeep(zcu)) return pt.undefRef(resolved_type);
        }
        if (maybe_rhs_val) |rhs_val| {
            if (rhs_val.isUndefDeep(zcu)) return pt.undefRef(resolved_type);
        }
    } else {
        if (maybe_lhs_val) |lhs_val| {
            if (lhs_val.anyScalarIsUndef(zcu)) return sema.failWithUseOfUndef(block, lhs_src);
        }
        if (maybe_rhs_val) |rhs_val| {
            if (rhs_val.anyScalarIsUndef(zcu)) return sema.failWithUseOfUndef(block, rhs_src);
        }
    }

    if (block.wantSafety() and want_safety and scalar_tag == .int) {
        if (air_tag != air_tag_safe and zcu.backendSupportsFeature(.panic_fn)) {
            _ = try sema.preparePanicId(src, .integer_overflow);
        }
        return block.addBinOp(air_tag_safe, casted_lhs, casted_rhs);
    }
    return block.addBinOp(air_tag, casted_lhs, casted_rhs);
}

fn analyzePtrArithmetic(
    sema: *Sema,
    block: *Block,
    op_src: LazySrcLoc,
    ptr: Air.Inst.Ref,
    uncasted_offset: Air.Inst.Ref,
    air_tag: Air.Inst.Tag,
    ptr_src: LazySrcLoc,
    offset_src: LazySrcLoc,
) CompileError!Air.Inst.Ref {
    // TODO if the operand is comptime-known to be negative, or is a negative int,
    // coerce to isize instead of usize.
    const offset = try sema.coerce(block, .usize, uncasted_offset, offset_src);
    const pt = sema.pt;
    const zcu = pt.zcu;
    const opt_ptr_val = try sema.resolveValue(ptr);
    const opt_off_val = try sema.resolveDefinedValue(block, offset_src, offset);
    const ptr_ty = sema.typeOf(ptr);
    const ptr_info = ptr_ty.ptrInfo(zcu);
    assert(ptr_info.flags.size == .many or ptr_info.flags.size == .c);

    const new_ptr_ty = t: {
        // Calculate the new pointer alignment.
        // This code is duplicated in `Type.elemPtrType`.
        if (ptr_info.flags.alignment == .none) {
            // ABI-aligned pointer. Any pointer arithmetic maintains the same ABI-alignedness.
            break :t ptr_ty;
        }
        // If the addend is not a comptime-known value we can still count on
        // it being a multiple of the type size.
        const elem_size = try Type.fromInterned(ptr_info.child).abiSizeSema(pt);
        const addend = if (opt_off_val) |off_val| a: {
            const off_int = try sema.usizeCast(block, offset_src, try off_val.toUnsignedIntSema(pt));
            break :a elem_size * off_int;
        } else elem_size;

        // The resulting pointer is aligned to the lcd between the offset (an
        // arbitrary number) and the alignment factor (always a power of two,
        // non zero).
        const new_align: Alignment = @enumFromInt(@min(
            @ctz(addend),
            @intFromEnum(ptr_info.flags.alignment),
        ));
        assert(new_align != .none);

        break :t try pt.ptrTypeSema(.{
            .child = ptr_info.child,
            .sentinel = ptr_info.sentinel,
            .flags = .{
                .size = ptr_info.flags.size,
                .alignment = new_align,
                .is_const = ptr_info.flags.is_const,
                .is_volatile = ptr_info.flags.is_volatile,
                .is_allowzero = ptr_info.flags.is_allowzero,
                .address_space = ptr_info.flags.address_space,
            },
        });
    };

    const runtime_src = rs: {
        if (opt_ptr_val) |ptr_val| {
            if (opt_off_val) |offset_val| {
                if (ptr_val.isUndef(zcu)) return pt.undefRef(new_ptr_ty);

                const offset_int = try sema.usizeCast(block, offset_src, try offset_val.toUnsignedIntSema(pt));
                if (offset_int == 0) return ptr;
                if (air_tag == .ptr_sub) {
                    const elem_size = try Type.fromInterned(ptr_info.child).abiSizeSema(pt);
                    const new_ptr_val = try sema.ptrSubtract(block, op_src, ptr_val, offset_int * elem_size, new_ptr_ty);
                    return Air.internedToRef(new_ptr_val.toIntern());
                } else {
                    const new_ptr_val = try pt.getCoerced(try ptr_val.ptrElem(offset_int, pt), new_ptr_ty);
                    return Air.internedToRef(new_ptr_val.toIntern());
                }
            } else break :rs offset_src;
        } else break :rs ptr_src;
    };

    try sema.requireRuntimeBlock(block, op_src, runtime_src);
    try sema.checkLogicalPtrOperation(block, op_src, ptr_ty);

    return block.addInst(.{
        .tag = air_tag,
        .data = .{ .ty_pl = .{
            .ty = Air.internedToRef(new_ptr_ty.toIntern()),
            .payload = try sema.addExtra(Air.Bin{
                .lhs = ptr,
                .rhs = offset,
            }),
        } },
    });
}

fn zirLoad(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    const tracy = trace(@src());
    defer tracy.end();

    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].un_node;
    const src = block.nodeOffset(inst_data.src_node);
    const ptr_src = src; // TODO better source location
    const ptr = try sema.resolveInst(inst_data.operand);
    return sema.analyzeLoad(block, src, ptr, ptr_src);
}

fn zirAsm(
    sema: *Sema,
    block: *Block,
    extended: Zir.Inst.Extended.InstData,
    tmpl_is_expr: bool,
) CompileError!Air.Inst.Ref {
    const tracy = trace(@src());
    defer tracy.end();

    const pt = sema.pt;
    const zcu = pt.zcu;
    const extra = sema.code.extraData(Zir.Inst.Asm, extended.operand);
    const src = block.nodeOffset(extra.data.src_node);
    const ret_ty_src = block.src(.{ .node_offset_asm_ret_ty = extra.data.src_node });
    const outputs_len: u4 = @truncate(extended.small);
    const inputs_len: u5 = @truncate(extended.small >> 4);
    const clobbers_len: u6 = @truncate(extended.small >> 9);
    const is_volatile = @as(u1, @truncate(extended.small >> 15)) != 0;
    const is_global_assembly = sema.func_index == .none;
    const zir_tags = sema.code.instructions.items(.tag);

    const asm_source: []const u8 = if (tmpl_is_expr) s: {
        const tmpl: Zir.Inst.Ref = @enumFromInt(@intFromEnum(extra.data.asm_source));
        break :s try sema.resolveConstString(block, src, tmpl, .{ .simple = .inline_assembly_code });
    } else sema.code.nullTerminatedString(extra.data.asm_source);

    if (is_global_assembly) {
        if (outputs_len != 0) {
            return sema.fail(block, src, "module-level assembly does not support outputs", .{});
        }
        if (inputs_len != 0) {
            return sema.fail(block, src, "module-level assembly does not support inputs", .{});
        }
        if (clobbers_len != 0) {
            return sema.fail(block, src, "module-level assembly does not support clobbers", .{});
        }
        if (is_volatile) {
            return sema.fail(block, src, "volatile keyword is redundant on module-level assembly", .{});
        }
        try zcu.addGlobalAssembly(sema.owner, asm_source);
        return .void_value;
    }

    try sema.requireRuntimeBlock(block, src, null);

    var extra_i = extra.end;
    var output_type_bits = extra.data.output_type_bits;
    var needed_capacity: usize = @typeInfo(Air.Asm).@"struct".fields.len + outputs_len + inputs_len;

    const ConstraintName = struct { c: []const u8, n: []const u8 };
    const out_args = try sema.arena.alloc(Air.Inst.Ref, outputs_len);
    const outputs = try sema.arena.alloc(ConstraintName, outputs_len);
    var expr_ty = Air.Inst.Ref.void_type;

    for (out_args, 0..) |*arg, out_i| {
        const output = sema.code.extraData(Zir.Inst.Asm.Output, extra_i);
        extra_i = output.end;

        const is_type = @as(u1, @truncate(output_type_bits)) != 0;
        output_type_bits >>= 1;

        if (is_type) {
            // Indicate the output is the asm instruction return value.
            arg.* = .none;
            const out_ty = try sema.resolveType(block, ret_ty_src, output.data.operand);
            expr_ty = Air.internedToRef(out_ty.toIntern());
        } else {
            arg.* = try sema.resolveInst(output.data.operand);
        }

        const constraint = sema.code.nullTerminatedString(output.data.constraint);
        const name = sema.code.nullTerminatedString(output.data.name);
        needed_capacity += (constraint.len + name.len + (2 + 3)) / 4;

        if (output.data.operand.toIndex()) |index| {
            if (zir_tags[@intFromEnum(index)] == .ref) {
                // TODO: better error location; it would be even nicer if there were notes that pointed at the output and the variable definition
                return sema.fail(block, src, "asm cannot output to const local '{s}'", .{name});
            }
        }

        outputs[out_i] = .{ .c = constraint, .n = name };
    }

    const args = try sema.arena.alloc(Air.Inst.Ref, inputs_len);
    const inputs = try sema.arena.alloc(ConstraintName, inputs_len);

    for (args, 0..) |*arg, arg_i| {
        const input = sema.code.extraData(Zir.Inst.Asm.Input, extra_i);
        extra_i = input.end;

        const uncasted_arg = try sema.resolveInst(input.data.operand);
        const uncasted_arg_ty = sema.typeOf(uncasted_arg);
        switch (uncasted_arg_ty.zigTypeTag(zcu)) {
            .comptime_int => arg.* = try sema.coerce(block, .usize, uncasted_arg, src),
            .comptime_float => arg.* = try sema.coerce(block, .f64, uncasted_arg, src),
            else => {
                arg.* = uncasted_arg;
            },
        }

        const constraint = sema.code.nullTerminatedString(input.data.constraint);
        const name = sema.code.nullTerminatedString(input.data.name);
        needed_capacity += (constraint.len + name.len + (2 + 3)) / 4;
        inputs[arg_i] = .{ .c = constraint, .n = name };
    }

    const clobbers = try sema.arena.alloc([]const u8, clobbers_len);
    for (clobbers) |*name| {
        const name_index: Zir.NullTerminatedString = @enumFromInt(sema.code.extra[extra_i]);
        name.* = sema.code.nullTerminatedString(name_index);
        extra_i += 1;

        needed_capacity += name.*.len / 4 + 1;
    }

    needed_capacity += (asm_source.len + 3) / 4;

    const gpa = sema.gpa;
    try sema.air_extra.ensureUnusedCapacity(gpa, needed_capacity);
    const asm_air = try block.addInst(.{
        .tag = .assembly,
        .data = .{ .ty_pl = .{
            .ty = expr_ty,
            .payload = sema.addExtraAssumeCapacity(Air.Asm{
                .source_len = @intCast(asm_source.len),
                .outputs_len = outputs_len,
                .inputs_len = @intCast(args.len),
                .flags = (@as(u32, @intFromBool(is_volatile)) << 31) | @as(u32, @intCast(clobbers.len)),
            }),
        } },
    });
    sema.appendRefsAssumeCapacity(out_args);
    sema.appendRefsAssumeCapacity(args);
    for (outputs) |o| {
        const buffer = mem.sliceAsBytes(sema.air_extra.unusedCapacitySlice());
        @memcpy(buffer[0..o.c.len], o.c);
        buffer[o.c.len] = 0;
        @memcpy(buffer[o.c.len + 1 ..][0..o.n.len], o.n);
        buffer[o.c.len + 1 + o.n.len] = 0;
        sema.air_extra.items.len += (o.c.len + o.n.len + (2 + 3)) / 4;
    }
    for (inputs) |input| {
        const buffer = mem.sliceAsBytes(sema.air_extra.unusedCapacitySlice());
        @memcpy(buffer[0..input.c.len], input.c);
        buffer[input.c.len] = 0;
        @memcpy(buffer[input.c.len + 1 ..][0..input.n.len], input.n);
        buffer[input.c.len + 1 + input.n.len] = 0;
        sema.air_extra.items.len += (input.c.len + input.n.len + (2 + 3)) / 4;
    }
    for (clobbers) |clobber| {
        const buffer = mem.sliceAsBytes(sema.air_extra.unusedCapacitySlice());
        @memcpy(buffer[0..clobber.len], clobber);
        buffer[clobber.len] = 0;
        sema.air_extra.items.len += clobber.len / 4 + 1;
    }
    {
        const buffer = mem.sliceAsBytes(sema.air_extra.unusedCapacitySlice());
        @memcpy(buffer[0..asm_source.len], asm_source);
        sema.air_extra.items.len += (asm_source.len + 3) / 4;
    }
    return asm_air;
}

/// Only called for equality operators. See also `zirCmp`.
fn zirCmpEq(
    sema: *Sema,
    block: *Block,
    inst: Zir.Inst.Index,
    op: std.math.CompareOperator,
    air_tag: Air.Inst.Tag,
) CompileError!Air.Inst.Ref {
    const tracy = trace(@src());
    defer tracy.end();

    const pt = sema.pt;
    const zcu = pt.zcu;
    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].pl_node;
    const extra = sema.code.extraData(Zir.Inst.Bin, inst_data.payload_index).data;
    const src: LazySrcLoc = block.nodeOffset(inst_data.src_node);
    const lhs_src = block.src(.{ .node_offset_bin_lhs = inst_data.src_node });
    const rhs_src = block.src(.{ .node_offset_bin_rhs = inst_data.src_node });
    const lhs = try sema.resolveInst(extra.lhs);
    const rhs = try sema.resolveInst(extra.rhs);

    const lhs_ty = sema.typeOf(lhs);
    const rhs_ty = sema.typeOf(rhs);
    const lhs_ty_tag = lhs_ty.zigTypeTag(zcu);
    const rhs_ty_tag = rhs_ty.zigTypeTag(zcu);
    if (lhs_ty_tag == .null and rhs_ty_tag == .null) {
        // null == null, null != null
        return if (op == .eq) .bool_true else .bool_false;
    }

    // comparing null with optionals
    if (lhs_ty_tag == .null and (rhs_ty_tag == .optional or rhs_ty.isCPtr(zcu))) {
        return sema.analyzeIsNull(block, rhs, op == .neq);
    }
    if (rhs_ty_tag == .null and (lhs_ty_tag == .optional or lhs_ty.isCPtr(zcu))) {
        return sema.analyzeIsNull(block, lhs, op == .neq);
    }

    if (lhs_ty_tag == .null or rhs_ty_tag == .null) {
        const non_null_type = if (lhs_ty_tag == .null) rhs_ty else lhs_ty;
        return sema.fail(block, src, "comparison of '{}' with null", .{non_null_type.fmt(pt)});
    }

    if (lhs_ty_tag == .@"union" and (rhs_ty_tag == .enum_literal or rhs_ty_tag == .@"enum")) {
        return sema.analyzeCmpUnionTag(block, src, lhs, lhs_src, rhs, rhs_src, op);
    }
    if (rhs_ty_tag == .@"union" and (lhs_ty_tag == .enum_literal or lhs_ty_tag == .@"enum")) {
        return sema.analyzeCmpUnionTag(block, src, rhs, rhs_src, lhs, lhs_src, op);
    }

    if (lhs_ty_tag == .error_set and rhs_ty_tag == .error_set) {
        const runtime_src: LazySrcLoc = src: {
            if (try sema.resolveValue(lhs)) |lval| {
                if (try sema.resolveValue(rhs)) |rval| {
                    if (lval.isUndef(zcu) or rval.isUndef(zcu)) return .undef_bool;
                    const lkey = zcu.intern_pool.indexToKey(lval.toIntern());
                    const rkey = zcu.intern_pool.indexToKey(rval.toIntern());
                    return if ((lkey.err.name == rkey.err.name) == (op == .eq))
                        .bool_true
                    else
                        .bool_false;
                } else {
                    break :src rhs_src;
                }
            } else {
                break :src lhs_src;
            }
        };
        try sema.requireRuntimeBlock(block, src, runtime_src);
        return block.addBinOp(air_tag, lhs, rhs);
    }
    if (lhs_ty_tag == .type and rhs_ty_tag == .type) {
        const lhs_as_type = try sema.analyzeAsType(block, lhs_src, lhs);
        const rhs_as_type = try sema.analyzeAsType(block, rhs_src, rhs);
        return if (lhs_as_type.eql(rhs_as_type, zcu) == (op == .eq)) .bool_true else .bool_false;
    }
    return sema.analyzeCmp(block, src, lhs, rhs, op, lhs_src, rhs_src, true);
}

fn analyzeCmpUnionTag(
    sema: *Sema,
    block: *Block,
    src: LazySrcLoc,
    un: Air.Inst.Ref,
    un_src: LazySrcLoc,
    tag: Air.Inst.Ref,
    tag_src: LazySrcLoc,
    op: std.math.CompareOperator,
) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const union_ty = sema.typeOf(un);
    try union_ty.resolveFields(pt);
    const union_tag_ty = union_ty.unionTagType(zcu) orelse {
        const msg = msg: {
            const msg = try sema.errMsg(un_src, "comparison of union and enum literal is only valid for tagged union types", .{});
            errdefer msg.destroy(sema.gpa);
            try sema.errNote(union_ty.srcLoc(zcu), msg, "union '{}' is not a tagged union", .{union_ty.fmt(pt)});
            break :msg msg;
        };
        return sema.failWithOwnedErrorMsg(block, msg);
    };
    // Coerce both the union and the tag to the union's tag type, and then execute the
    // enum comparison codepath.
    const coerced_tag = try sema.coerce(block, union_tag_ty, tag, tag_src);
    const coerced_union = try sema.coerce(block, union_tag_ty, un, un_src);

    if (try sema.resolveValue(coerced_tag)) |enum_val| {
        if (enum_val.isUndef(zcu)) return .undef_bool;
        const field_ty = union_ty.unionFieldType(enum_val, zcu).?;
        if (field_ty.zigTypeTag(zcu) == .noreturn) {
            return .bool_false;
        }
    }

    return sema.cmpSelf(block, src, coerced_union, coerced_tag, op, un_src, tag_src);
}

/// Only called for non-equality operators. See also `zirCmpEq`.
fn zirCmp(
    sema: *Sema,
    block: *Block,
    inst: Zir.Inst.Index,
    op: std.math.CompareOperator,
) CompileError!Air.Inst.Ref {
    const tracy = trace(@src());
    defer tracy.end();

    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].pl_node;
    const extra = sema.code.extraData(Zir.Inst.Bin, inst_data.payload_index).data;
    const src: LazySrcLoc = block.nodeOffset(inst_data.src_node);
    const lhs_src = block.src(.{ .node_offset_bin_lhs = inst_data.src_node });
    const rhs_src = block.src(.{ .node_offset_bin_rhs = inst_data.src_node });
    const lhs = try sema.resolveInst(extra.lhs);
    const rhs = try sema.resolveInst(extra.rhs);
    return sema.analyzeCmp(block, src, lhs, rhs, op, lhs_src, rhs_src, false);
}

fn analyzeCmp(
    sema: *Sema,
    block: *Block,
    src: LazySrcLoc,
    lhs: Air.Inst.Ref,
    rhs: Air.Inst.Ref,
    op: std.math.CompareOperator,
    lhs_src: LazySrcLoc,
    rhs_src: LazySrcLoc,
    is_equality_cmp: bool,
) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const lhs_ty = sema.typeOf(lhs);
    const rhs_ty = sema.typeOf(rhs);
    if (lhs_ty.zigTypeTag(zcu) != .optional and rhs_ty.zigTypeTag(zcu) != .optional) {
        try sema.checkVectorizableBinaryOperands(block, src, lhs_ty, rhs_ty, lhs_src, rhs_src);
    }

    if (lhs_ty.zigTypeTag(zcu) == .vector and rhs_ty.zigTypeTag(zcu) == .vector) {
        return sema.cmpVector(block, src, lhs, rhs, op, lhs_src, rhs_src);
    }
    if (lhs_ty.isNumeric(zcu) and rhs_ty.isNumeric(zcu)) {
        // This operation allows any combination of integer and float types, regardless of the
        // signed-ness, comptime-ness, and bit-width. So peer type resolution is incorrect for
        // numeric types.
        return sema.cmpNumeric(block, src, lhs, rhs, op, lhs_src, rhs_src);
    }
    if (is_equality_cmp and lhs_ty.zigTypeTag(zcu) == .error_union and rhs_ty.zigTypeTag(zcu) == .error_set) {
        if (try sema.resolveDefinedValue(block, lhs_src, lhs)) |lhs_val| {
            if (lhs_val.errorUnionIsPayload(zcu)) return .bool_false;
        }
        const casted_lhs = try sema.analyzeErrUnionCode(block, lhs_src, lhs);
        return sema.cmpSelf(block, src, casted_lhs, rhs, op, lhs_src, rhs_src);
    }
    if (is_equality_cmp and lhs_ty.zigTypeTag(zcu) == .error_set and rhs_ty.zigTypeTag(zcu) == .error_union) {
        if (try sema.resolveDefinedValue(block, rhs_src, rhs)) |rhs_val| {
            if (rhs_val.errorUnionIsPayload(zcu)) return .bool_false;
        }
        const casted_rhs = try sema.analyzeErrUnionCode(block, rhs_src, rhs);
        return sema.cmpSelf(block, src, lhs, casted_rhs, op, lhs_src, rhs_src);
    }
    const instructions = &[_]Air.Inst.Ref{ lhs, rhs };
    const resolved_type = try sema.resolvePeerTypes(block, src, instructions, .{ .override = &[_]?LazySrcLoc{ lhs_src, rhs_src } });
    if (!resolved_type.isSelfComparable(zcu, is_equality_cmp)) {
        return sema.fail(block, src, "operator {s} not allowed for type '{}'", .{
            compareOperatorName(op), resolved_type.fmt(pt),
        });
    }
    const casted_lhs = try sema.coerce(block, resolved_type, lhs, lhs_src);
    const casted_rhs = try sema.coerce(block, resolved_type, rhs, rhs_src);
    return sema.cmpSelf(block, src, casted_lhs, casted_rhs, op, lhs_src, rhs_src);
}

fn compareOperatorName(comp: std.math.CompareOperator) []const u8 {
    return switch (comp) {
        .lt => "<",
        .lte => "<=",
        .eq => "==",
        .gte => ">=",
        .gt => ">",
        .neq => "!=",
    };
}

fn cmpSelf(
    sema: *Sema,
    block: *Block,
    src: LazySrcLoc,
    casted_lhs: Air.Inst.Ref,
    casted_rhs: Air.Inst.Ref,
    op: std.math.CompareOperator,
    lhs_src: LazySrcLoc,
    rhs_src: LazySrcLoc,
) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const resolved_type = sema.typeOf(casted_lhs);

    const maybe_lhs_val = try sema.resolveValue(casted_lhs);
    const maybe_rhs_val = try sema.resolveValue(casted_rhs);
    if (maybe_lhs_val) |v| if (v.isUndef(zcu)) return .undef_bool;
    if (maybe_rhs_val) |v| if (v.isUndef(zcu)) return .undef_bool;

    const runtime_src: LazySrcLoc = src: {
        if (maybe_lhs_val) |lhs_val| {
            if (maybe_rhs_val) |rhs_val| {
                if (resolved_type.zigTypeTag(zcu) == .vector) {
                    const cmp_val = try sema.compareVector(lhs_val, op, rhs_val, resolved_type);
                    return Air.internedToRef(cmp_val.toIntern());
                }

                return if (try sema.compareAll(lhs_val, op, rhs_val, resolved_type))
                    .bool_true
                else
                    .bool_false;
            } else {
                if (resolved_type.zigTypeTag(zcu) == .bool) {
                    // We can lower bool eq/neq more efficiently.
                    return sema.runtimeBoolCmp(block, src, op, casted_rhs, lhs_val.toBool(), rhs_src);
                }
                break :src rhs_src;
            }
        } else {
            // For bools, we still check the other operand, because we can lower
            // bool eq/neq more efficiently.
            if (resolved_type.zigTypeTag(zcu) == .bool) {
                if (maybe_rhs_val) |rhs_val| {
                    return sema.runtimeBoolCmp(block, src, op, casted_lhs, rhs_val.toBool(), lhs_src);
                }
            }
            break :src lhs_src;
        }
    };
    try sema.requireRuntimeBlock(block, src, runtime_src);
    if (resolved_type.zigTypeTag(zcu) == .vector) {
        return block.addCmpVector(casted_lhs, casted_rhs, op);
    }
    const tag = Air.Inst.Tag.fromCmpOp(op, block.float_mode == .optimized);
    return block.addBinOp(tag, casted_lhs, casted_rhs);
}

/// cmp_eq (x, false) => not(x)
/// cmp_eq (x, true ) => x
/// cmp_neq(x, false) => x
/// cmp_neq(x, true ) => not(x)
fn runtimeBoolCmp(
    sema: *Sema,
    block: *Block,
    src: LazySrcLoc,
    op: std.math.CompareOperator,
    lhs: Air.Inst.Ref,
    rhs: bool,
    runtime_src: LazySrcLoc,
) CompileError!Air.Inst.Ref {
    if ((op == .neq) == rhs) {
        try sema.requireRuntimeBlock(block, src, runtime_src);
        return block.addTyOp(.not, .bool, lhs);
    } else {
        return lhs;
    }
}

fn zirSizeOf(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].un_node;
    const operand_src = block.builtinCallArgSrc(inst_data.src_node, 0);
    const ty = try sema.resolveType(block, operand_src, inst_data.operand);
    switch (ty.zigTypeTag(pt.zcu)) {
        .@"fn",
        .noreturn,
        .undefined,
        .null,
        .@"opaque",
        => return sema.fail(block, operand_src, "no size available for type '{}'", .{ty.fmt(pt)}),

        .type,
        .enum_literal,
        .comptime_float,
        .comptime_int,
        .void,
        => return .zero,

        .bool,
        .int,
        .float,
        .pointer,
        .array,
        .@"struct",
        .optional,
        .error_union,
        .error_set,
        .@"enum",
        .@"union",
        .vector,
        .frame,
        .@"anyframe",
        => {},
    }
    const val = try ty.abiSizeLazy(pt);
    return Air.internedToRef(val.toIntern());
}

fn zirBitSizeOf(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].un_node;
    const operand_src = block.builtinCallArgSrc(inst_data.src_node, 0);
    const operand_ty = try sema.resolveType(block, operand_src, inst_data.operand);
    switch (operand_ty.zigTypeTag(zcu)) {
        .@"fn",
        .noreturn,
        .undefined,
        .null,
        .@"opaque",
        => return sema.fail(block, operand_src, "no size available for type '{}'", .{operand_ty.fmt(pt)}),

        .type,
        .enum_literal,
        .comptime_float,
        .comptime_int,
        .void,
        => return .zero,

        .bool,
        .int,
        .float,
        .pointer,
        .array,
        .@"struct",
        .optional,
        .error_union,
        .error_set,
        .@"enum",
        .@"union",
        .vector,
        .frame,
        .@"anyframe",
        => {},
    }
    const bit_size = try operand_ty.bitSizeSema(pt);
    return pt.intRef(.comptime_int, bit_size);
}

fn zirThis(
    sema: *Sema,
    block: *Block,
    extended: Zir.Inst.Extended.InstData,
) CompileError!Air.Inst.Ref {
    _ = extended;
    const pt = sema.pt;
    const zcu = pt.zcu;
    const namespace = pt.zcu.namespacePtr(block.namespace);

    switch (pt.zcu.intern_pool.indexToKey(namespace.owner_type)) {
        .opaque_type => {
            // Opaque types are never outdated since they don't undergo type resolution, so nothing to do!
            return Air.internedToRef(namespace.owner_type);
        },
        .struct_type, .union_type => {
            const new_ty = try pt.ensureTypeUpToDate(namespace.owner_type);
            try sema.declareDependency(.{ .interned = new_ty });
            return Air.internedToRef(new_ty);
        },
        .enum_type => {
            const new_ty = try pt.ensureTypeUpToDate(namespace.owner_type);
            try sema.declareDependency(.{ .interned = new_ty });
            // Since this is an enum, it has to be resolved immediately.
            // `ensureTypeUpToDate` has resolved the new type if necessary.
            // We just need to check for resolution failures.
            const ty_unit: AnalUnit = .wrap(.{ .type = new_ty });
            if (zcu.failed_analysis.contains(ty_unit) or zcu.transitive_failed_analysis.contains(ty_unit)) {
                return error.AnalysisFail;
            }
            return Air.internedToRef(new_ty);
        },
        else => unreachable,
    }
}

fn zirClosureGet(sema: *Sema, block: *Block, extended: Zir.Inst.Extended.InstData) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const ip = &zcu.intern_pool;
    const captures = Type.fromInterned(zcu.namespacePtr(block.namespace).owner_type).getCaptures(zcu);

    const src_node: std.zig.Ast.Node.Offset = @enumFromInt(@as(i32, @bitCast(extended.operand)));
    const src = block.nodeOffset(src_node);

    const capture_ty = switch (captures.get(ip)[extended.small].unwrap()) {
        .@"comptime" => |index| return Air.internedToRef(index),
        .runtime => |index| index,
        .nav_val => |nav| return sema.analyzeNavVal(block, src, nav),
        .nav_ref => |nav| return sema.analyzeNavRef(block, src, nav),
    };

    // The comptime case is handled already above. Runtime case below.

    if (!block.is_typeof and sema.func_index == .none) {
        const msg = msg: {
            const name = name: {
                // TODO: we should probably store this name in the ZIR to avoid this complexity.
                const file, const src_base_node = Zcu.LazySrcLoc.resolveBaseNode(block.src_base_inst, zcu).?;
                const tree = file.getTree(zcu) catch |err| {
                    // In this case we emit a warning + a less precise source location.
                    log.warn("unable to load {}: {s}", .{
                        file.path.fmt(zcu.comp), @errorName(err),
                    });
                    break :name null;
                };
                const node = src_node.toAbsolute(src_base_node);
                const token = tree.nodeMainToken(node);
                break :name tree.tokenSlice(token);
            };

            const msg = if (name) |some|
                try sema.errMsg(src, "'{s}' not accessible outside function scope", .{some})
            else
                try sema.errMsg(src, "variable not accessible outside function scope", .{});
            errdefer msg.destroy(sema.gpa);

            // TODO add "declared here" note
            break :msg msg;
        };
        return sema.failWithOwnedErrorMsg(block, msg);
    }

    if (!block.is_typeof and !block.isComptime() and sema.func_index != .none) {
        const msg = msg: {
            const name = name: {
                const file, const src_base_node = Zcu.LazySrcLoc.resolveBaseNode(block.src_base_inst, zcu).?;
                const tree = file.getTree(zcu) catch |err| {
                    // In this case we emit a warning + a less precise source location.
                    log.warn("unable to load {}: {s}", .{
                        file.path.fmt(zcu.comp), @errorName(err),
                    });
                    break :name null;
                };
                const node = src_node.toAbsolute(src_base_node);
                const token = tree.nodeMainToken(node);
                break :name tree.tokenSlice(token);
            };

            const msg = if (name) |some|
                try sema.errMsg(src, "'{s}' not accessible from inner function", .{some})
            else
                try sema.errMsg(src, "variable not accessible from inner function", .{});
            errdefer msg.destroy(sema.gpa);

            try sema.errNote(block.nodeOffset(.zero), msg, "crossed function definition here", .{});

            // TODO add "declared here" note
            break :msg msg;
        };
        return sema.failWithOwnedErrorMsg(block, msg);
    }

    assert(block.is_typeof);
    // We need a dummy runtime instruction with the correct type.
    return block.addTy(.alloc, .fromInterned(capture_ty));
}

fn zirRetAddr(
    sema: *Sema,
    block: *Block,
    extended: Zir.Inst.Extended.InstData,
) CompileError!Air.Inst.Ref {
    _ = sema;
    _ = extended;
    if (block.isComptime()) {
        // TODO: we could give a meaningful lazy value here. #14938
        return .zero_usize;
    } else {
        return block.addNoOp(.ret_addr);
    }
}

fn zirFrameAddress(
    sema: *Sema,
    block: *Block,
    extended: Zir.Inst.Extended.InstData,
) CompileError!Air.Inst.Ref {
    const src_node: std.zig.Ast.Node.Offset = @enumFromInt(@as(i32, @bitCast(extended.operand)));
    const src = block.nodeOffset(src_node);
    try sema.requireRuntimeBlock(block, src, null);
    return try block.addNoOp(.frame_addr);
}

fn zirBuiltinSrc(
    sema: *Sema,
    block: *Block,
    extended: Zir.Inst.Extended.InstData,
) CompileError!Air.Inst.Ref {
    const tracy = trace(@src());
    defer tracy.end();

    const pt = sema.pt;
    const zcu = pt.zcu;
    const ip = &zcu.intern_pool;
    const extra = sema.code.extraData(Zir.Inst.Src, extended.operand).data;
    const fn_name = ip.getNav(zcu.funcInfo(sema.func_index).owner_nav).name;
    const gpa = sema.gpa;
    const file_scope = block.getFileScope(zcu);

    const func_name_val = v: {
        const func_name_len = fn_name.length(ip);
        const array_ty = try pt.intern(.{ .array_type = .{
            .len = func_name_len,
            .sentinel = .zero_u8,
            .child = .u8_type,
        } });
        break :v try pt.intern(.{ .slice = .{
            .ty = .slice_const_u8_sentinel_0_type,
            .ptr = try pt.intern(.{ .ptr = .{
                .ty = .manyptr_const_u8_sentinel_0_type,
                .base_addr = .{ .uav = .{
                    .orig_ty = .slice_const_u8_sentinel_0_type,
                    .val = try pt.intern(.{ .aggregate = .{
                        .ty = array_ty,
                        .storage = .{ .bytes = fn_name.toString() },
                    } }),
                } },
                .byte_offset = 0,
            } }),
            .len = (try pt.intValue(.usize, func_name_len)).toIntern(),
        } });
    };

    const module_name_val = v: {
        const module_name = file_scope.mod.?.fully_qualified_name;
        const array_ty = try pt.intern(.{ .array_type = .{
            .len = module_name.len,
            .sentinel = .zero_u8,
            .child = .u8_type,
        } });
        break :v try pt.intern(.{ .slice = .{
            .ty = .slice_const_u8_sentinel_0_type,
            .ptr = try pt.intern(.{ .ptr = .{
                .ty = .manyptr_const_u8_sentinel_0_type,
                .base_addr = .{ .uav = .{
                    .orig_ty = .slice_const_u8_sentinel_0_type,
                    .val = try pt.intern(.{ .aggregate = .{
                        .ty = array_ty,
                        .storage = .{
                            .bytes = try ip.getOrPutString(gpa, pt.tid, module_name, .maybe_embedded_nulls),
                        },
                    } }),
                } },
                .byte_offset = 0,
            } }),
            .len = (try pt.intValue(.usize, module_name.len)).toIntern(),
        } });
    };

    const file_name_val = v: {
        const file_name = file_scope.sub_file_path;
        const array_ty = try pt.intern(.{ .array_type = .{
            .len = file_name.len,
            .sentinel = .zero_u8,
            .child = .u8_type,
        } });
        break :v try pt.intern(.{ .slice = .{
            .ty = .slice_const_u8_sentinel_0_type,
            .ptr = try pt.intern(.{ .ptr = .{
                .ty = .manyptr_const_u8_sentinel_0_type,
                .base_addr = .{ .uav = .{
                    .orig_ty = .slice_const_u8_sentinel_0_type,
                    .val = try pt.intern(.{ .aggregate = .{
                        .ty = array_ty,
                        .storage = .{
                            .bytes = try ip.getOrPutString(gpa, pt.tid, file_name, .maybe_embedded_nulls),
                        },
                    } }),
                } },
                .byte_offset = 0,
            } }),
            .len = (try pt.intValue(.usize, file_name.len)).toIntern(),
        } });
    };

    const src_loc_ty = try sema.getBuiltinType(block.nodeOffset(.zero), .SourceLocation);
    const fields = .{
        // module: [:0]const u8,
        module_name_val,
        // file: [:0]const u8,
        file_name_val,
        // fn_name: [:0]const u8,
        func_name_val,
        // line: u32,
        (try pt.intValue(.u32, extra.line + 1)).toIntern(),
        // column: u32,
        (try pt.intValue(.u32, extra.column + 1)).toIntern(),
    };
    return Air.internedToRef((try pt.intern(.{ .aggregate = .{
        .ty = src_loc_ty.toIntern(),
        .storage = .{ .elems = &fields },
    } })));
}

fn zirTypeInfo(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const gpa = sema.gpa;
    const ip = &zcu.intern_pool;
    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].un_node;
    const src = block.nodeOffset(inst_data.src_node);
    const ty = try sema.resolveType(block, src, inst_data.operand);
    const type_info_ty = try sema.getBuiltinType(src, .Type);
    const type_info_tag_ty = type_info_ty.unionTagType(zcu).?;

    if (ty.typeDeclInst(zcu)) |type_decl_inst| {
        try sema.declareDependency(.{ .namespace = type_decl_inst });
    }

    switch (ty.zigTypeTag(zcu)) {
        .type,
        .void,
        .bool,
        .noreturn,
        .comptime_float,
        .comptime_int,
        .undefined,
        .null,
        .enum_literal,
        => |type_info_tag| return unionInitFromEnumTag(sema, block, src, type_info_ty, @intFromEnum(type_info_tag), .void_value),

        .@"fn" => {
            const fn_info_ty = try sema.getBuiltinType(src, .@"Type.Fn");
            const param_info_ty = try sema.getBuiltinType(src, .@"Type.Fn.Param");

            const func_ty_info = zcu.typeToFunc(ty).?;
            const param_vals = try sema.arena.alloc(InternPool.Index, func_ty_info.param_types.len);
            for (param_vals, 0..) |*param_val, i| {
                const param_ty = func_ty_info.param_types.get(ip)[i];
                const is_generic = param_ty == .generic_poison_type;
                const param_ty_val = try pt.intern(.{ .opt = .{
                    .ty = try pt.intern(.{ .opt_type = .type_type }),
                    .val = if (is_generic) .none else param_ty,
                } });

                const is_noalias = blk: {
                    const index = std.math.cast(u5, i) orelse break :blk false;
                    break :blk @as(u1, @truncate(func_ty_info.noalias_bits >> index)) != 0;
                };

                const param_fields = .{
                    // is_generic: bool,
                    Value.makeBool(is_generic).toIntern(),
                    // is_noalias: bool,
                    Value.makeBool(is_noalias).toIntern(),
                    // type: ?type,
                    param_ty_val,
                };
                param_val.* = try pt.intern(.{ .aggregate = .{
                    .ty = param_info_ty.toIntern(),
                    .storage = .{ .elems = &param_fields },
                } });
            }

            const args_val = v: {
                const new_decl_ty = try pt.arrayType(.{
                    .len = param_vals.len,
                    .child = param_info_ty.toIntern(),
                });
                const new_decl_val = try pt.intern(.{ .aggregate = .{
                    .ty = new_decl_ty.toIntern(),
                    .storage = .{ .elems = param_vals },
                } });
                const slice_ty = (try pt.ptrTypeSema(.{
                    .child = param_info_ty.toIntern(),
                    .flags = .{
                        .size = .slice,
                        .is_const = true,
                    },
                })).toIntern();
                const manyptr_ty = Type.fromInterned(slice_ty).slicePtrFieldType(zcu).toIntern();
                break :v try pt.intern(.{ .slice = .{
                    .ty = slice_ty,
                    .ptr = try pt.intern(.{ .ptr = .{
                        .ty = manyptr_ty,
                        .base_addr = .{ .uav = .{
                            .orig_ty = manyptr_ty,
                            .val = new_decl_val,
                        } },
                        .byte_offset = 0,
                    } }),
                    .len = (try pt.intValue(.usize, param_vals.len)).toIntern(),
                } });
            };

            const ret_ty_opt = try pt.intern(.{ .opt = .{
                .ty = try pt.intern(.{ .opt_type = .type_type }),
                .val = opt_val: {
                    const ret_ty: Type = .fromInterned(func_ty_info.return_type);
                    if (ret_ty.toIntern() == .generic_poison_type) break :opt_val .none;
                    if (ret_ty.zigTypeTag(zcu) == .error_union) {
                        if (ret_ty.errorUnionPayload(zcu).toIntern() == .generic_poison_type) {
                            break :opt_val .none;
                        }
                    }
                    break :opt_val ret_ty.toIntern();
                },
            } });

            const callconv_ty = try sema.getBuiltinType(src, .CallingConvention);
            const callconv_val = Value.uninterpret(func_ty_info.cc, callconv_ty, pt) catch |err| switch (err) {
                error.TypeMismatch => @panic("std.builtin is corrupt"),
                error.OutOfMemory => |e| return e,
            };

            const field_values: [5]InternPool.Index = .{
                // calling_convention: CallingConvention,
                callconv_val.toIntern(),
                // is_generic: bool,
                Value.makeBool(func_ty_info.is_generic).toIntern(),
                // is_var_args: bool,
                Value.makeBool(func_ty_info.is_var_args).toIntern(),
                // return_type: ?type,
                ret_ty_opt,
                // args: []const Fn.Param,
                args_val,
            };
            return Air.internedToRef((try pt.internUnion(.{
                .ty = type_info_ty.toIntern(),
                .tag = (try pt.enumValueFieldIndex(type_info_tag_ty, @intFromEnum(std.builtin.TypeId.@"fn"))).toIntern(),
                .val = try pt.intern(.{ .aggregate = .{
                    .ty = fn_info_ty.toIntern(),
                    .storage = .{ .elems = &field_values },
                } }),
            })));
        },
        .int => {
            const int_info_ty = try sema.getBuiltinType(src, .@"Type.Int");
            const signedness_ty = try sema.getBuiltinType(src, .Signedness);
            const info = ty.intInfo(zcu);
            const field_values = .{
                // signedness: Signedness,
                (try pt.enumValueFieldIndex(signedness_ty, @intFromEnum(info.signedness))).toIntern(),
                // bits: u16,
                (try pt.intValue(.u16, info.bits)).toIntern(),
            };
            return Air.internedToRef((try pt.internUnion(.{
                .ty = type_info_ty.toIntern(),
                .tag = (try pt.enumValueFieldIndex(type_info_tag_ty, @intFromEnum(std.builtin.TypeId.int))).toIntern(),
                .val = try pt.intern(.{ .aggregate = .{
                    .ty = int_info_ty.toIntern(),
                    .storage = .{ .elems = &field_values },
                } }),
            })));
        },
        .float => {
            const float_info_ty = try sema.getBuiltinType(src, .@"Type.Float");

            const field_vals = .{
                // bits: u16,
                (try pt.intValue(.u16, ty.bitSize(zcu))).toIntern(),
            };
            return Air.internedToRef((try pt.internUnion(.{
                .ty = type_info_ty.toIntern(),
                .tag = (try pt.enumValueFieldIndex(type_info_tag_ty, @intFromEnum(std.builtin.TypeId.float))).toIntern(),
                .val = try pt.intern(.{ .aggregate = .{
                    .ty = float_info_ty.toIntern(),
                    .storage = .{ .elems = &field_vals },
                } }),
            })));
        },
        .pointer => {
            const info = ty.ptrInfo(zcu);
            const alignment = if (info.flags.alignment.toByteUnits()) |alignment|
                try pt.intValue(.comptime_int, alignment)
            else
                try Type.fromInterned(info.child).lazyAbiAlignment(pt);

            const addrspace_ty = try sema.getBuiltinType(src, .AddressSpace);
            const pointer_ty = try sema.getBuiltinType(src, .@"Type.Pointer");
            const ptr_size_ty = try sema.getBuiltinType(src, .@"Type.Pointer.Size");

            const field_values = .{
                // size: Size,
                (try pt.enumValueFieldIndex(ptr_size_ty, @intFromEnum(info.flags.size))).toIntern(),
                // is_const: bool,
                Value.makeBool(info.flags.is_const).toIntern(),
                // is_volatile: bool,
                Value.makeBool(info.flags.is_volatile).toIntern(),
                // alignment: comptime_int,
                alignment.toIntern(),
                // address_space: AddressSpace
                (try pt.enumValueFieldIndex(addrspace_ty, @intFromEnum(info.flags.address_space))).toIntern(),
                // child: type,
                info.child,
                // is_allowzero: bool,
                Value.makeBool(info.flags.is_allowzero).toIntern(),
                // sentinel: ?*const anyopaque,
                (try sema.optRefValue(switch (info.sentinel) {
                    .none => null,
                    else => Value.fromInterned(info.sentinel),
                })).toIntern(),
            };
            return Air.internedToRef((try pt.internUnion(.{
                .ty = type_info_ty.toIntern(),
                .tag = (try pt.enumValueFieldIndex(type_info_tag_ty, @intFromEnum(std.builtin.TypeId.pointer))).toIntern(),
                .val = try pt.intern(.{ .aggregate = .{
                    .ty = pointer_ty.toIntern(),
                    .storage = .{ .elems = &field_values },
                } }),
            })));
        },
        .array => {
            const array_field_ty = try sema.getBuiltinType(src, .@"Type.Array");

            const info = ty.arrayInfo(zcu);
            const field_values = .{
                // len: comptime_int,
                (try pt.intValue(.comptime_int, info.len)).toIntern(),
                // child: type,
                info.elem_type.toIntern(),
                // sentinel: ?*const anyopaque,
                (try sema.optRefValue(info.sentinel)).toIntern(),
            };
            return Air.internedToRef((try pt.internUnion(.{
                .ty = type_info_ty.toIntern(),
                .tag = (try pt.enumValueFieldIndex(type_info_tag_ty, @intFromEnum(std.builtin.TypeId.array))).toIntern(),
                .val = try pt.intern(.{ .aggregate = .{
                    .ty = array_field_ty.toIntern(),
                    .storage = .{ .elems = &field_values },
                } }),
            })));
        },
        .vector => {
            const vector_field_ty = try sema.getBuiltinType(src, .@"Type.Vector");

            const info = ty.arrayInfo(zcu);
            const field_values = .{
                // len: comptime_int,
                (try pt.intValue(.comptime_int, info.len)).toIntern(),
                // child: type,
                info.elem_type.toIntern(),
            };
            return Air.internedToRef((try pt.internUnion(.{
                .ty = type_info_ty.toIntern(),
                .tag = (try pt.enumValueFieldIndex(type_info_tag_ty, @intFromEnum(std.builtin.TypeId.vector))).toIntern(),
                .val = try pt.intern(.{ .aggregate = .{
                    .ty = vector_field_ty.toIntern(),
                    .storage = .{ .elems = &field_values },
                } }),
            })));
        },
        .optional => {
            const optional_field_ty = try sema.getBuiltinType(src, .@"Type.Optional");

            const field_values = .{
                // child: type,
                ty.optionalChild(zcu).toIntern(),
            };
            return Air.internedToRef((try pt.internUnion(.{
                .ty = type_info_ty.toIntern(),
                .tag = (try pt.enumValueFieldIndex(type_info_tag_ty, @intFromEnum(std.builtin.TypeId.optional))).toIntern(),
                .val = try pt.intern(.{ .aggregate = .{
                    .ty = optional_field_ty.toIntern(),
                    .storage = .{ .elems = &field_values },
                } }),
            })));
        },
        .error_set => {
            // Get the Error type
            const error_field_ty = try sema.getBuiltinType(src, .@"Type.Error");

            // Build our list of Error values
            // Optional value is only null if anyerror
            // Value can be zero-length slice otherwise
            const error_field_vals = switch (try sema.resolveInferredErrorSetTy(block, src, ty.toIntern())) {
                .anyerror_type => null,
                else => |err_set_ty_index| blk: {
                    const names = ip.indexToKey(err_set_ty_index).error_set_type.names;
                    const vals = try sema.arena.alloc(InternPool.Index, names.len);
                    for (vals, 0..) |*field_val, error_index| {
                        const error_name = names.get(ip)[error_index];
                        const error_name_len = error_name.length(ip);
                        const error_name_val = v: {
                            const new_decl_ty = try pt.arrayType(.{
                                .len = error_name_len,
                                .sentinel = .zero_u8,
                                .child = .u8_type,
                            });
                            const new_decl_val = try pt.intern(.{ .aggregate = .{
                                .ty = new_decl_ty.toIntern(),
                                .storage = .{ .bytes = error_name.toString() },
                            } });
                            break :v try pt.intern(.{ .slice = .{
                                .ty = .slice_const_u8_sentinel_0_type,
                                .ptr = try pt.intern(.{ .ptr = .{
                                    .ty = .manyptr_const_u8_sentinel_0_type,
                                    .base_addr = .{ .uav = .{
                                        .val = new_decl_val,
                                        .orig_ty = .slice_const_u8_sentinel_0_type,
                                    } },
                                    .byte_offset = 0,
                                } }),
                                .len = (try pt.intValue(.usize, error_name_len)).toIntern(),
                            } });
                        };

                        const error_field_fields = .{
                            // name: [:0]const u8,
                            error_name_val,
                        };
                        field_val.* = try pt.intern(.{ .aggregate = .{
                            .ty = error_field_ty.toIntern(),
                            .storage = .{ .elems = &error_field_fields },
                        } });
                    }

                    break :blk vals;
                },
            };

            // Build our ?[]const Error value
            const slice_errors_ty = try pt.ptrTypeSema(.{
                .child = error_field_ty.toIntern(),
                .flags = .{
                    .size = .slice,
                    .is_const = true,
                },
            });
            const opt_slice_errors_ty = try pt.optionalType(slice_errors_ty.toIntern());
            const errors_payload_val: InternPool.Index = if (error_field_vals) |vals| v: {
                const array_errors_ty = try pt.arrayType(.{
                    .len = vals.len,
                    .child = error_field_ty.toIntern(),
                });
                const new_decl_val = try pt.intern(.{ .aggregate = .{
                    .ty = array_errors_ty.toIntern(),
                    .storage = .{ .elems = vals },
                } });
                const manyptr_errors_ty = slice_errors_ty.slicePtrFieldType(zcu).toIntern();
                break :v try pt.intern(.{ .slice = .{
                    .ty = slice_errors_ty.toIntern(),
                    .ptr = try pt.intern(.{ .ptr = .{
                        .ty = manyptr_errors_ty,
                        .base_addr = .{ .uav = .{
                            .orig_ty = manyptr_errors_ty,
                            .val = new_decl_val,
                        } },
                        .byte_offset = 0,
                    } }),
                    .len = (try pt.intValue(.usize, vals.len)).toIntern(),
                } });
            } else .none;
            const errors_val = try pt.intern(.{ .opt = .{
                .ty = opt_slice_errors_ty.toIntern(),
                .val = errors_payload_val,
            } });

            // Construct Type{ .error_set = errors_val }
            return Air.internedToRef((try pt.internUnion(.{
                .ty = type_info_ty.toIntern(),
                .tag = (try pt.enumValueFieldIndex(type_info_tag_ty, @intFromEnum(std.builtin.TypeId.error_set))).toIntern(),
                .val = errors_val,
            })));
        },
        .error_union => {
            const error_union_field_ty = try sema.getBuiltinType(src, .@"Type.ErrorUnion");

            const field_values = .{
                // error_set: type,
                ty.errorUnionSet(zcu).toIntern(),
                // payload: type,
                ty.errorUnionPayload(zcu).toIntern(),
            };
            return Air.internedToRef((try pt.internUnion(.{
                .ty = type_info_ty.toIntern(),
                .tag = (try pt.enumValueFieldIndex(type_info_tag_ty, @intFromEnum(std.builtin.TypeId.error_union))).toIntern(),
                .val = try pt.intern(.{ .aggregate = .{
                    .ty = error_union_field_ty.toIntern(),
                    .storage = .{ .elems = &field_values },
                } }),
            })));
        },
        .@"enum" => {
            const is_exhaustive = Value.makeBool(ip.loadEnumType(ty.toIntern()).tag_mode != .nonexhaustive);

            const enum_field_ty = try sema.getBuiltinType(src, .@"Type.EnumField");

            const enum_field_vals = try sema.arena.alloc(InternPool.Index, ip.loadEnumType(ty.toIntern()).names.len);
            for (enum_field_vals, 0..) |*field_val, tag_index| {
                const enum_type = ip.loadEnumType(ty.toIntern());
                const value_val = if (enum_type.values.len > 0)
                    try ip.getCoercedInts(
                        zcu.gpa,
                        pt.tid,
                        ip.indexToKey(enum_type.values.get(ip)[tag_index]).int,
                        .comptime_int_type,
                    )
                else
                    (try pt.intValue(.comptime_int, tag_index)).toIntern();

                // TODO: write something like getCoercedInts to avoid needing to dupe
                const name_val = v: {
                    const tag_name = enum_type.names.get(ip)[tag_index];
                    const tag_name_len = tag_name.length(ip);
                    const new_decl_ty = try pt.arrayType(.{
                        .len = tag_name_len,
                        .sentinel = .zero_u8,
                        .child = .u8_type,
                    });
                    const new_decl_val = try pt.intern(.{ .aggregate = .{
                        .ty = new_decl_ty.toIntern(),
                        .storage = .{ .bytes = tag_name.toString() },
                    } });
                    break :v try pt.intern(.{ .slice = .{
                        .ty = .slice_const_u8_sentinel_0_type,
                        .ptr = try pt.intern(.{ .ptr = .{
                            .ty = .manyptr_const_u8_sentinel_0_type,
                            .base_addr = .{ .uav = .{
                                .val = new_decl_val,
                                .orig_ty = .slice_const_u8_sentinel_0_type,
                            } },
                            .byte_offset = 0,
                        } }),
                        .len = (try pt.intValue(.usize, tag_name_len)).toIntern(),
                    } });
                };

                const enum_field_fields = .{
                    // name: [:0]const u8,
                    name_val,
                    // value: comptime_int,
                    value_val,
                };
                field_val.* = try pt.intern(.{ .aggregate = .{
                    .ty = enum_field_ty.toIntern(),
                    .storage = .{ .elems = &enum_field_fields },
                } });
            }

            const fields_val = v: {
                const fields_array_ty = try pt.arrayType(.{
                    .len = enum_field_vals.len,
                    .child = enum_field_ty.toIntern(),
                });
                const new_decl_val = try pt.intern(.{ .aggregate = .{
                    .ty = fields_array_ty.toIntern(),
                    .storage = .{ .elems = enum_field_vals },
                } });
                const slice_ty = (try pt.ptrTypeSema(.{
                    .child = enum_field_ty.toIntern(),
                    .flags = .{
                        .size = .slice,
                        .is_const = true,
                    },
                })).toIntern();
                const manyptr_ty = Type.fromInterned(slice_ty).slicePtrFieldType(zcu).toIntern();
                break :v try pt.intern(.{ .slice = .{
                    .ty = slice_ty,
                    .ptr = try pt.intern(.{ .ptr = .{
                        .ty = manyptr_ty,
                        .base_addr = .{ .uav = .{
                            .val = new_decl_val,
                            .orig_ty = manyptr_ty,
                        } },
                        .byte_offset = 0,
                    } }),
                    .len = (try pt.intValue(.usize, enum_field_vals.len)).toIntern(),
                } });
            };

            const decls_val = try sema.typeInfoDecls(block, src, ip.loadEnumType(ty.toIntern()).namespace.toOptional());

            const type_enum_ty = try sema.getBuiltinType(src, .@"Type.Enum");

            const field_values = .{
                // tag_type: type,
                ip.loadEnumType(ty.toIntern()).tag_ty,
                // fields: []const EnumField,
                fields_val,
                // decls: []const Declaration,
                decls_val,
                // is_exhaustive: bool,
                is_exhaustive.toIntern(),
            };
            return Air.internedToRef((try pt.internUnion(.{
                .ty = type_info_ty.toIntern(),
                .tag = (try pt.enumValueFieldIndex(type_info_tag_ty, @intFromEnum(std.builtin.TypeId.@"enum"))).toIntern(),
                .val = try pt.intern(.{ .aggregate = .{
                    .ty = type_enum_ty.toIntern(),
                    .storage = .{ .elems = &field_values },
                } }),
            })));
        },
        .@"union" => {
            const type_union_ty = try sema.getBuiltinType(src, .@"Type.Union");
            const union_field_ty = try sema.getBuiltinType(src, .@"Type.UnionField");

            try ty.resolveLayout(pt); // Getting alignment requires type layout
            const union_obj = zcu.typeToUnion(ty).?;
            const tag_type = union_obj.loadTagType(ip);
            const layout = union_obj.flagsUnordered(ip).layout;

            const union_field_vals = try gpa.alloc(InternPool.Index, tag_type.names.len);
            defer gpa.free(union_field_vals);

            for (union_field_vals, 0..) |*field_val, field_index| {
                const name_val = v: {
                    const field_name = tag_type.names.get(ip)[field_index];
                    const field_name_len = field_name.length(ip);
                    const new_decl_ty = try pt.arrayType(.{
                        .len = field_name_len,
                        .sentinel = .zero_u8,
                        .child = .u8_type,
                    });
                    const new_decl_val = try pt.intern(.{ .aggregate = .{
                        .ty = new_decl_ty.toIntern(),
                        .storage = .{ .bytes = field_name.toString() },
                    } });
                    break :v try pt.intern(.{ .slice = .{
                        .ty = .slice_const_u8_sentinel_0_type,
                        .ptr = try pt.intern(.{ .ptr = .{
                            .ty = .manyptr_const_u8_sentinel_0_type,
                            .base_addr = .{ .uav = .{
                                .val = new_decl_val,
                                .orig_ty = .slice_const_u8_sentinel_0_type,
                            } },
                            .byte_offset = 0,
                        } }),
                        .len = (try pt.intValue(.usize, field_name_len)).toIntern(),
                    } });
                };

                const alignment = switch (layout) {
                    .auto, .@"extern" => try ty.fieldAlignmentSema(field_index, pt),
                    .@"packed" => .none,
                };

                const field_ty = union_obj.field_types.get(ip)[field_index];
                const union_field_fields = .{
                    // name: [:0]const u8,
                    name_val,
                    // type: type,
                    field_ty,
                    // alignment: comptime_int,
                    (try pt.intValue(.comptime_int, alignment.toByteUnits() orelse 0)).toIntern(),
                };
                field_val.* = try pt.intern(.{ .aggregate = .{
                    .ty = union_field_ty.toIntern(),
                    .storage = .{ .elems = &union_field_fields },
                } });
            }

            const fields_val = v: {
                const array_fields_ty = try pt.arrayType(.{
                    .len = union_field_vals.len,
                    .child = union_field_ty.toIntern(),
                });
                const new_decl_val = try pt.intern(.{ .aggregate = .{
                    .ty = array_fields_ty.toIntern(),
                    .storage = .{ .elems = union_field_vals },
                } });
                const slice_ty = (try pt.ptrTypeSema(.{
                    .child = union_field_ty.toIntern(),
                    .flags = .{
                        .size = .slice,
                        .is_const = true,
                    },
                })).toIntern();
                const manyptr_ty = Type.fromInterned(slice_ty).slicePtrFieldType(zcu).toIntern();
                break :v try pt.intern(.{ .slice = .{
                    .ty = slice_ty,
                    .ptr = try pt.intern(.{ .ptr = .{
                        .ty = manyptr_ty,
                        .base_addr = .{ .uav = .{
                            .orig_ty = manyptr_ty,
                            .val = new_decl_val,
                        } },
                        .byte_offset = 0,
                    } }),
                    .len = (try pt.intValue(.usize, union_field_vals.len)).toIntern(),
                } });
            };

            const decls_val = try sema.typeInfoDecls(block, src, ty.getNamespaceIndex(zcu).toOptional());

            const enum_tag_ty_val = try pt.intern(.{ .opt = .{
                .ty = (try pt.optionalType(.type_type)).toIntern(),
                .val = if (ty.unionTagType(zcu)) |tag_ty| tag_ty.toIntern() else .none,
            } });

            const container_layout_ty = try sema.getBuiltinType(src, .@"Type.ContainerLayout");

            const field_values = .{
                // layout: ContainerLayout,
                (try pt.enumValueFieldIndex(container_layout_ty, @intFromEnum(layout))).toIntern(),

                // tag_type: ?type,
                enum_tag_ty_val,
                // fields: []const UnionField,
                fields_val,
                // decls: []const Declaration,
                decls_val,
            };
            return Air.internedToRef((try pt.internUnion(.{
                .ty = type_info_ty.toIntern(),
                .tag = (try pt.enumValueFieldIndex(type_info_tag_ty, @intFromEnum(std.builtin.TypeId.@"union"))).toIntern(),
                .val = try pt.intern(.{ .aggregate = .{
                    .ty = type_union_ty.toIntern(),
                    .storage = .{ .elems = &field_values },
                } }),
            })));
        },
        .@"struct" => {
            const type_struct_ty = try sema.getBuiltinType(src, .@"Type.Struct");
            const struct_field_ty = try sema.getBuiltinType(src, .@"Type.StructField");

            try ty.resolveLayout(pt); // Getting alignment requires type layout

            var struct_field_vals: []InternPool.Index = &.{};
            defer gpa.free(struct_field_vals);
            fv: {
                const struct_type = switch (ip.indexToKey(ty.toIntern())) {
                    .tuple_type => |tuple_type| {
                        struct_field_vals = try gpa.alloc(InternPool.Index, tuple_type.types.len);
                        for (struct_field_vals, 0..) |*struct_field_val, field_index| {
                            const field_ty = tuple_type.types.get(ip)[field_index];
                            const field_val = tuple_type.values.get(ip)[field_index];
                            const name_val = v: {
                                const field_name = try ip.getOrPutStringFmt(gpa, pt.tid, "{d}", .{field_index}, .no_embedded_nulls);
                                const field_name_len = field_name.length(ip);
                                const new_decl_ty = try pt.arrayType(.{
                                    .len = field_name_len,
                                    .sentinel = .zero_u8,
                                    .child = .u8_type,
                                });
                                const new_decl_val = try pt.intern(.{ .aggregate = .{
                                    .ty = new_decl_ty.toIntern(),
                                    .storage = .{ .bytes = field_name.toString() },
                                } });
                                break :v try pt.intern(.{ .slice = .{
                                    .ty = .slice_const_u8_sentinel_0_type,
                                    .ptr = try pt.intern(.{ .ptr = .{
                                        .ty = .manyptr_const_u8_sentinel_0_type,
                                        .base_addr = .{ .uav = .{
                                            .val = new_decl_val,
                                            .orig_ty = .slice_const_u8_sentinel_0_type,
                                        } },
                                        .byte_offset = 0,
                                    } }),
                                    .len = (try pt.intValue(.usize, field_name_len)).toIntern(),
                                } });
                            };

                            try Type.fromInterned(field_ty).resolveLayout(pt);

                            const is_comptime = field_val != .none;
                            const opt_default_val = if (is_comptime) Value.fromInterned(field_val) else null;
                            const default_val_ptr = try sema.optRefValue(opt_default_val);
                            const struct_field_fields = .{
                                // name: [:0]const u8,
                                name_val,
                                // type: type,
                                field_ty,
                                // default_value: ?*const anyopaque,
                                default_val_ptr.toIntern(),
                                // is_comptime: bool,
                                Value.makeBool(is_comptime).toIntern(),
                                // alignment: comptime_int,
                                (try pt.intValue(.comptime_int, Type.fromInterned(field_ty).abiAlignment(zcu).toByteUnits() orelse 0)).toIntern(),
                            };
                            struct_field_val.* = try pt.intern(.{ .aggregate = .{
                                .ty = struct_field_ty.toIntern(),
                                .storage = .{ .elems = &struct_field_fields },
                            } });
                        }
                        break :fv;
                    },
                    .struct_type => ip.loadStructType(ty.toIntern()),
                    else => unreachable,
                };
                struct_field_vals = try gpa.alloc(InternPool.Index, struct_type.field_types.len);

                try ty.resolveStructFieldInits(pt);

                for (struct_field_vals, 0..) |*field_val, field_index| {
                    const field_name = if (struct_type.fieldName(ip, field_index).unwrap()) |field_name|
                        field_name
                    else
                        try ip.getOrPutStringFmt(gpa, pt.tid, "{d}", .{field_index}, .no_embedded_nulls);
                    const field_name_len = field_name.length(ip);
                    const field_ty: Type = .fromInterned(struct_type.field_types.get(ip)[field_index]);
                    const field_init = struct_type.fieldInit(ip, field_index);
                    const field_is_comptime = struct_type.fieldIsComptime(ip, field_index);
                    const name_val = v: {
                        const new_decl_ty = try pt.arrayType(.{
                            .len = field_name_len,
                            .sentinel = .zero_u8,
                            .child = .u8_type,
                        });
                        const new_decl_val = try pt.intern(.{ .aggregate = .{
                            .ty = new_decl_ty.toIntern(),
                            .storage = .{ .bytes = field_name.toString() },
                        } });
                        break :v try pt.intern(.{ .slice = .{
                            .ty = .slice_const_u8_sentinel_0_type,
                            .ptr = try pt.intern(.{ .ptr = .{
                                .ty = .manyptr_const_u8_sentinel_0_type,
                                .base_addr = .{ .uav = .{
                                    .val = new_decl_val,
                                    .orig_ty = .slice_const_u8_sentinel_0_type,
                                } },
                                .byte_offset = 0,
                            } }),
                            .len = (try pt.intValue(.usize, field_name_len)).toIntern(),
                        } });
                    };

                    const opt_default_val = if (field_init == .none) null else Value.fromInterned(field_init);
                    const default_val_ptr = try sema.optRefValue(opt_default_val);
                    const alignment = switch (struct_type.layout) {
                        .@"packed" => .none,
                        else => try field_ty.structFieldAlignmentSema(
                            struct_type.fieldAlign(ip, field_index),
                            struct_type.layout,
                            pt,
                        ),
                    };

                    const struct_field_fields = .{
                        // name: [:0]const u8,
                        name_val,
                        // type: type,
                        field_ty.toIntern(),
                        // default_value: ?*const anyopaque,
                        default_val_ptr.toIntern(),
                        // is_comptime: bool,
                        Value.makeBool(field_is_comptime).toIntern(),
                        // alignment: comptime_int,
                        (try pt.intValue(.comptime_int, alignment.toByteUnits() orelse 0)).toIntern(),
                    };
                    field_val.* = try pt.intern(.{ .aggregate = .{
                        .ty = struct_field_ty.toIntern(),
                        .storage = .{ .elems = &struct_field_fields },
                    } });
                }
            }

            const fields_val = v: {
                const array_fields_ty = try pt.arrayType(.{
                    .len = struct_field_vals.len,
                    .child = struct_field_ty.toIntern(),
                });
                const new_decl_val = try pt.intern(.{ .aggregate = .{
                    .ty = array_fields_ty.toIntern(),
                    .storage = .{ .elems = struct_field_vals },
                } });
                const slice_ty = (try pt.ptrTypeSema(.{
                    .child = struct_field_ty.toIntern(),
                    .flags = .{
                        .size = .slice,
                        .is_const = true,
                    },
                })).toIntern();
                const manyptr_ty = Type.fromInterned(slice_ty).slicePtrFieldType(zcu).toIntern();
                break :v try pt.intern(.{ .slice = .{
                    .ty = slice_ty,
                    .ptr = try pt.intern(.{ .ptr = .{
                        .ty = manyptr_ty,
                        .base_addr = .{ .uav = .{
                            .orig_ty = manyptr_ty,
                            .val = new_decl_val,
                        } },
                        .byte_offset = 0,
                    } }),
                    .len = (try pt.intValue(.usize, struct_field_vals.len)).toIntern(),
                } });
            };

            const decls_val = try sema.typeInfoDecls(block, src, ty.getNamespace(zcu));

            const backing_integer_val = try pt.intern(.{ .opt = .{
                .ty = (try pt.optionalType(.type_type)).toIntern(),
                .val = if (zcu.typeToPackedStruct(ty)) |packed_struct| val: {
                    assert(Type.fromInterned(packed_struct.backingIntTypeUnordered(ip)).isInt(zcu));
                    break :val packed_struct.backingIntTypeUnordered(ip);
                } else .none,
            } });

            const container_layout_ty = try sema.getBuiltinType(src, .@"Type.ContainerLayout");

            const layout = ty.containerLayout(zcu);

            const field_values = [_]InternPool.Index{
                // layout: ContainerLayout,
                (try pt.enumValueFieldIndex(container_layout_ty, @intFromEnum(layout))).toIntern(),
                // backing_integer: ?type,
                backing_integer_val,
                // fields: []const StructField,
                fields_val,
                // decls: []const Declaration,
                decls_val,
                // is_tuple: bool,
                Value.makeBool(ty.isTuple(zcu)).toIntern(),
            };
            return Air.internedToRef((try pt.internUnion(.{
                .ty = type_info_ty.toIntern(),
                .tag = (try pt.enumValueFieldIndex(type_info_tag_ty, @intFromEnum(std.builtin.TypeId.@"struct"))).toIntern(),
                .val = try pt.intern(.{ .aggregate = .{
                    .ty = type_struct_ty.toIntern(),
                    .storage = .{ .elems = &field_values },
                } }),
            })));
        },
        .@"opaque" => {
            const type_opaque_ty = try sema.getBuiltinType(src, .@"Type.Opaque");

            try ty.resolveFields(pt);
            const decls_val = try sema.typeInfoDecls(block, src, ty.getNamespace(zcu));

            const field_values = .{
                // decls: []const Declaration,
                decls_val,
            };
            return Air.internedToRef((try pt.internUnion(.{
                .ty = type_info_ty.toIntern(),
                .tag = (try pt.enumValueFieldIndex(type_info_tag_ty, @intFromEnum(std.builtin.TypeId.@"opaque"))).toIntern(),
                .val = try pt.intern(.{ .aggregate = .{
                    .ty = type_opaque_ty.toIntern(),
                    .storage = .{ .elems = &field_values },
                } }),
            })));
        },
        .frame => return sema.failWithUseOfAsync(block, src),
        .@"anyframe" => return sema.failWithUseOfAsync(block, src),
    }
}

fn typeInfoDecls(
    sema: *Sema,
    block: *Block,
    src: LazySrcLoc,
    opt_namespace: InternPool.OptionalNamespaceIndex,
) CompileError!InternPool.Index {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const gpa = sema.gpa;

    const declaration_ty = try sema.getBuiltinType(src, .@"Type.Declaration");

    var decl_vals = std.ArrayList(InternPool.Index).init(gpa);
    defer decl_vals.deinit();

    var seen_namespaces = std.AutoHashMap(*Namespace, void).init(gpa);
    defer seen_namespaces.deinit();

    try sema.typeInfoNamespaceDecls(block, src, opt_namespace, declaration_ty, &decl_vals, &seen_namespaces);

    const array_decl_ty = try pt.arrayType(.{
        .len = decl_vals.items.len,
        .child = declaration_ty.toIntern(),
    });
    const new_decl_val = try pt.intern(.{ .aggregate = .{
        .ty = array_decl_ty.toIntern(),
        .storage = .{ .elems = decl_vals.items },
    } });
    const slice_ty = (try pt.ptrTypeSema(.{
        .child = declaration_ty.toIntern(),
        .flags = .{
            .size = .slice,
            .is_const = true,
        },
    })).toIntern();
    const manyptr_ty = Type.fromInterned(slice_ty).slicePtrFieldType(zcu).toIntern();
    return try pt.intern(.{ .slice = .{
        .ty = slice_ty,
        .ptr = try pt.intern(.{ .ptr = .{
            .ty = manyptr_ty,
            .base_addr = .{ .uav = .{
                .orig_ty = manyptr_ty,
                .val = new_decl_val,
            } },
            .byte_offset = 0,
        } }),
        .len = (try pt.intValue(.usize, decl_vals.items.len)).toIntern(),
    } });
}

fn typeInfoNamespaceDecls(
    sema: *Sema,
    block: *Block,
    src: LazySrcLoc,
    opt_namespace_index: InternPool.OptionalNamespaceIndex,
    declaration_ty: Type,
    decl_vals: *std.ArrayList(InternPool.Index),
    seen_namespaces: *std.AutoHashMap(*Namespace, void),
) !void {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const ip = &zcu.intern_pool;

    const namespace_index = opt_namespace_index.unwrap() orelse return;
    try pt.ensureNamespaceUpToDate(namespace_index);
    const namespace = zcu.namespacePtr(namespace_index);

    const gop = try seen_namespaces.getOrPut(namespace);
    if (gop.found_existing) return;

    for (namespace.pub_decls.keys()) |nav| {
        const name = ip.getNav(nav).name;
        const name_val = name_val: {
            const name_len = name.length(ip);
            const array_ty = try pt.arrayType(.{
                .len = name_len,
                .sentinel = .zero_u8,
                .child = .u8_type,
            });
            const array_val = try pt.intern(.{ .aggregate = .{
                .ty = array_ty.toIntern(),
                .storage = .{ .bytes = name.toString() },
            } });
            break :name_val try pt.intern(.{
                .slice = .{
                    .ty = .slice_const_u8_sentinel_0_type, // [:0]const u8
                    .ptr = try pt.intern(.{
                        .ptr = .{
                            .ty = .manyptr_const_u8_sentinel_0_type, // [*:0]const u8
                            .base_addr = .{ .uav = .{
                                .orig_ty = .slice_const_u8_sentinel_0_type,
                                .val = array_val,
                            } },
                            .byte_offset = 0,
                        },
                    }),
                    .len = (try pt.intValue(.usize, name_len)).toIntern(),
                },
            });
        };
        const fields = [_]InternPool.Index{
            // name: [:0]const u8,
            name_val,
        };
        try decl_vals.append(try pt.intern(.{ .aggregate = .{
            .ty = declaration_ty.toIntern(),
            .storage = .{ .elems = &fields },
        } }));
    }

    for (namespace.pub_usingnamespace.items) |nav| {
        if (zcu.analysis_in_progress.contains(.wrap(.{ .nav_val = nav }))) {
            continue;
        }
        try sema.ensureNavResolved(block, src, nav, .fully);
        const namespace_ty: Type = .fromInterned(ip.getNav(nav).status.fully_resolved.val);
        try sema.typeInfoNamespaceDecls(block, src, namespace_ty.getNamespaceIndex(zcu).toOptional(), declaration_ty, decl_vals, seen_namespaces);
    }
}

fn zirTypeof(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    _ = block;
    const zir_datas = sema.code.instructions.items(.data);
    const inst_data = zir_datas[@intFromEnum(inst)].un_node;
    const operand = try sema.resolveInst(inst_data.operand);
    const operand_ty = sema.typeOf(operand);
    return Air.internedToRef(operand_ty.toIntern());
}

fn zirTypeofBuiltin(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    const pl_node = sema.code.instructions.items(.data)[@intFromEnum(inst)].pl_node;
    const extra = sema.code.extraData(Zir.Inst.Block, pl_node.payload_index);
    const body = sema.code.bodySlice(extra.end, extra.data.body_len);

    var child_block: Block = .{
        .parent = block,
        .sema = sema,
        .namespace = block.namespace,
        .instructions = .{},
        .inlining = block.inlining,
        .comptime_reason = null,
        .is_typeof = true,
        .want_safety = false,
        .error_return_trace_index = block.error_return_trace_index,
        .src_base_inst = block.src_base_inst,
        .type_name_ctx = block.type_name_ctx,
    };
    defer child_block.instructions.deinit(sema.gpa);

    const operand = try sema.resolveInlineBody(&child_block, body, inst);
    return Air.internedToRef(sema.typeOf(operand).toIntern());
}

fn zirTypeofLog2IntType(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].un_node;
    const src = block.nodeOffset(inst_data.src_node);
    const operand = try sema.resolveInst(inst_data.operand);
    const operand_ty = sema.typeOf(operand);
    const res_ty = try sema.log2IntType(block, operand_ty, src);
    return Air.internedToRef(res_ty.toIntern());
}

fn log2IntType(sema: *Sema, block: *Block, operand: Type, src: LazySrcLoc) CompileError!Type {
    const pt = sema.pt;
    const zcu = pt.zcu;
    switch (operand.zigTypeTag(zcu)) {
        .comptime_int => return .comptime_int,
        .int => {
            const bits = operand.bitSize(zcu);
            const count = if (bits == 0)
                0
            else blk: {
                var count: u16 = 0;
                var s = bits - 1;
                while (s != 0) : (s >>= 1) {
                    count += 1;
                }
                break :blk count;
            };
            return pt.intType(.unsigned, count);
        },
        .vector => {
            const elem_ty = operand.elemType2(zcu);
            const log2_elem_ty = try sema.log2IntType(block, elem_ty, src);
            return pt.vectorType(.{
                .len = operand.vectorLen(zcu),
                .child = log2_elem_ty.toIntern(),
            });
        },
        else => {},
    }
    return sema.fail(
        block,
        src,
        "bit shifting operation expected integer type, found '{}'",
        .{operand.fmt(pt)},
    );
}

fn zirTypeofPeer(
    sema: *Sema,
    block: *Block,
    extended: Zir.Inst.Extended.InstData,
    inst: Zir.Inst.Index,
) CompileError!Air.Inst.Ref {
    const tracy = trace(@src());
    defer tracy.end();

    const extra = sema.code.extraData(Zir.Inst.TypeOfPeer, extended.operand);
    const src = block.nodeOffset(extra.data.src_node);
    const body = sema.code.bodySlice(extra.data.body_index, extra.data.body_len);

    var child_block: Block = .{
        .parent = block,
        .sema = sema,
        .namespace = block.namespace,
        .instructions = .{},
        .inlining = block.inlining,
        .comptime_reason = null,
        .is_typeof = true,
        .runtime_cond = block.runtime_cond,
        .runtime_loop = block.runtime_loop,
        .runtime_index = block.runtime_index,
        .src_base_inst = block.src_base_inst,
        .type_name_ctx = block.type_name_ctx,
    };
    defer child_block.instructions.deinit(sema.gpa);
    // Ignore the result, we only care about the instructions in `args`.
    _ = try sema.analyzeInlineBody(&child_block, body, inst);

    const args = sema.code.refSlice(extra.end, extended.small);

    const inst_list = try sema.gpa.alloc(Air.Inst.Ref, args.len);
    defer sema.gpa.free(inst_list);

    for (args, 0..) |arg_ref, i| {
        inst_list[i] = try sema.resolveInst(arg_ref);
    }

    const result_type = try sema.resolvePeerTypes(block, src, inst_list, .{ .typeof_builtin_call_node_offset = extra.data.src_node });
    return Air.internedToRef(result_type.toIntern());
}

fn zirBoolNot(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    const tracy = trace(@src());
    defer tracy.end();

    const pt = sema.pt;
    const zcu = pt.zcu;
    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].un_node;
    const src = block.nodeOffset(inst_data.src_node);
    const operand_src = block.src(.{ .node_offset_un_op = inst_data.src_node });
    const uncasted_operand = try sema.resolveInst(inst_data.operand);

    const operand = try sema.coerce(block, .bool, uncasted_operand, operand_src);
    if (try sema.resolveValue(operand)) |val| {
        return if (val.isUndef(zcu)) .undef_bool else if (val.toBool()) .bool_false else .bool_true;
    }
    try sema.requireRuntimeBlock(block, src, null);
    return block.addTyOp(.not, .bool, operand);
}

fn zirBoolBr(
    sema: *Sema,
    parent_block: *Block,
    inst: Zir.Inst.Index,
    is_bool_or: bool,
) CompileError!Air.Inst.Ref {
    const tracy = trace(@src());
    defer tracy.end();

    const pt = sema.pt;
    const zcu = pt.zcu;
    const gpa = sema.gpa;

    const datas = sema.code.instructions.items(.data);
    const inst_data = datas[@intFromEnum(inst)].pl_node;
    const extra = sema.code.extraData(Zir.Inst.BoolBr, inst_data.payload_index);

    const uncoerced_lhs = try sema.resolveInst(extra.data.lhs);
    const body = sema.code.bodySlice(extra.end, extra.data.body_len);
    const lhs_src = parent_block.src(.{ .node_offset_bin_lhs = inst_data.src_node });
    const rhs_src = parent_block.src(.{ .node_offset_bin_rhs = inst_data.src_node });

    const lhs = try sema.coerce(parent_block, .bool, uncoerced_lhs, lhs_src);

    if (try sema.resolveDefinedValue(parent_block, lhs_src, lhs)) |lhs_val| {
        if (is_bool_or and lhs_val.toBool()) {
            return .bool_true;
        } else if (!is_bool_or and !lhs_val.toBool()) {
            return .bool_false;
        }
        // comptime-known left-hand side. No need for a block here; the result
        // is simply the rhs expression. Here we rely on there only being 1
        // break instruction (`break_inline`).
        const rhs_result = try sema.resolveInlineBody(parent_block, body, inst);
        if (sema.typeOf(rhs_result).isNoReturn(zcu)) {
            return rhs_result;
        }
        return sema.coerce(parent_block, .bool, rhs_result, rhs_src);
    }

    const block_inst: Air.Inst.Index = @enumFromInt(sema.air_instructions.len);
    try sema.air_instructions.append(gpa, .{
        .tag = .block,
        .data = .{ .ty_pl = .{
            .ty = .bool_type,
            .payload = undefined,
        } },
    });

    var child_block = parent_block.makeSubBlock();
    child_block.runtime_loop = null;
    child_block.runtime_cond = lhs_src;
    child_block.runtime_index.increment();
    defer child_block.instructions.deinit(gpa);

    var then_block = child_block.makeSubBlock();
    defer then_block.instructions.deinit(gpa);

    var else_block = child_block.makeSubBlock();
    defer else_block.instructions.deinit(gpa);

    const lhs_block = if (is_bool_or) &then_block else &else_block;
    const rhs_block = if (is_bool_or) &else_block else &then_block;

    const lhs_result: Air.Inst.Ref = if (is_bool_or) .bool_true else .bool_false;
    _ = try lhs_block.addBr(block_inst, lhs_result);

    const parent_hint = sema.branch_hint;
    defer sema.branch_hint = parent_hint;
    sema.branch_hint = null;

    const rhs_result = try sema.resolveInlineBody(rhs_block, body, inst);
    const rhs_noret = sema.typeOf(rhs_result).isNoReturn(zcu);
    const coerced_rhs_result = if (!rhs_noret) rhs: {
        const coerced_result = try sema.coerce(rhs_block, .bool, rhs_result, rhs_src);
        _ = try rhs_block.addBr(block_inst, coerced_result);
        break :rhs coerced_result;
    } else rhs_result;

    const rhs_hint = sema.branch_hint orelse .none;

    const result = try sema.finishCondBr(
        parent_block,
        &child_block,
        &then_block,
        &else_block,
        lhs,
        block_inst,
        if (is_bool_or) .{
            .true = .none,
            .false = rhs_hint,
            .then_cov = .poi,
            .else_cov = .poi,
        } else .{
            .true = rhs_hint,
            .false = .none,
            .then_cov = .poi,
            .else_cov = .poi,
        },
    );
    if (!rhs_noret) {
        if (try sema.resolveDefinedValue(rhs_block, rhs_src, coerced_rhs_result)) |rhs_val| {
            if (is_bool_or and rhs_val.toBool()) {
                return .bool_true;
            } else if (!is_bool_or and !rhs_val.toBool()) {
                return .bool_false;
            }
        }
    }

    return result;
}

fn finishCondBr(
    sema: *Sema,
    parent_block: *Block,
    child_block: *Block,
    then_block: *Block,
    else_block: *Block,
    cond: Air.Inst.Ref,
    block_inst: Air.Inst.Index,
    branch_hints: Air.CondBr.BranchHints,
) !Air.Inst.Ref {
    const gpa = sema.gpa;

    try sema.air_extra.ensureUnusedCapacity(gpa, @typeInfo(Air.CondBr).@"struct".fields.len +
        then_block.instructions.items.len + else_block.instructions.items.len +
        @typeInfo(Air.Block).@"struct".fields.len + child_block.instructions.items.len + 1);

    const cond_br_payload = sema.addExtraAssumeCapacity(Air.CondBr{
        .then_body_len = @intCast(then_block.instructions.items.len),
        .else_body_len = @intCast(else_block.instructions.items.len),
        .branch_hints = branch_hints,
    });
    sema.air_extra.appendSliceAssumeCapacity(@ptrCast(then_block.instructions.items));
    sema.air_extra.appendSliceAssumeCapacity(@ptrCast(else_block.instructions.items));

    _ = try child_block.addInst(.{ .tag = .cond_br, .data = .{ .pl_op = .{
        .operand = cond,
        .payload = cond_br_payload,
    } } });

    sema.air_instructions.items(.data)[@intFromEnum(block_inst)].ty_pl.payload = sema.addExtraAssumeCapacity(
        Air.Block{ .body_len = @intCast(child_block.instructions.items.len) },
    );
    sema.air_extra.appendSliceAssumeCapacity(@ptrCast(child_block.instructions.items));

    try parent_block.instructions.append(gpa, block_inst);
    return block_inst.toRef();
}

fn checkNullableType(sema: *Sema, block: *Block, src: LazySrcLoc, ty: Type) !void {
    const pt = sema.pt;
    const zcu = pt.zcu;
    switch (ty.zigTypeTag(zcu)) {
        .optional, .null, .undefined => return,
        .pointer => if (ty.isPtrLikeOptional(zcu)) return,
        else => {},
    }
    return sema.failWithExpectedOptionalType(block, src, ty);
}

fn checkSentinelType(sema: *Sema, block: *Block, src: LazySrcLoc, ty: Type) !void {
    const pt = sema.pt;
    const zcu = pt.zcu;
    if (!ty.isSelfComparable(zcu, true)) {
        return sema.fail(block, src, "non-scalar sentinel type '{}'", .{ty.fmt(pt)});
    }
}

fn zirIsNonNull(
    sema: *Sema,
    block: *Block,
    inst: Zir.Inst.Index,
) CompileError!Air.Inst.Ref {
    const tracy = trace(@src());
    defer tracy.end();

    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].un_node;
    const src = block.nodeOffset(inst_data.src_node);
    const operand = try sema.resolveInst(inst_data.operand);
    try sema.checkNullableType(block, src, sema.typeOf(operand));
    return sema.analyzeIsNull(block, operand, true);
}

fn zirIsNonNullPtr(
    sema: *Sema,
    block: *Block,
    inst: Zir.Inst.Index,
) CompileError!Air.Inst.Ref {
    const tracy = trace(@src());
    defer tracy.end();

    const pt = sema.pt;
    const zcu = pt.zcu;
    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].un_node;
    const src = block.nodeOffset(inst_data.src_node);
    const ptr = try sema.resolveInst(inst_data.operand);
    const ptr_ty = sema.typeOf(ptr);
    try sema.checkNullableType(block, src, sema.typeOf(ptr).elemType2(zcu));
    if (try sema.resolveValue(ptr)) |ptr_val| {
        if (try sema.pointerDeref(block, src, ptr_val, ptr_ty)) |loaded_val| {
            return sema.analyzeIsNull(block, Air.internedToRef(loaded_val.toIntern()), true);
        }
    }
    if (ptr_ty.childType(zcu).isNullFromType(zcu)) |is_null| {
        return if (is_null) .bool_false else .bool_true;
    }
    return block.addUnOp(.is_non_null_ptr, ptr);
}

fn checkErrorType(sema: *Sema, block: *Block, src: LazySrcLoc, ty: Type) !void {
    const pt = sema.pt;
    const zcu = pt.zcu;
    switch (ty.zigTypeTag(zcu)) {
        .error_set, .error_union, .undefined => return,
        else => return sema.fail(block, src, "expected error union type, found '{}'", .{
            ty.fmt(pt),
        }),
    }
}

fn zirIsNonErr(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    const tracy = trace(@src());
    defer tracy.end();

    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].un_node;
    const src = block.nodeOffset(inst_data.src_node);
    const operand = try sema.resolveInst(inst_data.operand);
    try sema.checkErrorType(block, src, sema.typeOf(operand));
    return sema.analyzeIsNonErr(block, src, operand);
}

fn zirIsNonErrPtr(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    const tracy = trace(@src());
    defer tracy.end();

    const pt = sema.pt;
    const zcu = pt.zcu;
    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].un_node;
    const src = block.nodeOffset(inst_data.src_node);
    const ptr = try sema.resolveInst(inst_data.operand);
    try sema.checkErrorType(block, src, sema.typeOf(ptr).elemType2(zcu));
    const loaded = try sema.analyzeLoad(block, src, ptr, src);
    return sema.analyzeIsNonErr(block, src, loaded);
}

fn zirRetIsNonErr(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    const tracy = trace(@src());
    defer tracy.end();

    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].un_node;
    const src = block.nodeOffset(inst_data.src_node);
    const operand = try sema.resolveInst(inst_data.operand);
    return sema.analyzeIsNonErr(block, src, operand);
}

fn zirCondbr(
    sema: *Sema,
    parent_block: *Block,
    inst: Zir.Inst.Index,
) CompileError!void {
    const tracy = trace(@src());
    defer tracy.end();

    const pt = sema.pt;
    const zcu = pt.zcu;
    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].pl_node;
    const cond_src = parent_block.src(.{ .node_offset_if_cond = inst_data.src_node });
    const extra = sema.code.extraData(Zir.Inst.CondBr, inst_data.payload_index);

    const then_body = sema.code.bodySlice(extra.end, extra.data.then_body_len);
    const else_body = sema.code.bodySlice(extra.end + then_body.len, extra.data.else_body_len);

    const uncasted_cond = try sema.resolveInst(extra.data.condition);
    const cond = try sema.coerce(parent_block, .bool, uncasted_cond, cond_src);

    if (try sema.resolveDefinedValue(parent_block, cond_src, cond)) |cond_val| {
        const body = if (cond_val.toBool()) then_body else else_body;

        // We can propagate `.cold` hints from this branch since it's comptime-known
        // to be taken from the parent branch.
        const parent_hint = sema.branch_hint;
        defer sema.branch_hint = parent_hint orelse if (sema.branch_hint == .cold) .cold else null;

        try sema.maybeErrorUnwrapCondbr(parent_block, body, extra.data.condition, cond_src);
        // We use `analyzeBodyInner` since we want to propagate any comptime control flow to the caller.
        return sema.analyzeBodyInner(parent_block, body);
    }

    const gpa = sema.gpa;

    // We'll re-use the sub block to save on memory bandwidth, and yank out the
    // instructions array in between using it for the then block and else block.
    var sub_block = parent_block.makeSubBlock();
    sub_block.runtime_loop = null;
    sub_block.runtime_cond = cond_src;
    sub_block.runtime_index.increment();
    sub_block.need_debug_scope = null; // this body is emitted regardless
    defer sub_block.instructions.deinit(gpa);

    const true_hint = try sema.analyzeBodyRuntimeBreak(&sub_block, then_body);
    const true_instructions = try sub_block.instructions.toOwnedSlice(gpa);
    defer gpa.free(true_instructions);

    const err_cond = blk: {
        const index = extra.data.condition.toIndex() orelse break :blk null;
        if (sema.code.instructions.items(.tag)[@intFromEnum(index)] != .is_non_err) break :blk null;

        const err_inst_data = sema.code.instructions.items(.data)[@intFromEnum(index)].un_node;
        const err_operand = try sema.resolveInst(err_inst_data.operand);
        const operand_ty = sema.typeOf(err_operand);
        assert(operand_ty.zigTypeTag(zcu) == .error_union);
        const result_ty = operand_ty.errorUnionSet(zcu);
        break :blk try sub_block.addTyOp(.unwrap_errunion_err, result_ty, err_operand);
    };

    const false_hint: std.builtin.BranchHint = if (err_cond != null and
        try sema.maybeErrorUnwrap(&sub_block, else_body, err_cond.?, cond_src, false))
    h: {
        // nothing to do here. weight against error branch
        break :h .unlikely;
    } else try sema.analyzeBodyRuntimeBreak(&sub_block, else_body);

    try sema.air_extra.ensureUnusedCapacity(gpa, @typeInfo(Air.CondBr).@"struct".fields.len +
        true_instructions.len + sub_block.instructions.items.len);
    _ = try parent_block.addInst(.{
        .tag = .cond_br,
        .data = .{
            .pl_op = .{
                .operand = cond,
                .payload = sema.addExtraAssumeCapacity(Air.CondBr{
                    .then_body_len = @intCast(true_instructions.len),
                    .else_body_len = @intCast(sub_block.instructions.items.len),
                    .branch_hints = .{
                        .true = true_hint,
                        .false = false_hint,
                        // Code coverage is desired for error handling.
                        .then_cov = .poi,
                        .else_cov = .poi,
                    },
                }),
            },
        },
    });
    sema.air_extra.appendSliceAssumeCapacity(@ptrCast(true_instructions));
    sema.air_extra.appendSliceAssumeCapacity(@ptrCast(sub_block.instructions.items));
}

fn zirTry(sema: *Sema, parent_block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].pl_node;
    const src = parent_block.nodeOffset(inst_data.src_node);
    const operand_src = parent_block.src(.{ .node_offset_try_operand = inst_data.src_node });
    const extra = sema.code.extraData(Zir.Inst.Try, inst_data.payload_index);
    const body = sema.code.bodySlice(extra.end, extra.data.body_len);
    const err_union = try sema.resolveInst(extra.data.operand);
    const err_union_ty = sema.typeOf(err_union);
    const pt = sema.pt;
    const zcu = pt.zcu;
    if (err_union_ty.zigTypeTag(zcu) != .error_union) {
        return sema.fail(parent_block, operand_src, "expected error union type, found '{}'", .{
            err_union_ty.fmt(pt),
        });
    }
    const is_non_err = try sema.analyzeIsNonErrComptimeOnly(parent_block, operand_src, err_union);
    if (is_non_err != .none) {
        // We can propagate `.cold` hints from this branch since it's comptime-known
        // to be taken from the parent branch.
        const parent_hint = sema.branch_hint;
        defer sema.branch_hint = parent_hint orelse if (sema.branch_hint == .cold) .cold else null;

        const is_non_err_val = (try sema.resolveDefinedValue(parent_block, operand_src, is_non_err)).?;
        if (is_non_err_val.toBool()) {
            return sema.analyzeErrUnionPayload(parent_block, src, err_union_ty, err_union, operand_src, false);
        }
        // We can analyze the body directly in the parent block because we know there are
        // no breaks from the body possible, and that the body is noreturn.
        try sema.analyzeBodyInner(parent_block, body);
        return .unreachable_value;
    }

    var sub_block = parent_block.makeSubBlock();
    defer sub_block.instructions.deinit(sema.gpa);

    const parent_hint = sema.branch_hint;
    defer sema.branch_hint = parent_hint;

    // This body is guaranteed to end with noreturn and has no breaks.
    try sema.analyzeBodyInner(&sub_block, body);

    // The only interesting hint here is `.cold`, which can come from e.g. `errdefer @panic`.
    const is_cold = sema.branch_hint == .cold;

    try sema.air_extra.ensureUnusedCapacity(sema.gpa, @typeInfo(Air.Try).@"struct".fields.len +
        sub_block.instructions.items.len);
    const try_inst = try parent_block.addInst(.{
        .tag = if (is_cold) .try_cold else .@"try",
        .data = .{ .pl_op = .{
            .operand = err_union,
            .payload = sema.addExtraAssumeCapacity(Air.Try{
                .body_len = @intCast(sub_block.instructions.items.len),
            }),
        } },
    });
    sema.air_extra.appendSliceAssumeCapacity(@ptrCast(sub_block.instructions.items));
    return try_inst;
}

fn zirTryPtr(sema: *Sema, parent_block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].pl_node;
    const src = parent_block.nodeOffset(inst_data.src_node);
    const operand_src = parent_block.src(.{ .node_offset_try_operand = inst_data.src_node });
    const extra = sema.code.extraData(Zir.Inst.Try, inst_data.payload_index);
    const body = sema.code.bodySlice(extra.end, extra.data.body_len);
    const operand = try sema.resolveInst(extra.data.operand);
    const err_union = try sema.analyzeLoad(parent_block, src, operand, operand_src);
    const err_union_ty = sema.typeOf(err_union);
    const pt = sema.pt;
    const zcu = pt.zcu;
    if (err_union_ty.zigTypeTag(zcu) != .error_union) {
        return sema.fail(parent_block, operand_src, "expected error union type, found '{}'", .{
            err_union_ty.fmt(pt),
        });
    }
    const is_non_err = try sema.analyzeIsNonErrComptimeOnly(parent_block, operand_src, err_union);
    if (is_non_err != .none) {
        // We can propagate `.cold` hints from this branch since it's comptime-known
        // to be taken from the parent branch.
        const parent_hint = sema.branch_hint;
        defer sema.branch_hint = parent_hint orelse if (sema.branch_hint == .cold) .cold else null;

        const is_non_err_val = (try sema.resolveDefinedValue(parent_block, operand_src, is_non_err)).?;
        if (is_non_err_val.toBool()) {
            return sema.analyzeErrUnionPayloadPtr(parent_block, src, operand, false, false);
        }
        // We can analyze the body directly in the parent block because we know there are
        // no breaks from the body possible, and that the body is noreturn.
        try sema.analyzeBodyInner(parent_block, body);
        return .unreachable_value;
    }

    var sub_block = parent_block.makeSubBlock();
    defer sub_block.instructions.deinit(sema.gpa);

    const parent_hint = sema.branch_hint;
    defer sema.branch_hint = parent_hint;

    // This body is guaranteed to end with noreturn and has no breaks.
    try sema.analyzeBodyInner(&sub_block, body);

    // The only interesting hint here is `.cold`, which can come from e.g. `errdefer @panic`.
    const is_cold = sema.branch_hint == .cold;

    const operand_ty = sema.typeOf(operand);
    const ptr_info = operand_ty.ptrInfo(zcu);
    const res_ty = try pt.ptrTypeSema(.{
        .child = err_union_ty.errorUnionPayload(zcu).toIntern(),
        .flags = .{
            .is_const = ptr_info.flags.is_const,
            .is_volatile = ptr_info.flags.is_volatile,
            .is_allowzero = ptr_info.flags.is_allowzero,
            .address_space = ptr_info.flags.address_space,
        },
    });
    const res_ty_ref = Air.internedToRef(res_ty.toIntern());
    try sema.air_extra.ensureUnusedCapacity(sema.gpa, @typeInfo(Air.TryPtr).@"struct".fields.len +
        sub_block.instructions.items.len);
    const try_inst = try parent_block.addInst(.{
        .tag = if (is_cold) .try_ptr_cold else .try_ptr,
        .data = .{ .ty_pl = .{
            .ty = res_ty_ref,
            .payload = sema.addExtraAssumeCapacity(Air.TryPtr{
                .ptr = operand,
                .body_len = @intCast(sub_block.instructions.items.len),
            }),
        } },
    });
    sema.air_extra.appendSliceAssumeCapacity(@ptrCast(sub_block.instructions.items));
    return try_inst;
}

fn ensurePostHoc(sema: *Sema, block: *Block, dest_block: Zir.Inst.Index) !*LabeledBlock {
    const gop = sema.inst_map.getOrPutAssumeCapacity(dest_block);
    if (gop.found_existing) existing: {
        // This may be a *result* from an earlier iteration of an inline loop.
        // In this case, there will not be a post-hoc block entry, and we can
        // continue with the logic below.
        const new_block_inst = gop.value_ptr.*.toIndex() orelse break :existing;
        return sema.post_hoc_blocks.get(new_block_inst) orelse break :existing;
    }

    try sema.post_hoc_blocks.ensureUnusedCapacity(sema.gpa, 1);

    const new_block_inst: Air.Inst.Index = @enumFromInt(sema.air_instructions.len);
    gop.value_ptr.* = new_block_inst.toRef();
    try sema.air_instructions.append(sema.gpa, .{
        .tag = .block,
        .data = undefined,
    });
    const labeled_block = try sema.gpa.create(LabeledBlock);
    labeled_block.* = .{
        .label = .{
            .zir_block = dest_block,
            .merges = .{
                .src_locs = .{},
                .results = .{},
                .br_list = .{},
                .block_inst = new_block_inst,
            },
        },
        .block = .{
            .parent = block,
            .sema = sema,
            .namespace = block.namespace,
            .instructions = .{},
            .label = &labeled_block.label,
            .inlining = block.inlining,
            .comptime_reason = block.comptime_reason,
            .src_base_inst = block.src_base_inst,
            .type_name_ctx = block.type_name_ctx,
        },
    };
    sema.post_hoc_blocks.putAssumeCapacityNoClobber(new_block_inst, labeled_block);
    return labeled_block;
}

/// A `break` statement is inside a runtime condition, but trying to
/// break from an inline loop. In such case we must convert it to
/// a runtime break.
fn addRuntimeBreak(sema: *Sema, child_block: *Block, block_inst: Zir.Inst.Index, break_operand: Zir.Inst.Ref) !void {
    const labeled_block = try sema.ensurePostHoc(child_block, block_inst);

    const operand = try sema.resolveInst(break_operand);
    const br_ref = try child_block.addBr(labeled_block.label.merges.block_inst, operand);

    try labeled_block.label.merges.results.append(sema.gpa, operand);
    try labeled_block.label.merges.br_list.append(sema.gpa, br_ref.toIndex().?);
    try labeled_block.label.merges.src_locs.append(sema.gpa, null);

    labeled_block.block.runtime_index.increment();
    if (labeled_block.block.runtime_cond == null and labeled_block.block.runtime_loop == null) {
        labeled_block.block.runtime_cond = child_block.runtime_cond orelse child_block.runtime_loop;
        labeled_block.block.runtime_loop = child_block.runtime_loop;
    }
}

fn zirUnreachable(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!void {
    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].@"unreachable";
    const src = block.nodeOffset(inst_data.src_node);

    if (block.isComptime()) {
        return sema.fail(block, src, "reached unreachable code", .{});
    }
    // TODO Add compile error for @optimizeFor occurring too late in a scope.
    sema.analyzeUnreachable(block, src, true) catch |err| switch (err) {
        error.AnalysisFail => {
            const msg = sema.err orelse return err;
            if (!mem.eql(u8, msg.msg, "runtime safety check not allowed in naked function")) return err;
            try sema.errNote(src, msg, "the end of a naked function is implicitly unreachable", .{});
            return err;
        },
        else => |e| return e,
    };
}

fn zirRetErrValue(
    sema: *Sema,
    block: *Block,
    inst: Zir.Inst.Index,
) CompileError!void {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].str_tok;
    const src = block.tokenOffset(inst_data.src_tok);
    const err_name = try zcu.intern_pool.getOrPutString(
        sema.gpa,
        pt.tid,
        inst_data.get(sema.code),
        .no_embedded_nulls,
    );
    _ = try pt.getErrorValue(err_name);
    // Return the error code from the function.
    const error_set_type = try pt.singleErrorSetType(err_name);
    const result_inst = Air.internedToRef((try pt.intern(.{ .err = .{
        .ty = error_set_type.toIntern(),
        .name = err_name,
    } })));
    return sema.analyzeRet(block, result_inst, src, src);
}

fn zirRetImplicit(
    sema: *Sema,
    block: *Block,
    inst: Zir.Inst.Index,
) CompileError!void {
    const tracy = trace(@src());
    defer tracy.end();

    const pt = sema.pt;
    const zcu = pt.zcu;
    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].un_tok;
    const r_brace_src = block.tokenOffset(inst_data.src_tok);
    if (block.inlining == null and sema.func_is_naked) {
        assert(!block.isComptime());
        if (block.wantSafety()) {
            // Calling a safety function from a naked function would not be legal.
            _ = try block.addNoOp(.trap);
        } else {
            try sema.analyzeUnreachable(block, r_brace_src, false);
        }
        return;
    }

    const operand = try sema.resolveInst(inst_data.operand);
    const ret_ty_src = block.src(.{ .node_offset_fn_type_ret_ty = .zero });
    const base_tag = sema.fn_ret_ty.baseZigTypeTag(zcu);
    if (base_tag == .noreturn) {
        const msg = msg: {
            const msg = try sema.errMsg(ret_ty_src, "function declared '{}' implicitly returns", .{
                sema.fn_ret_ty.fmt(pt),
            });
            errdefer msg.destroy(sema.gpa);
            try sema.errNote(r_brace_src, msg, "control flow reaches end of body here", .{});
            break :msg msg;
        };
        return sema.failWithOwnedErrorMsg(block, msg);
    } else if (base_tag != .void) {
        const msg = msg: {
            const msg = try sema.errMsg(ret_ty_src, "function with non-void return type '{}' implicitly returns", .{
                sema.fn_ret_ty.fmt(pt),
            });
            errdefer msg.destroy(sema.gpa);
            try sema.errNote(r_brace_src, msg, "control flow reaches end of body here", .{});
            break :msg msg;
        };
        return sema.failWithOwnedErrorMsg(block, msg);
    }

    return sema.analyzeRet(block, operand, r_brace_src, r_brace_src);
}

fn zirRetNode(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!void {
    const tracy = trace(@src());
    defer tracy.end();

    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].un_node;
    const operand = try sema.resolveInst(inst_data.operand);
    const src = block.nodeOffset(inst_data.src_node);

    return sema.analyzeRet(block, operand, src, block.src(.{ .node_offset_return_operand = inst_data.src_node }));
}

fn zirRetLoad(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!void {
    const tracy = trace(@src());
    defer tracy.end();

    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].un_node;
    const src = block.nodeOffset(inst_data.src_node);
    const ret_ptr = try sema.resolveInst(inst_data.operand);

    if (block.isComptime() or block.inlining != null or sema.func_is_naked) {
        const operand = try sema.analyzeLoad(block, src, ret_ptr, src);
        return sema.analyzeRet(block, operand, src, block.src(.{ .node_offset_return_operand = inst_data.src_node }));
    }

    if (sema.wantErrorReturnTracing(sema.fn_ret_ty)) {
        const is_non_err = try sema.analyzePtrIsNonErr(block, src, ret_ptr);
        return sema.retWithErrTracing(block, src, is_non_err, .ret_load, ret_ptr);
    }

    _ = try block.addUnOp(.ret_load, ret_ptr);
}

fn retWithErrTracing(
    sema: *Sema,
    block: *Block,
    src: LazySrcLoc,
    is_non_err: Air.Inst.Ref,
    ret_tag: Air.Inst.Tag,
    operand: Air.Inst.Ref,
) CompileError!void {
    const pt = sema.pt;
    const need_check = switch (is_non_err) {
        .bool_true => {
            _ = try block.addUnOp(ret_tag, operand);
            return;
        },
        .bool_false => false,
        else => true,
    };

    // This means we're returning something that might be an error!
    // This should only be possible with the `auto` cc, so we definitely have an error trace.
    assert(pt.zcu.intern_pool.funcAnalysisUnordered(sema.owner.unwrap().func).has_error_trace);

    const gpa = sema.gpa;
    const return_err_fn = Air.internedToRef(try sema.getBuiltin(src, .returnError));

    if (!need_check) {
        try sema.callBuiltin(block, src, return_err_fn, .never_tail, &.{}, .@"error return");
        _ = try block.addUnOp(ret_tag, operand);
        return;
    }

    var then_block = block.makeSubBlock();
    defer then_block.instructions.deinit(gpa);
    _ = try then_block.addUnOp(ret_tag, operand);

    var else_block = block.makeSubBlock();
    defer else_block.instructions.deinit(gpa);
    try sema.callBuiltin(&else_block, src, return_err_fn, .never_tail, &.{}, .@"error return");
    _ = try else_block.addUnOp(ret_tag, operand);

    try sema.air_extra.ensureUnusedCapacity(gpa, @typeInfo(Air.CondBr).@"struct".fields.len +
        then_block.instructions.items.len + else_block.instructions.items.len +
        @typeInfo(Air.Block).@"struct".fields.len + 1);

    const cond_br_payload = sema.addExtraAssumeCapacity(Air.CondBr{
        .then_body_len = @intCast(then_block.instructions.items.len),
        .else_body_len = @intCast(else_block.instructions.items.len),
        .branch_hints = .{
            // Weight against error branch.
            .true = .likely,
            .false = .unlikely,
            // Code coverage is not valuable on either branch.
            .then_cov = .none,
            .else_cov = .none,
        },
    });
    sema.air_extra.appendSliceAssumeCapacity(@ptrCast(then_block.instructions.items));
    sema.air_extra.appendSliceAssumeCapacity(@ptrCast(else_block.instructions.items));

    _ = try block.addInst(.{ .tag = .cond_br, .data = .{ .pl_op = .{
        .operand = is_non_err,
        .payload = cond_br_payload,
    } } });
}

fn wantErrorReturnTracing(sema: *Sema, fn_ret_ty: Type) bool {
    const pt = sema.pt;
    const zcu = pt.zcu;
    return fn_ret_ty.isError(zcu) and zcu.comp.config.any_error_tracing;
}

fn zirSaveErrRetIndex(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!void {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].save_err_ret_index;

    if (!block.ownerModule().error_tracing) return;

    // This is only relevant at runtime.
    if (block.isComptime() or block.is_typeof) return;

    const save_index = inst_data.operand == .none or b: {
        const operand = try sema.resolveInst(inst_data.operand);
        const operand_ty = sema.typeOf(operand);
        break :b operand_ty.isError(zcu);
    };

    if (save_index)
        block.error_return_trace_index = try sema.analyzeSaveErrRetIndex(block);
}

fn zirRestoreErrRetIndex(sema: *Sema, start_block: *Block, extended: Zir.Inst.Extended.InstData) CompileError!void {
    const extra = sema.code.extraData(Zir.Inst.RestoreErrRetIndex, extended.operand).data;
    return sema.restoreErrRetIndex(start_block, start_block.nodeOffset(extra.src_node), extra.block, extra.operand);
}

/// If `operand` is non-error (or is `none`), restores the error return trace to
/// its state at the point `block` was reached (or, if `block` is `none`, the
/// point this function began execution).
fn restoreErrRetIndex(sema: *Sema, start_block: *Block, src: LazySrcLoc, target_block: Zir.Inst.Ref, operand_zir: Zir.Inst.Ref) CompileError!void {
    const tracy = trace(@src());
    defer tracy.end();

    const pt = sema.pt;
    const zcu = pt.zcu;

    const saved_index = if (target_block.toIndexAllowNone()) |zir_block| b: {
        var block = start_block;
        while (true) {
            if (block.label) |label| {
                if (label.zir_block == zir_block) {
                    const target_trace_index = if (block.parent) |parent_block| tgt: {
                        break :tgt parent_block.error_return_trace_index;
                    } else sema.error_return_trace_index_on_fn_entry;

                    if (start_block.error_return_trace_index != target_trace_index)
                        break :b target_trace_index;

                    return; // No need to restore
                }
            }
            block = block.parent.?;
        }
    } else b: {
        if (start_block.error_return_trace_index != sema.error_return_trace_index_on_fn_entry)
            break :b sema.error_return_trace_index_on_fn_entry;

        return; // No need to restore
    };

    const operand = try sema.resolveInstAllowNone(operand_zir);

    if (start_block.isComptime() or start_block.is_typeof) {
        const is_non_error = if (operand != .none) blk: {
            const is_non_error_inst = try sema.analyzeIsNonErr(start_block, src, operand);
            const cond_val = try sema.resolveDefinedValue(start_block, src, is_non_error_inst);
            break :blk cond_val.?.toBool();
        } else true; // no operand means pop unconditionally

        if (is_non_error) return;

        const saved_index_val = try sema.resolveDefinedValue(start_block, src, saved_index);
        const saved_index_int = saved_index_val.?.toUnsignedInt(zcu);
        assert(saved_index_int <= sema.comptime_err_ret_trace.items.len);
        sema.comptime_err_ret_trace.items.len = @intCast(saved_index_int);
        return;
    }

    if (!zcu.intern_pool.funcAnalysisUnordered(sema.owner.unwrap().func).has_error_trace) return;
    if (!start_block.ownerModule().error_tracing) return;

    assert(saved_index != .none); // The .error_return_trace_index field was dropped somewhere

    return sema.popErrorReturnTrace(start_block, src, operand, saved_index);
}

fn addToInferredErrorSet(sema: *Sema, uncasted_operand: Air.Inst.Ref) !void {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const ip = &zcu.intern_pool;
    assert(sema.fn_ret_ty.zigTypeTag(zcu) == .error_union);
    const err_set_ty = sema.fn_ret_ty.errorUnionSet(zcu).toIntern();
    switch (err_set_ty) {
        .adhoc_inferred_error_set_type => {
            const ies = sema.fn_ret_ty_ies.?;
            assert(ies.func == .none);
            try sema.addToInferredErrorSetPtr(ies, sema.typeOf(uncasted_operand));
        },
        else => if (ip.isInferredErrorSetType(err_set_ty)) {
            const ies = sema.fn_ret_ty_ies.?;
            assert(ies.func == sema.owner.unwrap().func);
            try sema.addToInferredErrorSetPtr(ies, sema.typeOf(uncasted_operand));
        },
    }
}

fn addToInferredErrorSetPtr(sema: *Sema, ies: *InferredErrorSet, op_ty: Type) !void {
    const arena = sema.arena;
    const pt = sema.pt;
    const zcu = pt.zcu;
    const ip = &zcu.intern_pool;
    switch (op_ty.zigTypeTag(zcu)) {
        .error_set => try ies.addErrorSet(op_ty, ip, arena),
        .error_union => try ies.addErrorSet(op_ty.errorUnionSet(zcu), ip, arena),
        else => {},
    }
}

fn analyzeRet(
    sema: *Sema,
    block: *Block,
    uncasted_operand: Air.Inst.Ref,
    src: LazySrcLoc,
    operand_src: LazySrcLoc,
) CompileError!void {
    // Special case for returning an error to an inferred error set; we need to
    // add the error tag to the inferred error set of the in-scope function, so
    // that the coercion below works correctly.
    const pt = sema.pt;
    const zcu = pt.zcu;
    if (sema.fn_ret_ty_ies != null and sema.fn_ret_ty.zigTypeTag(zcu) == .error_union) {
        try sema.addToInferredErrorSet(uncasted_operand);
    }
    const operand = sema.coerceExtra(block, sema.fn_ret_ty, uncasted_operand, operand_src, .{ .is_ret = true }) catch |err| switch (err) {
        error.NotCoercible => unreachable,
        else => |e| return e,
    };

    if (block.inlining) |inlining| {
        assert(!inlining.is_generic_instantiation); // can't `return` in a generic param/ret ty expr
        if (block.isComptime()) {
            const ret_val = try sema.resolveConstValue(block, operand_src, operand, null);
            inlining.comptime_result = operand;

            if (sema.fn_ret_ty.isError(zcu) and ret_val.getErrorName(zcu) != .none) {
                try sema.comptime_err_ret_trace.append(src);
            }
            return error.ComptimeReturn;
        }
        // We are inlining a function call; rewrite the `ret` as a `break`.
        const br_inst = try block.addBr(inlining.merges.block_inst, operand);
        try inlining.merges.results.append(sema.gpa, operand);
        try inlining.merges.br_list.append(sema.gpa, br_inst.toIndex().?);
        try inlining.merges.src_locs.append(sema.gpa, operand_src);
        return;
    } else if (block.isComptime()) {
        return sema.fail(block, src, "function called at runtime cannot return value at comptime", .{});
    } else if (sema.func_is_naked) {
        const msg = msg: {
            const msg = try sema.errMsg(src, "cannot return from naked function", .{});
            errdefer msg.destroy(sema.gpa);

            try sema.errNote(src, msg, "can only return using assembly", .{});
            break :msg msg;
        };
        return sema.failWithOwnedErrorMsg(block, msg);
    }

    try sema.fn_ret_ty.resolveLayout(pt);

    try sema.validateRuntimeValue(block, operand_src, operand);

    const air_tag: Air.Inst.Tag = if (block.wantSafety()) .ret_safe else .ret;
    if (sema.wantErrorReturnTracing(sema.fn_ret_ty)) {
        // Avoid adding a frame to the error return trace in case the value is comptime-known
        // to be not an error.
        const is_non_err = try sema.analyzeIsNonErr(block, operand_src, operand);
        return sema.retWithErrTracing(block, src, is_non_err, air_tag, operand);
    }

    _ = try block.addUnOp(air_tag, operand);
}

fn floatOpAllowed(tag: Zir.Inst.Tag) bool {
    // extend this swich as additional operators are implemented
    return switch (tag) {
        .add, .sub, .mul, .div, .div_exact, .div_trunc, .div_floor, .mod, .rem, .mod_rem => true,
        else => false,
    };
}

fn zirPtrType(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    const tracy = trace(@src());
    defer tracy.end();

    const pt = sema.pt;
    const zcu = pt.zcu;
    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].ptr_type;
    const extra = sema.code.extraData(Zir.Inst.PtrType, inst_data.payload_index);
    const elem_ty_src = block.src(.{ .node_offset_ptr_elem = extra.data.src_node });
    const sentinel_src = block.src(.{ .node_offset_ptr_sentinel = extra.data.src_node });
    const align_src = block.src(.{ .node_offset_ptr_align = extra.data.src_node });
    const addrspace_src = block.src(.{ .node_offset_ptr_addrspace = extra.data.src_node });
    const bitoffset_src = block.src(.{ .node_offset_ptr_bitoffset = extra.data.src_node });
    const hostsize_src = block.src(.{ .node_offset_ptr_hostsize = extra.data.src_node });

    const elem_ty = blk: {
        const air_inst = try sema.resolveInst(extra.data.elem_type);
        const ty = sema.analyzeAsType(block, elem_ty_src, air_inst) catch |err| {
            if (err == error.AnalysisFail and sema.err != null and sema.typeOf(air_inst).isSinglePointer(zcu)) {
                try sema.errNote(elem_ty_src, sema.err.?, "use '.*' to dereference pointer", .{});
            }
            return err;
        };
        assert(!ty.isGenericPoison());
        break :blk ty;
    };

    if (elem_ty.zigTypeTag(zcu) == .noreturn)
        return sema.fail(block, elem_ty_src, "pointer to noreturn not allowed", .{});

    const target = zcu.getTarget();

    var extra_i = extra.end;

    const sentinel = if (inst_data.flags.has_sentinel) blk: {
        const ref: Zir.Inst.Ref = @enumFromInt(sema.code.extra[extra_i]);
        extra_i += 1;
        const coerced = try sema.coerce(block, elem_ty, try sema.resolveInst(ref), sentinel_src);
        const val = try sema.resolveConstDefinedValue(block, sentinel_src, coerced, .{ .simple = .pointer_sentinel });
        try checkSentinelType(sema, block, sentinel_src, elem_ty);
        break :blk val.toIntern();
    } else .none;

    const abi_align: Alignment = if (inst_data.flags.has_align) blk: {
        const ref: Zir.Inst.Ref = @enumFromInt(sema.code.extra[extra_i]);
        extra_i += 1;
        const coerced = try sema.coerce(block, .u32, try sema.resolveInst(ref), align_src);
        const val = try sema.resolveConstDefinedValue(block, align_src, coerced, .{ .simple = .@"align" });
        // Check if this happens to be the lazy alignment of our element type, in
        // which case we can make this 0 without resolving it.
        switch (zcu.intern_pool.indexToKey(val.toIntern())) {
            .int => |int| switch (int.storage) {
                .lazy_align => |lazy_ty| if (lazy_ty == elem_ty.toIntern()) break :blk .none,
                else => {},
            },
            else => {},
        }
        const align_bytes = (try val.getUnsignedIntSema(pt)).?;
        break :blk try sema.validateAlign(block, align_src, align_bytes);
    } else .none;

    const address_space: std.builtin.AddressSpace = if (inst_data.flags.has_addrspace) blk: {
        const ref: Zir.Inst.Ref = @enumFromInt(sema.code.extra[extra_i]);
        extra_i += 1;
        break :blk try sema.resolveAddressSpace(block, addrspace_src, ref, .pointer);
    } else if (elem_ty.zigTypeTag(zcu) == .@"fn" and target.cpu.arch == .avr) .flash else .generic;

    const bit_offset: u16 = if (inst_data.flags.has_bit_range) blk: {
        const ref: Zir.Inst.Ref = @enumFromInt(sema.code.extra[extra_i]);
        extra_i += 1;
        const bit_offset = try sema.resolveInt(block, bitoffset_src, ref, .u16, .{ .simple = .type });
        break :blk @intCast(bit_offset);
    } else 0;

    const host_size: u16 = if (inst_data.flags.has_bit_range) blk: {
        const ref: Zir.Inst.Ref = @enumFromInt(sema.code.extra[extra_i]);
        extra_i += 1;
        const host_size = try sema.resolveInt(block, hostsize_src, ref, .u16, .{ .simple = .type });
        break :blk @intCast(host_size);
    } else 0;

    if (host_size != 0) {
        if (bit_offset >= host_size * 8) {
            return sema.fail(block, bitoffset_src, "packed type '{}' at bit offset {} starts {} bits after the end of a {} byte host integer", .{
                elem_ty.fmt(pt), bit_offset, bit_offset - host_size * 8, host_size,
            });
        }
        const elem_bit_size = try elem_ty.bitSizeSema(pt);
        if (elem_bit_size > host_size * 8 - bit_offset) {
            return sema.fail(block, bitoffset_src, "packed type '{}' at bit offset {} ends {} bits after the end of a {} byte host integer", .{
                elem_ty.fmt(pt), bit_offset, elem_bit_size - (host_size * 8 - bit_offset), host_size,
            });
        }
    }

    if (elem_ty.zigTypeTag(zcu) == .@"fn") {
        if (inst_data.size != .one) {
            return sema.fail(block, elem_ty_src, "function pointers must be single pointers", .{});
        }
    } else if (inst_data.size == .many and elem_ty.zigTypeTag(zcu) == .@"opaque") {
        return sema.fail(block, elem_ty_src, "unknown-length pointer to opaque not allowed", .{});
    } else if (inst_data.size == .c) {
        if (!try sema.validateExternType(elem_ty, .other)) {
            const msg = msg: {
                const msg = try sema.errMsg(elem_ty_src, "C pointers cannot point to non-C-ABI-compatible type '{}'", .{elem_ty.fmt(pt)});
                errdefer msg.destroy(sema.gpa);

                try sema.explainWhyTypeIsNotExtern(msg, elem_ty_src, elem_ty, .other);

                try sema.addDeclaredHereNote(msg, elem_ty);
                break :msg msg;
            };
            return sema.failWithOwnedErrorMsg(block, msg);
        }
        if (elem_ty.zigTypeTag(zcu) == .@"opaque") {
            return sema.fail(block, elem_ty_src, "C pointers cannot point to opaque types", .{});
        }
    }

    if (host_size != 0 and !try sema.validatePackedType(elem_ty)) {
        return sema.failWithOwnedErrorMsg(block, msg: {
            const msg = try sema.errMsg(elem_ty_src, "bit-pointer cannot refer to value of type '{}'", .{elem_ty.fmt(pt)});
            errdefer msg.destroy(sema.gpa);
            try sema.explainWhyTypeIsNotPacked(msg, elem_ty_src, elem_ty);
            break :msg msg;
        });
    }

    const ty = try pt.ptrTypeSema(.{
        .child = elem_ty.toIntern(),
        .sentinel = sentinel,
        .flags = .{
            .alignment = abi_align,
            .address_space = address_space,
            .is_const = !inst_data.flags.is_mutable,
            .is_allowzero = inst_data.flags.is_allowzero,
            .is_volatile = inst_data.flags.is_volatile,
            .size = inst_data.size,
        },
        .packed_offset = .{
            .bit_offset = bit_offset,
            .host_size = host_size,
        },
    });
    return Air.internedToRef(ty.toIntern());
}

fn zirStructInitEmpty(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    const tracy = trace(@src());
    defer tracy.end();

    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].un_node;
    const src = block.nodeOffset(inst_data.src_node);
    const ty_src = block.src(.{ .node_offset_init_ty = inst_data.src_node });
    const obj_ty = try sema.resolveType(block, ty_src, inst_data.operand);
    const pt = sema.pt;
    const zcu = pt.zcu;

    switch (obj_ty.zigTypeTag(zcu)) {
        .@"struct" => return sema.structInitEmpty(block, obj_ty, src, src),
        .array, .vector => return sema.arrayInitEmpty(block, src, obj_ty),
        .void => return Air.internedToRef(Value.void.toIntern()),
        .@"union" => return sema.fail(block, src, "union initializer must initialize one field", .{}),
        else => return sema.failWithArrayInitNotSupported(block, src, obj_ty),
    }
}

fn zirStructInitEmptyResult(sema: *Sema, block: *Block, inst: Zir.Inst.Index, is_byref: bool) CompileError!Air.Inst.Ref {
    const tracy = trace(@src());
    defer tracy.end();

    const pt = sema.pt;
    const zcu = pt.zcu;
    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].un_node;
    const src = block.nodeOffset(inst_data.src_node);

    // Generic poison means this is an untyped anonymous empty struct/array init
    const ty_operand = try sema.resolveTypeOrPoison(block, src, inst_data.operand) orelse {
        if (is_byref) {
            return sema.uavRef(.empty_tuple);
        } else {
            return .empty_tuple;
        }
    };

    const init_ty = if (is_byref) ty: {
        const ptr_ty = ty_operand.optEuBaseType(zcu);
        assert(ptr_ty.zigTypeTag(zcu) == .pointer); // validated by a previous instruction
        switch (ptr_ty.ptrSize(zcu)) {
            // Use a zero-length array for a slice or many-ptr result
            .slice, .many => break :ty try pt.arrayType(.{
                .len = 0,
                .child = ptr_ty.childType(zcu).toIntern(),
                .sentinel = if (ptr_ty.sentinel(zcu)) |s| s.toIntern() else .none,
            }),
            // Just use the child type for a single-pointer or C-pointer result
            .one, .c => {
                const child = ptr_ty.childType(zcu);
                if (child.toIntern() == .anyopaque_type) {
                    // ...unless that child is anyopaque, in which case this is equivalent to an untyped init.
                    // `.{}` is an empty tuple.
                    if (is_byref) {
                        return sema.uavRef(.empty_tuple);
                    } else {
                        return .empty_tuple;
                    }
                }
                break :ty child;
            },
        }
        if (!ptr_ty.isSlice(zcu)) {
            break :ty ptr_ty.childType(zcu);
        }
        // To make `&.{}` a `[:s]T`, the init should be a `[0:s]T`.
        break :ty try pt.arrayType(.{
            .len = 0,
            .sentinel = if (ptr_ty.sentinel(zcu)) |s| s.toIntern() else .none,
            .child = ptr_ty.childType(zcu).toIntern(),
        });
    } else ty_operand;
    const obj_ty = init_ty.optEuBaseType(zcu);

    const empty_ref = switch (obj_ty.zigTypeTag(zcu)) {
        .@"struct" => try sema.structInitEmpty(block, obj_ty, src, src),
        .array, .vector => try sema.arrayInitEmpty(block, src, obj_ty),
        .@"union" => return sema.fail(block, src, "union initializer must initialize one field", .{}),
        else => return sema.failWithArrayInitNotSupported(block, src, obj_ty),
    };
    const init_ref = try sema.coerce(block, init_ty, empty_ref, src);

    if (is_byref) {
        const init_val = (try sema.resolveValue(init_ref)).?;
        return sema.uavRef(init_val.toIntern());
    } else {
        return init_ref;
    }
}

fn structInitEmpty(
    sema: *Sema,
    block: *Block,
    struct_ty: Type,
    dest_src: LazySrcLoc,
    init_src: LazySrcLoc,
) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const gpa = sema.gpa;
    // This logic must be synchronized with that in `zirStructInit`.
    try struct_ty.resolveFields(pt);

    // The init values to use for the struct instance.
    const field_inits = try gpa.alloc(Air.Inst.Ref, struct_ty.structFieldCount(zcu));
    defer gpa.free(field_inits);
    @memset(field_inits, .none);

    // Maps field index in the struct declaration to the field index in the initialization expression.
    const field_assign_idxs = try gpa.alloc(?usize, struct_ty.structFieldCount(zcu));
    defer gpa.free(field_assign_idxs);
    @memset(field_assign_idxs, null);

    return sema.finishStructInit(block, init_src, dest_src, field_inits, field_assign_idxs, struct_ty, struct_ty, false);
}

fn arrayInitEmpty(sema: *Sema, block: *Block, src: LazySrcLoc, obj_ty: Type) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const arr_len = obj_ty.arrayLen(zcu);
    if (arr_len != 0) {
        if (obj_ty.zigTypeTag(zcu) == .array) {
            return sema.fail(block, src, "expected {d} array elements; found 0", .{arr_len});
        } else {
            return sema.fail(block, src, "expected {d} vector elements; found 0", .{arr_len});
        }
    }
    return Air.internedToRef((try pt.intern(.{ .aggregate = .{
        .ty = obj_ty.toIntern(),
        .storage = .{ .elems = &.{} },
    } })));
}

fn zirUnionInit(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].pl_node;
    const ty_src = block.builtinCallArgSrc(inst_data.src_node, 0);
    const field_src = block.builtinCallArgSrc(inst_data.src_node, 1);
    const init_src = block.builtinCallArgSrc(inst_data.src_node, 2);
    const extra = sema.code.extraData(Zir.Inst.UnionInit, inst_data.payload_index).data;
    const union_ty = try sema.resolveType(block, ty_src, extra.union_type);
    if (union_ty.zigTypeTag(pt.zcu) != .@"union") {
        return sema.fail(block, ty_src, "expected union type, found '{}'", .{union_ty.fmt(pt)});
    }
    const field_name = try sema.resolveConstStringIntern(block, field_src, extra.field_name, .{ .simple = .union_field_name });
    const init = try sema.resolveInst(extra.init);
    return sema.unionInit(block, init, init_src, union_ty, ty_src, field_name, field_src);
}

fn unionInit(
    sema: *Sema,
    block: *Block,
    uncasted_init: Air.Inst.Ref,
    init_src: LazySrcLoc,
    union_ty: Type,
    union_ty_src: LazySrcLoc,
    field_name: InternPool.NullTerminatedString,
    field_src: LazySrcLoc,
) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const ip = &zcu.intern_pool;
    const field_index = try sema.unionFieldIndex(block, union_ty, field_name, field_src);
    const field_ty: Type = .fromInterned(zcu.typeToUnion(union_ty).?.field_types.get(ip)[field_index]);
    const init = try sema.coerce(block, field_ty, uncasted_init, init_src);
    _ = union_ty_src;
    return unionInitFromEnumTag(sema, block, init_src, union_ty, field_index, init);
}

fn unionInitFromEnumTag(
    sema: *Sema,
    block: *Block,
    init_src: LazySrcLoc,
    union_ty: Type,
    field_index: u32,
    init: Air.Inst.Ref,
) !Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;

    if (try sema.resolveValue(init)) |init_val| {
        const tag_ty = union_ty.unionTagTypeHypothetical(zcu);
        const tag_val = try pt.enumValueFieldIndex(tag_ty, field_index);
        return Air.internedToRef((try pt.internUnion(.{
            .ty = union_ty.toIntern(),
            .tag = tag_val.toIntern(),
            .val = init_val.toIntern(),
        })));
    }

    try sema.requireRuntimeBlock(block, init_src, null);
    return block.addUnionInit(union_ty, field_index, init);
}

fn zirStructInit(
    sema: *Sema,
    block: *Block,
    inst: Zir.Inst.Index,
    is_ref: bool,
) CompileError!Air.Inst.Ref {
    const gpa = sema.gpa;
    const zir_datas = sema.code.instructions.items(.data);
    const inst_data = zir_datas[@intFromEnum(inst)].pl_node;
    const extra = sema.code.extraData(Zir.Inst.StructInit, inst_data.payload_index);
    const src = block.nodeOffset(inst_data.src_node);

    const pt = sema.pt;
    const zcu = pt.zcu;
    const ip = &zcu.intern_pool;
    const first_item = sema.code.extraData(Zir.Inst.StructInit.Item, extra.end).data;
    const first_field_type_data = zir_datas[@intFromEnum(first_item.field_type)].pl_node;
    const first_field_type_extra = sema.code.extraData(Zir.Inst.FieldType, first_field_type_data.payload_index).data;
    const result_ty = try sema.resolveTypeOrPoison(block, src, first_field_type_extra.container_type) orelse {
        // The type wasn't actually known, so treat this as an anon struct init.
        return sema.structInitAnon(block, src, inst, .typed_init, extra.data, extra.end, is_ref);
    };
    const resolved_ty = result_ty.optEuBaseType(zcu);
    try resolved_ty.resolveLayout(pt);

    if (resolved_ty.zigTypeTag(zcu) == .@"struct") {
        // This logic must be synchronized with that in `zirStructInitEmpty`.

        // Maps field index to field_type index of where it was already initialized.
        // For making sure all fields are accounted for and no fields are duplicated.
        const found_fields = try gpa.alloc(Zir.Inst.Index, resolved_ty.structFieldCount(zcu));
        defer gpa.free(found_fields);

        // The init values to use for the struct instance.
        const field_inits = try gpa.alloc(Air.Inst.Ref, resolved_ty.structFieldCount(zcu));
        defer gpa.free(field_inits);
        @memset(field_inits, .none);

        // Maps field index in the struct declaration to the field index in the initialization expression.
        const field_assign_idxs = try gpa.alloc(?usize, resolved_ty.structFieldCount(zcu));
        defer gpa.free(field_assign_idxs);
        @memset(field_assign_idxs, null);

        var field_i: u32 = 0;
        var extra_index = extra.end;

        const is_packed = resolved_ty.containerLayout(zcu) == .@"packed";
        while (field_i < extra.data.fields_len) : (field_i += 1) {
            const item = sema.code.extraData(Zir.Inst.StructInit.Item, extra_index);
            extra_index = item.end;

            const field_type_data = zir_datas[@intFromEnum(item.data.field_type)].pl_node;
            const field_src = block.src(.{ .node_offset_initializer = field_type_data.src_node });
            const field_type_extra = sema.code.extraData(Zir.Inst.FieldType, field_type_data.payload_index).data;
            const field_name = try ip.getOrPutString(
                gpa,
                pt.tid,
                sema.code.nullTerminatedString(field_type_extra.name_start),
                .no_embedded_nulls,
            );
            const field_index = if (resolved_ty.isTuple(zcu))
                try sema.tupleFieldIndex(block, resolved_ty, field_name, field_src)
            else
                try sema.structFieldIndex(block, resolved_ty, field_name, field_src);
            assert(field_inits[field_index] == .none);
            field_assign_idxs[field_index] = field_i;
            found_fields[field_index] = item.data.field_type;
            const uncoerced_init = try sema.resolveInst(item.data.init);
            const field_ty = resolved_ty.fieldType(field_index, zcu);
            field_inits[field_index] = try sema.coerce(block, field_ty, uncoerced_init, field_src);
            if (!is_packed) {
                try resolved_ty.resolveStructFieldInits(pt);
                if (try resolved_ty.structFieldValueComptime(pt, field_index)) |default_value| {
                    const init_val = (try sema.resolveValue(field_inits[field_index])) orelse {
                        return sema.failWithNeededComptime(block, field_src, .{ .simple = .stored_to_comptime_field });
                    };

                    if (!init_val.eql(default_value, resolved_ty.fieldType(field_index, zcu), zcu)) {
                        return sema.failWithInvalidComptimeFieldStore(block, field_src, resolved_ty, field_index);
                    }
                }
            }
        }

        return sema.finishStructInit(block, src, src, field_inits, field_assign_idxs, resolved_ty, result_ty, is_ref);
    } else if (resolved_ty.zigTypeTag(zcu) == .@"union") {
        if (extra.data.fields_len != 1) {
            return sema.fail(block, src, "union initialization expects exactly one field", .{});
        }

        const item = sema.code.extraData(Zir.Inst.StructInit.Item, extra.end);

        const field_type_data = zir_datas[@intFromEnum(item.data.field_type)].pl_node;
        const field_src = block.src(.{ .node_offset_initializer = field_type_data.src_node });
        const field_type_extra = sema.code.extraData(Zir.Inst.FieldType, field_type_data.payload_index).data;
        const field_name = try ip.getOrPutString(
            gpa,
            pt.tid,
            sema.code.nullTerminatedString(field_type_extra.name_start),
            .no_embedded_nulls,
        );
        const field_index = try sema.unionFieldIndex(block, resolved_ty, field_name, field_src);
        const tag_ty = resolved_ty.unionTagTypeHypothetical(zcu);
        const tag_val = try pt.enumValueFieldIndex(tag_ty, field_index);
        const field_ty: Type = .fromInterned(zcu.typeToUnion(resolved_ty).?.field_types.get(ip)[field_index]);

        if (field_ty.zigTypeTag(zcu) == .noreturn) {
            return sema.failWithOwnedErrorMsg(block, msg: {
                const msg = try sema.errMsg(src, "cannot initialize 'noreturn' field of union", .{});
                errdefer msg.destroy(sema.gpa);

                try sema.addFieldErrNote(resolved_ty, field_index, msg, "field '{}' declared here", .{
                    field_name.fmt(ip),
                });
                try sema.addDeclaredHereNote(msg, resolved_ty);
                break :msg msg;
            });
        }

        const uncoerced_init_inst = try sema.resolveInst(item.data.init);
        const init_inst = try sema.coerce(block, field_ty, uncoerced_init_inst, field_src);

        if (try sema.resolveValue(init_inst)) |val| {
            const struct_val = Value.fromInterned(try pt.internUnion(.{
                .ty = resolved_ty.toIntern(),
                .tag = tag_val.toIntern(),
                .val = val.toIntern(),
            }));
            const final_val_inst = try sema.coerce(block, result_ty, Air.internedToRef(struct_val.toIntern()), src);
            const final_val = (try sema.resolveValue(final_val_inst)).?;
            return sema.addConstantMaybeRef(final_val.toIntern(), is_ref);
        }

        if (try resolved_ty.comptimeOnlySema(pt)) {
            return sema.failWithNeededComptime(block, field_src, .{ .comptime_only = .{
                .ty = resolved_ty,
                .msg = .union_init,
            } });
        }

        try sema.validateRuntimeValue(block, field_src, init_inst);

        if (is_ref) {
            const target = zcu.getTarget();
            const alloc_ty = try pt.ptrTypeSema(.{
                .child = result_ty.toIntern(),
                .flags = .{ .address_space = target_util.defaultAddressSpace(target, .local) },
            });
            const alloc = try block.addTy(.alloc, alloc_ty);
            const base_ptr = try sema.optEuBasePtrInit(block, alloc, src);
            const field_ptr = try sema.unionFieldPtr(block, field_src, base_ptr, field_name, field_src, resolved_ty, true);
            try sema.storePtr(block, src, field_ptr, init_inst);
            const new_tag = Air.internedToRef(tag_val.toIntern());
            _ = try block.addBinOp(.set_union_tag, base_ptr, new_tag);
            return sema.makePtrConst(block, alloc);
        }

        try sema.requireRuntimeBlock(block, src, null);
        const union_val = try block.addUnionInit(resolved_ty, field_index, init_inst);
        return sema.coerce(block, result_ty, union_val, src);
    }
    unreachable;
}

fn finishStructInit(
    sema: *Sema,
    block: *Block,
    init_src: LazySrcLoc,
    dest_src: LazySrcLoc,
    field_inits: []Air.Inst.Ref,
    field_assign_idxs: []?usize,
    struct_ty: Type,
    result_ty: Type,
    is_ref: bool,
) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const ip = &zcu.intern_pool;

    var root_msg: ?*Zcu.ErrorMsg = null;
    errdefer if (root_msg) |msg| msg.destroy(sema.gpa);

    switch (ip.indexToKey(struct_ty.toIntern())) {
        .tuple_type => |tuple| {
            // We can't get the slices, as the coercion may invalidate them.
            for (0..tuple.types.len) |i| {
                if (field_inits[i] != .none) {
                    // Coerce the init value to the field type.
                    const field_src = block.src(.{ .init_elem = .{
                        .init_node_offset = init_src.offset.node_offset.x,
                        .elem_index = @intCast(i),
                    } });
                    const field_ty: Type = .fromInterned(tuple.types.get(ip)[i]);
                    field_inits[i] = try sema.coerce(block, field_ty, field_inits[i], field_src);
                    continue;
                }

                const default_val = tuple.values.get(ip)[i];

                if (default_val == .none) {
                    const template = "missing tuple field with index {d}";
                    if (root_msg) |msg| {
                        try sema.errNote(init_src, msg, template, .{i});
                    } else {
                        root_msg = try sema.errMsg(init_src, template, .{i});
                    }
                } else {
                    field_inits[i] = Air.internedToRef(default_val);
                }
            }
        },
        .struct_type => {
            const struct_type = ip.loadStructType(struct_ty.toIntern());
            for (0..struct_type.field_types.len) |i| {
                if (field_inits[i] != .none) {
                    // Coerce the init value to the field type.
                    const field_src = block.src(.{ .init_elem = .{
                        .init_node_offset = init_src.offset.node_offset.x,
                        .elem_index = @intCast(i),
                    } });
                    const field_ty: Type = .fromInterned(struct_type.field_types.get(ip)[i]);
                    field_inits[i] = try sema.coerce(block, field_ty, field_inits[i], field_src);
                    continue;
                }

                try struct_ty.resolveStructFieldInits(pt);

                const field_init = struct_type.fieldInit(ip, i);
                if (field_init == .none) {
                    const field_name = struct_type.field_names.get(ip)[i];
                    const template = "missing struct field: {}";
                    const args = .{field_name.fmt(ip)};
                    if (root_msg) |msg| {
                        try sema.errNote(init_src, msg, template, args);
                    } else {
                        root_msg = try sema.errMsg(init_src, template, args);
                    }
                } else {
                    field_inits[i] = Air.internedToRef(field_init);
                }
            }
        },
        else => unreachable,
    }

    if (root_msg) |msg| {
        try sema.addDeclaredHereNote(msg, struct_ty);
        root_msg = null;
        return sema.failWithOwnedErrorMsg(block, msg);
    }

    // Find which field forces the expression to be runtime, if any.
    const opt_runtime_index = for (field_inits, field_assign_idxs) |field_init, field_assign| {
        if (!(try sema.isComptimeKnown(field_init))) {
            break field_assign;
        }
    } else null;

    const runtime_index = opt_runtime_index orelse {
        const elems = try sema.arena.alloc(InternPool.Index, field_inits.len);
        for (elems, field_inits) |*elem, field_init| {
            elem.* = (sema.resolveValue(field_init) catch unreachable).?.toIntern();
        }
        const struct_val = try pt.intern(.{ .aggregate = .{
            .ty = struct_ty.toIntern(),
            .storage = .{ .elems = elems },
        } });
        const final_val_inst = try sema.coerce(block, result_ty, Air.internedToRef(struct_val), init_src);
        const final_val = (try sema.resolveValue(final_val_inst)).?;
        return sema.addConstantMaybeRef(final_val.toIntern(), is_ref);
    };

    if (try struct_ty.comptimeOnlySema(pt)) {
        return sema.failWithNeededComptime(block, block.src(.{ .init_elem = .{
            .init_node_offset = init_src.offset.node_offset.x,
            .elem_index = @intCast(runtime_index),
        } }), .{ .comptime_only = .{
            .ty = struct_ty,
            .msg = .struct_init,
        } });
    }

    for (field_inits) |field_init| {
        try sema.validateRuntimeValue(block, dest_src, field_init);
    }

    if (is_ref) {
        try struct_ty.resolveLayout(pt);
        const target = zcu.getTarget();
        const alloc_ty = try pt.ptrTypeSema(.{
            .child = result_ty.toIntern(),
            .flags = .{ .address_space = target_util.defaultAddressSpace(target, .local) },
        });
        const alloc = try block.addTy(.alloc, alloc_ty);
        const base_ptr = try sema.optEuBasePtrInit(block, alloc, init_src);
        for (field_inits, 0..) |field_init, i_usize| {
            const i: u32 = @intCast(i_usize);
            const field_ptr = try sema.structFieldPtrByIndex(block, dest_src, base_ptr, i, struct_ty);
            try sema.storePtr(block, dest_src, field_ptr, field_init);
        }

        return sema.makePtrConst(block, alloc);
    }

    try sema.requireRuntimeBlock(block, dest_src, block.src(.{ .init_elem = .{
        .init_node_offset = init_src.offset.node_offset.x,
        .elem_index = @intCast(runtime_index),
    } }));
    try struct_ty.resolveStructFieldInits(pt);
    const struct_val = try block.addAggregateInit(struct_ty, field_inits);
    return sema.coerce(block, result_ty, struct_val, init_src);
}

fn zirStructInitAnon(
    sema: *Sema,
    block: *Block,
    inst: Zir.Inst.Index,
) CompileError!Air.Inst.Ref {
    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].pl_node;
    const src = block.nodeOffset(inst_data.src_node);
    const extra = sema.code.extraData(Zir.Inst.StructInitAnon, inst_data.payload_index);
    return sema.structInitAnon(block, src, inst, .anon_init, extra.data, extra.end, false);
}

fn structInitAnon(
    sema: *Sema,
    block: *Block,
    src: LazySrcLoc,
    inst: Zir.Inst.Index,
    /// It is possible for a typed struct_init to be downgraded to an anonymous init due to a
    /// generic poison type. In this case, we need to know to interpret the extra data differently.
    comptime kind: enum { anon_init, typed_init },
    extra_data: switch (kind) {
        .anon_init => Zir.Inst.StructInitAnon,
        .typed_init => Zir.Inst.StructInit,
    },
    extra_end: usize,
    is_ref: bool,
) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const gpa = sema.gpa;
    const ip = &zcu.intern_pool;
    const zir_datas = sema.code.instructions.items(.data);

    const types = try sema.arena.alloc(InternPool.Index, extra_data.fields_len);
    const values = try sema.arena.alloc(InternPool.Index, types.len);
    const names = try sema.arena.alloc(InternPool.NullTerminatedString, types.len);

    var any_values = false;

    // Find which field forces the expression to be runtime, if any.
    const opt_runtime_index = rs: {
        var runtime_index: ?usize = null;
        var extra_index = extra_end;
        for (types, values, names, 0..) |*field_ty, *field_val, *field_name, i_usize| {
            const item = switch (kind) {
                .anon_init => sema.code.extraData(Zir.Inst.StructInitAnon.Item, extra_index),
                .typed_init => sema.code.extraData(Zir.Inst.StructInit.Item, extra_index),
            };
            extra_index = item.end;

            const name = switch (kind) {
                .anon_init => sema.code.nullTerminatedString(item.data.field_name),
                .typed_init => name: {
                    // `item.data.field_type` references a `field_type` instruction
                    const field_type_data = zir_datas[@intFromEnum(item.data.field_type)].pl_node;
                    const field_type_extra = sema.code.extraData(Zir.Inst.FieldType, field_type_data.payload_index);
                    break :name sema.code.nullTerminatedString(field_type_extra.data.name_start);
                },
            };

            field_name.* = try zcu.intern_pool.getOrPutString(gpa, pt.tid, name, .no_embedded_nulls);

            const init = try sema.resolveInst(item.data.init);
            field_ty.* = sema.typeOf(init).toIntern();
            if (Type.fromInterned(field_ty.*).zigTypeTag(zcu) == .@"opaque") {
                const msg = msg: {
                    const field_src = block.src(.{ .init_elem = .{
                        .init_node_offset = src.offset.node_offset.x,
                        .elem_index = @intCast(i_usize),
                    } });
                    const msg = try sema.errMsg(field_src, "opaque types have unknown size and therefore cannot be directly embedded in structs", .{});
                    errdefer msg.destroy(sema.gpa);

                    try sema.addDeclaredHereNote(msg, .fromInterned(field_ty.*));
                    break :msg msg;
                };
                return sema.failWithOwnedErrorMsg(block, msg);
            }
            if (try sema.resolveValue(init)) |init_val| {
                field_val.* = init_val.toIntern();
                any_values = true;
            } else {
                field_val.* = .none;
                runtime_index = @intCast(i_usize);
            }
        }
        break :rs runtime_index;
    };

    // We treat anonymous struct types as reified types, because there are similarities:
    // * They use a form of structural equivalence, which we can easily model using a custom hash
    // * They do not have captures
    // * They immediately have their fields resolved
    // In general, other code should treat anon struct types and reified struct types identically,
    // so there's no point having a separate `InternPool.NamespaceType` field for them.
    const type_hash: u64 = hash: {
        var hasher = std.hash.Wyhash.init(0);
        hasher.update(std.mem.sliceAsBytes(types));
        hasher.update(std.mem.sliceAsBytes(values));
        hasher.update(std.mem.sliceAsBytes(names));
        break :hash hasher.final();
    };
    const tracked_inst = try block.trackZir(inst);
    const struct_ty = switch (try ip.getStructType(gpa, pt.tid, .{
        .layout = .auto,
        .fields_len = extra_data.fields_len,
        .known_non_opv = false,
        .requires_comptime = .unknown,
        .any_comptime_fields = any_values,
        .any_default_inits = any_values,
        .inits_resolved = true,
        .any_aligned_fields = false,
        .key = .{ .reified = .{
            .zir_index = tracked_inst,
            .type_hash = type_hash,
        } },
    }, false)) {
        .wip => |wip| ty: {
            errdefer wip.cancel(ip, pt.tid);
            wip.setName(ip, try sema.createTypeName(block, .anon, "struct", inst, wip.index));

            const struct_type = ip.loadStructType(wip.index);

            for (names, values, 0..) |name, init_val, field_idx| {
                assert(struct_type.addFieldName(ip, name) == null);
                if (init_val != .none) struct_type.setFieldComptime(ip, field_idx);
            }

            @memcpy(struct_type.field_types.get(ip), types);
            if (any_values) {
                @memcpy(struct_type.field_inits.get(ip), values);
            }

            const new_namespace_index = try pt.createNamespace(.{
                .parent = block.namespace.toOptional(),
                .owner_type = wip.index,
                .file_scope = block.getFileScopeIndex(zcu),
                .generation = zcu.generation,
            });
            try zcu.comp.queueJob(.{ .resolve_type_fully = wip.index });
            codegen_type: {
                if (zcu.comp.config.use_llvm) break :codegen_type;
                if (block.ownerModule().strip) break :codegen_type;
                try zcu.comp.queueJob(.{ .codegen_type = wip.index });
            }
            if (zcu.comp.debugIncremental()) try zcu.incremental_debug_state.newType(zcu, wip.index);
            break :ty wip.finish(ip, new_namespace_index);
        },
        .existing => |ty| ty,
    };
    try sema.declareDependency(.{ .interned = struct_ty });
    try sema.addTypeReferenceEntry(src, struct_ty);

    const runtime_index = opt_runtime_index orelse {
        const struct_val = try pt.intern(.{ .aggregate = .{
            .ty = struct_ty,
            .storage = .{ .elems = values },
        } });
        return sema.addConstantMaybeRef(struct_val, is_ref);
    };

    try sema.requireRuntimeBlock(block, LazySrcLoc.unneeded, block.src(.{ .init_elem = .{
        .init_node_offset = src.offset.node_offset.x,
        .elem_index = @intCast(runtime_index),
    } }));

    if (is_ref) {
        const target = zcu.getTarget();
        const alloc_ty = try pt.ptrTypeSema(.{
            .child = struct_ty,
            .flags = .{ .address_space = target_util.defaultAddressSpace(target, .local) },
        });
        const alloc = try block.addTy(.alloc, alloc_ty);
        var extra_index = extra_end;
        for (types, 0..) |field_ty, i_usize| {
            const i: u32 = @intCast(i_usize);
            const item = switch (kind) {
                .anon_init => sema.code.extraData(Zir.Inst.StructInitAnon.Item, extra_index),
                .typed_init => sema.code.extraData(Zir.Inst.StructInit.Item, extra_index),
            };
            extra_index = item.end;

            const field_ptr_ty = try pt.ptrTypeSema(.{
                .child = field_ty,
                .flags = .{ .address_space = target_util.defaultAddressSpace(target, .local) },
            });
            if (values[i] == .none) {
                const init = try sema.resolveInst(item.data.init);
                const field_ptr = try block.addStructFieldPtr(alloc, i, field_ptr_ty);
                _ = try block.addBinOp(.store, field_ptr, init);
            }
        }

        return sema.makePtrConst(block, alloc);
    }

    const element_refs = try sema.arena.alloc(Air.Inst.Ref, types.len);
    var extra_index = extra_end;
    for (types, 0..) |_, i| {
        const item = switch (kind) {
            .anon_init => sema.code.extraData(Zir.Inst.StructInitAnon.Item, extra_index),
            .typed_init => sema.code.extraData(Zir.Inst.StructInit.Item, extra_index),
        };
        extra_index = item.end;
        element_refs[i] = try sema.resolveInst(item.data.init);
    }

    return block.addAggregateInit(.fromInterned(struct_ty), element_refs);
}

fn zirArrayInit(
    sema: *Sema,
    block: *Block,
    inst: Zir.Inst.Index,
    is_ref: bool,
) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const gpa = sema.gpa;
    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].pl_node;
    const src = block.nodeOffset(inst_data.src_node);

    const extra = sema.code.extraData(Zir.Inst.MultiOp, inst_data.payload_index);
    const args = sema.code.refSlice(extra.end, extra.data.operands_len);
    assert(args.len >= 2); // array_ty + at least one element

    const result_ty = try sema.resolveTypeOrPoison(block, src, args[0]) orelse {
        // The type wasn't actually known, so treat this as an anon array init.
        return sema.arrayInitAnon(block, src, args[1..], is_ref);
    };
    const array_ty = result_ty.optEuBaseType(zcu);
    const is_tuple = array_ty.zigTypeTag(zcu) == .@"struct";
    const sentinel_val = array_ty.sentinel(zcu);

    var root_msg: ?*Zcu.ErrorMsg = null;
    errdefer if (root_msg) |msg| msg.destroy(sema.gpa);

    const final_len = try sema.usizeCast(block, src, array_ty.arrayLenIncludingSentinel(zcu));
    const resolved_args = try gpa.alloc(Air.Inst.Ref, final_len);
    defer gpa.free(resolved_args);
    for (resolved_args, 0..) |*dest, i| {
        const elem_src = block.src(.{ .init_elem = .{
            .init_node_offset = src.offset.node_offset.x,
            .elem_index = @intCast(i),
        } });
        // Less inits than needed.
        if (i + 2 > args.len) if (is_tuple) {
            const default_val = array_ty.structFieldDefaultValue(i, zcu).toIntern();
            if (default_val == .unreachable_value) {
                const template = "missing tuple field with index {d}";
                if (root_msg) |msg| {
                    try sema.errNote(src, msg, template, .{i});
                } else {
                    root_msg = try sema.errMsg(src, template, .{i});
                }
            } else {
                dest.* = Air.internedToRef(default_val);
            }
            continue;
        } else {
            dest.* = Air.internedToRef(sentinel_val.?.toIntern());
            break;
        };

        const arg = args[i + 1];
        const resolved_arg = try sema.resolveInst(arg);
        const elem_ty = if (is_tuple)
            array_ty.fieldType(i, zcu)
        else
            array_ty.elemType2(zcu);
        dest.* = try sema.coerce(block, elem_ty, resolved_arg, elem_src);
        if (is_tuple) {
            if (array_ty.structFieldIsComptime(i, zcu))
                try array_ty.resolveStructFieldInits(pt);
            if (try array_ty.structFieldValueComptime(pt, i)) |field_val| {
                const init_val = try sema.resolveConstValue(block, elem_src, dest.*, .{ .simple = .stored_to_comptime_field });
                if (!field_val.eql(init_val, elem_ty, zcu)) {
                    return sema.failWithInvalidComptimeFieldStore(block, elem_src, array_ty, i);
                }
            }
        }
    }

    if (root_msg) |msg| {
        try sema.addDeclaredHereNote(msg, array_ty);
        root_msg = null;
        return sema.failWithOwnedErrorMsg(block, msg);
    }

    const opt_runtime_index: ?u32 = for (resolved_args, 0..) |arg, i| {
        const comptime_known = try sema.isComptimeKnown(arg);
        if (!comptime_known) break @intCast(i);
    } else null;

    const runtime_index = opt_runtime_index orelse {
        const elem_vals = try sema.arena.alloc(InternPool.Index, resolved_args.len);
        for (elem_vals, resolved_args) |*val, arg| {
            // We checked that all args are comptime above.
            val.* = (sema.resolveValue(arg) catch unreachable).?.toIntern();
        }
        const arr_val = try pt.intern(.{ .aggregate = .{
            .ty = array_ty.toIntern(),
            .storage = .{ .elems = elem_vals },
        } });
        const result_ref = try sema.coerce(block, result_ty, Air.internedToRef(arr_val), src);
        const result_val = (try sema.resolveValue(result_ref)).?;
        return sema.addConstantMaybeRef(result_val.toIntern(), is_ref);
    };

    try sema.requireRuntimeBlock(block, LazySrcLoc.unneeded, block.src(.{ .init_elem = .{
        .init_node_offset = src.offset.node_offset.x,
        .elem_index = runtime_index,
    } }));

    if (is_ref) {
        const target = zcu.getTarget();
        const alloc_ty = try pt.ptrTypeSema(.{
            .child = result_ty.toIntern(),
            .flags = .{ .address_space = target_util.defaultAddressSpace(target, .local) },
        });
        const alloc = try block.addTy(.alloc, alloc_ty);
        const base_ptr = try sema.optEuBasePtrInit(block, alloc, src);

        if (is_tuple) {
            for (resolved_args, 0..) |arg, i| {
                const elem_ptr_ty = try pt.ptrTypeSema(.{
                    .child = array_ty.fieldType(i, zcu).toIntern(),
                    .flags = .{ .address_space = target_util.defaultAddressSpace(target, .local) },
                });
                const elem_ptr_ty_ref = Air.internedToRef(elem_ptr_ty.toIntern());

                const index = try pt.intRef(.usize, i);
                const elem_ptr = try block.addPtrElemPtrTypeRef(base_ptr, index, elem_ptr_ty_ref);
                _ = try block.addBinOp(.store, elem_ptr, arg);
            }
            return sema.makePtrConst(block, alloc);
        }

        const elem_ptr_ty = try pt.ptrTypeSema(.{
            .child = array_ty.elemType2(zcu).toIntern(),
            .flags = .{ .address_space = target_util.defaultAddressSpace(target, .local) },
        });
        const elem_ptr_ty_ref = Air.internedToRef(elem_ptr_ty.toIntern());

        for (resolved_args, 0..) |arg, i| {
            const index = try pt.intRef(.usize, i);
            const elem_ptr = try block.addPtrElemPtrTypeRef(base_ptr, index, elem_ptr_ty_ref);
            _ = try block.addBinOp(.store, elem_ptr, arg);
        }
        return sema.makePtrConst(block, alloc);
    }

    const arr_ref = try block.addAggregateInit(array_ty, resolved_args);
    return sema.coerce(block, result_ty, arr_ref, src);
}

fn zirArrayInitAnon(
    sema: *Sema,
    block: *Block,
    inst: Zir.Inst.Index,
) CompileError!Air.Inst.Ref {
    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].pl_node;
    const src = block.nodeOffset(inst_data.src_node);
    const extra = sema.code.extraData(Zir.Inst.MultiOp, inst_data.payload_index);
    const operands = sema.code.refSlice(extra.end, extra.data.operands_len);
    return sema.arrayInitAnon(block, src, operands, false);
}

fn arrayInitAnon(
    sema: *Sema,
    block: *Block,
    src: LazySrcLoc,
    operands: []const Zir.Inst.Ref,
    is_ref: bool,
) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const gpa = sema.gpa;
    const ip = &zcu.intern_pool;

    const types = try sema.arena.alloc(InternPool.Index, operands.len);
    const values = try sema.arena.alloc(InternPool.Index, operands.len);

    const opt_runtime_src = rs: {
        var runtime_src: ?LazySrcLoc = null;
        for (operands, 0..) |operand, i| {
            const operand_src = src; // TODO better source location
            const elem = try sema.resolveInst(operand);
            types[i] = sema.typeOf(elem).toIntern();
            if (Type.fromInterned(types[i]).zigTypeTag(zcu) == .@"opaque") {
                const msg = msg: {
                    const msg = try sema.errMsg(operand_src, "opaque types have unknown size and therefore cannot be directly embedded in structs", .{});
                    errdefer msg.destroy(gpa);

                    try sema.addDeclaredHereNote(msg, .fromInterned(types[i]));
                    break :msg msg;
                };
                return sema.failWithOwnedErrorMsg(block, msg);
            }
            if (try sema.resolveValue(elem)) |val| {
                values[i] = val.toIntern();
            } else {
                values[i] = .none;
                runtime_src = operand_src;
            }
        }
        break :rs runtime_src;
    };

    const tuple_ty = try ip.getTupleType(gpa, pt.tid, .{
        .types = types,
        .values = values,
    });

    const runtime_src = opt_runtime_src orelse {
        const tuple_val = try pt.intern(.{ .aggregate = .{
            .ty = tuple_ty,
            .storage = .{ .elems = values },
        } });
        return sema.addConstantMaybeRef(tuple_val, is_ref);
    };

    try sema.requireRuntimeBlock(block, src, runtime_src);

    if (is_ref) {
        const target = sema.pt.zcu.getTarget();
        const alloc_ty = try pt.ptrTypeSema(.{
            .child = tuple_ty,
            .flags = .{ .address_space = target_util.defaultAddressSpace(target, .local) },
        });
        const alloc = try block.addTy(.alloc, alloc_ty);
        for (operands, 0..) |operand, i_usize| {
            const i: u32 = @intCast(i_usize);
            const field_ptr_ty = try pt.ptrTypeSema(.{
                .child = types[i],
                .flags = .{ .address_space = target_util.defaultAddressSpace(target, .local) },
            });
            if (values[i] == .none) {
                const field_ptr = try block.addStructFieldPtr(alloc, i, field_ptr_ty);
                _ = try block.addBinOp(.store, field_ptr, try sema.resolveInst(operand));
            }
        }

        return sema.makePtrConst(block, alloc);
    }

    const element_refs = try sema.arena.alloc(Air.Inst.Ref, operands.len);
    for (operands, 0..) |operand, i| {
        element_refs[i] = try sema.resolveInst(operand);
    }

    return block.addAggregateInit(.fromInterned(tuple_ty), element_refs);
}

fn addConstantMaybeRef(sema: *Sema, val: InternPool.Index, is_ref: bool) !Air.Inst.Ref {
    return if (is_ref) sema.uavRef(val) else Air.internedToRef(val);
}

fn zirFieldTypeRef(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].pl_node;
    const extra = sema.code.extraData(Zir.Inst.FieldTypeRef, inst_data.payload_index).data;
    const ty_src = block.builtinCallArgSrc(inst_data.src_node, 0);
    const field_src = block.builtinCallArgSrc(inst_data.src_node, 1);
    const aggregate_ty = try sema.resolveType(block, ty_src, extra.container_type);
    const field_name = try sema.resolveConstStringIntern(block, field_src, extra.field_name, .{ .simple = .field_name });
    return sema.fieldType(block, aggregate_ty, field_name, field_src, ty_src);
}

fn zirStructInitFieldType(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const ip = &zcu.intern_pool;
    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].pl_node;
    const extra = sema.code.extraData(Zir.Inst.FieldType, inst_data.payload_index).data;
    const ty_src = block.nodeOffset(inst_data.src_node);
    const field_name_src = block.src(.{ .node_offset_field_name_init = inst_data.src_node });
    const wrapped_aggregate_ty = try sema.resolveTypeOrPoison(block, ty_src, extra.container_type) orelse return .generic_poison_type;
    const aggregate_ty = wrapped_aggregate_ty.optEuBaseType(zcu);
    const zir_field_name = sema.code.nullTerminatedString(extra.name_start);
    const field_name = try ip.getOrPutString(sema.gpa, pt.tid, zir_field_name, .no_embedded_nulls);
    return sema.fieldType(block, aggregate_ty, field_name, field_name_src, ty_src);
}

fn fieldType(
    sema: *Sema,
    block: *Block,
    aggregate_ty: Type,
    field_name: InternPool.NullTerminatedString,
    field_src: LazySrcLoc,
    ty_src: LazySrcLoc,
) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const ip = &zcu.intern_pool;
    var cur_ty = aggregate_ty;
    while (true) {
        try cur_ty.resolveFields(pt);
        switch (cur_ty.zigTypeTag(zcu)) {
            .@"struct" => switch (ip.indexToKey(cur_ty.toIntern())) {
                .tuple_type => |tuple| {
                    const field_index = try sema.tupleFieldIndex(block, cur_ty, field_name, field_src);
                    return Air.internedToRef(tuple.types.get(ip)[field_index]);
                },
                .struct_type => {
                    const struct_type = ip.loadStructType(cur_ty.toIntern());
                    const field_index = struct_type.nameIndex(ip, field_name) orelse
                        return sema.failWithBadStructFieldAccess(block, cur_ty, struct_type, field_src, field_name);
                    const field_ty = struct_type.field_types.get(ip)[field_index];
                    return Air.internedToRef(field_ty);
                },
                else => unreachable,
            },
            .@"union" => {
                const union_obj = zcu.typeToUnion(cur_ty).?;
                const field_index = union_obj.loadTagType(ip).nameIndex(ip, field_name) orelse
                    return sema.failWithBadUnionFieldAccess(block, cur_ty, union_obj, field_src, field_name);
                const field_ty = union_obj.field_types.get(ip)[field_index];
                return Air.internedToRef(field_ty);
            },
            .optional => {
                // Struct/array init through optional requires the child type to not be a pointer.
                // If the child of .optional is a pointer it'll error on the next loop.
                cur_ty = .fromInterned(ip.indexToKey(cur_ty.toIntern()).opt_type);
                continue;
            },
            .error_union => {
                cur_ty = cur_ty.errorUnionPayload(zcu);
                continue;
            },
            else => {},
        }
        return sema.fail(block, ty_src, "expected struct or union; found '{}'", .{
            cur_ty.fmt(pt),
        });
    }
}

fn zirErrorReturnTrace(sema: *Sema, block: *Block) CompileError!Air.Inst.Ref {
    return sema.getErrorReturnTrace(block);
}

fn getErrorReturnTrace(sema: *Sema, block: *Block) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const ip = &zcu.intern_pool;
    const stack_trace_ty = try sema.getBuiltinType(block.nodeOffset(.zero), .StackTrace);
    try stack_trace_ty.resolveFields(pt);
    const ptr_stack_trace_ty = try pt.singleMutPtrType(stack_trace_ty);
    const opt_ptr_stack_trace_ty = try pt.optionalType(ptr_stack_trace_ty.toIntern());

    switch (sema.owner.unwrap()) {
        .func => |func| if (ip.funcAnalysisUnordered(func).has_error_trace and block.ownerModule().error_tracing) {
            return block.addTy(.err_return_trace, opt_ptr_stack_trace_ty);
        },
        .@"comptime", .nav_ty, .nav_val, .type, .memoized_state => {},
    }
    return Air.internedToRef(try pt.intern(.{ .opt = .{
        .ty = opt_ptr_stack_trace_ty.toIntern(),
        .val = .none,
    } }));
}

fn zirFrame(
    sema: *Sema,
    block: *Block,
    extended: Zir.Inst.Extended.InstData,
) CompileError!Air.Inst.Ref {
    const src_node: std.zig.Ast.Node.Offset = @enumFromInt(@as(i32, @bitCast(extended.operand)));
    const src = block.nodeOffset(src_node);
    return sema.failWithUseOfAsync(block, src);
}

fn zirAlignOf(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    const zcu = sema.pt.zcu;
    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].un_node;
    const operand_src = block.builtinCallArgSrc(inst_data.src_node, 0);
    const ty = try sema.resolveType(block, operand_src, inst_data.operand);
    if (ty.isNoReturn(zcu)) {
        return sema.fail(block, operand_src, "no align available for type '{}'", .{ty.fmt(sema.pt)});
    }
    const val = try ty.lazyAbiAlignment(sema.pt);
    return Air.internedToRef(val.toIntern());
}

fn zirIntFromBool(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].un_node;
    const src = block.nodeOffset(inst_data.src_node);
    const operand = try sema.resolveInst(inst_data.operand);
    const operand_ty = sema.typeOf(operand);
    const is_vector = operand_ty.zigTypeTag(zcu) == .vector;
    const operand_scalar_ty = operand_ty.scalarType(zcu);
    if (operand_scalar_ty.toIntern() != .bool_type) {
        return sema.fail(block, src, "expected 'bool', found '{}'", .{operand_scalar_ty.zigTypeTag(zcu)});
    }
    const len = if (is_vector) operand_ty.vectorLen(zcu) else undefined;
    const dest_ty: Type = if (is_vector) try pt.vectorType(.{ .child = .u1_type, .len = len }) else .u1;
    if (try sema.resolveValue(operand)) |val| {
        if (!is_vector) {
            return if (val.isUndef(zcu)) .undef_u1 else if (val.toBool()) .one_u1 else .zero_u1;
        }
        if (val.isUndef(zcu)) return pt.undefRef(dest_ty);
        const new_elems = try sema.arena.alloc(InternPool.Index, len);
        for (new_elems, 0..) |*new_elem, i| {
            const old_elem = try val.elemValue(pt, i);
            new_elem.* = if (old_elem.isUndef(zcu))
                .undef_u1
            else if (old_elem.toBool())
                .one_u1
            else
                .zero_u1;
        }
        return Air.internedToRef(try pt.intern(.{ .aggregate = .{
            .ty = dest_ty.toIntern(),
            .storage = .{ .elems = new_elems },
        } }));
    }
    return block.addBitCast(dest_ty, operand);
}

fn zirErrorName(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].un_node;
    const operand_src = block.builtinCallArgSrc(inst_data.src_node, 0);
    const uncoerced_operand = try sema.resolveInst(inst_data.operand);
    const operand = try sema.coerce(block, .anyerror, uncoerced_operand, operand_src);

    if (try sema.resolveDefinedValue(block, operand_src, operand)) |val| {
        const err_name = sema.pt.zcu.intern_pool.indexToKey(val.toIntern()).err.name;
        return sema.addNullTerminatedStrLit(err_name);
    }

    // Similar to zirTagName, we have special AIR instruction for the error name in case an optimimzation pass
    // might be able to resolve the result at compile time.
    return block.addUnOp(.error_name, operand);
}

fn zirAbs(
    sema: *Sema,
    block: *Block,
    inst: Zir.Inst.Index,
) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].un_node;
    const operand = try sema.resolveInst(inst_data.operand);
    const operand_src = block.builtinCallArgSrc(inst_data.src_node, 0);
    const operand_ty = sema.typeOf(operand);
    const scalar_ty = operand_ty.scalarType(zcu);

    const result_ty = switch (scalar_ty.zigTypeTag(zcu)) {
        .comptime_float, .float, .comptime_int => operand_ty,
        .int => if (scalar_ty.isSignedInt(zcu)) try operand_ty.toUnsigned(pt) else return operand,
        else => return sema.fail(
            block,
            operand_src,
            "expected integer, float, or vector of either integers or floats, found '{}'",
            .{operand_ty.fmt(pt)},
        ),
    };

    return (try sema.maybeConstantUnaryMath(operand, result_ty, Value.abs)) orelse {
        try sema.requireRuntimeBlock(block, operand_src, null);
        return block.addTyOp(.abs, result_ty, operand);
    };
}

fn maybeConstantUnaryMath(
    sema: *Sema,
    operand: Air.Inst.Ref,
    result_ty: Type,
    comptime eval: fn (Value, Type, Allocator, Zcu.PerThread) Allocator.Error!Value,
) CompileError!?Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    switch (result_ty.zigTypeTag(zcu)) {
        .vector => if (try sema.resolveValue(operand)) |val| {
            const scalar_ty = result_ty.scalarType(zcu);
            const vec_len = result_ty.vectorLen(zcu);
            if (val.isUndef(zcu))
                return try pt.undefRef(result_ty);

            const elems = try sema.arena.alloc(InternPool.Index, vec_len);
            for (elems, 0..) |*elem, i| {
                const elem_val = try val.elemValue(pt, i);
                elem.* = (try eval(elem_val, scalar_ty, sema.arena, pt)).toIntern();
            }
            return Air.internedToRef((try pt.intern(.{ .aggregate = .{
                .ty = result_ty.toIntern(),
                .storage = .{ .elems = elems },
            } })));
        },
        else => if (try sema.resolveValue(operand)) |operand_val| {
            if (operand_val.isUndef(zcu))
                return try pt.undefRef(result_ty);
            const result_val = try eval(operand_val, result_ty, sema.arena, pt);
            return Air.internedToRef(result_val.toIntern());
        },
    }
    return null;
}

fn zirUnaryMath(
    sema: *Sema,
    block: *Block,
    inst: Zir.Inst.Index,
    air_tag: Air.Inst.Tag,
    comptime eval: fn (Value, Type, Allocator, Zcu.PerThread) Allocator.Error!Value,
) CompileError!Air.Inst.Ref {
    const tracy = trace(@src());
    defer tracy.end();

    const pt = sema.pt;
    const zcu = pt.zcu;
    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].un_node;
    const operand = try sema.resolveInst(inst_data.operand);
    const operand_src = block.builtinCallArgSrc(inst_data.src_node, 0);
    const operand_ty = sema.typeOf(operand);
    const scalar_ty = operand_ty.scalarType(zcu);

    switch (scalar_ty.zigTypeTag(zcu)) {
        .comptime_float, .float => {},
        else => return sema.fail(
            block,
            operand_src,
            "expected vector of floats or float type, found '{}'",
            .{operand_ty.fmt(pt)},
        ),
    }

    return (try sema.maybeConstantUnaryMath(operand, operand_ty, eval)) orelse {
        try sema.requireRuntimeBlock(block, operand_src, null);
        return block.addUnOp(air_tag, operand);
    };
}

fn zirTagName(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].un_node;
    const operand_src = block.builtinCallArgSrc(inst_data.src_node, 0);
    const src = block.nodeOffset(inst_data.src_node);
    const operand = try sema.resolveInst(inst_data.operand);
    const operand_ty = sema.typeOf(operand);
    const pt = sema.pt;
    const zcu = pt.zcu;
    const ip = &zcu.intern_pool;
    try operand_ty.resolveLayout(pt);
    const enum_ty = switch (operand_ty.zigTypeTag(zcu)) {
        .enum_literal => {
            const val = (try sema.resolveDefinedValue(block, operand_src, operand)).?;
            const tag_name = ip.indexToKey(val.toIntern()).enum_literal;
            return sema.addNullTerminatedStrLit(tag_name);
        },
        .@"enum" => operand_ty,
        .@"union" => operand_ty.unionTagType(zcu) orelse
            return sema.fail(block, src, "union '{}' is untagged", .{operand_ty.fmt(pt)}),
        else => return sema.fail(block, operand_src, "expected enum or union; found '{}'", .{
            operand_ty.fmt(pt),
        }),
    };
    if (enum_ty.enumFieldCount(zcu) == 0) {
        // TODO I don't think this is the correct way to handle this but
        // it prevents a crash.
        // https://github.com/ziglang/zig/issues/15909
        return sema.fail(block, operand_src, "cannot get @tagName of empty enum '{}'", .{
            enum_ty.fmt(pt),
        });
    }
    const casted_operand = try sema.coerce(block, enum_ty, operand, operand_src);
    if (try sema.resolveDefinedValue(block, operand_src, casted_operand)) |val| {
        const field_index = enum_ty.enumTagFieldIndex(val, zcu) orelse {
            const msg = msg: {
                const msg = try sema.errMsg(src, "no field with value '{}' in enum '{}'", .{
                    val.fmtValueSema(pt, sema), enum_ty.fmt(pt),
                });
                errdefer msg.destroy(sema.gpa);
                try sema.errNote(enum_ty.srcLoc(zcu), msg, "declared here", .{});
                break :msg msg;
            };
            return sema.failWithOwnedErrorMsg(block, msg);
        };
        // TODO: write something like getCoercedInts to avoid needing to dupe
        const field_name = enum_ty.enumFieldName(field_index, zcu);
        return sema.addNullTerminatedStrLit(field_name);
    }
    try sema.requireRuntimeBlock(block, src, operand_src);
    if (block.wantSafety() and zcu.backendSupportsFeature(.is_named_enum_value)) {
        const ok = try block.addUnOp(.is_named_enum_value, casted_operand);
        try sema.addSafetyCheck(block, src, ok, .invalid_enum_value);
    }
    // In case the value is runtime-known, we have an AIR instruction for this instead
    // of trying to lower it in Sema because an optimization pass may result in the operand
    // being comptime-known, which would let us elide the `tag_name` AIR instruction.
    return block.addUnOp(.tag_name, casted_operand);
}

fn zirReify(
    sema: *Sema,
    block: *Block,
    extended: Zir.Inst.Extended.InstData,
    inst: Zir.Inst.Index,
) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const gpa = sema.gpa;
    const ip = &zcu.intern_pool;
    const name_strategy: Zir.Inst.NameStrategy = @enumFromInt(extended.small);
    const extra = sema.code.extraData(Zir.Inst.Reify, extended.operand).data;
    const tracked_inst = try block.trackZir(inst);
    const src: LazySrcLoc = .{
        .base_node_inst = tracked_inst,
        .offset = LazySrcLoc.Offset.nodeOffset(.zero),
    };
    const operand_src: LazySrcLoc = .{
        .base_node_inst = tracked_inst,
        .offset = .{
            .node_offset_builtin_call_arg = .{
                .builtin_call_node = .zero, // `tracked_inst` is precisely the `reify` instruction, so offset is 0
                .arg_index = 0,
            },
        },
    };
    const type_info_ty = try sema.getBuiltinType(src, .Type);
    const uncasted_operand = try sema.resolveInst(extra.operand);
    const type_info = try sema.coerce(block, type_info_ty, uncasted_operand, operand_src);
    const val = try sema.resolveConstDefinedValue(block, operand_src, type_info, .{ .simple = .operand_Type });
    const union_val = ip.indexToKey(val.toIntern()).un;
    if (try sema.anyUndef(block, operand_src, Value.fromInterned(union_val.val))) {
        return sema.failWithUseOfUndef(block, operand_src);
    }
    const tag_index = type_info_ty.unionTagFieldIndex(Value.fromInterned(union_val.tag), zcu).?;
    switch (@as(std.builtin.TypeId, @enumFromInt(tag_index))) {
        .type => return .type_type,
        .void => return .void_type,
        .bool => return .bool_type,
        .noreturn => return .noreturn_type,
        .comptime_float => return .comptime_float_type,
        .comptime_int => return .comptime_int_type,
        .undefined => return .undefined_type,
        .null => return .null_type,
        .@"anyframe" => return sema.failWithUseOfAsync(block, src),
        .enum_literal => return .enum_literal_type,
        .int => {
            const int = try sema.interpretBuiltinType(block, operand_src, .fromInterned(union_val.val), std.builtin.Type.Int);
            const ty = try pt.intType(int.signedness, int.bits);
            return Air.internedToRef(ty.toIntern());
        },
        .vector => {
            const struct_type = ip.loadStructType(ip.typeOf(union_val.val));
            const len_val = try Value.fromInterned(union_val.val).fieldValue(pt, struct_type.nameIndex(
                ip,
                try ip.getOrPutString(gpa, pt.tid, "len", .no_embedded_nulls),
            ).?);
            const child_val = try Value.fromInterned(union_val.val).fieldValue(pt, struct_type.nameIndex(
                ip,
                try ip.getOrPutString(gpa, pt.tid, "child", .no_embedded_nulls),
            ).?);

            const len: u32 = @intCast(try len_val.toUnsignedIntSema(pt));
            const child_ty = child_val.toType();

            try sema.checkVectorElemType(block, src, child_ty);

            const ty = try pt.vectorType(.{
                .len = len,
                .child = child_ty.toIntern(),
            });
            return Air.internedToRef(ty.toIntern());
        },
        .float => {
            const float = try sema.interpretBuiltinType(block, operand_src, .fromInterned(union_val.val), std.builtin.Type.Float);

            const ty: Type = switch (float.bits) {
                16 => .f16,
                32 => .f32,
                64 => .f64,
                80 => .f80,
                128 => .f128,
                else => return sema.fail(block, src, "{}-bit float unsupported", .{float.bits}),
            };
            return Air.internedToRef(ty.toIntern());
        },
        .pointer => {
            const struct_type = ip.loadStructType(ip.typeOf(union_val.val));
            const size_val = try Value.fromInterned(union_val.val).fieldValue(pt, struct_type.nameIndex(
                ip,
                try ip.getOrPutString(gpa, pt.tid, "size", .no_embedded_nulls),
            ).?);
            const is_const_val = try Value.fromInterned(union_val.val).fieldValue(pt, struct_type.nameIndex(
                ip,
                try ip.getOrPutString(gpa, pt.tid, "is_const", .no_embedded_nulls),
            ).?);
            const is_volatile_val = try Value.fromInterned(union_val.val).fieldValue(pt, struct_type.nameIndex(
                ip,
                try ip.getOrPutString(gpa, pt.tid, "is_volatile", .no_embedded_nulls),
            ).?);
            const alignment_val = try Value.fromInterned(union_val.val).fieldValue(pt, struct_type.nameIndex(
                ip,
                try ip.getOrPutString(gpa, pt.tid, "alignment", .no_embedded_nulls),
            ).?);
            const address_space_val = try Value.fromInterned(union_val.val).fieldValue(pt, struct_type.nameIndex(
                ip,
                try ip.getOrPutString(gpa, pt.tid, "address_space", .no_embedded_nulls),
            ).?);
            const child_val = try Value.fromInterned(union_val.val).fieldValue(pt, struct_type.nameIndex(
                ip,
                try ip.getOrPutString(gpa, pt.tid, "child", .no_embedded_nulls),
            ).?);
            const is_allowzero_val = try Value.fromInterned(union_val.val).fieldValue(pt, struct_type.nameIndex(
                ip,
                try ip.getOrPutString(gpa, pt.tid, "is_allowzero", .no_embedded_nulls),
            ).?);
            const sentinel_val = try Value.fromInterned(union_val.val).fieldValue(pt, struct_type.nameIndex(
                ip,
                try ip.getOrPutString(gpa, pt.tid, "sentinel_ptr", .no_embedded_nulls),
            ).?);

            if (!try sema.intFitsInType(alignment_val, .u32, null)) {
                return sema.fail(block, src, "alignment must fit in 'u32'", .{});
            }

            const alignment_val_int = try alignment_val.toUnsignedIntSema(pt);
            if (alignment_val_int > 0 and !math.isPowerOfTwo(alignment_val_int)) {
                return sema.fail(block, src, "alignment value '{d}' is not a power of two or zero", .{alignment_val_int});
            }
            const abi_align = Alignment.fromByteUnits(alignment_val_int);

            const elem_ty = child_val.toType();
            if (abi_align != .none) {
                try elem_ty.resolveLayout(pt);
            }

            const ptr_size = try sema.interpretBuiltinType(block, operand_src, size_val, std.builtin.Type.Pointer.Size);

            const actual_sentinel: InternPool.Index = s: {
                if (!sentinel_val.isNull(zcu)) {
                    if (ptr_size == .one or ptr_size == .c) {
                        return sema.fail(block, src, "sentinels are only allowed on slices and unknown-length pointers", .{});
                    }
                    const sentinel_ptr_val = sentinel_val.optionalValue(zcu).?;
                    const ptr_ty = try pt.singleMutPtrType(elem_ty);
                    const sent_val = (try sema.pointerDeref(block, src, sentinel_ptr_val, ptr_ty)).?;
                    try sema.checkSentinelType(block, src, elem_ty);
                    break :s sent_val.toIntern();
                }
                break :s .none;
            };

            if (elem_ty.zigTypeTag(zcu) == .noreturn) {
                return sema.fail(block, src, "pointer to noreturn not allowed", .{});
            } else if (elem_ty.zigTypeTag(zcu) == .@"fn") {
                if (ptr_size != .one) {
                    return sema.fail(block, src, "function pointers must be single pointers", .{});
                }
            } else if (ptr_size == .many and elem_ty.zigTypeTag(zcu) == .@"opaque") {
                return sema.fail(block, src, "unknown-length pointer to opaque not allowed", .{});
            } else if (ptr_size == .c) {
                if (!try sema.validateExternType(elem_ty, .other)) {
                    const msg = msg: {
                        const msg = try sema.errMsg(src, "C pointers cannot point to non-C-ABI-compatible type '{}'", .{elem_ty.fmt(pt)});
                        errdefer msg.destroy(gpa);

                        try sema.explainWhyTypeIsNotExtern(msg, src, elem_ty, .other);

                        try sema.addDeclaredHereNote(msg, elem_ty);
                        break :msg msg;
                    };
                    return sema.failWithOwnedErrorMsg(block, msg);
                }
                if (elem_ty.zigTypeTag(zcu) == .@"opaque") {
                    return sema.fail(block, src, "C pointers cannot point to opaque types", .{});
                }
            }

            const ty = try pt.ptrTypeSema(.{
                .child = elem_ty.toIntern(),
                .sentinel = actual_sentinel,
                .flags = .{
                    .size = ptr_size,
                    .is_const = is_const_val.toBool(),
                    .is_volatile = is_volatile_val.toBool(),
                    .alignment = abi_align,
                    .address_space = try sema.interpretBuiltinType(block, operand_src, address_space_val, std.builtin.AddressSpace),
                    .is_allowzero = is_allowzero_val.toBool(),
                },
            });
            return Air.internedToRef(ty.toIntern());
        },
        .array => {
            const struct_type = ip.loadStructType(ip.typeOf(union_val.val));
            const len_val = try Value.fromInterned(union_val.val).fieldValue(pt, struct_type.nameIndex(
                ip,
                try ip.getOrPutString(gpa, pt.tid, "len", .no_embedded_nulls),
            ).?);
            const child_val = try Value.fromInterned(union_val.val).fieldValue(pt, struct_type.nameIndex(
                ip,
                try ip.getOrPutString(gpa, pt.tid, "child", .no_embedded_nulls),
            ).?);
            const sentinel_val = try Value.fromInterned(union_val.val).fieldValue(pt, struct_type.nameIndex(
                ip,
                try ip.getOrPutString(gpa, pt.tid, "sentinel_ptr", .no_embedded_nulls),
            ).?);

            const len = try len_val.toUnsignedIntSema(pt);
            const child_ty = child_val.toType();
            const sentinel = if (sentinel_val.optionalValue(zcu)) |p| blk: {
                const ptr_ty = try pt.singleMutPtrType(child_ty);
                try sema.checkSentinelType(block, src, child_ty);
                break :blk (try sema.pointerDeref(block, src, p, ptr_ty)).?;
            } else null;

            const ty = try pt.arrayType(.{
                .len = len,
                .sentinel = if (sentinel) |s| s.toIntern() else .none,
                .child = child_ty.toIntern(),
            });
            return Air.internedToRef(ty.toIntern());
        },
        .optional => {
            const struct_type = ip.loadStructType(ip.typeOf(union_val.val));
            const child_val = try Value.fromInterned(union_val.val).fieldValue(pt, struct_type.nameIndex(
                ip,
                try ip.getOrPutString(gpa, pt.tid, "child", .no_embedded_nulls),
            ).?);

            const child_ty = child_val.toType();

            const ty = try pt.optionalType(child_ty.toIntern());
            return Air.internedToRef(ty.toIntern());
        },
        .error_union => {
            const struct_type = ip.loadStructType(ip.typeOf(union_val.val));
            const error_set_val = try Value.fromInterned(union_val.val).fieldValue(pt, struct_type.nameIndex(
                ip,
                try ip.getOrPutString(gpa, pt.tid, "error_set", .no_embedded_nulls),
            ).?);
            const payload_val = try Value.fromInterned(union_val.val).fieldValue(pt, struct_type.nameIndex(
                ip,
                try ip.getOrPutString(gpa, pt.tid, "payload", .no_embedded_nulls),
            ).?);

            const error_set_ty = error_set_val.toType();
            const payload_ty = payload_val.toType();

            if (error_set_ty.zigTypeTag(zcu) != .error_set) {
                return sema.fail(block, src, "Type.ErrorUnion.error_set must be an error set type", .{});
            }

            const ty = try pt.errorUnionType(error_set_ty, payload_ty);
            return Air.internedToRef(ty.toIntern());
        },
        .error_set => {
            const payload_val = Value.fromInterned(union_val.val).optionalValue(zcu) orelse
                return .anyerror_type;

            const names_val = try sema.derefSliceAsArray(block, src, payload_val, .{ .simple = .error_set_contents });

            const len = try sema.usizeCast(block, src, names_val.typeOf(zcu).arrayLen(zcu));
            var names: InferredErrorSet.NameMap = .{};
            try names.ensureUnusedCapacity(sema.arena, len);
            for (0..len) |i| {
                const elem_val = try names_val.elemValue(pt, i);
                const elem_struct_type = ip.loadStructType(ip.typeOf(elem_val.toIntern()));
                const name_val = try elem_val.fieldValue(pt, elem_struct_type.nameIndex(
                    ip,
                    try ip.getOrPutString(gpa, pt.tid, "name", .no_embedded_nulls),
                ).?);

                const name = try sema.sliceToIpString(block, src, name_val, .{ .simple = .error_set_contents });
                _ = try pt.getErrorValue(name);
                const gop = names.getOrPutAssumeCapacity(name);
                if (gop.found_existing) {
                    return sema.fail(block, src, "duplicate error '{}'", .{
                        name.fmt(ip),
                    });
                }
            }

            const ty = try pt.errorSetFromUnsortedNames(names.keys());
            return Air.internedToRef(ty.toIntern());
        },
        .@"struct" => {
            const struct_type = ip.loadStructType(ip.typeOf(union_val.val));
            const layout_val = try Value.fromInterned(union_val.val).fieldValue(pt, struct_type.nameIndex(
                ip,
                try ip.getOrPutString(gpa, pt.tid, "layout", .no_embedded_nulls),
            ).?);
            const backing_integer_val = try Value.fromInterned(union_val.val).fieldValue(pt, struct_type.nameIndex(
                ip,
                try ip.getOrPutString(gpa, pt.tid, "backing_integer", .no_embedded_nulls),
            ).?);
            const fields_val = try Value.fromInterned(union_val.val).fieldValue(pt, struct_type.nameIndex(
                ip,
                try ip.getOrPutString(gpa, pt.tid, "fields", .no_embedded_nulls),
            ).?);
            const decls_val = try Value.fromInterned(union_val.val).fieldValue(pt, struct_type.nameIndex(
                ip,
                try ip.getOrPutString(gpa, pt.tid, "decls", .no_embedded_nulls),
            ).?);
            const is_tuple_val = try Value.fromInterned(union_val.val).fieldValue(pt, struct_type.nameIndex(
                ip,
                try ip.getOrPutString(gpa, pt.tid, "is_tuple", .no_embedded_nulls),
            ).?);

            const layout = try sema.interpretBuiltinType(block, operand_src, layout_val, std.builtin.Type.ContainerLayout);

            // Decls
            if (try decls_val.sliceLen(pt) > 0) {
                return sema.fail(block, src, "reified structs must have no decls", .{});
            }

            if (layout != .@"packed" and !backing_integer_val.isNull(zcu)) {
                return sema.fail(block, src, "non-packed struct does not support backing integer type", .{});
            }

            const fields_arr = try sema.derefSliceAsArray(block, operand_src, fields_val, .{ .simple = .struct_fields });

            if (is_tuple_val.toBool()) {
                switch (layout) {
                    .@"extern" => return sema.fail(block, src, "extern tuples are not supported", .{}),
                    .@"packed" => return sema.fail(block, src, "packed tuples are not supported", .{}),
                    .auto => {},
                }
                return sema.reifyTuple(block, src, fields_arr);
            } else {
                return sema.reifyStruct(block, inst, src, layout, backing_integer_val, fields_arr, name_strategy);
            }
        },
        .@"enum" => {
            const struct_type = ip.loadStructType(ip.typeOf(union_val.val));
            const tag_type_val = try Value.fromInterned(union_val.val).fieldValue(pt, struct_type.nameIndex(
                ip,
                try ip.getOrPutString(gpa, pt.tid, "tag_type", .no_embedded_nulls),
            ).?);
            const fields_val = try Value.fromInterned(union_val.val).fieldValue(pt, struct_type.nameIndex(
                ip,
                try ip.getOrPutString(gpa, pt.tid, "fields", .no_embedded_nulls),
            ).?);
            const decls_val = try Value.fromInterned(union_val.val).fieldValue(pt, struct_type.nameIndex(
                ip,
                try ip.getOrPutString(gpa, pt.tid, "decls", .no_embedded_nulls),
            ).?);
            const is_exhaustive_val = try Value.fromInterned(union_val.val).fieldValue(pt, struct_type.nameIndex(
                ip,
                try ip.getOrPutString(gpa, pt.tid, "is_exhaustive", .no_embedded_nulls),
            ).?);

            if (try decls_val.sliceLen(pt) > 0) {
                return sema.fail(block, src, "reified enums must have no decls", .{});
            }

            const fields_arr = try sema.derefSliceAsArray(block, operand_src, fields_val, .{ .simple = .enum_fields });

            return sema.reifyEnum(block, inst, src, tag_type_val.toType(), is_exhaustive_val.toBool(), fields_arr, name_strategy);
        },
        .@"opaque" => {
            const struct_type = ip.loadStructType(ip.typeOf(union_val.val));
            const decls_val = try Value.fromInterned(union_val.val).fieldValue(pt, struct_type.nameIndex(
                ip,
                try ip.getOrPutString(gpa, pt.tid, "decls", .no_embedded_nulls),
            ).?);

            // Decls
            if (try decls_val.sliceLen(pt) > 0) {
                return sema.fail(block, src, "reified opaque must have no decls", .{});
            }

            const wip_ty = switch (try ip.getOpaqueType(gpa, pt.tid, .{
                .key = .{ .reified = .{
                    .zir_index = try block.trackZir(inst),
                } },
            })) {
                .existing => |ty| {
                    try sema.addTypeReferenceEntry(src, ty);
                    return Air.internedToRef(ty);
                },
                .wip => |wip| wip,
            };
            errdefer wip_ty.cancel(ip, pt.tid);

            wip_ty.setName(ip, try sema.createTypeName(
                block,
                name_strategy,
                "opaque",
                inst,
                wip_ty.index,
            ));

            const new_namespace_index = try pt.createNamespace(.{
                .parent = block.namespace.toOptional(),
                .owner_type = wip_ty.index,
                .file_scope = block.getFileScopeIndex(zcu),
                .generation = zcu.generation,
            });

            try sema.addTypeReferenceEntry(src, wip_ty.index);
            if (zcu.comp.debugIncremental()) try zcu.incremental_debug_state.newType(zcu, wip_ty.index);
            return Air.internedToRef(wip_ty.finish(ip, new_namespace_index));
        },
        .@"union" => {
            const struct_type = ip.loadStructType(ip.typeOf(union_val.val));
            const layout_val = try Value.fromInterned(union_val.val).fieldValue(pt, struct_type.nameIndex(
                ip,
                try ip.getOrPutString(gpa, pt.tid, "layout", .no_embedded_nulls),
            ).?);
            const tag_type_val = try Value.fromInterned(union_val.val).fieldValue(pt, struct_type.nameIndex(
                ip,
                try ip.getOrPutString(gpa, pt.tid, "tag_type", .no_embedded_nulls),
            ).?);
            const fields_val = try Value.fromInterned(union_val.val).fieldValue(pt, struct_type.nameIndex(
                ip,
                try ip.getOrPutString(gpa, pt.tid, "fields", .no_embedded_nulls),
            ).?);
            const decls_val = try Value.fromInterned(union_val.val).fieldValue(pt, struct_type.nameIndex(
                ip,
                try ip.getOrPutString(gpa, pt.tid, "decls", .no_embedded_nulls),
            ).?);

            if (try decls_val.sliceLen(pt) > 0) {
                return sema.fail(block, src, "reified unions must have no decls", .{});
            }
            const layout = try sema.interpretBuiltinType(block, operand_src, layout_val, std.builtin.Type.ContainerLayout);

            const fields_arr = try sema.derefSliceAsArray(block, operand_src, fields_val, .{ .simple = .union_fields });

            return sema.reifyUnion(block, inst, src, layout, tag_type_val, fields_arr, name_strategy);
        },
        .@"fn" => {
            const struct_type = ip.loadStructType(ip.typeOf(union_val.val));
            const calling_convention_val = try Value.fromInterned(union_val.val).fieldValue(pt, struct_type.nameIndex(
                ip,
                try ip.getOrPutString(gpa, pt.tid, "calling_convention", .no_embedded_nulls),
            ).?);
            const is_generic_val = try Value.fromInterned(union_val.val).fieldValue(pt, struct_type.nameIndex(
                ip,
                try ip.getOrPutString(gpa, pt.tid, "is_generic", .no_embedded_nulls),
            ).?);
            const is_var_args_val = try Value.fromInterned(union_val.val).fieldValue(pt, struct_type.nameIndex(
                ip,
                try ip.getOrPutString(gpa, pt.tid, "is_var_args", .no_embedded_nulls),
            ).?);
            const return_type_val = try Value.fromInterned(union_val.val).fieldValue(pt, struct_type.nameIndex(
                ip,
                try ip.getOrPutString(gpa, pt.tid, "return_type", .no_embedded_nulls),
            ).?);
            const params_slice_val = try Value.fromInterned(union_val.val).fieldValue(pt, struct_type.nameIndex(
                ip,
                try ip.getOrPutString(gpa, pt.tid, "params", .no_embedded_nulls),
            ).?);

            const is_generic = is_generic_val.toBool();
            if (is_generic) {
                return sema.fail(block, src, "Type.Fn.is_generic must be false for @Type", .{});
            }

            const is_var_args = is_var_args_val.toBool();
            const cc = try sema.analyzeValueAsCallconv(block, src, calling_convention_val);
            if (is_var_args) {
                try sema.checkCallConvSupportsVarArgs(block, src, cc);
            }

            const return_type = return_type_val.optionalValue(zcu) orelse
                return sema.fail(block, src, "Type.Fn.return_type must be non-null for @Type", .{});

            const params_val = try sema.derefSliceAsArray(block, operand_src, params_slice_val, .{ .simple = .function_parameters });

            const args_len = try sema.usizeCast(block, src, params_val.typeOf(zcu).arrayLen(zcu));
            const param_types = try sema.arena.alloc(InternPool.Index, args_len);

            var noalias_bits: u32 = 0;
            for (param_types, 0..) |*param_type, i| {
                const elem_val = try params_val.elemValue(pt, i);
                const elem_struct_type = ip.loadStructType(ip.typeOf(elem_val.toIntern()));
                const param_is_generic_val = try elem_val.fieldValue(pt, elem_struct_type.nameIndex(
                    ip,
                    try ip.getOrPutString(gpa, pt.tid, "is_generic", .no_embedded_nulls),
                ).?);
                const param_is_noalias_val = try elem_val.fieldValue(pt, elem_struct_type.nameIndex(
                    ip,
                    try ip.getOrPutString(gpa, pt.tid, "is_noalias", .no_embedded_nulls),
                ).?);
                const opt_param_type_val = try elem_val.fieldValue(pt, elem_struct_type.nameIndex(
                    ip,
                    try ip.getOrPutString(gpa, pt.tid, "type", .no_embedded_nulls),
                ).?);

                if (param_is_generic_val.toBool()) {
                    return sema.fail(block, src, "Type.Fn.Param.is_generic must be false for @Type", .{});
                }

                const param_type_val = opt_param_type_val.optionalValue(zcu) orelse
                    return sema.fail(block, src, "Type.Fn.Param.type must be non-null for @Type", .{});
                param_type.* = param_type_val.toIntern();

                if (param_is_noalias_val.toBool()) {
                    if (!Type.fromInterned(param_type.*).isPtrAtRuntime(zcu)) {
                        return sema.fail(block, src, "non-pointer parameter declared noalias", .{});
                    }
                    noalias_bits |= @as(u32, 1) << (std.math.cast(u5, i) orelse
                        return sema.fail(block, src, "this compiler implementation only supports 'noalias' on the first 32 parameters", .{}));
                }
            }

            const ty = try pt.funcType(.{
                .param_types = param_types,
                .noalias_bits = noalias_bits,
                .return_type = return_type.toIntern(),
                .cc = cc,
                .is_var_args = is_var_args,
            });
            return Air.internedToRef(ty.toIntern());
        },
        .frame => return sema.failWithUseOfAsync(block, src),
    }
}

fn reifyEnum(
    sema: *Sema,
    block: *Block,
    inst: Zir.Inst.Index,
    src: LazySrcLoc,
    tag_ty: Type,
    is_exhaustive: bool,
    fields_val: Value,
    name_strategy: Zir.Inst.NameStrategy,
) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const gpa = sema.gpa;
    const ip = &zcu.intern_pool;

    // This logic must stay in sync with the structure of `std.builtin.Type.Enum` - search for `fieldValue`.

    const fields_len: u32 = @intCast(fields_val.typeOf(zcu).arrayLen(zcu));

    // The validation work here is non-trivial, and it's possible the type already exists.
    // So in this first pass, let's just construct a hash to optimize for this case. If the
    // inputs turn out to be invalid, we can cancel the WIP type later.

    // For deduplication purposes, we must create a hash including all details of this type.
    // TODO: use a longer hash!
    var hasher = std.hash.Wyhash.init(0);
    std.hash.autoHash(&hasher, tag_ty.toIntern());
    std.hash.autoHash(&hasher, is_exhaustive);
    std.hash.autoHash(&hasher, fields_len);

    for (0..fields_len) |field_idx| {
        const field_info = try fields_val.elemValue(pt, field_idx);

        const field_name_val = try field_info.fieldValue(pt, 0);
        const field_value_val = try sema.resolveLazyValue(try field_info.fieldValue(pt, 1));

        const field_name = try sema.sliceToIpString(block, src, field_name_val, .{ .simple = .enum_field_name });

        std.hash.autoHash(&hasher, .{
            field_name,
            field_value_val.toIntern(),
        });
    }

    const tracked_inst = try block.trackZir(inst);

    const wip_ty = switch (try ip.getEnumType(gpa, pt.tid, .{
        .has_values = true,
        .tag_mode = if (is_exhaustive) .explicit else .nonexhaustive,
        .fields_len = fields_len,
        .key = .{ .reified = .{
            .zir_index = tracked_inst,
            .type_hash = hasher.final(),
        } },
    }, false)) {
        .wip => |wip| wip,
        .existing => |ty| {
            try sema.declareDependency(.{ .interned = ty });
            try sema.addTypeReferenceEntry(src, ty);
            return Air.internedToRef(ty);
        },
    };
    var done = false;
    errdefer if (!done) wip_ty.cancel(ip, pt.tid);

    if (tag_ty.zigTypeTag(zcu) != .int) {
        return sema.fail(block, src, "Type.Enum.tag_type must be an integer type", .{});
    }

    wip_ty.setName(ip, try sema.createTypeName(
        block,
        name_strategy,
        "enum",
        inst,
        wip_ty.index,
    ));

    const new_namespace_index = try pt.createNamespace(.{
        .parent = block.namespace.toOptional(),
        .owner_type = wip_ty.index,
        .file_scope = block.getFileScopeIndex(zcu),
        .generation = zcu.generation,
    });

    try sema.declareDependency(.{ .interned = wip_ty.index });
    try sema.addTypeReferenceEntry(src, wip_ty.index);
    if (zcu.comp.debugIncremental()) try zcu.incremental_debug_state.newType(zcu, wip_ty.index);
    wip_ty.prepare(ip, new_namespace_index);
    wip_ty.setTagTy(ip, tag_ty.toIntern());
    done = true;

    for (0..fields_len) |field_idx| {
        const field_info = try fields_val.elemValue(pt, field_idx);

        const field_name_val = try field_info.fieldValue(pt, 0);
        const field_value_val = try sema.resolveLazyValue(try field_info.fieldValue(pt, 1));

        // Don't pass a reason; first loop acts as an assertion that this is valid.
        const field_name = try sema.sliceToIpString(block, src, field_name_val, undefined);

        if (!try sema.intFitsInType(field_value_val, tag_ty, null)) {
            // TODO: better source location
            return sema.fail(block, src, "field '{}' with enumeration value '{}' is too large for backing int type '{}'", .{
                field_name.fmt(ip),
                field_value_val.fmtValueSema(pt, sema),
                tag_ty.fmt(pt),
            });
        }

        const coerced_field_val = try pt.getCoerced(field_value_val, tag_ty);
        if (wip_ty.nextField(ip, field_name, coerced_field_val.toIntern())) |conflict| {
            return sema.failWithOwnedErrorMsg(block, switch (conflict.kind) {
                .name => msg: {
                    const msg = try sema.errMsg(src, "duplicate enum field '{}'", .{field_name.fmt(ip)});
                    errdefer msg.destroy(gpa);
                    _ = conflict.prev_field_idx; // TODO: this note is incorrect
                    try sema.errNote(src, msg, "other field here", .{});
                    break :msg msg;
                },
                .value => msg: {
                    const msg = try sema.errMsg(src, "enum tag value {} already taken", .{field_value_val.fmtValueSema(pt, sema)});
                    errdefer msg.destroy(gpa);
                    _ = conflict.prev_field_idx; // TODO: this note is incorrect
                    try sema.errNote(src, msg, "other enum tag value here", .{});
                    break :msg msg;
                },
            });
        }
    }

    if (!is_exhaustive and fields_len > 1 and std.math.log2_int(u64, fields_len) == tag_ty.bitSize(zcu)) {
        return sema.fail(block, src, "non-exhaustive enum specified every value", .{});
    }

    codegen_type: {
        if (zcu.comp.config.use_llvm) break :codegen_type;
        if (block.ownerModule().strip) break :codegen_type;
        // This job depends on any resolve_type_fully jobs queued up before it.
        try zcu.comp.queueJob(.{ .codegen_type = wip_ty.index });
    }
    return Air.internedToRef(wip_ty.index);
}

fn reifyUnion(
    sema: *Sema,
    block: *Block,
    inst: Zir.Inst.Index,
    src: LazySrcLoc,
    layout: std.builtin.Type.ContainerLayout,
    opt_tag_type_val: Value,
    fields_val: Value,
    name_strategy: Zir.Inst.NameStrategy,
) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const gpa = sema.gpa;
    const ip = &zcu.intern_pool;

    // This logic must stay in sync with the structure of `std.builtin.Type.Union` - search for `fieldValue`.

    const fields_len: u32 = @intCast(fields_val.typeOf(zcu).arrayLen(zcu));

    // The validation work here is non-trivial, and it's possible the type already exists.
    // So in this first pass, let's just construct a hash to optimize for this case. If the
    // inputs turn out to be invalid, we can cancel the WIP type later.

    // For deduplication purposes, we must create a hash including all details of this type.
    // TODO: use a longer hash!
    var hasher = std.hash.Wyhash.init(0);
    std.hash.autoHash(&hasher, layout);
    std.hash.autoHash(&hasher, opt_tag_type_val.toIntern());
    std.hash.autoHash(&hasher, fields_len);

    var any_aligns = false;

    for (0..fields_len) |field_idx| {
        const field_info = try fields_val.elemValue(pt, field_idx);

        const field_name_val = try field_info.fieldValue(pt, 0);
        const field_type_val = try field_info.fieldValue(pt, 1);
        const field_align_val = try sema.resolveLazyValue(try field_info.fieldValue(pt, 2));

        const field_name = try sema.sliceToIpString(block, src, field_name_val, .{ .simple = .union_field_name });

        std.hash.autoHash(&hasher, .{
            field_name,
            field_type_val.toIntern(),
            field_align_val.toIntern(),
        });

        if (field_align_val.toUnsignedInt(zcu) != 0) {
            any_aligns = true;
        }
    }

    const tracked_inst = try block.trackZir(inst);

    const wip_ty = switch (try ip.getUnionType(gpa, pt.tid, .{
        .flags = .{
            .layout = layout,
            .status = .none,
            .runtime_tag = if (opt_tag_type_val.optionalValue(zcu) != null)
                .tagged
            else if (layout != .auto)
                .none
            else switch (block.wantSafeTypes()) {
                true => .safety,
                false => .none,
            },
            .any_aligned_fields = any_aligns,
            .requires_comptime = .unknown,
            .assumed_runtime_bits = false,
            .assumed_pointer_aligned = false,
            .alignment = .none,
        },
        .fields_len = fields_len,
        .enum_tag_ty = .none, // set later because not yet validated
        .field_types = &.{}, // set later
        .field_aligns = &.{}, // set later
        .key = .{ .reified = .{
            .zir_index = tracked_inst,
            .type_hash = hasher.final(),
        } },
    }, false)) {
        .wip => |wip| wip,
        .existing => |ty| {
            try sema.declareDependency(.{ .interned = ty });
            try sema.addTypeReferenceEntry(src, ty);
            return Air.internedToRef(ty);
        },
    };
    errdefer wip_ty.cancel(ip, pt.tid);

    const type_name = try sema.createTypeName(
        block,
        name_strategy,
        "union",
        inst,
        wip_ty.index,
    );
    wip_ty.setName(ip, type_name);

    const field_types = try sema.arena.alloc(InternPool.Index, fields_len);
    const field_aligns = if (any_aligns) try sema.arena.alloc(InternPool.Alignment, fields_len) else undefined;

    const enum_tag_ty, const has_explicit_tag = if (opt_tag_type_val.optionalValue(zcu)) |tag_type_val| tag_ty: {
        switch (ip.indexToKey(tag_type_val.toIntern())) {
            .enum_type => {},
            else => return sema.fail(block, src, "Type.Union.tag_type must be an enum type", .{}),
        }
        const enum_tag_ty = tag_type_val.toType();

        // We simply track which fields of the tag type have been seen.
        const tag_ty_fields_len = enum_tag_ty.enumFieldCount(zcu);
        var seen_tags = try std.DynamicBitSetUnmanaged.initEmpty(sema.arena, tag_ty_fields_len);

        for (field_types, 0..) |*field_ty, field_idx| {
            const field_info = try fields_val.elemValue(pt, field_idx);

            const field_name_val = try field_info.fieldValue(pt, 0);
            const field_type_val = try field_info.fieldValue(pt, 1);

            // Don't pass a reason; first loop acts as an assertion that this is valid.
            const field_name = try sema.sliceToIpString(block, src, field_name_val, undefined);

            const enum_index = enum_tag_ty.enumFieldIndex(field_name, zcu) orelse {
                // TODO: better source location
                return sema.fail(block, src, "no field named '{}' in enum '{}'", .{
                    field_name.fmt(ip), enum_tag_ty.fmt(pt),
                });
            };
            if (seen_tags.isSet(enum_index)) {
                // TODO: better source location
                return sema.fail(block, src, "duplicate union field {}", .{field_name.fmt(ip)});
            }
            seen_tags.set(enum_index);

            field_ty.* = field_type_val.toIntern();
            if (any_aligns) {
                const byte_align = try (try field_info.fieldValue(pt, 2)).toUnsignedIntSema(pt);
                if (byte_align > 0 and !math.isPowerOfTwo(byte_align)) {
                    // TODO: better source location
                    return sema.fail(block, src, "alignment value '{d}' is not a power of two or zero", .{byte_align});
                }
                field_aligns[field_idx] = Alignment.fromByteUnits(byte_align);
            }
        }

        if (tag_ty_fields_len > fields_len) return sema.failWithOwnedErrorMsg(block, msg: {
            const msg = try sema.errMsg(src, "enum fields missing in union", .{});
            errdefer msg.destroy(gpa);
            var it = seen_tags.iterator(.{ .kind = .unset });
            while (it.next()) |enum_index| {
                const field_name = enum_tag_ty.enumFieldName(enum_index, zcu);
                try sema.addFieldErrNote(enum_tag_ty, enum_index, msg, "field '{}' missing, declared here", .{
                    field_name.fmt(ip),
                });
            }
            try sema.addDeclaredHereNote(msg, enum_tag_ty);
            break :msg msg;
        });

        break :tag_ty .{ enum_tag_ty.toIntern(), true };
    } else tag_ty: {
        // We must track field names and set up the tag type ourselves.
        var field_names: std.AutoArrayHashMapUnmanaged(InternPool.NullTerminatedString, void) = .empty;
        try field_names.ensureTotalCapacity(sema.arena, fields_len);

        for (field_types, 0..) |*field_ty, field_idx| {
            const field_info = try fields_val.elemValue(pt, field_idx);

            const field_name_val = try field_info.fieldValue(pt, 0);
            const field_type_val = try field_info.fieldValue(pt, 1);

            // Don't pass a reason; first loop acts as an assertion that this is valid.
            const field_name = try sema.sliceToIpString(block, src, field_name_val, undefined);
            const gop = field_names.getOrPutAssumeCapacity(field_name);
            if (gop.found_existing) {
                // TODO: better source location
                return sema.fail(block, src, "duplicate union field {}", .{field_name.fmt(ip)});
            }

            field_ty.* = field_type_val.toIntern();
            if (any_aligns) {
                const byte_align = try (try field_info.fieldValue(pt, 2)).toUnsignedIntSema(pt);
                if (byte_align > 0 and !math.isPowerOfTwo(byte_align)) {
                    // TODO: better source location
                    return sema.fail(block, src, "alignment value '{d}' is not a power of two or zero", .{byte_align});
                }
                field_aligns[field_idx] = Alignment.fromByteUnits(byte_align);
            }
        }

        const enum_tag_ty = try sema.generateUnionTagTypeSimple(block, field_names.keys(), wip_ty.index, type_name);
        break :tag_ty .{ enum_tag_ty, false };
    };
    errdefer if (!has_explicit_tag) ip.remove(pt.tid, enum_tag_ty); // remove generated tag type on error

    for (field_types) |field_ty_ip| {
        const field_ty: Type = .fromInterned(field_ty_ip);
        if (field_ty.zigTypeTag(zcu) == .@"opaque") {
            return sema.failWithOwnedErrorMsg(block, msg: {
                const msg = try sema.errMsg(src, "opaque types have unknown size and therefore cannot be directly embedded in unions", .{});
                errdefer msg.destroy(gpa);

                try sema.addDeclaredHereNote(msg, field_ty);
                break :msg msg;
            });
        }
        if (layout == .@"extern" and !try sema.validateExternType(field_ty, .union_field)) {
            return sema.failWithOwnedErrorMsg(block, msg: {
                const msg = try sema.errMsg(src, "extern unions cannot contain fields of type '{}'", .{field_ty.fmt(pt)});
                errdefer msg.destroy(gpa);

                try sema.explainWhyTypeIsNotExtern(msg, src, field_ty, .union_field);

                try sema.addDeclaredHereNote(msg, field_ty);
                break :msg msg;
            });
        } else if (layout == .@"packed" and !try sema.validatePackedType(field_ty)) {
            return sema.failWithOwnedErrorMsg(block, msg: {
                const msg = try sema.errMsg(src, "packed unions cannot contain fields of type '{}'", .{field_ty.fmt(pt)});
                errdefer msg.destroy(gpa);

                try sema.explainWhyTypeIsNotPacked(msg, src, field_ty);

                try sema.addDeclaredHereNote(msg, field_ty);
                break :msg msg;
            });
        }
    }

    const loaded_union = ip.loadUnionType(wip_ty.index);
    loaded_union.setFieldTypes(ip, field_types);
    if (any_aligns) {
        loaded_union.setFieldAligns(ip, field_aligns);
    }
    loaded_union.setTagType(ip, enum_tag_ty);
    loaded_union.setStatus(ip, .have_field_types);

    const new_namespace_index = try pt.createNamespace(.{
        .parent = block.namespace.toOptional(),
        .owner_type = wip_ty.index,
        .file_scope = block.getFileScopeIndex(zcu),
        .generation = zcu.generation,
    });

    try zcu.comp.queueJob(.{ .resolve_type_fully = wip_ty.index });
    codegen_type: {
        if (zcu.comp.config.use_llvm) break :codegen_type;
        if (block.ownerModule().strip) break :codegen_type;
        // This job depends on any resolve_type_fully jobs queued up before it.
        try zcu.comp.queueJob(.{ .codegen_type = wip_ty.index });
    }
    try sema.declareDependency(.{ .interned = wip_ty.index });
    try sema.addTypeReferenceEntry(src, wip_ty.index);
    if (zcu.comp.debugIncremental()) try zcu.incremental_debug_state.newType(zcu, wip_ty.index);
    return Air.internedToRef(wip_ty.finish(ip, new_namespace_index));
}

fn reifyTuple(
    sema: *Sema,
    block: *Block,
    src: LazySrcLoc,
    fields_val: Value,
) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const gpa = sema.gpa;
    const ip = &zcu.intern_pool;

    const fields_len: u32 = @intCast(fields_val.typeOf(zcu).arrayLen(zcu));

    const types = try sema.arena.alloc(InternPool.Index, fields_len);
    const inits = try sema.arena.alloc(InternPool.Index, fields_len);

    for (types, inits, 0..) |*field_ty, *field_init, field_idx| {
        const field_info = try fields_val.elemValue(pt, field_idx);

        const field_name_val = try field_info.fieldValue(pt, 0);
        const field_type_val = try field_info.fieldValue(pt, 1);
        const field_default_value_val = try field_info.fieldValue(pt, 2);
        const field_is_comptime_val = try field_info.fieldValue(pt, 3);
        const field_alignment_val = try sema.resolveLazyValue(try field_info.fieldValue(pt, 4));

        const field_name = try sema.sliceToIpString(block, src, field_name_val, .{ .simple = .tuple_field_name });
        const field_type = field_type_val.toType();
        const field_default_value: InternPool.Index = if (field_default_value_val.optionalValue(zcu)) |ptr_val| d: {
            const ptr_ty = try pt.singleConstPtrType(field_type_val.toType());
            // We need to do this deref here, so we won't check for this error case later on.
            const val = try sema.pointerDeref(block, src, ptr_val, ptr_ty) orelse return sema.failWithNeededComptime(
                block,
                src,
                .{ .simple = .tuple_field_default_value },
            );
            // Resolve the value so that lazy values do not create distinct types.
            break :d (try sema.resolveLazyValue(val)).toIntern();
        } else .none;

        const field_name_index = field_name.toUnsigned(ip) orelse return sema.fail(
            block,
            src,
            "tuple cannot have non-numeric field '{}'",
            .{field_name.fmt(ip)},
        );
        if (field_name_index != field_idx) {
            return sema.fail(
                block,
                src,
                "tuple field name '{}' does not match field index {}",
                .{ field_name_index, field_idx },
            );
        }

        try sema.validateTupleFieldType(block, field_type, src);

        {
            const alignment_ok = ok: {
                if (field_alignment_val.toIntern() == .zero) break :ok true;
                const given_align = try field_alignment_val.getUnsignedIntSema(pt) orelse break :ok false;
                const abi_align = (try field_type.abiAlignmentSema(pt)).toByteUnits() orelse 0;
                break :ok abi_align == given_align;
            };
            if (!alignment_ok) {
                return sema.fail(block, src, "tuple fields cannot specify alignment", .{});
            }
        }

        if (field_is_comptime_val.toBool() and field_default_value == .none) {
            return sema.fail(block, src, "comptime field without default initialization value", .{});
        }

        if (!field_is_comptime_val.toBool() and field_default_value != .none) {
            return sema.fail(block, src, "non-comptime tuple fields cannot specify default initialization value", .{});
        }

        const default_or_opv: InternPool.Index = default: {
            if (field_default_value != .none) {
                break :default field_default_value;
            }
            if (try sema.typeHasOnePossibleValue(field_type)) |opv| {
                break :default opv.toIntern();
            }
            break :default .none;
        };

        field_ty.* = field_type.toIntern();
        field_init.* = default_or_opv;
    }

    return Air.internedToRef(try zcu.intern_pool.getTupleType(gpa, pt.tid, .{
        .types = types,
        .values = inits,
    }));
}

fn reifyStruct(
    sema: *Sema,
    block: *Block,
    inst: Zir.Inst.Index,
    src: LazySrcLoc,
    layout: std.builtin.Type.ContainerLayout,
    opt_backing_int_val: Value,
    fields_val: Value,
    name_strategy: Zir.Inst.NameStrategy,
) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const gpa = sema.gpa;
    const ip = &zcu.intern_pool;

    // This logic must stay in sync with the structure of `std.builtin.Type.Struct` - search for `fieldValue`.

    const fields_len: u32 = @intCast(fields_val.typeOf(zcu).arrayLen(zcu));

    // The validation work here is non-trivial, and it's possible the type already exists.
    // So in this first pass, let's just construct a hash to optimize for this case. If the
    // inputs turn out to be invalid, we can cancel the WIP type later.

    // For deduplication purposes, we must create a hash including all details of this type.
    // TODO: use a longer hash!
    var hasher = std.hash.Wyhash.init(0);
    std.hash.autoHash(&hasher, layout);
    std.hash.autoHash(&hasher, opt_backing_int_val.toIntern());
    std.hash.autoHash(&hasher, fields_len);

    var any_comptime_fields = false;
    var any_default_inits = false;
    var any_aligned_fields = false;

    for (0..fields_len) |field_idx| {
        const field_info = try fields_val.elemValue(pt, field_idx);

        const field_name_val = try field_info.fieldValue(pt, 0);
        const field_type_val = try field_info.fieldValue(pt, 1);
        const field_default_value_val = try field_info.fieldValue(pt, 2);
        const field_is_comptime_val = try field_info.fieldValue(pt, 3);
        const field_alignment_val = try sema.resolveLazyValue(try field_info.fieldValue(pt, 4));

        const field_name = try sema.sliceToIpString(block, src, field_name_val, .{ .simple = .struct_field_name });
        const field_is_comptime = field_is_comptime_val.toBool();
        const field_default_value: InternPool.Index = if (field_default_value_val.optionalValue(zcu)) |ptr_val| d: {
            const ptr_ty = try pt.singleConstPtrType(field_type_val.toType());
            // We need to do this deref here, so we won't check for this error case later on.
            const val = try sema.pointerDeref(block, src, ptr_val, ptr_ty) orelse return sema.failWithNeededComptime(
                block,
                src,
                .{ .simple = .struct_field_default_value },
            );
            // Resolve the value so that lazy values do not create distinct types.
            break :d (try sema.resolveLazyValue(val)).toIntern();
        } else .none;

        std.hash.autoHash(&hasher, .{
            field_name,
            field_type_val.toIntern(),
            field_default_value,
            field_is_comptime,
            field_alignment_val.toIntern(),
        });

        if (field_is_comptime) any_comptime_fields = true;
        if (field_default_value != .none) any_default_inits = true;
        switch (try field_alignment_val.orderAgainstZeroSema(pt)) {
            .eq => {},
            .gt => any_aligned_fields = true,
            .lt => unreachable,
        }
    }

    const tracked_inst = try block.trackZir(inst);

    const wip_ty = switch (try ip.getStructType(gpa, pt.tid, .{
        .layout = layout,
        .fields_len = fields_len,
        .known_non_opv = false,
        .requires_comptime = .unknown,
        .any_comptime_fields = any_comptime_fields,
        .any_default_inits = any_default_inits,
        .any_aligned_fields = any_aligned_fields,
        .inits_resolved = true,
        .key = .{ .reified = .{
            .zir_index = tracked_inst,
            .type_hash = hasher.final(),
        } },
    }, false)) {
        .wip => |wip| wip,
        .existing => |ty| {
            try sema.declareDependency(.{ .interned = ty });
            try sema.addTypeReferenceEntry(src, ty);
            return Air.internedToRef(ty);
        },
    };
    errdefer wip_ty.cancel(ip, pt.tid);

    wip_ty.setName(ip, try sema.createTypeName(
        block,
        name_strategy,
        "struct",
        inst,
        wip_ty.index,
    ));

    const struct_type = ip.loadStructType(wip_ty.index);

    for (0..fields_len) |field_idx| {
        const field_info = try fields_val.elemValue(pt, field_idx);

        const field_name_val = try field_info.fieldValue(pt, 0);
        const field_type_val = try field_info.fieldValue(pt, 1);
        const field_default_value_val = try field_info.fieldValue(pt, 2);
        const field_is_comptime_val = try field_info.fieldValue(pt, 3);
        const field_alignment_val = try field_info.fieldValue(pt, 4);

        const field_ty = field_type_val.toType();
        // Don't pass a reason; first loop acts as an assertion that this is valid.
        const field_name = try sema.sliceToIpString(block, src, field_name_val, undefined);
        if (struct_type.addFieldName(ip, field_name)) |prev_index| {
            _ = prev_index; // TODO: better source location
            return sema.fail(block, src, "duplicate struct field name {}", .{field_name.fmt(ip)});
        }

        if (any_aligned_fields) {
            if (!try sema.intFitsInType(field_alignment_val, .u32, null)) {
                return sema.fail(block, src, "alignment must fit in 'u32'", .{});
            }

            const byte_align = try field_alignment_val.toUnsignedIntSema(pt);
            if (byte_align == 0) {
                if (layout != .@"packed") {
                    struct_type.field_aligns.get(ip)[field_idx] = .none;
                }
            } else {
                if (layout == .@"packed") return sema.fail(block, src, "alignment in a packed struct field must be set to 0", .{});
                if (!math.isPowerOfTwo(byte_align)) return sema.fail(block, src, "alignment value '{d}' is not a power of two or zero", .{byte_align});
                struct_type.field_aligns.get(ip)[field_idx] = Alignment.fromNonzeroByteUnits(byte_align);
            }
        }

        const field_is_comptime = field_is_comptime_val.toBool();
        if (field_is_comptime) {
            assert(any_comptime_fields);
            switch (layout) {
                .@"extern" => return sema.fail(block, src, "extern struct fields cannot be marked comptime", .{}),
                .@"packed" => return sema.fail(block, src, "packed struct fields cannot be marked comptime", .{}),
                .auto => struct_type.setFieldComptime(ip, field_idx),
            }
        }

        const field_default: InternPool.Index = d: {
            if (!any_default_inits) break :d .none;
            const ptr_val = field_default_value_val.optionalValue(zcu) orelse break :d .none;
            const ptr_ty = try pt.singleConstPtrType(field_ty);
            // Asserted comptime-dereferencable above.
            const val = (try sema.pointerDeref(block, src, ptr_val, ptr_ty)).?;
            // We already resolved this for deduplication, so we may as well do it now.
            break :d (try sema.resolveLazyValue(val)).toIntern();
        };

        if (field_is_comptime and field_default == .none) {
            return sema.fail(block, src, "comptime field without default initialization value", .{});
        }

        struct_type.field_types.get(ip)[field_idx] = field_type_val.toIntern();
        if (field_default != .none) {
            struct_type.field_inits.get(ip)[field_idx] = field_default;
        }

        if (field_ty.zigTypeTag(zcu) == .@"opaque") {
            return sema.failWithOwnedErrorMsg(block, msg: {
                const msg = try sema.errMsg(src, "opaque types have unknown size and therefore cannot be directly embedded in structs", .{});
                errdefer msg.destroy(gpa);

                try sema.addDeclaredHereNote(msg, field_ty);
                break :msg msg;
            });
        }
        if (field_ty.zigTypeTag(zcu) == .noreturn) {
            return sema.failWithOwnedErrorMsg(block, msg: {
                const msg = try sema.errMsg(src, "struct fields cannot be 'noreturn'", .{});
                errdefer msg.destroy(gpa);

                try sema.addDeclaredHereNote(msg, field_ty);
                break :msg msg;
            });
        }
        if (layout == .@"extern" and !try sema.validateExternType(field_ty, .struct_field)) {
            return sema.failWithOwnedErrorMsg(block, msg: {
                const msg = try sema.errMsg(src, "extern structs cannot contain fields of type '{}'", .{field_ty.fmt(pt)});
                errdefer msg.destroy(gpa);

                try sema.explainWhyTypeIsNotExtern(msg, src, field_ty, .struct_field);

                try sema.addDeclaredHereNote(msg, field_ty);
                break :msg msg;
            });
        } else if (layout == .@"packed" and !try sema.validatePackedType(field_ty)) {
            return sema.failWithOwnedErrorMsg(block, msg: {
                const msg = try sema.errMsg(src, "packed structs cannot contain fields of type '{}'", .{field_ty.fmt(pt)});
                errdefer msg.destroy(gpa);

                try sema.explainWhyTypeIsNotPacked(msg, src, field_ty);

                try sema.addDeclaredHereNote(msg, field_ty);
                break :msg msg;
            });
        }
    }

    if (layout == .@"packed") {
        var fields_bit_sum: u64 = 0;
        for (0..struct_type.field_types.len) |field_idx| {
            const field_ty: Type = .fromInterned(struct_type.field_types.get(ip)[field_idx]);
            field_ty.resolveLayout(pt) catch |err| switch (err) {
                error.AnalysisFail => {
                    const msg = sema.err orelse return err;
                    try sema.errNote(src, msg, "while checking a field of this struct", .{});
                    return err;
                },
                else => return err,
            };
            fields_bit_sum += field_ty.bitSize(zcu);
        }

        if (opt_backing_int_val.optionalValue(zcu)) |backing_int_val| {
            const backing_int_ty = backing_int_val.toType();
            try sema.checkBackingIntType(block, src, backing_int_ty, fields_bit_sum);
            struct_type.setBackingIntType(ip, backing_int_ty.toIntern());
        } else {
            const backing_int_ty = try pt.intType(.unsigned, @intCast(fields_bit_sum));
            struct_type.setBackingIntType(ip, backing_int_ty.toIntern());
        }
    }

    const new_namespace_index = try pt.createNamespace(.{
        .parent = block.namespace.toOptional(),
        .owner_type = wip_ty.index,
        .file_scope = block.getFileScopeIndex(zcu),
        .generation = zcu.generation,
    });

    try zcu.comp.queueJob(.{ .resolve_type_fully = wip_ty.index });
    codegen_type: {
        if (zcu.comp.config.use_llvm) break :codegen_type;
        if (block.ownerModule().strip) break :codegen_type;
        // This job depends on any resolve_type_fully jobs queued up before it.
        try zcu.comp.queueJob(.{ .codegen_type = wip_ty.index });
    }
    try sema.declareDependency(.{ .interned = wip_ty.index });
    try sema.addTypeReferenceEntry(src, wip_ty.index);
    if (zcu.comp.debugIncremental()) try zcu.incremental_debug_state.newType(zcu, wip_ty.index);
    return Air.internedToRef(wip_ty.finish(ip, new_namespace_index));
}

fn resolveVaListRef(sema: *Sema, block: *Block, src: LazySrcLoc, zir_ref: Zir.Inst.Ref) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const va_list_ty = try sema.getBuiltinType(src, .VaList);
    const va_list_ptr = try pt.singleMutPtrType(va_list_ty);

    const inst = try sema.resolveInst(zir_ref);
    return sema.coerce(block, va_list_ptr, inst, src);
}

fn zirCVaArg(sema: *Sema, block: *Block, extended: Zir.Inst.Extended.InstData) CompileError!Air.Inst.Ref {
    const extra = sema.code.extraData(Zir.Inst.BinNode, extended.operand).data;
    const src = block.nodeOffset(extra.node);
    const va_list_src = block.builtinCallArgSrc(extra.node, 0);
    const ty_src = block.builtinCallArgSrc(extra.node, 1);

    const va_list_ref = try sema.resolveVaListRef(block, va_list_src, extra.lhs);
    const arg_ty = try sema.resolveType(block, ty_src, extra.rhs);

    if (!try sema.validateExternType(arg_ty, .param_ty)) {
        const msg = msg: {
            const msg = try sema.errMsg(ty_src, "cannot get '{}' from variadic argument", .{arg_ty.fmt(sema.pt)});
            errdefer msg.destroy(sema.gpa);

            try sema.explainWhyTypeIsNotExtern(msg, ty_src, arg_ty, .param_ty);

            try sema.addDeclaredHereNote(msg, arg_ty);
            break :msg msg;
        };
        return sema.failWithOwnedErrorMsg(block, msg);
    }

    try sema.requireRuntimeBlock(block, src, null);
    return block.addTyOp(.c_va_arg, arg_ty, va_list_ref);
}

fn zirCVaCopy(sema: *Sema, block: *Block, extended: Zir.Inst.Extended.InstData) CompileError!Air.Inst.Ref {
    const extra = sema.code.extraData(Zir.Inst.UnNode, extended.operand).data;
    const src = block.nodeOffset(extra.node);
    const va_list_src = block.builtinCallArgSrc(extra.node, 0);

    const va_list_ref = try sema.resolveVaListRef(block, va_list_src, extra.operand);
    const va_list_ty = try sema.getBuiltinType(src, .VaList);

    try sema.requireRuntimeBlock(block, src, null);
    return block.addTyOp(.c_va_copy, va_list_ty, va_list_ref);
}

fn zirCVaEnd(sema: *Sema, block: *Block, extended: Zir.Inst.Extended.InstData) CompileError!Air.Inst.Ref {
    const extra = sema.code.extraData(Zir.Inst.UnNode, extended.operand).data;
    const src = block.nodeOffset(extra.node);
    const va_list_src = block.builtinCallArgSrc(extra.node, 0);

    const va_list_ref = try sema.resolveVaListRef(block, va_list_src, extra.operand);

    try sema.requireRuntimeBlock(block, src, null);
    return block.addUnOp(.c_va_end, va_list_ref);
}

fn zirCVaStart(sema: *Sema, block: *Block, extended: Zir.Inst.Extended.InstData) CompileError!Air.Inst.Ref {
    const src_node: std.zig.Ast.Node.Offset = @enumFromInt(@as(i32, @bitCast(extended.operand)));
    const src = block.nodeOffset(src_node);

    const va_list_ty = try sema.getBuiltinType(src, .VaList);
    try sema.requireRuntimeBlock(block, src, null);
    return block.addInst(.{
        .tag = .c_va_start,
        .data = .{ .ty = va_list_ty },
    });
}

fn zirTypeName(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const ip = &zcu.intern_pool;

    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].un_node;
    const ty_src = block.builtinCallArgSrc(inst_data.src_node, 0);
    const ty = try sema.resolveType(block, ty_src, inst_data.operand);

    const type_name = try ip.getOrPutStringFmt(sema.gpa, pt.tid, "{}", .{ty.fmt(pt)}, .no_embedded_nulls);
    return sema.addNullTerminatedStrLit(type_name);
}

fn zirFrameType(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].un_node;
    const src = block.nodeOffset(inst_data.src_node);
    return sema.failWithUseOfAsync(block, src);
}

fn zirFrameSize(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].un_node;
    const src = block.nodeOffset(inst_data.src_node);
    return sema.failWithUseOfAsync(block, src);
}

fn zirIntFromFloat(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].pl_node;
    const src = block.nodeOffset(inst_data.src_node);
    const extra = sema.code.extraData(Zir.Inst.Bin, inst_data.payload_index).data;
    const operand_src = block.builtinCallArgSrc(inst_data.src_node, 0);
    const dest_ty = try sema.resolveDestType(block, src, extra.lhs, .remove_eu_opt, "@intFromFloat");
    const operand = try sema.resolveInst(extra.rhs);
    const operand_ty = sema.typeOf(operand);

    try sema.checkVectorizableBinaryOperands(block, operand_src, dest_ty, operand_ty, src, operand_src);
    const is_vector = dest_ty.zigTypeTag(zcu) == .vector;

    const dest_scalar_ty = dest_ty.scalarType(zcu);
    const operand_scalar_ty = operand_ty.scalarType(zcu);

    _ = try sema.checkIntType(block, src, dest_scalar_ty);
    try sema.checkFloatType(block, operand_src, operand_scalar_ty);

    if (try sema.resolveValue(operand)) |operand_val| {
        const result_val = try sema.intFromFloat(block, operand_src, operand_val, operand_ty, dest_ty, .truncate);
        return Air.internedToRef(result_val.toIntern());
    } else if (dest_scalar_ty.zigTypeTag(zcu) == .comptime_int) {
        return sema.failWithNeededComptime(block, operand_src, .{ .simple = .casted_to_comptime_int });
    }

    try sema.requireRuntimeBlock(block, src, operand_src);
    if (dest_scalar_ty.intInfo(zcu).bits == 0) {
        if (!is_vector) {
            if (block.wantSafety()) {
                const ok = try block.addBinOp(if (block.float_mode == .optimized) .cmp_eq_optimized else .cmp_eq, operand, Air.internedToRef((try pt.floatValue(operand_ty, 0.0)).toIntern()));
                try sema.addSafetyCheck(block, src, ok, .integer_part_out_of_bounds);
            }
            return Air.internedToRef((try pt.intValue(dest_ty, 0)).toIntern());
        }
        if (block.wantSafety()) {
            const len = dest_ty.vectorLen(zcu);
            for (0..len) |i| {
                const idx_ref = try pt.intRef(.usize, i);
                const elem_ref = try block.addBinOp(.array_elem_val, operand, idx_ref);
                const ok = try block.addBinOp(if (block.float_mode == .optimized) .cmp_eq_optimized else .cmp_eq, elem_ref, Air.internedToRef((try pt.floatValue(operand_scalar_ty, 0.0)).toIntern()));
                try sema.addSafetyCheck(block, src, ok, .integer_part_out_of_bounds);
            }
        }
        return Air.internedToRef(try pt.intern(.{ .aggregate = .{
            .ty = dest_ty.toIntern(),
            .storage = .{ .repeated_elem = (try pt.intValue(dest_scalar_ty, 0)).toIntern() },
        } }));
    }
    const result = try block.addTyOp(if (block.float_mode == .optimized) .int_from_float_optimized else .int_from_float, dest_ty, operand);
    if (block.wantSafety()) {
        const back = try block.addTyOp(.float_from_int, operand_ty, result);
        const diff = try block.addBinOp(if (block.float_mode == .optimized) .sub_optimized else .sub, operand, back);
        const ok = if (is_vector) ok: {
            const ok_pos = try block.addCmpVector(diff, Air.internedToRef((try sema.splat(operand_ty, try pt.floatValue(operand_scalar_ty, 1.0))).toIntern()), .lt);
            const ok_neg = try block.addCmpVector(diff, Air.internedToRef((try sema.splat(operand_ty, try pt.floatValue(operand_scalar_ty, -1.0))).toIntern()), .gt);
            const ok = try block.addBinOp(.bit_and, ok_pos, ok_neg);
            break :ok try block.addReduce(ok, .And);
        } else ok: {
            const ok_pos = try block.addBinOp(if (block.float_mode == .optimized) .cmp_lt_optimized else .cmp_lt, diff, Air.internedToRef((try pt.floatValue(operand_ty, 1.0)).toIntern()));
            const ok_neg = try block.addBinOp(if (block.float_mode == .optimized) .cmp_gt_optimized else .cmp_gt, diff, Air.internedToRef((try pt.floatValue(operand_ty, -1.0)).toIntern()));
            break :ok try block.addBinOp(.bool_and, ok_pos, ok_neg);
        };
        try sema.addSafetyCheck(block, src, ok, .integer_part_out_of_bounds);
    }
    return result;
}

fn zirFloatFromInt(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].pl_node;
    const src = block.nodeOffset(inst_data.src_node);
    const extra = sema.code.extraData(Zir.Inst.Bin, inst_data.payload_index).data;
    const operand_src = block.builtinCallArgSrc(inst_data.src_node, 0);
    const dest_ty = try sema.resolveDestType(block, src, extra.lhs, .remove_eu_opt, "@floatFromInt");
    const operand = try sema.resolveInst(extra.rhs);
    const operand_ty = sema.typeOf(operand);

    try sema.checkVectorizableBinaryOperands(block, operand_src, dest_ty, operand_ty, src, operand_src);

    const dest_scalar_ty = dest_ty.scalarType(zcu);
    const operand_scalar_ty = operand_ty.scalarType(zcu);

    try sema.checkFloatType(block, src, dest_scalar_ty);
    _ = try sema.checkIntType(block, operand_src, operand_scalar_ty);

    if (try sema.resolveValue(operand)) |operand_val| {
        const result_val = try operand_val.floatFromIntAdvanced(sema.arena, operand_ty, dest_ty, pt, .sema);
        return Air.internedToRef(result_val.toIntern());
    } else if (dest_scalar_ty.zigTypeTag(zcu) == .comptime_float) {
        return sema.failWithNeededComptime(block, operand_src, .{ .simple = .casted_to_comptime_float });
    }

    try sema.requireRuntimeBlock(block, src, operand_src);
    return block.addTyOp(.float_from_int, dest_ty, operand);
}

fn zirPtrFromInt(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].pl_node;
    const src = block.nodeOffset(inst_data.src_node);

    const extra = sema.code.extraData(Zir.Inst.Bin, inst_data.payload_index).data;

    const operand_src = block.builtinCallArgSrc(inst_data.src_node, 0);
    const operand_res = try sema.resolveInst(extra.rhs);

    const uncoerced_operand_ty = sema.typeOf(operand_res);
    const dest_ty = try sema.resolveDestType(block, src, extra.lhs, .remove_eu, "@ptrFromInt");
    try sema.checkVectorizableBinaryOperands(block, operand_src, dest_ty, uncoerced_operand_ty, src, operand_src);

    const is_vector = dest_ty.zigTypeTag(zcu) == .vector;
    const operand_ty: Type = if (is_vector) operand_ty: {
        const len = dest_ty.vectorLen(zcu);
        break :operand_ty try pt.vectorType(.{ .child = .usize_type, .len = len });
    } else .usize;

    const operand_coerced = try sema.coerce(block, operand_ty, operand_res, operand_src);

    const ptr_ty = dest_ty.scalarType(zcu);
    try sema.checkPtrType(block, src, ptr_ty, true);

    const elem_ty = ptr_ty.elemType2(zcu);
    const ptr_align = try ptr_ty.ptrAlignmentSema(pt);

    if (ptr_ty.isSlice(zcu)) {
        const msg = msg: {
            const msg = try sema.errMsg(src, "integer cannot be converted to slice type '{}'", .{ptr_ty.fmt(pt)});
            errdefer msg.destroy(sema.gpa);
            try sema.errNote(src, msg, "slice length cannot be inferred from address", .{});
            break :msg msg;
        };
        return sema.failWithOwnedErrorMsg(block, msg);
    }

    if (try sema.resolveDefinedValue(block, operand_src, operand_coerced)) |val| {
        if (!is_vector) {
            const ptr_val = try sema.ptrFromIntVal(block, operand_src, val, ptr_ty, ptr_align);
            return Air.internedToRef(ptr_val.toIntern());
        }
        const len = dest_ty.vectorLen(zcu);
        const new_elems = try sema.arena.alloc(InternPool.Index, len);
        for (new_elems, 0..) |*new_elem, i| {
            const elem = try val.elemValue(pt, i);
            const ptr_val = try sema.ptrFromIntVal(block, operand_src, elem, ptr_ty, ptr_align);
            new_elem.* = ptr_val.toIntern();
        }
        return Air.internedToRef(try pt.intern(.{ .aggregate = .{
            .ty = dest_ty.toIntern(),
            .storage = .{ .elems = new_elems },
        } }));
    }
    if (try ptr_ty.comptimeOnlySema(pt)) {
        return sema.failWithOwnedErrorMsg(block, msg: {
            const msg = try sema.errMsg(src, "pointer to comptime-only type '{}' must be comptime-known, but operand is runtime-known", .{ptr_ty.fmt(pt)});
            errdefer msg.destroy(sema.gpa);

            try sema.explainWhyTypeIsComptime(msg, src, ptr_ty);
            break :msg msg;
        });
    }
    try sema.requireRuntimeBlock(block, src, operand_src);
    try sema.checkLogicalPtrOperation(block, src, ptr_ty);
    if (block.wantSafety() and (try elem_ty.hasRuntimeBitsSema(pt) or elem_ty.zigTypeTag(zcu) == .@"fn")) {
        if (!ptr_ty.isAllowzeroPtr(zcu)) {
            const is_non_zero = if (is_vector) all_non_zero: {
                const zero_usize = Air.internedToRef((try sema.splat(operand_ty, .zero_usize)).toIntern());
                const is_non_zero = try block.addCmpVector(operand_coerced, zero_usize, .neq);
                break :all_non_zero try block.addReduce(is_non_zero, .And);
            } else try block.addBinOp(.cmp_neq, operand_coerced, .zero_usize);
            try sema.addSafetyCheck(block, src, is_non_zero, .cast_to_null);
        }
        if (ptr_align.compare(.gt, .@"1")) {
            const align_bytes_minus_1 = ptr_align.toByteUnits().? - 1;
            const align_mask = Air.internedToRef((try sema.splat(operand_ty, try pt.intValue(
                .usize,
                if (elem_ty.fnPtrMaskOrNull(zcu)) |mask|
                    align_bytes_minus_1 & mask
                else
                    align_bytes_minus_1,
            ))).toIntern());
            const remainder = try block.addBinOp(.bit_and, operand_coerced, align_mask);
            const is_aligned = if (is_vector) all_aligned: {
                const splat_zero_usize = Air.internedToRef((try sema.splat(operand_ty, .zero_usize)).toIntern());
                const is_aligned = try block.addCmpVector(remainder, splat_zero_usize, .eq);
                break :all_aligned try block.addReduce(is_aligned, .And);
            } else try block.addBinOp(.cmp_eq, remainder, .zero_usize);
            try sema.addSafetyCheck(block, src, is_aligned, .incorrect_alignment);
        }
    }
    return block.addBitCast(dest_ty, operand_coerced);
}

fn ptrFromIntVal(
    sema: *Sema,
    block: *Block,
    operand_src: LazySrcLoc,
    operand_val: Value,
    ptr_ty: Type,
    ptr_align: Alignment,
) !Value {
    const pt = sema.pt;
    const zcu = pt.zcu;
    if (operand_val.isUndef(zcu)) {
        if (ptr_ty.isAllowzeroPtr(zcu) and ptr_align == .@"1") {
            return pt.undefValue(ptr_ty);
        }
        return sema.failWithUseOfUndef(block, operand_src);
    }
    const addr = try operand_val.toUnsignedIntSema(pt);
    if (!ptr_ty.isAllowzeroPtr(zcu) and addr == 0)
        return sema.fail(block, operand_src, "pointer type '{}' does not allow address zero", .{ptr_ty.fmt(pt)});
    if (addr != 0 and ptr_align != .none) {
        const masked_addr = if (ptr_ty.childType(zcu).fnPtrMaskOrNull(zcu)) |mask|
            addr & mask
        else
            addr;

        if (!ptr_align.check(masked_addr)) {
            return sema.fail(block, operand_src, "pointer type '{}' requires aligned address", .{ptr_ty.fmt(pt)});
        }
    }

    return switch (ptr_ty.zigTypeTag(zcu)) {
        .optional => Value.fromInterned(try pt.intern(.{ .opt = .{
            .ty = ptr_ty.toIntern(),
            .val = if (addr == 0) .none else (try pt.ptrIntValue(ptr_ty.childType(zcu), addr)).toIntern(),
        } })),
        .pointer => try pt.ptrIntValue(ptr_ty, addr),
        else => unreachable,
    };
}

fn zirErrorCast(sema: *Sema, block: *Block, extended: Zir.Inst.Extended.InstData) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const ip = &zcu.intern_pool;
    const extra = sema.code.extraData(Zir.Inst.BinNode, extended.operand).data;
    const src = block.nodeOffset(extra.node);
    const operand_src = block.builtinCallArgSrc(extra.node, 0);
    const dest_ty = try sema.resolveDestType(block, src, extra.lhs, .remove_opt, "@errorCast");
    const operand = try sema.resolveInst(extra.rhs);
    const operand_ty = sema.typeOf(operand);

    const dest_tag = dest_ty.zigTypeTag(zcu);
    const operand_tag = operand_ty.zigTypeTag(zcu);

    if (dest_tag != .error_set and dest_tag != .error_union) {
        return sema.fail(block, src, "expected error set or error union type, found '{s}'", .{@tagName(dest_tag)});
    }
    if (operand_tag != .error_set and operand_tag != .error_union) {
        return sema.fail(block, src, "expected error set or error union type, found '{s}'", .{@tagName(operand_tag)});
    }
    if (dest_tag == .error_set and operand_tag == .error_union) {
        return sema.fail(block, src, "cannot cast an error union type to error set", .{});
    }
    if (dest_tag == .error_union and operand_tag == .error_union and
        dest_ty.errorUnionPayload(zcu).toIntern() != operand_ty.errorUnionPayload(zcu).toIntern())
    {
        return sema.failWithOwnedErrorMsg(block, msg: {
            const msg = try sema.errMsg(src, "payload types of error unions must match", .{});
            errdefer msg.destroy(sema.gpa);
            const dest_payload_ty = dest_ty.errorUnionPayload(zcu);
            const operand_payload_ty = operand_ty.errorUnionPayload(zcu);
            try sema.errNote(src, msg, "destination payload is '{}'", .{dest_payload_ty.fmt(pt)});
            try sema.errNote(src, msg, "operand payload is '{}'", .{operand_payload_ty.fmt(pt)});
            try addDeclaredHereNote(sema, msg, dest_ty);
            try addDeclaredHereNote(sema, msg, operand_ty);
            break :msg msg;
        });
    }
    const dest_err_ty = switch (dest_tag) {
        .error_union => dest_ty.errorUnionSet(zcu),
        .error_set => dest_ty,
        else => unreachable,
    };
    const operand_err_ty = switch (operand_tag) {
        .error_union => operand_ty.errorUnionSet(zcu),
        .error_set => operand_ty,
        else => unreachable,
    };

    const disjoint = disjoint: {
        // Try avoiding resolving inferred error sets if we can
        if (!dest_err_ty.isAnyError(zcu) and dest_err_ty.errorSetIsEmpty(zcu)) break :disjoint true;
        if (!operand_err_ty.isAnyError(zcu) and operand_err_ty.errorSetIsEmpty(zcu)) break :disjoint true;
        if (dest_err_ty.isAnyError(zcu)) break :disjoint false;
        if (operand_err_ty.isAnyError(zcu)) break :disjoint false;
        const dest_err_names = dest_err_ty.errorSetNames(zcu);
        for (0..dest_err_names.len) |dest_err_index| {
            if (Type.errorSetHasFieldIp(ip, operand_err_ty.toIntern(), dest_err_names.get(ip)[dest_err_index]))
                break :disjoint false;
        }

        if (!ip.isInferredErrorSetType(dest_err_ty.toIntern()) and
            !ip.isInferredErrorSetType(operand_err_ty.toIntern()))
        {
            break :disjoint true;
        }

        _ = try sema.resolveInferredErrorSetTy(block, src, dest_err_ty.toIntern());
        _ = try sema.resolveInferredErrorSetTy(block, operand_src, operand_err_ty.toIntern());
        for (0..dest_err_names.len) |dest_err_index| {
            if (Type.errorSetHasFieldIp(ip, operand_err_ty.toIntern(), dest_err_names.get(ip)[dest_err_index]))
                break :disjoint false;
        }

        break :disjoint true;
    };
    if (disjoint and !(operand_tag == .error_union and dest_tag == .error_union)) {
        return sema.fail(block, src, "error sets '{}' and '{}' have no common errors", .{
            operand_err_ty.fmt(pt), dest_err_ty.fmt(pt),
        });
    }

    // operand must be defined since it can be an invalid error value
    if (try sema.resolveDefinedValue(block, operand_src, operand)) |operand_val| {
        const err_name: InternPool.NullTerminatedString = switch (operand_tag) {
            .error_set => ip.indexToKey(operand_val.toIntern()).err.name,
            .error_union => switch (ip.indexToKey(operand_val.toIntern()).error_union.val) {
                .err_name => |name| name,
                .payload => |payload_val| {
                    assert(dest_tag == .error_union); // should be guaranteed from the type checks above
                    return sema.coerce(block, dest_ty, Air.internedToRef(payload_val), operand_src);
                },
            },
            else => unreachable,
        };

        if (!dest_err_ty.isAnyError(zcu) and !Type.errorSetHasFieldIp(ip, dest_err_ty.toIntern(), err_name)) {
            return sema.fail(block, src, "'error.{}' not a member of error set '{}'", .{
                err_name.fmt(ip), dest_err_ty.fmt(pt),
            });
        }

        return Air.internedToRef(try pt.intern(switch (dest_tag) {
            .error_set => .{ .err = .{
                .ty = dest_ty.toIntern(),
                .name = err_name,
            } },
            .error_union => .{ .error_union = .{
                .ty = dest_ty.toIntern(),
                .val = .{ .err_name = err_name },
            } },
            else => unreachable,
        }));
    }

    const err_int_ty = try pt.errorIntType();
    if (block.wantSafety() and !dest_err_ty.isAnyError(zcu) and
        dest_err_ty.toIntern() != .adhoc_inferred_error_set_type and
        zcu.backendSupportsFeature(.error_set_has_value))
    {
        const err_code_inst = switch (operand_tag) {
            .error_set => operand,
            .error_union => try block.addTyOp(.unwrap_errunion_err, operand_err_ty, operand),
            else => unreachable,
        };
        const err_int_inst = try block.addBitCast(err_int_ty, err_code_inst);

        if (dest_tag == .error_union) {
            const zero_err = try pt.intRef(err_int_ty, 0);
            const is_zero = try block.addBinOp(.cmp_eq, err_int_inst, zero_err);
            if (disjoint) {
                // Error must be zero.
                try sema.addSafetyCheck(block, src, is_zero, .invalid_error_code);
            } else {
                // Error must be in destination set or zero.
                const has_value = try block.addTyOp(.error_set_has_value, dest_err_ty, err_int_inst);
                const ok = try block.addBinOp(.bool_or, has_value, is_zero);
                try sema.addSafetyCheck(block, src, ok, .invalid_error_code);
            }
        } else {
            const ok = try block.addTyOp(.error_set_has_value, dest_err_ty, err_int_inst);
            try sema.addSafetyCheck(block, src, ok, .invalid_error_code);
        }
    }

    if (operand_tag == .error_set and dest_tag == .error_union) {
        const err_val = try block.addBitCast(dest_err_ty, operand);
        return block.addTyOp(.wrap_errunion_err, dest_ty, err_val);
    } else {
        return block.addBitCast(dest_ty, operand);
    }
}

fn zirPtrCastFull(sema: *Sema, block: *Block, extended: Zir.Inst.Extended.InstData) CompileError!Air.Inst.Ref {
    const FlagsInt = @typeInfo(Zir.Inst.FullPtrCastFlags).@"struct".backing_integer.?;
    const flags: Zir.Inst.FullPtrCastFlags = @bitCast(@as(FlagsInt, @truncate(extended.small)));
    const extra = sema.code.extraData(Zir.Inst.BinNode, extended.operand).data;
    const src = block.nodeOffset(extra.node);
    const operand_src = block.src(.{ .node_offset_ptrcast_operand = extra.node });
    const operand = try sema.resolveInst(extra.rhs);
    const dest_ty = try sema.resolveDestType(block, src, extra.lhs, .remove_eu, flags.needResultTypeBuiltinName());
    return sema.ptrCastFull(
        block,
        flags,
        src,
        operand,
        operand_src,
        dest_ty,
        flags.needResultTypeBuiltinName(),
    );
}

fn zirPtrCast(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].pl_node;
    const src = block.nodeOffset(inst_data.src_node);
    const operand_src = block.builtinCallArgSrc(inst_data.src_node, 0);
    const extra = sema.code.extraData(Zir.Inst.Bin, inst_data.payload_index).data;
    const dest_ty = try sema.resolveDestType(block, src, extra.lhs, .remove_eu, "@ptrCast");
    const operand = try sema.resolveInst(extra.rhs);

    return sema.ptrCastFull(
        block,
        .{ .ptr_cast = true },
        src,
        operand,
        operand_src,
        dest_ty,
        "@ptrCast",
    );
}

fn ptrCastFull(
    sema: *Sema,
    block: *Block,
    flags: Zir.Inst.FullPtrCastFlags,
    src: LazySrcLoc,
    operand: Air.Inst.Ref,
    operand_src: LazySrcLoc,
    dest_ty: Type,
    operation: []const u8,
) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const operand_ty = sema.typeOf(operand);

    try sema.checkPtrType(block, src, dest_ty, true);
    try sema.checkPtrOperand(block, operand_src, operand_ty);

    const src_info = operand_ty.ptrInfo(zcu);
    const dest_info = dest_ty.ptrInfo(zcu);

    try Type.fromInterned(src_info.child).resolveLayout(pt);
    try Type.fromInterned(dest_info.child).resolveLayout(pt);

    const DestSliceLen = union(enum) {
        undef,
        constant: u64,
        equal_runtime_src_slice,
        change_runtime_src_slice: struct {
            bytes_per_src: u64,
            bytes_per_dest: u64,
        },
    };
    // Populated iff the destination type is a slice.
    const dest_slice_len: ?DestSliceLen = len: {
        switch (dest_info.flags.size) {
            .slice => {},
            .many, .c, .one => break :len null,
        }
        // `null` means the operand is a runtime-known slice (so the length is runtime-known).
        const opt_src_len: ?u64 = switch (src_info.flags.size) {
            .one => 1,
            .slice => src_len: {
                const operand_val = try sema.resolveValue(operand) orelse break :src_len null;
                if (operand_val.isUndef(zcu)) break :len .undef;
                const slice_val = switch (operand_ty.zigTypeTag(zcu)) {
                    .optional => operand_val.optionalValue(zcu) orelse break :len .undef,
                    .pointer => operand_val,
                    else => unreachable,
                };
                const slice_len_resolved = try sema.resolveLazyValue(.fromInterned(zcu.intern_pool.sliceLen(slice_val.toIntern())));
                if (slice_len_resolved.isUndef(zcu)) break :len .undef;
                break :src_len slice_len_resolved.toUnsignedInt(zcu);
            },
            .many, .c => {
                return sema.fail(block, src, "cannot infer length of slice from {s}", .{pointerSizeString(src_info.flags.size)});
            },
        };
        const dest_elem_ty: Type = .fromInterned(dest_info.child);
        const src_elem_ty: Type = .fromInterned(src_info.child);
        if (dest_elem_ty.toIntern() == src_elem_ty.toIntern()) {
            break :len if (opt_src_len) |l| .{ .constant = l } else .equal_runtime_src_slice;
        }
        if (!src_elem_ty.comptimeOnly(zcu) and !dest_elem_ty.comptimeOnly(zcu)) {
            const src_elem_size = src_elem_ty.abiSize(zcu);
            const dest_elem_size = dest_elem_ty.abiSize(zcu);
            if (dest_elem_size == 0) {
                return sema.fail(block, src, "cannot infer length of slice of zero-bit '{}' from '{}'", .{ dest_elem_ty.fmt(pt), operand_ty.fmt(pt) });
            }
            if (opt_src_len) |src_len| {
                const bytes = src_len * src_elem_size;
                const dest_len = std.math.divExact(u64, bytes, dest_elem_size) catch switch (src_info.flags.size) {
                    .slice => return sema.fail(block, src, "slice length '{d}' does not divide exactly into destination elements", .{src_len}),
                    .one => return sema.fail(block, src, "type '{}' does not divide exactly into destination elements", .{src_elem_ty.fmt(pt)}),
                    else => unreachable,
                };
                break :len .{ .constant = dest_len };
            }
            assert(src_info.flags.size == .slice);
            break :len .{ .change_runtime_src_slice = .{
                .bytes_per_src = src_elem_size,
                .bytes_per_dest = dest_elem_size,
            } };
        }
        // We apply rules for comptime memory consistent with comptime loads/stores, where arrays of
        // comptime-only types can be "restructured".
        const dest_base_ty: Type, const dest_base_per_elem: u64 = dest_elem_ty.arrayBase(zcu);
        const src_base_ty: Type, const src_base_per_elem: u64 = src_elem_ty.arrayBase(zcu);
        // The source value has `src_len * src_base_per_elem` values of type `src_base_ty`.
        // The result value will have `dest_len * dest_base_per_elem` values of type `dest_base_ty`.
        if (dest_base_ty.toIntern() != src_base_ty.toIntern()) {
            return sema.fail(block, src, "cannot infer length of comptime-only '{}' from incompatible '{}'", .{ dest_ty.fmt(pt), operand_ty.fmt(pt) });
        }
        // `src_base_ty` is comptime-only, so `src_elem_ty` is comptime-only, so `operand_ty` is
        // comptime-only, so `operand` is comptime-known, so `opt_src_len` is non-`null`.
        const src_len = opt_src_len.?;
        const base_len = src_len * src_base_per_elem;
        const dest_len = std.math.divExact(u64, base_len, dest_base_per_elem) catch switch (src_info.flags.size) {
            .slice => return sema.fail(block, src, "slice length '{d}' does not divide exactly into destination elements", .{src_len}),
            .one => return sema.fail(block, src, "type '{}' does not divide exactly into destination elements", .{src_elem_ty.fmt(pt)}),
            else => unreachable,
        };
        break :len .{ .constant = dest_len };
    };

    // The checking logic in this function must stay in sync with Sema.coerceInMemoryAllowedPtrs

    if (!flags.ptr_cast) {
        const is_array_ptr_to_slice = b: {
            if (dest_info.flags.size != .slice) break :b false;
            if (src_info.flags.size != .one) break :b false;
            const src_pointer_child: Type = .fromInterned(src_info.child);
            if (src_pointer_child.zigTypeTag(zcu) != .array) break :b false;
            const src_elem = src_pointer_child.childType(zcu);
            break :b src_elem.toIntern() == dest_info.child;
        };

        check_size: {
            if (src_info.flags.size == dest_info.flags.size) break :check_size;
            if (is_array_ptr_to_slice) break :check_size;
            if (src_info.flags.size == .c) break :check_size;
            if (dest_info.flags.size == .c) break :check_size;
            return sema.failWithOwnedErrorMsg(block, msg: {
                const msg = try sema.errMsg(src, "cannot implicitly convert {s} to {s}", .{
                    pointerSizeString(src_info.flags.size),
                    pointerSizeString(dest_info.flags.size),
                });
                errdefer msg.destroy(sema.gpa);
                if (dest_info.flags.size == .many and
                    (src_info.flags.size == .slice or
                        (src_info.flags.size == .one and Type.fromInterned(src_info.child).zigTypeTag(zcu) == .array)))
                {
                    try sema.errNote(src, msg, "use 'ptr' field to convert slice to many pointer", .{});
                } else {
                    try sema.errNote(src, msg, "use @ptrCast to change pointer size", .{});
                }
                break :msg msg;
            });
        }

        check_child: {
            const src_child: Type = if (dest_info.flags.size == .slice and src_info.flags.size == .one) blk: {
                // *[n]T -> []T
                break :blk Type.fromInterned(src_info.child).childType(zcu);
            } else .fromInterned(src_info.child);

            const dest_child: Type = .fromInterned(dest_info.child);

            const imc_res = try sema.coerceInMemoryAllowed(
                block,
                dest_child,
                src_child,
                !dest_info.flags.is_const,
                zcu.getTarget(),
                src,
                operand_src,
                null,
            );
            if (imc_res == .ok) break :check_child;
            return sema.failWithOwnedErrorMsg(block, msg: {
                const msg = try sema.errMsg(src, "pointer element type '{}' cannot coerce into element type '{}'", .{
                    src_child.fmt(pt), dest_child.fmt(pt),
                });
                errdefer msg.destroy(sema.gpa);
                try imc_res.report(sema, src, msg);
                try sema.errNote(src, msg, "use @ptrCast to cast pointer element type", .{});
                break :msg msg;
            });
        }

        check_sent: {
            if (dest_info.sentinel == .none) break :check_sent;
            if (src_info.flags.size == .c) break :check_sent;
            if (src_info.sentinel != .none) {
                const coerced_sent = try zcu.intern_pool.getCoerced(sema.gpa, pt.tid, src_info.sentinel, dest_info.child);
                if (dest_info.sentinel == coerced_sent) break :check_sent;
            }
            if (is_array_ptr_to_slice) {
                // [*]nT -> []T
                const arr_ty: Type = .fromInterned(src_info.child);
                if (arr_ty.sentinel(zcu)) |src_sentinel| {
                    const coerced_sent = try zcu.intern_pool.getCoerced(sema.gpa, pt.tid, src_sentinel.toIntern(), dest_info.child);
                    if (dest_info.sentinel == coerced_sent) break :check_sent;
                }
            }
            return sema.failWithOwnedErrorMsg(block, msg: {
                const msg = if (src_info.sentinel == .none) blk: {
                    break :blk try sema.errMsg(src, "destination pointer requires '{}' sentinel", .{
                        Value.fromInterned(dest_info.sentinel).fmtValueSema(pt, sema),
                    });
                } else blk: {
                    break :blk try sema.errMsg(src, "pointer sentinel '{}' cannot coerce into pointer sentinel '{}'", .{
                        Value.fromInterned(src_info.sentinel).fmtValueSema(pt, sema),
                        Value.fromInterned(dest_info.sentinel).fmtValueSema(pt, sema),
                    });
                };
                errdefer msg.destroy(sema.gpa);
                try sema.errNote(src, msg, "use @ptrCast to cast pointer sentinel", .{});
                break :msg msg;
            });
        }

        if (src_info.packed_offset.host_size != dest_info.packed_offset.host_size) {
            return sema.failWithOwnedErrorMsg(block, msg: {
                const msg = try sema.errMsg(src, "pointer host size '{}' cannot coerce into pointer host size '{}'", .{
                    src_info.packed_offset.host_size,
                    dest_info.packed_offset.host_size,
                });
                errdefer msg.destroy(sema.gpa);
                try sema.errNote(src, msg, "use @ptrCast to cast pointer host size", .{});
                break :msg msg;
            });
        }

        if (src_info.packed_offset.bit_offset != dest_info.packed_offset.bit_offset) {
            return sema.failWithOwnedErrorMsg(block, msg: {
                const msg = try sema.errMsg(src, "pointer bit offset '{}' cannot coerce into pointer bit offset '{}'", .{
                    src_info.packed_offset.bit_offset,
                    dest_info.packed_offset.bit_offset,
                });
                errdefer msg.destroy(sema.gpa);
                try sema.errNote(src, msg, "use @ptrCast to cast pointer bit offset", .{});
                break :msg msg;
            });
        }

        check_allowzero: {
            const src_allows_zero = operand_ty.ptrAllowsZero(zcu);
            const dest_allows_zero = dest_ty.ptrAllowsZero(zcu);
            if (!src_allows_zero) break :check_allowzero;
            if (dest_allows_zero) break :check_allowzero;

            return sema.failWithOwnedErrorMsg(block, msg: {
                const msg = try sema.errMsg(src, "'{}' could have null values which are illegal in type '{}'", .{
                    operand_ty.fmt(pt),
                    dest_ty.fmt(pt),
                });
                errdefer msg.destroy(sema.gpa);
                try sema.errNote(src, msg, "use @ptrCast to assert the pointer is not null", .{});
                break :msg msg;
            });
        }

        // TODO: vector index?
    }

    const src_align = if (src_info.flags.alignment != .none)
        src_info.flags.alignment
    else
        Type.fromInterned(src_info.child).abiAlignment(zcu);

    const dest_align = if (dest_info.flags.alignment != .none)
        dest_info.flags.alignment
    else
        Type.fromInterned(dest_info.child).abiAlignment(zcu);

    if (!flags.align_cast) {
        if (dest_align.compare(.gt, src_align)) {
            return sema.failWithOwnedErrorMsg(block, msg: {
                const msg = try sema.errMsg(src, "{s} increases pointer alignment", .{operation});
                errdefer msg.destroy(sema.gpa);
                try sema.errNote(operand_src, msg, "'{}' has alignment '{d}'", .{
                    operand_ty.fmt(pt), src_align.toByteUnits() orelse 0,
                });
                try sema.errNote(src, msg, "'{}' has alignment '{d}'", .{
                    dest_ty.fmt(pt), dest_align.toByteUnits() orelse 0,
                });
                try sema.errNote(src, msg, "use @alignCast to assert pointer alignment", .{});
                break :msg msg;
            });
        }
    }

    if (!flags.addrspace_cast) {
        if (src_info.flags.address_space != dest_info.flags.address_space) {
            return sema.failWithOwnedErrorMsg(block, msg: {
                const msg = try sema.errMsg(src, "{s} changes pointer address space", .{operation});
                errdefer msg.destroy(sema.gpa);
                try sema.errNote(operand_src, msg, "'{}' has address space '{s}'", .{
                    operand_ty.fmt(pt), @tagName(src_info.flags.address_space),
                });
                try sema.errNote(src, msg, "'{}' has address space '{s}'", .{
                    dest_ty.fmt(pt), @tagName(dest_info.flags.address_space),
                });
                try sema.errNote(src, msg, "use @addrSpaceCast to cast pointer address space", .{});
                break :msg msg;
            });
        }
    } else {
        // Some address space casts are always disallowed
        if (!target_util.addrSpaceCastIsValid(zcu.getTarget(), src_info.flags.address_space, dest_info.flags.address_space)) {
            return sema.failWithOwnedErrorMsg(block, msg: {
                const msg = try sema.errMsg(src, "invalid address space cast", .{});
                errdefer msg.destroy(sema.gpa);
                try sema.errNote(operand_src, msg, "address space '{s}' is not compatible with address space '{s}'", .{
                    @tagName(src_info.flags.address_space),
                    @tagName(dest_info.flags.address_space),
                });
                break :msg msg;
            });
        }
    }

    if (!flags.const_cast) {
        if (src_info.flags.is_const and !dest_info.flags.is_const) {
            return sema.failWithOwnedErrorMsg(block, msg: {
                const msg = try sema.errMsg(src, "{s} discards const qualifier", .{operation});
                errdefer msg.destroy(sema.gpa);
                try sema.errNote(src, msg, "use @constCast to discard const qualifier", .{});
                break :msg msg;
            });
        }
    }

    if (!flags.volatile_cast) {
        if (src_info.flags.is_volatile and !dest_info.flags.is_volatile) {
            return sema.failWithOwnedErrorMsg(block, msg: {
                const msg = try sema.errMsg(src, "{s} discards volatile qualifier", .{operation});
                errdefer msg.destroy(sema.gpa);
                try sema.errNote(src, msg, "use @volatileCast to discard volatile qualifier", .{});
                break :msg msg;
            });
        }
    }

    // Type validation done -- this cast is okay. Let's do it!
    //
    // `operand` is a maybe-optional pointer or slice.
    // `dest_ty` is a maybe-optional pointer or slice.
    //
    // We have a few safety checks:
    // * if the destination does not allow zero, check the operand is not null / 0
    // * if the destination is more aligned than the operand, check the pointer alignment
    // * if `slice_needs_len_change`, check the element count divides neatly

    ct: {
        if (flags.addrspace_cast) break :ct; // cannot `@addrSpaceCast` at comptime
        const operand_val = try sema.resolveValue(operand) orelse break :ct;

        if (operand_val.isUndef(zcu)) {
            if (!dest_ty.ptrAllowsZero(zcu)) {
                return sema.failWithUseOfUndef(block, operand_src);
            }
            return pt.undefRef(dest_ty);
        }

        if (operand_val.isNull(zcu)) {
            if (!dest_ty.ptrAllowsZero(zcu)) {
                return sema.fail(block, operand_src, "null pointer casted to type '{}'", .{dest_ty.fmt(pt)});
            }
            if (dest_ty.zigTypeTag(zcu) == .optional) {
                return Air.internedToRef((try pt.nullValue(dest_ty)).toIntern());
            } else {
                return Air.internedToRef((try pt.ptrIntValue(dest_ty, 0)).toIntern());
            }
        }

        const ptr_val: Value = switch (src_info.flags.size) {
            .slice => .fromInterned(zcu.intern_pool.indexToKey(operand_val.toIntern()).slice.ptr),
            .one, .many, .c => operand_val,
        };

        if (dest_align.compare(.gt, src_align)) {
            if (try ptr_val.getUnsignedIntSema(pt)) |addr| {
                const masked_addr = if (Type.fromInterned(dest_info.child).fnPtrMaskOrNull(zcu)) |mask|
                    addr & mask
                else
                    addr;

                if (!dest_align.check(masked_addr)) {
                    return sema.fail(block, operand_src, "pointer address 0x{X} is not aligned to {d} bytes", .{
                        addr,
                        dest_align.toByteUnits().?,
                    });
                }
            }
        }

        if (dest_info.flags.size == .slice) {
            // Because the operand is comptime-known and not `null`, the slice length has already been computed:
            const len: Value = switch (dest_slice_len.?) {
                .undef => .undef_usize,
                .constant => |n| try pt.intValue(.usize, n),
                .equal_runtime_src_slice => unreachable,
                .change_runtime_src_slice => unreachable,
            };
            return Air.internedToRef(try pt.intern(.{ .slice = .{
                .ty = dest_ty.toIntern(),
                .ptr = (try pt.getCoerced(ptr_val, dest_ty.slicePtrFieldType(zcu))).toIntern(),
                .len = len.toIntern(),
            } }));
        } else {
            // Any to non-slice
            const new_ptr_val = try pt.getCoerced(ptr_val, dest_ty);
            return Air.internedToRef(new_ptr_val.toIntern());
        }
    }

    try sema.validateRuntimeValue(block, operand_src, operand);

    const can_cast_to_int = !target_util.arePointersLogical(zcu.getTarget(), operand_ty.ptrAddressSpace(zcu));
    const need_null_check = can_cast_to_int and block.wantSafety() and operand_ty.ptrAllowsZero(zcu) and !dest_ty.ptrAllowsZero(zcu);
    const need_align_check = can_cast_to_int and block.wantSafety() and dest_align.compare(.gt, src_align);

    const slice_needs_len_change = if (dest_slice_len) |l| switch (l) {
        .undef, .equal_runtime_src_slice => false,
        .constant, .change_runtime_src_slice => true,
    } else false;

    // `operand` might be a slice. If `need_operand_ptr`, we'll populate `operand_ptr` with the raw pointer.
    const need_operand_ptr = src_info.flags.size != .slice or // we already have it
        dest_info.flags.size != .slice or // the result is a raw pointer
        need_null_check or // safety check happens on pointer
        need_align_check or // safety check happens on pointer
        flags.addrspace_cast or // AIR addrspace_cast acts on a pointer
        slice_needs_len_change; // to change the length, we reconstruct the slice

    // This is not quite just the pointer part of `operand` -- it's also had the address space cast done already.
    const operand_ptr: Air.Inst.Ref = ptr: {
        if (!need_operand_ptr) break :ptr .none;
        // First, just get the pointer.
        const pre_addrspace_cast = inner: {
            if (src_info.flags.size != .slice) break :inner operand;
            if (operand_ty.zigTypeTag(zcu) == .optional) {
                break :inner try sema.analyzeOptionalSlicePtr(block, operand_src, operand, operand_ty);
            } else {
                break :inner try sema.analyzeSlicePtr(block, operand_src, operand, operand_ty);
            }
        };
        // Now, do an addrspace cast if necessary!
        if (!flags.addrspace_cast) break :ptr pre_addrspace_cast;

        const intermediate_ptr_ty = try pt.ptrTypeSema(info: {
            var info = src_info;
            info.flags.address_space = dest_info.flags.address_space;
            break :info info;
        });
        const intermediate_ty = if (operand_ty.zigTypeTag(zcu) == .optional) blk: {
            break :blk try pt.optionalType(intermediate_ptr_ty.toIntern());
        } else intermediate_ptr_ty;
        break :ptr try block.addInst(.{
            .tag = .addrspace_cast,
            .data = .{ .ty_op = .{
                .ty = Air.internedToRef(intermediate_ty.toIntern()),
                .operand = pre_addrspace_cast,
            } },
        });
    };

    // Whether we need to know if the (slice) operand has `len == 0`.
    const need_operand_len_is_zero = src_info.flags.size == .slice and
        dest_info.flags.size == .slice and
        (need_null_check or need_align_check);
    // Whether we need to get the (slice) operand's `len`.
    const need_operand_len = need_len: {
        if (src_info.flags.size != .slice) break :need_len false;
        if (dest_info.flags.size != .slice) break :need_len false;
        if (need_operand_len_is_zero) break :need_len true;
        if (flags.addrspace_cast or slice_needs_len_change) break :need_len true;
        break :need_len false;
    };
    // `.none` if `!need_operand_len`.
    const operand_len: Air.Inst.Ref = len: {
        if (!need_operand_len) break :len .none;
        break :len try block.addTyOp(.slice_len, .usize, operand);
    };
    // `.none` if `!need_operand_len_is_zero`.
    const operand_len_is_zero: Air.Inst.Ref = zero: {
        if (!need_operand_len_is_zero) break :zero .none;
        assert(need_operand_len);
        break :zero try block.addBinOp(.cmp_eq, operand_len, .zero_usize);
    };

    // `operand_ptr` converted to an integer, for safety checks.
    const operand_ptr_int: Air.Inst.Ref = if (need_null_check or need_align_check) i: {
        assert(need_operand_ptr);
        break :i try block.addBitCast(.usize, operand_ptr);
    } else .none;

    if (need_null_check) {
        assert(operand_ptr_int != .none);
        const ptr_is_non_zero = try block.addBinOp(.cmp_neq, operand_ptr_int, .zero_usize);
        const ok = if (src_info.flags.size == .slice and dest_info.flags.size == .slice) ok: {
            break :ok try block.addBinOp(.bool_or, operand_len_is_zero, ptr_is_non_zero);
        } else ptr_is_non_zero;
        try sema.addSafetyCheck(block, src, ok, .cast_to_null);
    }
    if (need_align_check) {
        assert(operand_ptr_int != .none);
        const align_mask = try pt.intRef(.usize, mask: {
            const target_ptr_mask = Type.fromInterned(dest_info.child).fnPtrMaskOrNull(zcu) orelse ~@as(u64, 0);
            break :mask (dest_align.toByteUnits().? - 1) & target_ptr_mask;
        });
        const ptr_masked = try block.addBinOp(.bit_and, operand_ptr_int, align_mask);
        const is_aligned = try block.addBinOp(.cmp_eq, ptr_masked, .zero_usize);
        const ok = if (src_info.flags.size == .slice and dest_info.flags.size == .slice) ok: {
            break :ok try block.addBinOp(.bool_or, operand_len_is_zero, is_aligned);
        } else is_aligned;
        try sema.addSafetyCheck(block, src, ok, .incorrect_alignment);
    }

    if (dest_info.flags.size == .slice) {
        if (src_info.flags.size == .slice and !flags.addrspace_cast and !slice_needs_len_change) {
            // Fast path: just bitcast!
            return block.addBitCast(dest_ty, operand);
        }

        // We need to deconstruct the slice (if applicable) and reconstruct it.
        assert(need_operand_ptr);

        const result_len: Air.Inst.Ref = switch (dest_slice_len.?) {
            .undef => .undef_usize,
            .constant => |n| try pt.intRef(.usize, n),
            .equal_runtime_src_slice => len: {
                assert(need_operand_len);
                break :len operand_len;
            },
            .change_runtime_src_slice => |change| len: {
                assert(need_operand_len);
                // If `mul / div` is a whole number, then just multiply the length by it.
                if (std.math.divExact(u64, change.bytes_per_src, change.bytes_per_dest)) |dest_per_src| {
                    const multiplier = try pt.intRef(.usize, dest_per_src);
                    break :len try block.addBinOp(.mul, operand_len, multiplier);
                } else |err| switch (err) {
                    error.DivisionByZero => unreachable,
                    error.UnexpectedRemainder => {}, // fall through to code below
                }
                // If `div / mul` is a whole number, then just divide the length by it.
                // This incurs a safety check.
                if (std.math.divExact(u64, change.bytes_per_dest, change.bytes_per_src)) |src_per_dest| {
                    const divisor = try pt.intRef(.usize, src_per_dest);
                    if (block.wantSafety()) {
                        // Check that the element count divides neatly.
                        const remainder = try block.addBinOp(.rem, operand_len, divisor);
                        const ok = try block.addBinOp(.cmp_eq, remainder, .zero_usize);
                        try sema.addSafetyCheckCall(block, src, ok, .@"panic.sliceCastLenRemainder", &.{operand_len});
                    }
                    break :len try block.addBinOp(.div_exact, operand_len, divisor);
                } else |err| switch (err) {
                    error.DivisionByZero => unreachable,
                    error.UnexpectedRemainder => {}, // fall through to code below
                }
                // Fallback: the elements don't divide easily. We'll multiply *and* divide. This incurs a safety check.
                const total_bytes_ref = try block.addBinOp(.mul, operand_len, try pt.intRef(.usize, change.bytes_per_src));
                const bytes_per_dest_ref = try pt.intRef(.usize, change.bytes_per_dest);
                if (block.wantSafety()) {
                    // Check that `total_bytes_ref` divides neatly into `bytes_per_dest_ref`.
                    const remainder = try block.addBinOp(.rem, total_bytes_ref, bytes_per_dest_ref);
                    const ok = try block.addBinOp(.cmp_eq, remainder, .zero_usize);
                    try sema.addSafetyCheckCall(block, src, ok, .@"panic.sliceCastLenRemainder", &.{operand_len});
                }
                break :len try block.addBinOp(.div_exact, total_bytes_ref, bytes_per_dest_ref);
            },
        };

        const operand_ptr_ty = sema.typeOf(operand_ptr);
        const want_ptr_ty = switch (dest_ty.zigTypeTag(zcu)) {
            .optional => try pt.optionalType(dest_ty.childType(zcu).slicePtrFieldType(zcu).toIntern()),
            .pointer => dest_ty.slicePtrFieldType(zcu),
            else => unreachable,
        };
        const coerced_ptr = if (operand_ptr_ty.toIntern() != want_ptr_ty.toIntern()) ptr: {
            break :ptr try block.addBitCast(want_ptr_ty, operand_ptr);
        } else operand_ptr;

        return block.addInst(.{
            .tag = .slice,
            .data = .{ .ty_pl = .{
                .ty = Air.internedToRef(dest_ty.toIntern()),
                .payload = try sema.addExtra(Air.Bin{
                    .lhs = coerced_ptr,
                    .rhs = result_len,
                }),
            } },
        });
    } else {
        assert(need_operand_ptr);
        // We just need to bitcast the pointer, if necessary.
        // It might not be necessary, since we might have just needed the `addrspace_cast`.
        const result = if (sema.typeOf(operand_ptr).toIntern() == dest_ty.toIntern())
            operand_ptr
        else
            try block.addBitCast(dest_ty, operand_ptr);

        try sema.checkKnownAllocPtr(block, operand, result);
        return result;
    }
}

fn zirPtrCastNoDest(sema: *Sema, block: *Block, extended: Zir.Inst.Extended.InstData) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const FlagsInt = @typeInfo(Zir.Inst.FullPtrCastFlags).@"struct".backing_integer.?;
    const flags: Zir.Inst.FullPtrCastFlags = @bitCast(@as(FlagsInt, @truncate(extended.small)));
    const extra = sema.code.extraData(Zir.Inst.UnNode, extended.operand).data;
    const src = block.nodeOffset(extra.node);
    const operand_src = block.src(.{ .node_offset_ptrcast_operand = extra.node });
    const operand = try sema.resolveInst(extra.operand);
    const operand_ty = sema.typeOf(operand);
    try sema.checkPtrOperand(block, operand_src, operand_ty);

    var ptr_info = operand_ty.ptrInfo(zcu);
    if (flags.const_cast) ptr_info.flags.is_const = false;
    if (flags.volatile_cast) ptr_info.flags.is_volatile = false;

    const dest_ty = blk: {
        const dest_ty = try pt.ptrTypeSema(ptr_info);
        if (operand_ty.zigTypeTag(zcu) == .optional) {
            break :blk try pt.optionalType(dest_ty.toIntern());
        }
        break :blk dest_ty;
    };

    if (try sema.resolveValue(operand)) |operand_val| {
        return Air.internedToRef((try pt.getCoerced(operand_val, dest_ty)).toIntern());
    }

    try sema.requireRuntimeBlock(block, src, null);
    const new_ptr = try block.addBitCast(dest_ty, operand);
    try sema.checkKnownAllocPtr(block, operand, new_ptr);
    return new_ptr;
}

fn zirTruncate(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].pl_node;
    const src = block.nodeOffset(inst_data.src_node);
    const operand_src = block.builtinCallArgSrc(inst_data.src_node, 0);
    const extra = sema.code.extraData(Zir.Inst.Bin, inst_data.payload_index).data;
    const dest_ty = try sema.resolveDestType(block, src, extra.lhs, .remove_eu_opt, "@truncate");
    const dest_scalar_ty = try sema.checkIntOrVectorAllowComptime(block, dest_ty, src);
    const operand = try sema.resolveInst(extra.rhs);
    const operand_ty = sema.typeOf(operand);
    const operand_scalar_ty = try sema.checkIntOrVectorAllowComptime(block, operand_ty, operand_src);

    const operand_is_vector = operand_ty.zigTypeTag(zcu) == .vector;
    const dest_is_vector = dest_ty.zigTypeTag(zcu) == .vector;
    if (operand_is_vector != dest_is_vector) {
        return sema.fail(block, operand_src, "expected type '{}', found '{}'", .{ dest_ty.fmt(pt), operand_ty.fmt(pt) });
    }

    if (dest_scalar_ty.zigTypeTag(zcu) == .comptime_int) {
        return sema.coerce(block, dest_ty, operand, operand_src);
    }

    const dest_info = dest_scalar_ty.intInfo(zcu);

    if (try sema.typeHasOnePossibleValue(dest_ty)) |val| {
        return Air.internedToRef(val.toIntern());
    }

    if (operand_scalar_ty.zigTypeTag(zcu) != .comptime_int) {
        const operand_info = operand_ty.intInfo(zcu);
        if (try sema.typeHasOnePossibleValue(operand_ty)) |val| {
            return Air.internedToRef(val.toIntern());
        }

        if (operand_info.signedness != dest_info.signedness) {
            return sema.fail(block, operand_src, "expected {s} integer type, found '{}'", .{
                @tagName(dest_info.signedness), operand_ty.fmt(pt),
            });
        }
        switch (std.math.order(dest_info.bits, operand_info.bits)) {
            .gt => {
                const msg = msg: {
                    const msg = try sema.errMsg(
                        src,
                        "destination type '{}' has more bits than source type '{}'",
                        .{ dest_ty.fmt(pt), operand_ty.fmt(pt) },
                    );
                    errdefer msg.destroy(sema.gpa);
                    try sema.errNote(src, msg, "destination type has {d} bits", .{
                        dest_info.bits,
                    });
                    try sema.errNote(operand_src, msg, "operand type has {d} bits", .{
                        operand_info.bits,
                    });
                    break :msg msg;
                };
                return sema.failWithOwnedErrorMsg(block, msg);
            },
            .eq => return operand,
            .lt => {},
        }
    }

    if (try sema.resolveValueResolveLazy(operand)) |val| {
        if (val.isUndef(zcu)) return pt.undefRef(dest_ty);
        if (!dest_is_vector) {
            return Air.internedToRef((try pt.getCoerced(
                try val.intTrunc(operand_ty, sema.arena, dest_info.signedness, dest_info.bits, pt),
                dest_ty,
            )).toIntern());
        }
        const elems = try sema.arena.alloc(InternPool.Index, operand_ty.vectorLen(zcu));
        for (elems, 0..) |*elem, i| {
            const elem_val = try val.elemValue(pt, i);
            const uncoerced_elem = try elem_val.intTrunc(operand_scalar_ty, sema.arena, dest_info.signedness, dest_info.bits, pt);
            elem.* = (try pt.getCoerced(uncoerced_elem, dest_scalar_ty)).toIntern();
        }
        return Air.internedToRef((try pt.intern(.{ .aggregate = .{
            .ty = dest_ty.toIntern(),
            .storage = .{ .elems = elems },
        } })));
    }

    try sema.requireRuntimeBlock(block, src, operand_src);
    return block.addTyOp(.trunc, dest_ty, operand);
}

fn zirBitCount(
    sema: *Sema,
    block: *Block,
    inst: Zir.Inst.Index,
    air_tag: Air.Inst.Tag,
    comptime comptimeOp: fn (val: Value, ty: Type, zcu: *Zcu) u64,
) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].un_node;
    const src = block.nodeOffset(inst_data.src_node);
    const operand_src = block.builtinCallArgSrc(inst_data.src_node, 0);
    const operand = try sema.resolveInst(inst_data.operand);
    const operand_ty = sema.typeOf(operand);
    _ = try sema.checkIntOrVector(block, operand, operand_src);
    const bits = operand_ty.intInfo(zcu).bits;

    if (try sema.typeHasOnePossibleValue(operand_ty)) |val| {
        return Air.internedToRef(val.toIntern());
    }

    const result_scalar_ty = try pt.smallestUnsignedInt(bits);
    switch (operand_ty.zigTypeTag(zcu)) {
        .vector => {
            const vec_len = operand_ty.vectorLen(zcu);
            const result_ty = try pt.vectorType(.{
                .len = vec_len,
                .child = result_scalar_ty.toIntern(),
            });
            if (try sema.resolveValue(operand)) |val| {
                if (val.isUndef(zcu)) return pt.undefRef(result_ty);

                const elems = try sema.arena.alloc(InternPool.Index, vec_len);
                const scalar_ty = operand_ty.scalarType(zcu);
                for (elems, 0..) |*elem, i| {
                    const elem_val = try val.elemValue(pt, i);
                    const count = comptimeOp(elem_val, scalar_ty, zcu);
                    elem.* = (try pt.intValue(result_scalar_ty, count)).toIntern();
                }
                return Air.internedToRef((try pt.intern(.{ .aggregate = .{
                    .ty = result_ty.toIntern(),
                    .storage = .{ .elems = elems },
                } })));
            } else {
                try sema.requireRuntimeBlock(block, src, operand_src);
                return block.addTyOp(air_tag, result_ty, operand);
            }
        },
        .int => {
            if (try sema.resolveValueResolveLazy(operand)) |val| {
                if (val.isUndef(zcu)) return pt.undefRef(result_scalar_ty);
                return pt.intRef(result_scalar_ty, comptimeOp(val, operand_ty, zcu));
            } else {
                try sema.requireRuntimeBlock(block, src, operand_src);
                return block.addTyOp(air_tag, result_scalar_ty, operand);
            }
        },
        else => unreachable,
    }
}

fn zirByteSwap(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].un_node;
    const src = block.nodeOffset(inst_data.src_node);
    const operand_src = block.builtinCallArgSrc(inst_data.src_node, 0);
    const operand = try sema.resolveInst(inst_data.operand);
    const operand_ty = sema.typeOf(operand);
    const scalar_ty = try sema.checkIntOrVector(block, operand, operand_src);
    const bits = scalar_ty.intInfo(zcu).bits;
    if (bits % 8 != 0) {
        return sema.fail(
            block,
            operand_src,
            "@byteSwap requires the number of bits to be evenly divisible by 8, but {} has {} bits",
            .{ scalar_ty.fmt(pt), bits },
        );
    }

    if (try sema.typeHasOnePossibleValue(operand_ty)) |val| {
        return Air.internedToRef(val.toIntern());
    }

    switch (operand_ty.zigTypeTag(zcu)) {
        .int => {
            const runtime_src = if (try sema.resolveValue(operand)) |val| {
                if (val.isUndef(zcu)) return pt.undefRef(operand_ty);
                const result_val = try val.byteSwap(operand_ty, pt, sema.arena);
                return Air.internedToRef(result_val.toIntern());
            } else operand_src;

            try sema.requireRuntimeBlock(block, src, runtime_src);
            return block.addTyOp(.byte_swap, operand_ty, operand);
        },
        .vector => {
            const runtime_src = if (try sema.resolveValue(operand)) |val| {
                if (val.isUndef(zcu))
                    return pt.undefRef(operand_ty);

                const vec_len = operand_ty.vectorLen(zcu);
                const elems = try sema.arena.alloc(InternPool.Index, vec_len);
                for (elems, 0..) |*elem, i| {
                    const elem_val = try val.elemValue(pt, i);
                    elem.* = (try elem_val.byteSwap(scalar_ty, pt, sema.arena)).toIntern();
                }
                return Air.internedToRef((try pt.intern(.{ .aggregate = .{
                    .ty = operand_ty.toIntern(),
                    .storage = .{ .elems = elems },
                } })));
            } else operand_src;

            try sema.requireRuntimeBlock(block, src, runtime_src);
            return block.addTyOp(.byte_swap, operand_ty, operand);
        },
        else => unreachable,
    }
}

fn zirBitReverse(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].un_node;
    const src = block.nodeOffset(inst_data.src_node);
    const operand_src = block.builtinCallArgSrc(inst_data.src_node, 0);
    const operand = try sema.resolveInst(inst_data.operand);
    const operand_ty = sema.typeOf(operand);
    const scalar_ty = try sema.checkIntOrVector(block, operand, operand_src);

    if (try sema.typeHasOnePossibleValue(operand_ty)) |val| {
        return Air.internedToRef(val.toIntern());
    }

    const pt = sema.pt;
    const zcu = pt.zcu;
    switch (operand_ty.zigTypeTag(zcu)) {
        .int => {
            const runtime_src = if (try sema.resolveValue(operand)) |val| {
                if (val.isUndef(zcu)) return pt.undefRef(operand_ty);
                const result_val = try val.bitReverse(operand_ty, pt, sema.arena);
                return Air.internedToRef(result_val.toIntern());
            } else operand_src;

            try sema.requireRuntimeBlock(block, src, runtime_src);
            return block.addTyOp(.bit_reverse, operand_ty, operand);
        },
        .vector => {
            const runtime_src = if (try sema.resolveValue(operand)) |val| {
                if (val.isUndef(zcu))
                    return pt.undefRef(operand_ty);

                const vec_len = operand_ty.vectorLen(zcu);
                const elems = try sema.arena.alloc(InternPool.Index, vec_len);
                for (elems, 0..) |*elem, i| {
                    const elem_val = try val.elemValue(pt, i);
                    elem.* = (try elem_val.bitReverse(scalar_ty, pt, sema.arena)).toIntern();
                }
                return Air.internedToRef((try pt.intern(.{ .aggregate = .{
                    .ty = operand_ty.toIntern(),
                    .storage = .{ .elems = elems },
                } })));
            } else operand_src;

            try sema.requireRuntimeBlock(block, src, runtime_src);
            return block.addTyOp(.bit_reverse, operand_ty, operand);
        },
        else => unreachable,
    }
}

fn zirBitOffsetOf(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    const offset = try sema.bitOffsetOf(block, inst);
    return sema.pt.intRef(.comptime_int, offset);
}

fn zirOffsetOf(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    const offset = try sema.bitOffsetOf(block, inst);
    // TODO reminder to make this a compile error for packed structs
    return sema.pt.intRef(.comptime_int, offset / 8);
}

fn bitOffsetOf(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!u64 {
    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].pl_node;
    const src = block.src(.{ .node_offset_bin_op = inst_data.src_node });
    const ty_src = block.builtinCallArgSrc(inst_data.src_node, 0);
    const field_name_src = block.builtinCallArgSrc(inst_data.src_node, 1);
    const extra = sema.code.extraData(Zir.Inst.Bin, inst_data.payload_index).data;

    const ty = try sema.resolveType(block, ty_src, extra.lhs);
    const field_name = try sema.resolveConstStringIntern(block, field_name_src, extra.rhs, .{ .simple = .field_name });

    const pt = sema.pt;
    const zcu = pt.zcu;
    const ip = &zcu.intern_pool;
    try ty.resolveLayout(pt);
    switch (ty.zigTypeTag(zcu)) {
        .@"struct" => {},
        else => return sema.fail(block, ty_src, "expected struct type, found '{}'", .{ty.fmt(pt)}),
    }

    const field_index = if (ty.isTuple(zcu)) blk: {
        if (field_name.eqlSlice("len", ip)) {
            return sema.fail(block, src, "no offset available for 'len' field of tuple", .{});
        }
        break :blk try sema.tupleFieldIndex(block, ty, field_name, field_name_src);
    } else try sema.structFieldIndex(block, ty, field_name, field_name_src);

    if (ty.structFieldIsComptime(field_index, zcu)) {
        return sema.fail(block, src, "no offset available for comptime field", .{});
    }

    switch (ty.containerLayout(zcu)) {
        .@"packed" => {
            var bit_sum: u64 = 0;
            const struct_type = ip.loadStructType(ty.toIntern());
            for (0..struct_type.field_types.len) |i| {
                if (i == field_index) {
                    return bit_sum;
                }
                const field_ty: Type = .fromInterned(struct_type.field_types.get(ip)[i]);
                bit_sum += field_ty.bitSize(zcu);
            } else unreachable;
        },
        else => return ty.structFieldOffset(field_index, zcu) * 8,
    }
}

fn checkNamespaceType(sema: *Sema, block: *Block, src: LazySrcLoc, ty: Type) CompileError!void {
    const pt = sema.pt;
    const zcu = pt.zcu;
    switch (ty.zigTypeTag(zcu)) {
        .@"struct", .@"enum", .@"union", .@"opaque" => return,
        else => return sema.fail(block, src, "expected struct, enum, union, or opaque; found '{}'", .{ty.fmt(pt)}),
    }
}

/// Returns `true` if the type was a comptime_int.
fn checkIntType(sema: *Sema, block: *Block, src: LazySrcLoc, ty: Type) CompileError!bool {
    const pt = sema.pt;
    const zcu = pt.zcu;
    switch (ty.zigTypeTag(zcu)) {
        .comptime_int => return true,
        .int => return false,
        else => return sema.fail(block, src, "expected integer type, found '{}'", .{ty.fmt(pt)}),
    }
}

fn checkInvalidPtrIntArithmetic(
    sema: *Sema,
    block: *Block,
    src: LazySrcLoc,
    ty: Type,
) CompileError!void {
    const pt = sema.pt;
    const zcu = pt.zcu;
    switch (ty.zigTypeTag(zcu)) {
        .pointer => switch (ty.ptrSize(zcu)) {
            .one, .slice => return,
            .many, .c => return sema.failWithInvalidPtrArithmetic(block, src, "pointer-integer", "addition and subtraction"),
        },
        else => return,
    }
}

fn checkArithmeticOp(
    sema: *Sema,
    block: *Block,
    src: LazySrcLoc,
    scalar_tag: std.builtin.TypeId,
    lhs_zig_ty_tag: std.builtin.TypeId,
    rhs_zig_ty_tag: std.builtin.TypeId,
    zir_tag: Zir.Inst.Tag,
) CompileError!void {
    const is_int = scalar_tag == .int or scalar_tag == .comptime_int;
    const is_float = scalar_tag == .float or scalar_tag == .comptime_float;

    if (!is_int and !(is_float and floatOpAllowed(zir_tag))) {
        return sema.fail(block, src, "invalid operands to binary expression: '{s}' and '{s}'", .{
            @tagName(lhs_zig_ty_tag), @tagName(rhs_zig_ty_tag),
        });
    }
}

fn checkPtrOperand(
    sema: *Sema,
    block: *Block,
    ty_src: LazySrcLoc,
    ty: Type,
) CompileError!void {
    const pt = sema.pt;
    const zcu = pt.zcu;
    switch (ty.zigTypeTag(zcu)) {
        .pointer => return,
        .@"fn" => {
            const msg = msg: {
                const msg = try sema.errMsg(
                    ty_src,
                    "expected pointer, found '{}'",
                    .{ty.fmt(pt)},
                );
                errdefer msg.destroy(sema.gpa);

                try sema.errNote(ty_src, msg, "use '&' to obtain a function pointer", .{});

                break :msg msg;
            };
            return sema.failWithOwnedErrorMsg(block, msg);
        },
        .optional => if (ty.childType(zcu).zigTypeTag(zcu) == .pointer) return,
        else => {},
    }
    return sema.fail(block, ty_src, "expected pointer type, found '{}'", .{ty.fmt(pt)});
}

fn checkPtrType(
    sema: *Sema,
    block: *Block,
    ty_src: LazySrcLoc,
    ty: Type,
    allow_slice: bool,
) CompileError!void {
    const pt = sema.pt;
    const zcu = pt.zcu;
    switch (ty.zigTypeTag(zcu)) {
        .pointer => if (allow_slice or !ty.isSlice(zcu)) return,
        .@"fn" => {
            const msg = msg: {
                const msg = try sema.errMsg(
                    ty_src,
                    "expected pointer type, found '{}'",
                    .{ty.fmt(pt)},
                );
                errdefer msg.destroy(sema.gpa);

                try sema.errNote(ty_src, msg, "use '*const ' to make a function pointer type", .{});

                break :msg msg;
            };
            return sema.failWithOwnedErrorMsg(block, msg);
        },
        .optional => if (ty.childType(zcu).zigTypeTag(zcu) == .pointer) return,
        else => {},
    }
    return sema.fail(block, ty_src, "expected pointer type, found '{}'", .{ty.fmt(pt)});
}

fn checkLogicalPtrOperation(sema: *Sema, block: *Block, src: LazySrcLoc, ty: Type) !void {
    const pt = sema.pt;
    const zcu = pt.zcu;
    if (zcu.intern_pool.indexToKey(ty.toIntern()) == .ptr_type) {
        const target = zcu.getTarget();
        const as = ty.ptrAddressSpace(zcu);
        if (target_util.arePointersLogical(target, as)) {
            return sema.failWithOwnedErrorMsg(block, msg: {
                const msg = try sema.errMsg(src, "illegal operation on logical pointer of type '{}'", .{ty.fmt(pt)});
                errdefer msg.destroy(sema.gpa);
                try sema.errNote(
                    src,
                    msg,
                    "cannot perform arithmetic on pointers with address space '{s}' on target {s}-{s}",
                    .{
                        @tagName(as),
                        @tagName(target.cpu.arch.family()),
                        @tagName(target.os.tag),
                    },
                );
                break :msg msg;
            });
        }
    }
}

fn checkVectorElemType(
    sema: *Sema,
    block: *Block,
    ty_src: LazySrcLoc,
    ty: Type,
) CompileError!void {
    const pt = sema.pt;
    const zcu = pt.zcu;
    switch (ty.zigTypeTag(zcu)) {
        .int, .float, .bool => return,
        .optional, .pointer => if (ty.isPtrAtRuntime(zcu)) return,
        else => {},
    }
    return sema.fail(block, ty_src, "expected integer, float, bool, or pointer for the vector element type; found '{}'", .{ty.fmt(pt)});
}

fn checkFloatType(
    sema: *Sema,
    block: *Block,
    ty_src: LazySrcLoc,
    ty: Type,
) CompileError!void {
    const pt = sema.pt;
    const zcu = pt.zcu;
    switch (ty.zigTypeTag(zcu)) {
        .comptime_int, .comptime_float, .float => {},
        else => return sema.fail(block, ty_src, "expected float type, found '{}'", .{ty.fmt(pt)}),
    }
}

fn checkNumericType(
    sema: *Sema,
    block: *Block,
    ty_src: LazySrcLoc,
    ty: Type,
) CompileError!void {
    const pt = sema.pt;
    const zcu = pt.zcu;
    switch (ty.zigTypeTag(zcu)) {
        .comptime_float, .float, .comptime_int, .int => {},
        .vector => switch (ty.childType(zcu).zigTypeTag(zcu)) {
            .comptime_float, .float, .comptime_int, .int => {},
            else => |t| return sema.fail(block, ty_src, "expected number, found '{}'", .{t}),
        },
        else => return sema.fail(block, ty_src, "expected number, found '{}'", .{ty.fmt(pt)}),
    }
}

/// Returns the casted pointer.
fn checkAtomicPtrOperand(
    sema: *Sema,
    block: *Block,
    elem_ty: Type,
    elem_ty_src: LazySrcLoc,
    ptr: Air.Inst.Ref,
    ptr_src: LazySrcLoc,
    ptr_const: bool,
) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    var diag: Zcu.AtomicPtrAlignmentDiagnostics = .{};
    const alignment = zcu.atomicPtrAlignment(elem_ty, &diag) catch |err| switch (err) {
        error.OutOfMemory => return error.OutOfMemory,
        error.FloatTooBig => return sema.fail(
            block,
            elem_ty_src,
            "expected {d}-bit float type or smaller; found {d}-bit float type",
            .{ diag.max_bits, diag.bits },
        ),
        error.IntTooBig => return sema.fail(
            block,
            elem_ty_src,
            "expected {d}-bit integer type or smaller; found {d}-bit integer type",
            .{ diag.max_bits, diag.bits },
        ),
        error.BadType => return sema.fail(
            block,
            elem_ty_src,
            "expected bool, integer, float, enum, packed struct, or pointer type; found '{}'",
            .{elem_ty.fmt(pt)},
        ),
    };

    var wanted_ptr_data: InternPool.Key.PtrType = .{
        .child = elem_ty.toIntern(),
        .flags = .{
            .alignment = alignment,
            .is_const = ptr_const,
        },
    };

    const ptr_ty = sema.typeOf(ptr);
    const ptr_data = switch (ptr_ty.zigTypeTag(zcu)) {
        .pointer => ptr_ty.ptrInfo(zcu),
        else => {
            const wanted_ptr_ty = try pt.ptrTypeSema(wanted_ptr_data);
            _ = try sema.coerce(block, wanted_ptr_ty, ptr, ptr_src);
            unreachable;
        },
    };

    wanted_ptr_data.flags.address_space = ptr_data.flags.address_space;
    wanted_ptr_data.flags.is_allowzero = ptr_data.flags.is_allowzero;
    wanted_ptr_data.flags.is_volatile = ptr_data.flags.is_volatile;

    const wanted_ptr_ty = try pt.ptrTypeSema(wanted_ptr_data);
    const casted_ptr = try sema.coerce(block, wanted_ptr_ty, ptr, ptr_src);

    return casted_ptr;
}

fn checkPtrIsNotComptimeMutable(
    sema: *Sema,
    block: *Block,
    ptr_val: Value,
    ptr_src: LazySrcLoc,
    operand_src: LazySrcLoc,
) CompileError!void {
    _ = operand_src;
    if (sema.isComptimeMutablePtr(ptr_val)) {
        return sema.fail(block, ptr_src, "cannot store runtime value in compile time variable", .{});
    }
}

fn checkIntOrVector(
    sema: *Sema,
    block: *Block,
    operand: Air.Inst.Ref,
    operand_src: LazySrcLoc,
) CompileError!Type {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const operand_ty = sema.typeOf(operand);
    switch (operand_ty.zigTypeTag(zcu)) {
        .int => return operand_ty,
        .vector => {
            const elem_ty = operand_ty.childType(zcu);
            switch (elem_ty.zigTypeTag(zcu)) {
                .int => return elem_ty,
                else => return sema.fail(block, operand_src, "expected vector of integers; found vector of '{}'", .{
                    elem_ty.fmt(pt),
                }),
            }
        },
        else => return sema.fail(block, operand_src, "expected integer or vector, found '{}'", .{
            operand_ty.fmt(pt),
        }),
    }
}

fn checkIntOrVectorAllowComptime(
    sema: *Sema,
    block: *Block,
    operand_ty: Type,
    operand_src: LazySrcLoc,
) CompileError!Type {
    const pt = sema.pt;
    const zcu = pt.zcu;
    switch (operand_ty.zigTypeTag(zcu)) {
        .int, .comptime_int => return operand_ty,
        .vector => {
            const elem_ty = operand_ty.childType(zcu);
            switch (elem_ty.zigTypeTag(zcu)) {
                .int, .comptime_int => return elem_ty,
                else => return sema.fail(block, operand_src, "expected vector of integers; found vector of '{}'", .{
                    elem_ty.fmt(pt),
                }),
            }
        },
        else => return sema.fail(block, operand_src, "expected integer or vector, found '{}'", .{
            operand_ty.fmt(pt),
        }),
    }
}

const SimdBinOp = struct {
    len: ?usize,
    /// Coerced to `result_ty`.
    lhs: Air.Inst.Ref,
    /// Coerced to `result_ty`.
    rhs: Air.Inst.Ref,
    lhs_val: ?Value,
    rhs_val: ?Value,
    /// Only different than `scalar_ty` when it is a vector operation.
    result_ty: Type,
    scalar_ty: Type,
};

fn checkSimdBinOp(
    sema: *Sema,
    block: *Block,
    src: LazySrcLoc,
    uncasted_lhs: Air.Inst.Ref,
    uncasted_rhs: Air.Inst.Ref,
    lhs_src: LazySrcLoc,
    rhs_src: LazySrcLoc,
) CompileError!SimdBinOp {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const lhs_ty = sema.typeOf(uncasted_lhs);
    const rhs_ty = sema.typeOf(uncasted_rhs);

    try sema.checkVectorizableBinaryOperands(block, src, lhs_ty, rhs_ty, lhs_src, rhs_src);
    const vec_len: ?usize = if (lhs_ty.zigTypeTag(zcu) == .vector) lhs_ty.vectorLen(zcu) else null;
    const result_ty = try sema.resolvePeerTypes(block, src, &.{ uncasted_lhs, uncasted_rhs }, .{
        .override = &[_]?LazySrcLoc{ lhs_src, rhs_src },
    });
    const lhs = try sema.coerce(block, result_ty, uncasted_lhs, lhs_src);
    const rhs = try sema.coerce(block, result_ty, uncasted_rhs, rhs_src);

    return SimdBinOp{
        .len = vec_len,
        .lhs = lhs,
        .rhs = rhs,
        .lhs_val = try sema.resolveValue(lhs),
        .rhs_val = try sema.resolveValue(rhs),
        .result_ty = result_ty,
        .scalar_ty = result_ty.scalarType(zcu),
    };
}

fn checkVectorizableBinaryOperands(
    sema: *Sema,
    block: *Block,
    src: LazySrcLoc,
    lhs_ty: Type,
    rhs_ty: Type,
    lhs_src: LazySrcLoc,
    rhs_src: LazySrcLoc,
) CompileError!void {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const lhs_zig_ty_tag = lhs_ty.zigTypeTag(zcu);
    const rhs_zig_ty_tag = rhs_ty.zigTypeTag(zcu);
    if (lhs_zig_ty_tag != .vector and rhs_zig_ty_tag != .vector) return;

    const lhs_is_vector = switch (lhs_zig_ty_tag) {
        .vector, .array => true,
        else => false,
    };
    const rhs_is_vector = switch (rhs_zig_ty_tag) {
        .vector, .array => true,
        else => false,
    };

    if (lhs_is_vector and rhs_is_vector) {
        const lhs_len = lhs_ty.arrayLen(zcu);
        const rhs_len = rhs_ty.arrayLen(zcu);
        if (lhs_len != rhs_len) {
            const msg = msg: {
                const msg = try sema.errMsg(src, "vector length mismatch", .{});
                errdefer msg.destroy(sema.gpa);
                try sema.errNote(lhs_src, msg, "length {d} here", .{lhs_len});
                try sema.errNote(rhs_src, msg, "length {d} here", .{rhs_len});
                break :msg msg;
            };
            return sema.failWithOwnedErrorMsg(block, msg);
        }
    } else {
        const msg = msg: {
            const msg = try sema.errMsg(src, "mixed scalar and vector operands: '{}' and '{}'", .{
                lhs_ty.fmt(pt), rhs_ty.fmt(pt),
            });
            errdefer msg.destroy(sema.gpa);
            if (lhs_is_vector) {
                try sema.errNote(lhs_src, msg, "vector here", .{});
                try sema.errNote(rhs_src, msg, "scalar here", .{});
            } else {
                try sema.errNote(lhs_src, msg, "scalar here", .{});
                try sema.errNote(rhs_src, msg, "vector here", .{});
            }
            break :msg msg;
        };
        return sema.failWithOwnedErrorMsg(block, msg);
    }
}

fn resolveExportOptions(
    sema: *Sema,
    block: *Block,
    src: LazySrcLoc,
    zir_ref: Zir.Inst.Ref,
) CompileError!Zcu.Export.Options {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const gpa = sema.gpa;
    const ip = &zcu.intern_pool;
    const export_options_ty = try sema.getBuiltinType(src, .ExportOptions);
    const air_ref = try sema.resolveInst(zir_ref);
    const options = try sema.coerce(block, export_options_ty, air_ref, src);

    const name_src = block.src(.{ .init_field_name = src.offset.node_offset_builtin_call_arg.builtin_call_node });
    const linkage_src = block.src(.{ .init_field_linkage = src.offset.node_offset_builtin_call_arg.builtin_call_node });
    const section_src = block.src(.{ .init_field_section = src.offset.node_offset_builtin_call_arg.builtin_call_node });
    const visibility_src = block.src(.{ .init_field_visibility = src.offset.node_offset_builtin_call_arg.builtin_call_node });

    const name_operand = try sema.fieldVal(block, src, options, try ip.getOrPutString(gpa, pt.tid, "name", .no_embedded_nulls), name_src);
    const name = try sema.toConstString(block, name_src, name_operand, .{ .simple = .export_options });

    const linkage_operand = try sema.fieldVal(block, src, options, try ip.getOrPutString(gpa, pt.tid, "linkage", .no_embedded_nulls), linkage_src);
    const linkage_val = try sema.resolveConstDefinedValue(block, linkage_src, linkage_operand, .{ .simple = .export_options });
    const linkage = try sema.interpretBuiltinType(block, linkage_src, linkage_val, std.builtin.GlobalLinkage);

    const section_operand = try sema.fieldVal(block, src, options, try ip.getOrPutString(gpa, pt.tid, "section", .no_embedded_nulls), section_src);
    const section_opt_val = try sema.resolveConstDefinedValue(block, section_src, section_operand, .{ .simple = .export_options });
    const section = if (section_opt_val.optionalValue(zcu)) |section_val|
        try sema.toConstString(block, section_src, Air.internedToRef(section_val.toIntern()), .{ .simple = .export_options })
    else
        null;

    const visibility_operand = try sema.fieldVal(block, src, options, try ip.getOrPutString(gpa, pt.tid, "visibility", .no_embedded_nulls), visibility_src);
    const visibility_val = try sema.resolveConstDefinedValue(block, visibility_src, visibility_operand, .{ .simple = .export_options });
    const visibility = try sema.interpretBuiltinType(block, visibility_src, visibility_val, std.builtin.SymbolVisibility);

    if (name.len < 1) {
        return sema.fail(block, name_src, "exported symbol name cannot be empty", .{});
    }

    if (visibility != .default and linkage == .internal) {
        return sema.fail(block, visibility_src, "symbol '{s}' exported with internal linkage has non-default visibility {s}", .{
            name, @tagName(visibility),
        });
    }

    return .{
        .name = try ip.getOrPutString(gpa, pt.tid, name, .no_embedded_nulls),
        .linkage = linkage,
        .section = try ip.getOrPutStringOpt(gpa, pt.tid, section, .no_embedded_nulls),
        .visibility = visibility,
    };
}

fn resolveBuiltinEnum(
    sema: *Sema,
    block: *Block,
    src: LazySrcLoc,
    zir_ref: Zir.Inst.Ref,
    comptime name: Zcu.BuiltinDecl,
    reason: ComptimeReason,
) CompileError!@field(std.builtin, @tagName(name)) {
    const ty = try sema.getBuiltinType(src, name);
    const air_ref = try sema.resolveInst(zir_ref);
    const coerced = try sema.coerce(block, ty, air_ref, src);
    const val = try sema.resolveConstDefinedValue(block, src, coerced, reason);
    return sema.interpretBuiltinType(block, src, val, @field(std.builtin, @tagName(name)));
}

fn resolveAtomicOrder(
    sema: *Sema,
    block: *Block,
    src: LazySrcLoc,
    zir_ref: Zir.Inst.Ref,
    reason: ComptimeReason,
) CompileError!std.builtin.AtomicOrder {
    return sema.resolveBuiltinEnum(block, src, zir_ref, .AtomicOrder, reason);
}

fn resolveAtomicRmwOp(
    sema: *Sema,
    block: *Block,
    src: LazySrcLoc,
    zir_ref: Zir.Inst.Ref,
) CompileError!std.builtin.AtomicRmwOp {
    return sema.resolveBuiltinEnum(block, src, zir_ref, .AtomicRmwOp, .{ .simple = .operand_atomicRmw_operation });
}

fn zirCmpxchg(
    sema: *Sema,
    block: *Block,
    extended: Zir.Inst.Extended.InstData,
) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const extra = sema.code.extraData(Zir.Inst.Cmpxchg, extended.operand).data;
    const air_tag: Air.Inst.Tag = switch (extended.small) {
        0 => .cmpxchg_weak,
        1 => .cmpxchg_strong,
        else => unreachable,
    };
    const src = block.nodeOffset(extra.node);
    // zig fmt: off
    const elem_ty_src       = block.builtinCallArgSrc(extra.node, 0);
    const ptr_src           = block.builtinCallArgSrc(extra.node, 1);
    const expected_src      = block.builtinCallArgSrc(extra.node, 2);
    const new_value_src     = block.builtinCallArgSrc(extra.node, 3);
    const success_order_src = block.builtinCallArgSrc(extra.node, 4);
    const failure_order_src = block.builtinCallArgSrc(extra.node, 5);
    // zig fmt: on
    const expected_value = try sema.resolveInst(extra.expected_value);
    const elem_ty = sema.typeOf(expected_value);
    if (elem_ty.zigTypeTag(zcu) == .float) {
        return sema.fail(
            block,
            elem_ty_src,
            "expected bool, integer, enum, packed struct, or pointer type; found '{}'",
            .{elem_ty.fmt(pt)},
        );
    }
    const uncasted_ptr = try sema.resolveInst(extra.ptr);
    const ptr = try sema.checkAtomicPtrOperand(block, elem_ty, elem_ty_src, uncasted_ptr, ptr_src, false);
    const new_value = try sema.coerce(block, elem_ty, try sema.resolveInst(extra.new_value), new_value_src);
    const success_order = try sema.resolveAtomicOrder(block, success_order_src, extra.success_order, .{ .simple = .atomic_order });
    const failure_order = try sema.resolveAtomicOrder(block, failure_order_src, extra.failure_order, .{ .simple = .atomic_order });

    if (@intFromEnum(success_order) < @intFromEnum(std.builtin.AtomicOrder.monotonic)) {
        return sema.fail(block, success_order_src, "success atomic ordering must be monotonic or stricter", .{});
    }
    if (@intFromEnum(failure_order) < @intFromEnum(std.builtin.AtomicOrder.monotonic)) {
        return sema.fail(block, failure_order_src, "failure atomic ordering must be monotonic or stricter", .{});
    }
    if (@intFromEnum(failure_order) > @intFromEnum(success_order)) {
        return sema.fail(block, failure_order_src, "failure atomic ordering must be no stricter than success", .{});
    }
    if (failure_order == .release or failure_order == .acq_rel) {
        return sema.fail(block, failure_order_src, "failure atomic ordering must not be release or acq_rel", .{});
    }

    const result_ty = try pt.optionalType(elem_ty.toIntern());

    // special case zero bit types
    if ((try sema.typeHasOnePossibleValue(elem_ty)) != null) {
        return Air.internedToRef((try pt.intern(.{ .opt = .{
            .ty = result_ty.toIntern(),
            .val = .none,
        } })));
    }

    const runtime_src = if (try sema.resolveDefinedValue(block, ptr_src, ptr)) |ptr_val| rs: {
        if (try sema.resolveValue(expected_value)) |expected_val| {
            if (try sema.resolveValue(new_value)) |new_val| {
                if (expected_val.isUndef(zcu) or new_val.isUndef(zcu)) {
                    // TODO: this should probably cause the memory stored at the pointer
                    // to become undef as well
                    return pt.undefRef(result_ty);
                }
                const ptr_ty = sema.typeOf(ptr);
                const stored_val = (try sema.pointerDeref(block, ptr_src, ptr_val, ptr_ty)) orelse break :rs ptr_src;
                const result_val = try pt.intern(.{ .opt = .{
                    .ty = result_ty.toIntern(),
                    .val = if (stored_val.eql(expected_val, elem_ty, zcu)) blk: {
                        try sema.storePtr(block, src, ptr, new_value);
                        break :blk .none;
                    } else stored_val.toIntern(),
                } });
                return Air.internedToRef(result_val);
            } else break :rs new_value_src;
        } else break :rs expected_src;
    } else ptr_src;

    const flags: u32 = @as(u32, @intFromEnum(success_order)) |
        (@as(u32, @intFromEnum(failure_order)) << 3);

    try sema.requireRuntimeBlock(block, src, runtime_src);
    return block.addInst(.{
        .tag = air_tag,
        .data = .{ .ty_pl = .{
            .ty = Air.internedToRef(result_ty.toIntern()),
            .payload = try sema.addExtra(Air.Cmpxchg{
                .ptr = ptr,
                .expected_value = expected_value,
                .new_value = new_value,
                .flags = flags,
            }),
        } },
    });
}

fn zirSplat(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].pl_node;
    const extra = sema.code.extraData(Zir.Inst.Bin, inst_data.payload_index).data;
    const src = block.nodeOffset(inst_data.src_node);
    const scalar_src = block.builtinCallArgSrc(inst_data.src_node, 0);
    const dest_ty = try sema.resolveDestType(block, src, extra.lhs, .remove_eu_opt, "@splat");

    switch (dest_ty.zigTypeTag(zcu)) {
        .array, .vector => {},
        else => return sema.fail(block, src, "expected array or vector type, found '{}'", .{dest_ty.fmt(pt)}),
    }

    const operand = try sema.resolveInst(extra.rhs);
    const scalar_ty = dest_ty.childType(zcu);
    const scalar = try sema.coerce(block, scalar_ty, operand, scalar_src);

    const len = try sema.usizeCast(block, src, dest_ty.arrayLen(zcu));

    if (try sema.typeHasOnePossibleValue(dest_ty)) |val| {
        return Air.internedToRef(val.toIntern());
    }

    // We also need this case because `[0:s]T` is not OPV.
    if (len == 0) {
        const empty_aggregate = try pt.intern(.{ .aggregate = .{
            .ty = dest_ty.toIntern(),
            .storage = .{ .elems = &.{} },
        } });
        return Air.internedToRef(empty_aggregate);
    }

    const maybe_sentinel = dest_ty.sentinel(zcu);

    if (try sema.resolveValue(scalar)) |scalar_val| {
        if (scalar_val.isUndef(zcu) and maybe_sentinel == null) {
            return pt.undefRef(dest_ty);
        }
        // TODO: I didn't want to put `.aggregate` on a separate line here; `zig fmt` bugs have forced my hand
        return Air.internedToRef(try pt.intern(.{
            .aggregate = .{
                .ty = dest_ty.toIntern(),
                .storage = s: {
                    full: {
                        if (dest_ty.zigTypeTag(zcu) == .vector) break :full;
                        const sentinel = maybe_sentinel orelse break :full;
                        if (sentinel.toIntern() == scalar_val.toIntern()) break :full;
                        // This is a array with non-zero length and a sentinel which does not match the element.
                        // We have to use the full `elems` representation.
                        const elems = try sema.arena.alloc(InternPool.Index, len + 1);
                        @memset(elems[0..len], scalar_val.toIntern());
                        elems[len] = sentinel.toIntern();
                        break :s .{ .elems = elems };
                    }
                    break :s .{ .repeated_elem = scalar_val.toIntern() };
                },
            },
        }));
    }

    try sema.requireRuntimeBlock(block, src, scalar_src);

    switch (dest_ty.zigTypeTag(zcu)) {
        .array => {
            const elems = try sema.arena.alloc(Air.Inst.Ref, len + @intFromBool(maybe_sentinel != null));
            @memset(elems[0..len], scalar);
            if (maybe_sentinel) |s| elems[len] = Air.internedToRef(s.toIntern());
            return block.addAggregateInit(dest_ty, elems);
        },
        .vector => return block.addTyOp(.splat, dest_ty, scalar),
        else => unreachable,
    }
}

fn zirReduce(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].pl_node;
    const extra = sema.code.extraData(Zir.Inst.Bin, inst_data.payload_index).data;
    const op_src = block.builtinCallArgSrc(inst_data.src_node, 0);
    const operand_src = block.builtinCallArgSrc(inst_data.src_node, 1);
    const operation = try sema.resolveBuiltinEnum(block, op_src, extra.lhs, .ReduceOp, .{ .simple = .operand_reduce_operation });
    const operand = try sema.resolveInst(extra.rhs);
    const operand_ty = sema.typeOf(operand);
    const pt = sema.pt;
    const zcu = pt.zcu;

    if (operand_ty.zigTypeTag(zcu) != .vector) {
        return sema.fail(block, operand_src, "expected vector, found '{}'", .{operand_ty.fmt(pt)});
    }

    const scalar_ty = operand_ty.childType(zcu);

    // Type-check depending on operation.
    switch (operation) {
        .And, .Or, .Xor => switch (scalar_ty.zigTypeTag(zcu)) {
            .int, .bool => {},
            else => return sema.fail(block, operand_src, "@reduce operation '{s}' requires integer or boolean operand; found '{}'", .{
                @tagName(operation), operand_ty.fmt(pt),
            }),
        },
        .Min, .Max, .Add, .Mul => switch (scalar_ty.zigTypeTag(zcu)) {
            .int, .float => {},
            else => return sema.fail(block, operand_src, "@reduce operation '{s}' requires integer or float operand; found '{}'", .{
                @tagName(operation), operand_ty.fmt(pt),
            }),
        },
    }

    const vec_len = operand_ty.vectorLen(zcu);
    if (vec_len == 0) {
        // TODO re-evaluate if we should introduce a "neutral value" for some operations,
        // e.g. zero for add and one for mul.
        return sema.fail(block, operand_src, "@reduce operation requires a vector with nonzero length", .{});
    }

    if (try sema.resolveValue(operand)) |operand_val| {
        if (operand_val.isUndef(zcu)) return pt.undefRef(scalar_ty);

        var accum: Value = try operand_val.elemValue(pt, 0);
        var i: u32 = 1;
        while (i < vec_len) : (i += 1) {
            const elem_val = try operand_val.elemValue(pt, i);
            switch (operation) {
                .And => accum = try accum.bitwiseAnd(elem_val, scalar_ty, sema.arena, pt),
                .Or => accum = try accum.bitwiseOr(elem_val, scalar_ty, sema.arena, pt),
                .Xor => accum = try accum.bitwiseXor(elem_val, scalar_ty, sema.arena, pt),
                .Min => accum = accum.numberMin(elem_val, zcu),
                .Max => accum = accum.numberMax(elem_val, zcu),
                .Add => accum = try arith.addMaybeWrap(sema, scalar_ty, accum, elem_val),
                .Mul => accum = try arith.mulMaybeWrap(sema, scalar_ty, accum, elem_val),
            }
        }
        return Air.internedToRef(accum.toIntern());
    }

    try sema.requireRuntimeBlock(block, block.nodeOffset(inst_data.src_node), operand_src);
    return block.addReduce(operand, operation);
}

fn zirShuffle(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].pl_node;
    const extra = sema.code.extraData(Zir.Inst.Shuffle, inst_data.payload_index).data;
    const elem_ty_src = block.builtinCallArgSrc(inst_data.src_node, 0);
    const mask_src = block.builtinCallArgSrc(inst_data.src_node, 3);

    const elem_ty = try sema.resolveType(block, elem_ty_src, extra.elem_type);
    try sema.checkVectorElemType(block, elem_ty_src, elem_ty);
    const a = try sema.resolveInst(extra.a);
    const b = try sema.resolveInst(extra.b);
    var mask = try sema.resolveInst(extra.mask);
    var mask_ty = sema.typeOf(mask);

    const mask_len = switch (sema.typeOf(mask).zigTypeTag(zcu)) {
        .array, .vector => sema.typeOf(mask).arrayLen(zcu),
        else => return sema.fail(block, mask_src, "expected vector or array, found '{}'", .{sema.typeOf(mask).fmt(pt)}),
    };
    mask_ty = try pt.vectorType(.{
        .len = @intCast(mask_len),
        .child = .i32_type,
    });
    mask = try sema.coerce(block, mask_ty, mask, mask_src);
    const mask_val = try sema.resolveConstValue(block, mask_src, mask, .{ .simple = .operand_shuffle_mask });
    return sema.analyzeShuffle(block, inst_data.src_node, elem_ty, a, b, mask_val, @intCast(mask_len));
}

fn analyzeShuffle(
    sema: *Sema,
    block: *Block,
    src_node: std.zig.Ast.Node.Offset,
    elem_ty: Type,
    a_uncoerced: Air.Inst.Ref,
    b_uncoerced: Air.Inst.Ref,
    mask: Value,
    mask_len: u32,
) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const a_src = block.builtinCallArgSrc(src_node, 1);
    const b_src = block.builtinCallArgSrc(src_node, 2);
    const mask_src = block.builtinCallArgSrc(src_node, 3);

    // If the type of `a` is `@Type(.undefined)`, i.e. the argument is untyped, this is 0, because it is an error to index into this vector.
    const a_len: u32 = switch (sema.typeOf(a_uncoerced).zigTypeTag(zcu)) {
        .array, .vector => @intCast(sema.typeOf(a_uncoerced).arrayLen(zcu)),
        .undefined => 0,
        else => return sema.fail(block, a_src, "expected vector of '{}', found '{}'", .{ elem_ty.fmt(pt), sema.typeOf(a_uncoerced).fmt(pt) }),
    };
    const a_ty = try pt.vectorType(.{ .len = a_len, .child = elem_ty.toIntern() });
    const a_coerced = try sema.coerce(block, a_ty, a_uncoerced, a_src);

    // If the type of `b` is `@Type(.undefined)`, i.e. the argument is untyped, this is 0, because it is an error to index into this vector.
    const b_len: u32 = switch (sema.typeOf(b_uncoerced).zigTypeTag(zcu)) {
        .array, .vector => @intCast(sema.typeOf(b_uncoerced).arrayLen(zcu)),
        .undefined => 0,
        else => return sema.fail(block, b_src, "expected vector of '{}', found '{}'", .{ elem_ty.fmt(pt), sema.typeOf(b_uncoerced).fmt(pt) }),
    };
    const b_ty = try pt.vectorType(.{ .len = b_len, .child = elem_ty.toIntern() });
    const b_coerced = try sema.coerce(block, b_ty, b_uncoerced, b_src);

    const result_ty = try pt.vectorType(.{ .len = mask_len, .child = elem_ty.toIntern() });

    // We're going to pre-emptively reserve space in `sema.air_extra`. The reason for this is we need
    // a `u32` buffer of length `mask_len` anyway, and putting it in `sema.air_extra` avoids a copy
    // in the runtime case. If the result is comptime-known, we'll shrink `air_extra` back.
    const air_extra_idx: u32 = @intCast(sema.air_extra.items.len);
    const air_mask_buf = try sema.air_extra.addManyAsSlice(sema.gpa, mask_len);

    // We want to interpret that buffer in `air_extra` in a few ways. Initially, we'll consider its
    // elements as `Air.Inst.ShuffleTwoMask`, essentially representing the raw mask values; then, we'll
    // convert it to `InternPool.Index` or `Air.Inst.ShuffleOneMask` if there are comptime-known operands.
    const mask_ip_index: []InternPool.Index = @ptrCast(air_mask_buf);
    const mask_shuffle_one: []Air.ShuffleOneMask = @ptrCast(air_mask_buf);
    const mask_shuffle_two: []Air.ShuffleTwoMask = @ptrCast(air_mask_buf);

    // Initial loop: check mask elements, populate `mask_shuffle_two`.
    var a_used = false;
    var b_used = false;
    for (mask_shuffle_two, 0..mask_len) |*out, mask_idx| {
        const mask_val = try mask.elemValue(pt, mask_idx);
        if (mask_val.isUndef(zcu)) {
            out.* = .undef;
            continue;
        }
        // Safe because mask elements are `i32` and we already checked for undef:
        const raw = (try sema.resolveLazyValue(mask_val)).toSignedInt(zcu);
        if (raw >= 0) {
            const idx: u32 = @intCast(raw);
            a_used = true;
            out.* = .aElem(idx);
            if (idx >= a_len) return sema.failWithOwnedErrorMsg(block, msg: {
                const msg = try sema.errMsg(mask_src, "mask element at index '{d}' selects out-of-bounds index", .{mask_idx});
                errdefer msg.destroy(sema.gpa);
                try sema.errNote(a_src, msg, "index '{d}' exceeds bounds of '{}' given here", .{ idx, a_ty.fmt(pt) });
                if (idx < b_len) {
                    try sema.errNote(b_src, msg, "use '~@as(u32, {d})' to index into second vector given here", .{idx});
                }
                break :msg msg;
            });
        } else {
            const idx: u32 = @intCast(~raw);
            b_used = true;
            out.* = .bElem(idx);
            if (idx >= b_len) return sema.failWithOwnedErrorMsg(block, msg: {
                const msg = try sema.errMsg(mask_src, "mask element at index '{d}' selects out-of-bounds index", .{mask_idx});
                errdefer msg.destroy(sema.gpa);
                try sema.errNote(b_src, msg, "index '{d}' exceeds bounds of '{}' given here", .{ idx, b_ty.fmt(pt) });
                break :msg msg;
            });
        }
    }

    const maybe_a_val = try sema.resolveValue(a_coerced);
    const maybe_b_val = try sema.resolveValue(b_coerced);

    const a_rt = a_used and maybe_a_val == null;
    const b_rt = b_used and maybe_b_val == null;

    if (a_rt and b_rt) {
        // Both operands are needed and runtime-known. We need a `[]ShuffleTwomask`... which is
        // exactly what we already have in `mask_shuffle_two`! So, we're basically done already.
        // We just need to append the two operands.
        try sema.air_extra.ensureUnusedCapacity(sema.gpa, 2);
        sema.appendRefsAssumeCapacity(&.{ a_coerced, b_coerced });
        return block.addInst(.{
            .tag = .shuffle_two,
            .data = .{ .ty_pl = .{
                .ty = Air.internedToRef(result_ty.toIntern()),
                .payload = air_extra_idx,
            } },
        });
    } else if (a_rt) {
        // We need to convert the `ShuffleTwoMask` values to `ShuffleOneMask`.
        for (mask_shuffle_two, mask_shuffle_one) |in, *out| {
            out.* = switch (in.unwrap()) {
                .undef => .value(try pt.undefValue(elem_ty)),
                .a_elem => |idx| .elem(idx),
                .b_elem => |idx| .value(try maybe_b_val.?.elemValue(pt, idx)),
            };
        }
        // Now just append our single runtime operand, and we're done.
        try sema.air_extra.ensureUnusedCapacity(sema.gpa, 1);
        sema.appendRefsAssumeCapacity(&.{a_coerced});
        return block.addInst(.{
            .tag = .shuffle_one,
            .data = .{ .ty_pl = .{
                .ty = Air.internedToRef(result_ty.toIntern()),
                .payload = air_extra_idx,
            } },
        });
    } else if (b_rt) {
        // We need to convert the `ShuffleTwoMask` values to `ShuffleOneMask`.
        for (mask_shuffle_two, mask_shuffle_one) |in, *out| {
            out.* = switch (in.unwrap()) {
                .undef => .value(try pt.undefValue(elem_ty)),
                .a_elem => |idx| .value(try maybe_a_val.?.elemValue(pt, idx)),
                .b_elem => |idx| .elem(idx),
            };
        }
        // Now just append our single runtime operand, and we're done.
        try sema.air_extra.ensureUnusedCapacity(sema.gpa, 1);
        sema.appendRefsAssumeCapacity(&.{b_coerced});
        return block.addInst(.{
            .tag = .shuffle_one,
            .data = .{ .ty_pl = .{
                .ty = Air.internedToRef(result_ty.toIntern()),
                .payload = air_extra_idx,
            } },
        });
    } else {
        // The result will be comptime-known. We must convert the `ShuffleTwoMask` values to
        // `InternPool.Index` values using the known operands.
        for (mask_shuffle_two, mask_ip_index) |in, *out| {
            const val: Value = switch (in.unwrap()) {
                .undef => try pt.undefValue(elem_ty),
                .a_elem => |idx| try maybe_a_val.?.elemValue(pt, idx),
                .b_elem => |idx| try maybe_b_val.?.elemValue(pt, idx),
            };
            out.* = val.toIntern();
        }
        const res = try pt.intern(.{ .aggregate = .{
            .ty = result_ty.toIntern(),
            .storage = .{ .elems = mask_ip_index },
        } });
        // We have a comptime-known result, so didn't need `air_mask_buf` -- remove it from `sema.air_extra`.
        assert(sema.air_extra.items.len == air_extra_idx + air_mask_buf.len);
        sema.air_extra.shrinkRetainingCapacity(air_extra_idx);
        return Air.internedToRef(res);
    }
}

fn zirSelect(sema: *Sema, block: *Block, extended: Zir.Inst.Extended.InstData) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const extra = sema.code.extraData(Zir.Inst.Select, extended.operand).data;

    const src = block.nodeOffset(extra.node);
    const elem_ty_src = block.builtinCallArgSrc(extra.node, 0);
    const pred_src = block.builtinCallArgSrc(extra.node, 1);
    const a_src = block.builtinCallArgSrc(extra.node, 2);
    const b_src = block.builtinCallArgSrc(extra.node, 3);

    const elem_ty = try sema.resolveType(block, elem_ty_src, extra.elem_type);
    try sema.checkVectorElemType(block, elem_ty_src, elem_ty);
    const pred_uncoerced = try sema.resolveInst(extra.pred);
    const pred_ty = sema.typeOf(pred_uncoerced);

    const vec_len_u64 = switch (pred_ty.zigTypeTag(zcu)) {
        .vector, .array => pred_ty.arrayLen(zcu),
        else => return sema.fail(block, pred_src, "expected vector or array, found '{}'", .{pred_ty.fmt(pt)}),
    };
    const vec_len: u32 = @intCast(try sema.usizeCast(block, pred_src, vec_len_u64));

    const bool_vec_ty = try pt.vectorType(.{
        .len = vec_len,
        .child = .bool_type,
    });
    const pred = try sema.coerce(block, bool_vec_ty, pred_uncoerced, pred_src);

    const vec_ty = try pt.vectorType(.{
        .len = vec_len,
        .child = elem_ty.toIntern(),
    });
    const a = try sema.coerce(block, vec_ty, try sema.resolveInst(extra.a), a_src);
    const b = try sema.coerce(block, vec_ty, try sema.resolveInst(extra.b), b_src);

    const maybe_pred = try sema.resolveValue(pred);
    const maybe_a = try sema.resolveValue(a);
    const maybe_b = try sema.resolveValue(b);

    const runtime_src = if (maybe_pred) |pred_val| rs: {
        if (pred_val.isUndef(zcu)) return pt.undefRef(vec_ty);

        if (maybe_a) |a_val| {
            if (a_val.isUndef(zcu)) return pt.undefRef(vec_ty);

            if (maybe_b) |b_val| {
                if (b_val.isUndef(zcu)) return pt.undefRef(vec_ty);

                const elems = try sema.gpa.alloc(InternPool.Index, vec_len);
                defer sema.gpa.free(elems);
                for (elems, 0..) |*elem, i| {
                    const pred_elem_val = try pred_val.elemValue(pt, i);
                    const should_choose_a = pred_elem_val.toBool();
                    elem.* = (try (if (should_choose_a) a_val else b_val).elemValue(pt, i)).toIntern();
                }

                return Air.internedToRef((try pt.intern(.{ .aggregate = .{
                    .ty = vec_ty.toIntern(),
                    .storage = .{ .elems = elems },
                } })));
            } else {
                break :rs b_src;
            }
        } else {
            if (maybe_b) |b_val| {
                if (b_val.isUndef(zcu)) return pt.undefRef(vec_ty);
            }
            break :rs a_src;
        }
    } else rs: {
        if (maybe_a) |a_val| {
            if (a_val.isUndef(zcu)) return pt.undefRef(vec_ty);
        }
        if (maybe_b) |b_val| {
            if (b_val.isUndef(zcu)) return pt.undefRef(vec_ty);
        }
        break :rs pred_src;
    };

    try sema.requireRuntimeBlock(block, src, runtime_src);
    return block.addInst(.{
        .tag = .select,
        .data = .{ .pl_op = .{
            .operand = pred,
            .payload = try block.sema.addExtra(Air.Bin{
                .lhs = a,
                .rhs = b,
            }),
        } },
    });
}

fn zirAtomicLoad(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].pl_node;
    const extra = sema.code.extraData(Zir.Inst.AtomicLoad, inst_data.payload_index).data;
    // zig fmt: off
    const elem_ty_src = block.builtinCallArgSrc(inst_data.src_node, 0);
    const ptr_src     = block.builtinCallArgSrc(inst_data.src_node, 1);
    const order_src   = block.builtinCallArgSrc(inst_data.src_node, 2);
    // zig fmt: on
    const elem_ty = try sema.resolveType(block, elem_ty_src, extra.elem_type);
    const uncasted_ptr = try sema.resolveInst(extra.ptr);
    const ptr = try sema.checkAtomicPtrOperand(block, elem_ty, elem_ty_src, uncasted_ptr, ptr_src, true);
    const order = try sema.resolveAtomicOrder(block, order_src, extra.ordering, .{ .simple = .atomic_order });

    switch (order) {
        .release, .acq_rel => {
            return sema.fail(
                block,
                order_src,
                "@atomicLoad atomic ordering must not be release or acq_rel",
                .{},
            );
        },
        else => {},
    }

    if (try sema.typeHasOnePossibleValue(elem_ty)) |val| {
        return Air.internedToRef(val.toIntern());
    }

    if (try sema.resolveDefinedValue(block, ptr_src, ptr)) |ptr_val| {
        if (try sema.pointerDeref(block, ptr_src, ptr_val, sema.typeOf(ptr))) |elem_val| {
            return Air.internedToRef(elem_val.toIntern());
        }
    }

    try sema.requireRuntimeBlock(block, block.nodeOffset(inst_data.src_node), ptr_src);
    return block.addInst(.{
        .tag = .atomic_load,
        .data = .{ .atomic_load = .{
            .ptr = ptr,
            .order = order,
        } },
    });
}

fn zirAtomicRmw(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].pl_node;
    const extra = sema.code.extraData(Zir.Inst.AtomicRmw, inst_data.payload_index).data;
    const src = block.nodeOffset(inst_data.src_node);
    // zig fmt: off
    const elem_ty_src    = block.builtinCallArgSrc(inst_data.src_node, 0);
    const ptr_src        = block.builtinCallArgSrc(inst_data.src_node, 1);
    const op_src         = block.builtinCallArgSrc(inst_data.src_node, 2);
    const operand_src    = block.builtinCallArgSrc(inst_data.src_node, 3);
    const order_src      = block.builtinCallArgSrc(inst_data.src_node, 4);
    // zig fmt: on
    const operand = try sema.resolveInst(extra.operand);
    const elem_ty = sema.typeOf(operand);
    const uncasted_ptr = try sema.resolveInst(extra.ptr);
    const ptr = try sema.checkAtomicPtrOperand(block, elem_ty, elem_ty_src, uncasted_ptr, ptr_src, false);
    const op = try sema.resolveAtomicRmwOp(block, op_src, extra.operation);

    switch (elem_ty.zigTypeTag(zcu)) {
        .@"enum" => if (op != .Xchg) {
            return sema.fail(block, op_src, "@atomicRmw with enum only allowed with .Xchg", .{});
        },
        .bool => if (op != .Xchg) {
            return sema.fail(block, op_src, "@atomicRmw with bool only allowed with .Xchg", .{});
        },
        .float => switch (op) {
            .Xchg, .Add, .Sub, .Max, .Min => {},
            else => return sema.fail(block, op_src, "@atomicRmw with float only allowed with .Xchg, .Add, .Sub, .Max, and .Min", .{}),
        },
        else => {},
    }
    const order = try sema.resolveAtomicOrder(block, order_src, extra.ordering, .{ .simple = .atomic_order });

    if (order == .unordered) {
        return sema.fail(block, order_src, "@atomicRmw atomic ordering must not be unordered", .{});
    }

    // special case zero bit types
    if (try sema.typeHasOnePossibleValue(elem_ty)) |val| {
        return Air.internedToRef(val.toIntern());
    }

    const runtime_src = if (try sema.resolveDefinedValue(block, ptr_src, ptr)) |ptr_val| rs: {
        const maybe_operand_val = try sema.resolveValue(operand);
        const operand_val = maybe_operand_val orelse {
            try sema.checkPtrIsNotComptimeMutable(block, ptr_val, ptr_src, operand_src);
            break :rs operand_src;
        };
        if (sema.isComptimeMutablePtr(ptr_val)) {
            const ptr_ty = sema.typeOf(ptr);
            const stored_val = (try sema.pointerDeref(block, ptr_src, ptr_val, ptr_ty)) orelse break :rs ptr_src;
            const new_val = switch (op) {
                // zig fmt: off
                .Xchg => operand_val,
                .Add  => try arith.addMaybeWrap(sema, elem_ty, stored_val, operand_val),
                .Sub  => try arith.subMaybeWrap(sema, elem_ty, stored_val, operand_val),
                .And  => try stored_val.bitwiseAnd   (operand_val, elem_ty, sema.arena, pt ),
                .Nand => try stored_val.bitwiseNand  (operand_val, elem_ty, sema.arena, pt ),
                .Or   => try stored_val.bitwiseOr    (operand_val, elem_ty, sema.arena, pt ),
                .Xor  => try stored_val.bitwiseXor   (operand_val, elem_ty, sema.arena, pt ),
                .Max  =>     stored_val.numberMax    (operand_val,                      zcu),
                .Min  =>     stored_val.numberMin    (operand_val,                      zcu),
                // zig fmt: on
            };
            try sema.storePtrVal(block, src, ptr_val, new_val, elem_ty);
            return Air.internedToRef(stored_val.toIntern());
        } else break :rs ptr_src;
    } else ptr_src;

    const flags: u32 = @as(u32, @intFromEnum(order)) | (@as(u32, @intFromEnum(op)) << 3);

    try sema.requireRuntimeBlock(block, src, runtime_src);
    return block.addInst(.{
        .tag = .atomic_rmw,
        .data = .{ .pl_op = .{
            .operand = ptr,
            .payload = try sema.addExtra(Air.AtomicRmw{
                .operand = operand,
                .flags = flags,
            }),
        } },
    });
}

fn zirAtomicStore(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!void {
    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].pl_node;
    const extra = sema.code.extraData(Zir.Inst.AtomicStore, inst_data.payload_index).data;
    const src = block.nodeOffset(inst_data.src_node);
    // zig fmt: off
    const elem_ty_src = block.builtinCallArgSrc(inst_data.src_node, 0);
    const ptr_src     = block.builtinCallArgSrc(inst_data.src_node, 1);
    const operand_src = block.builtinCallArgSrc(inst_data.src_node, 2);
    const order_src   = block.builtinCallArgSrc(inst_data.src_node, 3);
    // zig fmt: on
    const operand = try sema.resolveInst(extra.operand);
    const elem_ty = sema.typeOf(operand);
    const uncasted_ptr = try sema.resolveInst(extra.ptr);
    const ptr = try sema.checkAtomicPtrOperand(block, elem_ty, elem_ty_src, uncasted_ptr, ptr_src, false);
    const order = try sema.resolveAtomicOrder(block, order_src, extra.ordering, .{ .simple = .atomic_order });

    const air_tag: Air.Inst.Tag = switch (order) {
        .acquire, .acq_rel => {
            return sema.fail(
                block,
                order_src,
                "@atomicStore atomic ordering must not be acquire or acq_rel",
                .{},
            );
        },
        .unordered => .atomic_store_unordered,
        .monotonic => .atomic_store_monotonic,
        .release => .atomic_store_release,
        .seq_cst => .atomic_store_seq_cst,
    };

    return sema.storePtr2(block, src, ptr, ptr_src, operand, operand_src, air_tag);
}

fn zirMulAdd(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].pl_node;
    const extra = sema.code.extraData(Zir.Inst.MulAdd, inst_data.payload_index).data;
    const src = block.nodeOffset(inst_data.src_node);

    const mulend1_src = block.builtinCallArgSrc(inst_data.src_node, 1);
    const mulend2_src = block.builtinCallArgSrc(inst_data.src_node, 2);
    const addend_src = block.builtinCallArgSrc(inst_data.src_node, 3);

    const addend = try sema.resolveInst(extra.addend);
    const ty = sema.typeOf(addend);
    const mulend1 = try sema.coerce(block, ty, try sema.resolveInst(extra.mulend1), mulend1_src);
    const mulend2 = try sema.coerce(block, ty, try sema.resolveInst(extra.mulend2), mulend2_src);

    const maybe_mulend1 = try sema.resolveValue(mulend1);
    const maybe_mulend2 = try sema.resolveValue(mulend2);
    const maybe_addend = try sema.resolveValue(addend);
    const pt = sema.pt;
    const zcu = pt.zcu;

    switch (ty.scalarType(zcu).zigTypeTag(zcu)) {
        .comptime_float, .float => {},
        else => return sema.fail(block, src, "expected vector of floats or float type, found '{}'", .{ty.fmt(pt)}),
    }

    const runtime_src = if (maybe_mulend1) |mulend1_val| rs: {
        if (maybe_mulend2) |mulend2_val| {
            if (mulend2_val.isUndef(zcu)) return pt.undefRef(ty);

            if (maybe_addend) |addend_val| {
                if (addend_val.isUndef(zcu)) return pt.undefRef(ty);
                const result_val = try Value.mulAdd(ty, mulend1_val, mulend2_val, addend_val, sema.arena, pt);
                return Air.internedToRef(result_val.toIntern());
            } else {
                break :rs addend_src;
            }
        } else {
            if (maybe_addend) |addend_val| {
                if (addend_val.isUndef(zcu)) return pt.undefRef(ty);
            }
            break :rs mulend2_src;
        }
    } else rs: {
        if (maybe_mulend2) |mulend2_val| {
            if (mulend2_val.isUndef(zcu)) return pt.undefRef(ty);
        }
        if (maybe_addend) |addend_val| {
            if (addend_val.isUndef(zcu)) return pt.undefRef(ty);
        }
        break :rs mulend1_src;
    };

    try sema.requireRuntimeBlock(block, src, runtime_src);
    return block.addInst(.{
        .tag = .mul_add,
        .data = .{ .pl_op = .{
            .operand = addend,
            .payload = try sema.addExtra(Air.Bin{
                .lhs = mulend1,
                .rhs = mulend2,
            }),
        } },
    });
}

fn zirBuiltinCall(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    const tracy = trace(@src());
    defer tracy.end();

    const pt = sema.pt;
    const zcu = pt.zcu;
    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].pl_node;
    const modifier_src = block.builtinCallArgSrc(inst_data.src_node, 0);
    const func_src = block.builtinCallArgSrc(inst_data.src_node, 1);
    const args_src = block.builtinCallArgSrc(inst_data.src_node, 2);
    const call_src = block.nodeOffset(inst_data.src_node);

    const extra = sema.code.extraData(Zir.Inst.BuiltinCall, inst_data.payload_index).data;
    const func = try sema.resolveInst(extra.callee);

    const modifier_ty = try sema.getBuiltinType(call_src, .CallModifier);
    const air_ref = try sema.resolveInst(extra.modifier);
    const modifier_ref = try sema.coerce(block, modifier_ty, air_ref, modifier_src);
    const modifier_val = try sema.resolveConstDefinedValue(block, modifier_src, modifier_ref, .{ .simple = .call_modifier });
    var modifier = try sema.interpretBuiltinType(block, modifier_src, modifier_val, std.builtin.CallModifier);
    switch (modifier) {
        // These can be upgraded to comptime or nosuspend calls.
        .auto, .never_tail, .no_async => {
            if (block.isComptime()) {
                if (modifier == .never_tail) {
                    return sema.fail(block, modifier_src, "unable to perform 'never_tail' call at compile-time", .{});
                }
                modifier = .compile_time;
            } else if (extra.flags.is_nosuspend) {
                modifier = .no_async;
            }
        },
        // These can be upgraded to comptime. nosuspend bit can be safely ignored.
        .always_inline, .compile_time => {
            _ = (try sema.resolveDefinedValue(block, func_src, func)) orelse {
                return sema.fail(block, func_src, "modifier '{s}' requires a comptime-known function", .{@tagName(modifier)});
            };

            if (block.isComptime()) {
                modifier = .compile_time;
            }
        },
        .always_tail => {
            if (block.isComptime()) {
                modifier = .compile_time;
            }
        },
        .async_kw => {
            if (extra.flags.is_nosuspend) {
                return sema.fail(block, modifier_src, "modifier 'async_kw' cannot be used inside nosuspend block", .{});
            }
            if (block.isComptime()) {
                return sema.fail(block, modifier_src, "modifier 'async_kw' cannot be used in combination with comptime function call", .{});
            }
        },
        .never_inline => {
            if (block.isComptime()) {
                return sema.fail(block, modifier_src, "unable to perform 'never_inline' call at compile-time", .{});
            }
        },
    }

    const args = try sema.resolveInst(extra.args);

    const args_ty = sema.typeOf(args);
    if (!args_ty.isTuple(zcu)) {
        return sema.fail(block, args_src, "expected a tuple, found '{}'", .{args_ty.fmt(pt)});
    }

    const resolved_args: []Air.Inst.Ref = try sema.arena.alloc(Air.Inst.Ref, args_ty.structFieldCount(zcu));
    for (resolved_args, 0..) |*resolved, i| {
        resolved.* = try sema.tupleFieldValByIndex(block, args, @intCast(i), args_ty);
    }

    const callee_ty = sema.typeOf(func);
    const func_ty = try sema.checkCallArgumentCount(block, func, func_src, callee_ty, resolved_args.len, false);
    const ensure_result_used = extra.flags.ensure_result_used;
    return sema.analyzeCall(
        block,
        func,
        func_ty,
        func_src,
        call_src,
        modifier,
        ensure_result_used,
        .{ .call_builtin = .{
            .call_node_offset = inst_data.src_node,
            .args = resolved_args,
        } },
        null,
        .@"@call",
    );
}

fn zirFieldParentPtr(sema: *Sema, block: *Block, extended: Zir.Inst.Extended.InstData) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const ip = &zcu.intern_pool;

    const extra = sema.code.extraData(Zir.Inst.FieldParentPtr, extended.operand).data;
    const FlagsInt = @typeInfo(Zir.Inst.FullPtrCastFlags).@"struct".backing_integer.?;
    const flags: Zir.Inst.FullPtrCastFlags = @bitCast(@as(FlagsInt, @truncate(extended.small)));
    assert(!flags.ptr_cast);
    const inst_src = block.nodeOffset(extra.src_node);
    const field_name_src = block.builtinCallArgSrc(extra.src_node, 0);
    const field_ptr_src = block.builtinCallArgSrc(extra.src_node, 1);

    const parent_ptr_ty = try sema.resolveDestType(block, inst_src, extra.parent_ptr_type, .remove_eu, "@fieldParentPtr");
    try sema.checkPtrType(block, inst_src, parent_ptr_ty, true);
    const parent_ptr_info = parent_ptr_ty.ptrInfo(zcu);
    if (parent_ptr_info.flags.size != .one) {
        return sema.fail(block, inst_src, "expected single pointer type, found '{}'", .{parent_ptr_ty.fmt(pt)});
    }
    const parent_ty: Type = .fromInterned(parent_ptr_info.child);
    switch (parent_ty.zigTypeTag(zcu)) {
        .@"struct", .@"union" => {},
        else => return sema.fail(block, inst_src, "expected pointer to struct or union type, found '{}'", .{parent_ptr_ty.fmt(pt)}),
    }
    try parent_ty.resolveLayout(pt);

    const field_name = try sema.resolveConstStringIntern(block, field_name_src, extra.field_name, .{ .simple = .field_name });
    const field_index = switch (parent_ty.zigTypeTag(zcu)) {
        .@"struct" => blk: {
            if (parent_ty.isTuple(zcu)) {
                if (field_name.eqlSlice("len", ip)) {
                    return sema.fail(block, inst_src, "cannot get @fieldParentPtr of 'len' field of tuple", .{});
                }
                break :blk try sema.tupleFieldIndex(block, parent_ty, field_name, field_name_src);
            } else {
                break :blk try sema.structFieldIndex(block, parent_ty, field_name, field_name_src);
            }
        },
        .@"union" => try sema.unionFieldIndex(block, parent_ty, field_name, field_name_src),
        else => unreachable,
    };
    if (parent_ty.zigTypeTag(zcu) == .@"struct" and parent_ty.structFieldIsComptime(field_index, zcu)) {
        return sema.fail(block, field_name_src, "cannot get @fieldParentPtr of a comptime field", .{});
    }

    const field_ptr = try sema.resolveInst(extra.field_ptr);
    const field_ptr_ty = sema.typeOf(field_ptr);
    try sema.checkPtrOperand(block, field_ptr_src, field_ptr_ty);
    const field_ptr_info = field_ptr_ty.ptrInfo(zcu);

    var actual_parent_ptr_info: InternPool.Key.PtrType = .{
        .child = parent_ty.toIntern(),
        .flags = .{
            .alignment = try parent_ptr_ty.ptrAlignmentSema(pt),
            .is_const = field_ptr_info.flags.is_const,
            .is_volatile = field_ptr_info.flags.is_volatile,
            .is_allowzero = field_ptr_info.flags.is_allowzero,
            .address_space = field_ptr_info.flags.address_space,
        },
        .packed_offset = parent_ptr_info.packed_offset,
    };
    const field_ty = parent_ty.fieldType(field_index, zcu);
    var actual_field_ptr_info: InternPool.Key.PtrType = .{
        .child = field_ty.toIntern(),
        .flags = .{
            .alignment = try field_ptr_ty.ptrAlignmentSema(pt),
            .is_const = field_ptr_info.flags.is_const,
            .is_volatile = field_ptr_info.flags.is_volatile,
            .is_allowzero = field_ptr_info.flags.is_allowzero,
            .address_space = field_ptr_info.flags.address_space,
        },
        .packed_offset = field_ptr_info.packed_offset,
    };
    switch (parent_ty.containerLayout(zcu)) {
        .auto => {
            actual_parent_ptr_info.flags.alignment = actual_field_ptr_info.flags.alignment.minStrict(
                if (zcu.typeToStruct(parent_ty)) |struct_obj|
                    try field_ty.structFieldAlignmentSema(
                        struct_obj.fieldAlign(ip, field_index),
                        struct_obj.layout,
                        pt,
                    )
                else if (zcu.typeToUnion(parent_ty)) |union_obj|
                    try field_ty.unionFieldAlignmentSema(
                        union_obj.fieldAlign(ip, field_index),
                        union_obj.flagsUnordered(ip).layout,
                        pt,
                    )
                else
                    actual_field_ptr_info.flags.alignment,
            );

            actual_parent_ptr_info.packed_offset = .{ .bit_offset = 0, .host_size = 0 };
            actual_field_ptr_info.packed_offset = .{ .bit_offset = 0, .host_size = 0 };
        },
        .@"extern" => {
            const field_offset = parent_ty.structFieldOffset(field_index, zcu);
            actual_parent_ptr_info.flags.alignment = actual_field_ptr_info.flags.alignment.minStrict(if (field_offset > 0)
                Alignment.fromLog2Units(@ctz(field_offset))
            else
                actual_field_ptr_info.flags.alignment);

            actual_parent_ptr_info.packed_offset = .{ .bit_offset = 0, .host_size = 0 };
            actual_field_ptr_info.packed_offset = .{ .bit_offset = 0, .host_size = 0 };
        },
        .@"packed" => {
            const byte_offset = std.math.divExact(u32, @abs(@as(i32, actual_parent_ptr_info.packed_offset.bit_offset) +
                (if (zcu.typeToStruct(parent_ty)) |struct_obj| pt.structPackedFieldBitOffset(struct_obj, field_index) else 0) -
                actual_field_ptr_info.packed_offset.bit_offset), 8) catch
                return sema.fail(block, inst_src, "pointer bit-offset mismatch", .{});
            actual_parent_ptr_info.flags.alignment = actual_field_ptr_info.flags.alignment.minStrict(if (byte_offset > 0)
                Alignment.fromLog2Units(@ctz(byte_offset))
            else
                actual_field_ptr_info.flags.alignment);
        },
    }

    const actual_field_ptr_ty = try pt.ptrTypeSema(actual_field_ptr_info);
    const casted_field_ptr = try sema.coerce(block, actual_field_ptr_ty, field_ptr, field_ptr_src);
    const actual_parent_ptr_ty = try pt.ptrTypeSema(actual_parent_ptr_info);

    const result = if (try sema.resolveDefinedValue(block, field_ptr_src, casted_field_ptr)) |field_ptr_val| result: {
        switch (parent_ty.zigTypeTag(zcu)) {
            .@"struct" => switch (parent_ty.containerLayout(zcu)) {
                .auto => {},
                .@"extern" => {
                    const byte_offset = parent_ty.structFieldOffset(field_index, zcu);
                    const parent_ptr_val = try sema.ptrSubtract(block, field_ptr_src, field_ptr_val, byte_offset, actual_parent_ptr_ty);
                    break :result Air.internedToRef(parent_ptr_val.toIntern());
                },
                .@"packed" => {
                    // Logic lifted from type computation above - I'm just assuming it's correct.
                    // `catch unreachable` since error case handled above.
                    const byte_offset = std.math.divExact(u32, @abs(@as(i32, actual_parent_ptr_info.packed_offset.bit_offset) +
                        pt.structPackedFieldBitOffset(zcu.typeToStruct(parent_ty).?, field_index) -
                        actual_field_ptr_info.packed_offset.bit_offset), 8) catch unreachable;
                    const parent_ptr_val = try sema.ptrSubtract(block, field_ptr_src, field_ptr_val, byte_offset, actual_parent_ptr_ty);
                    break :result Air.internedToRef(parent_ptr_val.toIntern());
                },
            },
            .@"union" => switch (parent_ty.containerLayout(zcu)) {
                .auto => {},
                .@"extern", .@"packed" => {
                    // For an extern or packed union, just coerce the pointer.
                    const parent_ptr_val = try pt.getCoerced(field_ptr_val, actual_parent_ptr_ty);
                    break :result Air.internedToRef(parent_ptr_val.toIntern());
                },
            },
            else => unreachable,
        }

        const opt_field: ?InternPool.Key.Ptr.BaseAddr.BaseIndex = opt_field: {
            const ptr = switch (ip.indexToKey(field_ptr_val.toIntern())) {
                .ptr => |ptr| ptr,
                else => break :opt_field null,
            };
            if (ptr.byte_offset != 0) break :opt_field null;
            break :opt_field switch (ptr.base_addr) {
                .field => |field| field,
                else => null,
            };
        };

        const field = opt_field orelse {
            return sema.fail(block, field_ptr_src, "pointer value not based on parent struct", .{});
        };

        if (Value.fromInterned(field.base).typeOf(zcu).childType(zcu).toIntern() != parent_ty.toIntern()) {
            return sema.fail(block, field_ptr_src, "pointer value not based on parent struct", .{});
        }

        if (field.index != field_index) {
            return sema.fail(block, inst_src, "field '{}' has index '{d}' but pointer value is index '{d}' of struct '{}'", .{
                field_name.fmt(ip), field_index, field.index, parent_ty.fmt(pt),
            });
        }
        break :result try sema.coerce(block, actual_parent_ptr_ty, Air.internedToRef(field.base), inst_src);
    } else result: {
        try sema.requireRuntimeBlock(block, inst_src, field_ptr_src);
        break :result try block.addInst(.{
            .tag = .field_parent_ptr,
            .data = .{ .ty_pl = .{
                .ty = Air.internedToRef(actual_parent_ptr_ty.toIntern()),
                .payload = try block.sema.addExtra(Air.FieldParentPtr{
                    .field_ptr = casted_field_ptr,
                    .field_index = @intCast(field_index),
                }),
            } },
        });
    };
    return sema.ptrCastFull(block, flags, inst_src, result, inst_src, parent_ptr_ty, "@fieldParentPtr");
}

fn ptrSubtract(sema: *Sema, block: *Block, src: LazySrcLoc, ptr_val: Value, byte_subtract: u64, new_ty: Type) !Value {
    const pt = sema.pt;
    const zcu = pt.zcu;
    if (byte_subtract == 0) return pt.getCoerced(ptr_val, new_ty);
    var ptr = switch (zcu.intern_pool.indexToKey(ptr_val.toIntern())) {
        .undef => return sema.failWithUseOfUndef(block, src),
        .ptr => |ptr| ptr,
        else => unreachable,
    };
    if (ptr.byte_offset < byte_subtract) {
        return sema.failWithOwnedErrorMsg(block, msg: {
            const msg = try sema.errMsg(src, "pointer computation here causes illegal behavior", .{});
            errdefer msg.destroy(sema.gpa);
            try sema.errNote(src, msg, "resulting pointer exceeds bounds of containing value which may trigger overflow", .{});
            break :msg msg;
        });
    }
    ptr.byte_offset -= byte_subtract;
    ptr.ty = new_ty.toIntern();
    return Value.fromInterned(try pt.intern(.{ .ptr = ptr }));
}

fn zirMinMax(
    sema: *Sema,
    block: *Block,
    inst: Zir.Inst.Index,
    comptime air_tag: Air.Inst.Tag,
) CompileError!Air.Inst.Ref {
    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].pl_node;
    const extra = sema.code.extraData(Zir.Inst.Bin, inst_data.payload_index).data;
    const src = block.nodeOffset(inst_data.src_node);
    const lhs_src = block.builtinCallArgSrc(inst_data.src_node, 0);
    const rhs_src = block.builtinCallArgSrc(inst_data.src_node, 1);
    const lhs = try sema.resolveInst(extra.lhs);
    const rhs = try sema.resolveInst(extra.rhs);
    return sema.analyzeMinMax(block, src, air_tag, &.{ lhs, rhs }, &.{ lhs_src, rhs_src });
}

fn zirMinMaxMulti(
    sema: *Sema,
    block: *Block,
    extended: Zir.Inst.Extended.InstData,
    comptime air_tag: Air.Inst.Tag,
) CompileError!Air.Inst.Ref {
    const extra = sema.code.extraData(Zir.Inst.NodeMultiOp, extended.operand);
    const src_node = extra.data.src_node;
    const src = block.nodeOffset(src_node);
    const operands = sema.code.refSlice(extra.end, extended.small);

    const air_refs = try sema.arena.alloc(Air.Inst.Ref, operands.len);
    const operand_srcs = try sema.arena.alloc(LazySrcLoc, operands.len);

    for (operands, air_refs, operand_srcs, 0..) |zir_ref, *air_ref, *op_src, i| {
        op_src.* = block.builtinCallArgSrc(src_node, @intCast(i));
        air_ref.* = try sema.resolveInst(zir_ref);
    }

    return sema.analyzeMinMax(block, src, air_tag, air_refs, operand_srcs);
}

fn analyzeMinMax(
    sema: *Sema,
    block: *Block,
    src: LazySrcLoc,
    comptime air_tag: Air.Inst.Tag,
    operands: []const Air.Inst.Ref,
    operand_srcs: []const LazySrcLoc,
) CompileError!Air.Inst.Ref {
    assert(operands.len == operand_srcs.len);
    assert(operands.len > 0);

    const pt = sema.pt;
    const zcu = pt.zcu;

    // This function has the signature `fn (Value, Value, *Zcu) Value`.
    // It is only used on scalar values, although the values may have different types.
    // If either operand is undef, it returns undef.
    const opFunc = switch (air_tag) {
        .min => Value.numberMin,
        .max => Value.numberMax,
        else => comptime unreachable,
    };

    if (operands.len == 1) {
        try sema.checkNumericType(block, operand_srcs[0], sema.typeOf(operands[0]));
        return operands[0];
    }

    // First, basic type validation; we'll make sure all the operands are numeric and agree on vector length.
    // This value will be `null` for a scalar type, otherwise the length of the vector type.
    const vector_len: ?u64 = vec_len: {
        const first_operand_ty = sema.typeOf(operands[0]);
        try sema.checkNumericType(block, operand_srcs[0], first_operand_ty);
        if (first_operand_ty.zigTypeTag(zcu) == .vector) {
            const vec_len = first_operand_ty.vectorLen(zcu);
            for (operands[1..], operand_srcs[1..]) |operand, operand_src| {
                const operand_ty = sema.typeOf(operand);
                try sema.checkNumericType(block, operand_src, operand_ty);
                if (operand_ty.zigTypeTag(zcu) != .vector) {
                    return sema.failWithOwnedErrorMsg(block, msg: {
                        const msg = try sema.errMsg(operand_src, "expected vector, found '{}'", .{operand_ty.fmt(pt)});
                        errdefer msg.destroy(zcu.gpa);
                        try sema.errNote(operand_srcs[0], msg, "vector operand here", .{});
                        break :msg msg;
                    });
                }
                if (operand_ty.vectorLen(zcu) != vec_len) {
                    return sema.failWithOwnedErrorMsg(block, msg: {
                        const msg = try sema.errMsg(operand_src, "expected vector of length '{d}', found '{}'", .{ vec_len, operand_ty.fmt(pt) });
                        errdefer msg.destroy(zcu.gpa);
                        try sema.errNote(operand_srcs[0], msg, "vector of length '{d}' here", .{vec_len});
                        break :msg msg;
                    });
                }
            }
            break :vec_len vec_len;
        } else {
            for (operands[1..], operand_srcs[1..]) |operand, operand_src| {
                const operand_ty = sema.typeOf(operand);
                if (operand_ty.zigTypeTag(zcu) == .vector) {
                    return sema.failWithOwnedErrorMsg(block, msg: {
                        const msg = try sema.errMsg(operand_srcs[0], "expected vector, found '{}'", .{first_operand_ty.fmt(pt)});
                        errdefer msg.destroy(zcu.gpa);
                        try sema.errNote(operand_src, msg, "vector operand here", .{});
                        break :msg msg;
                    });
                }
            }
            break :vec_len null;
        }
    };

    // Now we want to look at the scalar types. If any is a float, our result will be a float. This
    // union is in "priority" order: `float` overrides `comptime_float` overrides `int`.
    const TypeStrat = union(enum) {
        float: Type,
        comptime_float,
        int: struct {
            /// If this is still `true` at the end, we will just use a `comptime_int`.
            all_comptime_int: bool,
            // These two fields tells us about the *result* type, which is refined based on operand types.
            // e.g. `@max(u32, i64)` results in a `u63`, because the result is >=0 and <=maxInt(i64).
            result_min: Value,
            result_max: Value,
            // These two fields tell us the *intermediate* type to use for actually computing the min/max.
            // e.g. `@max(u32, i64)` uses an intermediate `i64`, because it can fit all our operands.
            operand_min: Value,
            operand_max: Value,
        },
        none,
    };
    var cur_strat: TypeStrat = .none;
    for (operands) |operand| {
        const operand_scalar_ty = sema.typeOf(operand).scalarType(zcu);
        const want_strat: TypeStrat = switch (operand_scalar_ty.zigTypeTag(zcu)) {
            .comptime_int => s: {
                const val = (try sema.resolveValueResolveLazy(operand)).?;
                if (val.isUndef(zcu)) break :s .none;
                break :s .{ .int = .{
                    .all_comptime_int = true,
                    .result_min = val,
                    .result_max = val,
                    .operand_min = val,
                    .operand_max = val,
                } };
            },
            .comptime_float => .comptime_float,
            .float => .{ .float = operand_scalar_ty },
            .int => s: {
                // If the *value* is comptime-known, we will use that to get tighter bounds. If #3806
                // is accepted and implemented, so that integer literals have a tightly-bounded ranged
                // integer type (and `comptime_int` ceases to exist), this block should probably go away
                // (replaced with just the simple calls to `Type.minInt`/`Type.maxInt`) so that we only
                // use the input *types* to determine the result type.
                const min: Value, const max: Value = bounds: {
                    if (try sema.resolveValueResolveLazy(operand)) |operand_val| {
                        if (vector_len) |len| {
                            var min = try operand_val.elemValue(pt, 0);
                            var max = min;
                            for (1..@intCast(len)) |elem_idx| {
                                const elem_val = try operand_val.elemValue(pt, elem_idx);
                                min = Value.numberMin(min, elem_val, zcu);
                                max = Value.numberMax(max, elem_val, zcu);
                            }
                            if (!min.isUndef(zcu) and !max.isUndef(zcu)) {
                                break :bounds .{ min, max };
                            }
                        } else {
                            if (!operand_val.isUndef(zcu)) {
                                break :bounds .{ operand_val, operand_val };
                            }
                        }
                    }
                    break :bounds .{
                        try operand_scalar_ty.minInt(pt, operand_scalar_ty),
                        try operand_scalar_ty.maxInt(pt, operand_scalar_ty),
                    };
                };
                break :s .{ .int = .{
                    .all_comptime_int = false,
                    .result_min = min,
                    .result_max = max,
                    .operand_min = min,
                    .operand_max = max,
                } };
            },
            else => unreachable,
        };
        if (@intFromEnum(want_strat) < @intFromEnum(cur_strat)) {
            // `want_strat` overrides `cur_strat`.
            cur_strat = want_strat;
        } else if (@intFromEnum(want_strat) == @intFromEnum(cur_strat)) {
            // The behavior depends on the tag.
            switch (cur_strat) {
                .none, .comptime_float => {}, // no payload, so nop
                .float => |cur_float| {
                    const want_float = want_strat.float;
                    // Select the larger bit size. If the bit size is the same, select whichever is not c_longdouble.
                    const cur_bits = cur_float.floatBits(zcu.getTarget());
                    const want_bits = want_float.floatBits(zcu.getTarget());
                    if (want_bits > cur_bits or
                        (want_bits == cur_bits and
                            cur_float.toIntern() == .c_longdouble_type and
                            want_float.toIntern() != .c_longdouble_type))
                    {
                        cur_strat = want_strat;
                    }
                },
                .int => |*cur_int| {
                    const want_int = want_strat.int;
                    if (!want_int.all_comptime_int) cur_int.all_comptime_int = false;
                    cur_int.result_min = opFunc(cur_int.result_min, want_int.result_min, zcu);
                    cur_int.result_max = opFunc(cur_int.result_max, want_int.result_max, zcu);
                    cur_int.operand_min = Value.numberMin(cur_int.operand_min, want_int.operand_min, zcu);
                    cur_int.operand_max = Value.numberMax(cur_int.operand_max, want_int.operand_max, zcu);
                },
            }
        }
    }

    // Use `cur_strat` to actually resolve the result type (and intermediate type).
    const result_scalar_ty: Type, const intermediate_scalar_ty: Type = switch (cur_strat) {
        .float => |ty| .{ ty, ty },
        .comptime_float => .{ .comptime_float, .comptime_float },
        .int => |int| if (int.all_comptime_int) .{
            .comptime_int,
            .comptime_int,
        } else .{
            try pt.intFittingRange(int.result_min, int.result_max),
            try pt.intFittingRange(int.operand_min, int.operand_max),
        },
        .none => .{ .comptime_int, .comptime_int }, // all undef comptime ints
    };
    const result_ty: Type = if (vector_len) |l| try pt.vectorType(.{
        .len = @intCast(l),
        .child = result_scalar_ty.toIntern(),
    }) else result_scalar_ty;
    const intermediate_ty: Type = if (vector_len) |l| try pt.vectorType(.{
        .len = @intCast(l),
        .child = intermediate_scalar_ty.toIntern(),
    }) else intermediate_scalar_ty;

    // This value, if not `null`, will have type `intermediate_ty`.
    const comptime_part: ?Value = ct: {
        // Contains the comptime-known scalar result values.
        // Values are scalars with no particular type.
        // `elems.len` is `vector_len orelse 1`.
        const elems: []InternPool.Index = try sema.arena.alloc(
            InternPool.Index,
            try sema.usizeCast(block, src, vector_len orelse 1),
        );
        // If `false`, we've not seen any comptime-known operand yet, so `elems` contains `undefined`.
        // Otherwise, `elems` is populated with the comptime-known results so far.
        var elems_populated = false;
        // Populated when we see a runtime-known operand.
        var opt_runtime_src: ?LazySrcLoc = null;

        for (operands, operand_srcs) |operand, operand_src| {
            const operand_val = try sema.resolveValueResolveLazy(operand) orelse {
                if (opt_runtime_src == null) opt_runtime_src = operand_src;
                continue;
            };
            if (vector_len) |len| {
                // Vector case; apply `opFunc` to each element.
                if (elems_populated) {
                    for (elems, 0..@intCast(len)) |*elem, elem_idx| {
                        const new_elem = try operand_val.elemValue(pt, elem_idx);
                        elem.* = opFunc(.fromInterned(elem.*), new_elem, zcu).toIntern();
                    }
                } else {
                    elems_populated = true;
                    for (elems, 0..@intCast(len)) |*elem_out, elem_idx| {
                        elem_out.* = (try operand_val.elemValue(pt, elem_idx)).toIntern();
                    }
                }
            } else {
                // Scalar case; just apply `opFunc`.
                if (elems_populated) {
                    elems[0] = opFunc(.fromInterned(elems[0]), operand_val, zcu).toIntern();
                } else {
                    elems_populated = true;
                    elems[0] = operand_val.toIntern();
                }
            }
        }
        const runtime_src = opt_runtime_src orelse {
            // The result is comptime-known. Coerce each element to its scalar type.
            assert(elems_populated);
            for (elems) |*elem| {
                if (Value.fromInterned(elem.*).isUndef(zcu)) {
                    elem.* = (try pt.undefValue(result_scalar_ty)).toIntern();
                } else {
                    // This coercion will always succeed, because `result_scalar_ty` can definitely hold the result.
                    const coerced_ref = try sema.coerce(block, result_scalar_ty, Air.internedToRef(elem.*), .unneeded);
                    elem.* = coerced_ref.toInterned().?;
                }
            }
            if (vector_len == null) return Air.internedToRef(elems[0]);
            return Air.internedToRef(try pt.intern(.{ .aggregate = .{
                .ty = result_ty.toIntern(),
                .storage = .{ .elems = elems },
            } }));
        };
        _ = runtime_src;
        // The result is runtime-known.
        // Coerce each element to the intermediate scalar type, unless there were no comptime-known operands.
        if (!elems_populated) break :ct null;
        for (elems) |*elem| {
            if (Value.fromInterned(elem.*).isUndef(zcu)) {
                elem.* = (try pt.undefValue(intermediate_scalar_ty)).toIntern();
            } else {
                // This coercion will always succeed, because `intermediate_scalar_ty` can definitely hold all operands.
                const coerced_ref = try sema.coerce(block, intermediate_scalar_ty, Air.internedToRef(elem.*), .unneeded);
                elem.* = coerced_ref.toInterned().?;
            }
        }
        break :ct .fromInterned(if (vector_len != null) try pt.intern(.{ .aggregate = .{
            .ty = intermediate_ty.toIntern(),
            .storage = .{ .elems = elems },
        } }) else elems[0]);
    };

    // Time to emit the runtime operations. All runtime-known peers are coerced to `intermediate_ty`, and we cast down to `result_ty` at the end.

    // `.none` indicates no result so far.
    var cur_result: Air.Inst.Ref = if (comptime_part) |val| Air.internedToRef(val.toIntern()) else .none;
    for (operands, operand_srcs) |operand, operand_src| {
        if (try sema.isComptimeKnown(operand)) continue; // already in `comptime_part`
        // This coercion could fail; e.g. coercing a runtime integer peer to a `comptime_float` in a case like `@min(runtime_int, 1.5)`.
        const operand_coerced = try sema.coerce(block, intermediate_ty, operand, operand_src);
        if (cur_result == .none) {
            cur_result = operand_coerced;
        } else {
            cur_result = try block.addBinOp(air_tag, cur_result, operand_coerced);
        }
    }

    assert(cur_result != .none);
    assert(sema.typeOf(cur_result).toIntern() == intermediate_ty.toIntern());

    // If there is a comptime-known undef operand, we actually return comptime-known undef -- but we had to do the runtime stuff to check for coercion errors.
    if (comptime_part) |val| {
        if (val.isUndefDeep(zcu)) {
            return pt.undefRef(result_ty);
        }
    }

    if (result_ty.toIntern() == intermediate_ty.toIntern()) {
        // No final cast needed; we're all done.
        return cur_result;
    }

    // A final cast is needed. The only case where `intermediate_ty` is different is for integers,
    // where we have refined the range, so we should be doing an intcast.
    assert(intermediate_scalar_ty.zigTypeTag(zcu) == .int);
    assert(result_scalar_ty.zigTypeTag(zcu) == .int);
    return block.addTyOp(.intcast, result_ty, cur_result);
}

fn upgradeToArrayPtr(sema: *Sema, block: *Block, ptr: Air.Inst.Ref, len: u64) !Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const ptr_ty = sema.typeOf(ptr);
    const info = ptr_ty.ptrInfo(zcu);
    if (info.flags.size == .one) {
        // Already an array pointer.
        return ptr;
    }
    const new_ty = try pt.ptrTypeSema(.{
        .child = (try pt.arrayType(.{
            .len = len,
            .sentinel = info.sentinel,
            .child = info.child,
        })).toIntern(),
        .flags = .{
            .alignment = info.flags.alignment,
            .is_const = info.flags.is_const,
            .is_volatile = info.flags.is_volatile,
            .is_allowzero = info.flags.is_allowzero,
            .address_space = info.flags.address_space,
        },
    });
    const non_slice_ptr = if (info.flags.size == .slice)
        try block.addTyOp(.slice_ptr, ptr_ty.slicePtrFieldType(zcu), ptr)
    else
        ptr;
    return block.addBitCast(new_ty, non_slice_ptr);
}

fn zirMemcpy(
    sema: *Sema,
    block: *Block,
    inst: Zir.Inst.Index,
    air_tag: Air.Inst.Tag,
    check_aliasing: bool,
) CompileError!void {
    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].pl_node;
    const extra = sema.code.extraData(Zir.Inst.Bin, inst_data.payload_index).data;
    const src = block.nodeOffset(inst_data.src_node);
    const dest_src = block.builtinCallArgSrc(inst_data.src_node, 0);
    const src_src = block.builtinCallArgSrc(inst_data.src_node, 1);
    const dest_ptr = try sema.resolveInst(extra.lhs);
    const src_ptr = try sema.resolveInst(extra.rhs);
    const dest_ty = sema.typeOf(dest_ptr);
    const src_ty = sema.typeOf(src_ptr);
    const dest_len = try indexablePtrLenOrNone(sema, block, dest_src, dest_ptr);
    const src_len = try indexablePtrLenOrNone(sema, block, src_src, src_ptr);
    const pt = sema.pt;
    const zcu = pt.zcu;

    if (dest_ty.isConstPtr(zcu)) {
        return sema.fail(block, dest_src, "cannot copy to constant pointer", .{});
    }

    if (dest_len == .none and src_len == .none) {
        const msg = msg: {
            const msg = try sema.errMsg(src, "unknown copy length", .{});
            errdefer msg.destroy(sema.gpa);
            try sema.errNote(dest_src, msg, "destination type '{}' provides no length", .{
                dest_ty.fmt(pt),
            });
            try sema.errNote(src_src, msg, "source type '{}' provides no length", .{
                src_ty.fmt(pt),
            });
            break :msg msg;
        };
        return sema.failWithOwnedErrorMsg(block, msg);
    }

    const dest_elem_ty = dest_ty.indexablePtrElem(zcu);
    const src_elem_ty = src_ty.indexablePtrElem(zcu);

    const imc = try sema.coerceInMemoryAllowed(
        block,
        dest_elem_ty,
        src_elem_ty,
        false,
        zcu.getTarget(),
        dest_src,
        src_src,
        null,
    );
    if (imc != .ok) return sema.failWithOwnedErrorMsg(block, msg: {
        const msg = try sema.errMsg(
            src,
            "pointer element type '{}' cannot coerce into element type '{}'",
            .{ src_elem_ty.fmt(pt), dest_elem_ty.fmt(pt) },
        );
        errdefer msg.destroy(sema.gpa);
        try imc.report(sema, src, msg);
        break :msg msg;
    });

    var len_val: ?Value = null;

    if (dest_len != .none and src_len != .none) check: {
        // If we can check at compile-time, no need for runtime safety.
        if (try sema.resolveDefinedValue(block, dest_src, dest_len)) |dest_len_val| {
            len_val = dest_len_val;
            if (try sema.resolveDefinedValue(block, src_src, src_len)) |src_len_val| {
                if (!(try sema.valuesEqual(dest_len_val, src_len_val, .usize))) {
                    const msg = msg: {
                        const msg = try sema.errMsg(src, "non-matching copy lengths", .{});
                        errdefer msg.destroy(sema.gpa);
                        try sema.errNote(dest_src, msg, "length {} here", .{
                            dest_len_val.fmtValueSema(pt, sema),
                        });
                        try sema.errNote(src_src, msg, "length {} here", .{
                            src_len_val.fmtValueSema(pt, sema),
                        });
                        break :msg msg;
                    };
                    return sema.failWithOwnedErrorMsg(block, msg);
                }
                break :check;
            }
        } else if (try sema.resolveDefinedValue(block, src_src, src_len)) |src_len_val| {
            len_val = src_len_val;
        }

        if (block.wantSafety()) {
            const ok = try block.addBinOp(.cmp_eq, dest_len, src_len);
            try sema.addSafetyCheck(block, src, ok, .copy_len_mismatch);
        }
    } else if (dest_len != .none) {
        if (try sema.resolveDefinedValue(block, dest_src, dest_len)) |dest_len_val| {
            len_val = dest_len_val;
        }
    } else if (src_len != .none) {
        if (try sema.resolveDefinedValue(block, src_src, src_len)) |src_len_val| {
            len_val = src_len_val;
        }
    }

    zero_bit: {
        const src_comptime = try src_elem_ty.comptimeOnlySema(pt);
        const dest_comptime = try dest_elem_ty.comptimeOnlySema(pt);
        assert(src_comptime == dest_comptime); // IMC
        if (src_comptime) break :zero_bit;

        const src_has_bits = try src_elem_ty.hasRuntimeBitsIgnoreComptimeSema(pt);
        const dest_has_bits = try dest_elem_ty.hasRuntimeBitsIgnoreComptimeSema(pt);
        assert(src_has_bits == dest_has_bits); // IMC
        if (src_has_bits) break :zero_bit;

        // The element type is zero-bit. We've done all validation (aside from the aliasing check,
        // which we must skip) so we're done.
        return;
    }

    const runtime_src = rs: {
        const dest_ptr_val = try sema.resolveDefinedValue(block, dest_src, dest_ptr) orelse break :rs dest_src;
        const src_ptr_val = try sema.resolveDefinedValue(block, src_src, src_ptr) orelse break :rs src_src;

        const raw_dest_ptr = if (dest_ty.isSlice(zcu)) dest_ptr_val.slicePtr(zcu) else dest_ptr_val;
        const raw_src_ptr = if (src_ty.isSlice(zcu)) src_ptr_val.slicePtr(zcu) else src_ptr_val;

        const len_u64 = try len_val.?.toUnsignedIntSema(pt);

        if (check_aliasing) {
            if (Value.doPointersOverlap(
                raw_src_ptr,
                raw_dest_ptr,
                len_u64,
                zcu,
            )) return sema.fail(block, src, "'@memcpy' arguments alias", .{});
        }

        if (!sema.isComptimeMutablePtr(dest_ptr_val)) break :rs dest_src;

        // Because comptime pointer access is a somewhat expensive operation, we implement @memcpy
        // as one load and store of an array, rather than N loads and stores of individual elements.

        const array_ty = try pt.arrayType(.{
            .child = dest_elem_ty.toIntern(),
            .len = len_u64,
        });

        const dest_array_ptr_ty = try pt.ptrType(info: {
            var info = dest_ty.ptrInfo(zcu);
            info.flags.size = .one;
            info.child = array_ty.toIntern();
            info.sentinel = .none;
            break :info info;
        });
        const src_array_ptr_ty = try pt.ptrType(info: {
            var info = src_ty.ptrInfo(zcu);
            info.flags.size = .one;
            info.child = array_ty.toIntern();
            info.sentinel = .none;
            break :info info;
        });

        const coerced_dest_ptr = try pt.getCoerced(raw_dest_ptr, dest_array_ptr_ty);
        const coerced_src_ptr = try pt.getCoerced(raw_src_ptr, src_array_ptr_ty);

        const array_val = try sema.pointerDeref(block, src_src, coerced_src_ptr, src_array_ptr_ty) orelse break :rs src_src;
        try sema.storePtrVal(block, dest_src, coerced_dest_ptr, array_val, array_ty);
        return;
    };

    // If the length is comptime-known, then upgrade src and destination types
    // into pointer-to-array. At this point we know they are both pointers
    // already.
    var new_dest_ptr = dest_ptr;
    var new_src_ptr = src_ptr;
    if (len_val) |val| {
        const len = try val.toUnsignedIntSema(pt);
        if (len == 0) {
            // This AIR instruction guarantees length > 0 if it is comptime-known.
            return;
        }
        new_dest_ptr = try upgradeToArrayPtr(sema, block, dest_ptr, len);
        new_src_ptr = try upgradeToArrayPtr(sema, block, src_ptr, len);
    }

    if (dest_len != .none) {
        // Change the src from slice to a many pointer, to avoid multiple ptr
        // slice extractions in AIR instructions.
        const new_src_ptr_ty = sema.typeOf(new_src_ptr);
        if (new_src_ptr_ty.isSlice(zcu)) {
            new_src_ptr = try sema.analyzeSlicePtr(block, src_src, new_src_ptr, new_src_ptr_ty);
        }
    } else if (dest_len == .none and len_val == null) {
        // Change the dest to a slice, since its type must have the length.
        const dest_ptr_ptr = try sema.analyzeRef(block, dest_src, new_dest_ptr);
        new_dest_ptr = try sema.analyzeSlice(block, dest_src, dest_ptr_ptr, .zero, src_len, .none, LazySrcLoc.unneeded, dest_src, dest_src, dest_src, false);
        const new_src_ptr_ty = sema.typeOf(new_src_ptr);
        if (new_src_ptr_ty.isSlice(zcu)) {
            new_src_ptr = try sema.analyzeSlicePtr(block, src_src, new_src_ptr, new_src_ptr_ty);
        }
    }

    try sema.requireRuntimeBlock(block, src, runtime_src);
    try sema.validateRuntimeValue(block, dest_src, dest_ptr);
    try sema.validateRuntimeValue(block, src_src, src_ptr);

    // Aliasing safety check.
    if (check_aliasing and block.wantSafety()) {
        const len = if (len_val) |v|
            Air.internedToRef(v.toIntern())
        else if (dest_len != .none)
            dest_len
        else
            src_len;

        // Extract raw pointer from dest slice. The AIR instructions could support them, but
        // it would cause redundant machine code instructions.
        const new_dest_ptr_ty = sema.typeOf(new_dest_ptr);
        const raw_dest_ptr = if (new_dest_ptr_ty.isSlice(zcu))
            try sema.analyzeSlicePtr(block, dest_src, new_dest_ptr, new_dest_ptr_ty)
        else if (new_dest_ptr_ty.ptrSize(zcu) == .one) ptr: {
            var dest_manyptr_ty_key = zcu.intern_pool.indexToKey(new_dest_ptr_ty.toIntern()).ptr_type;
            assert(dest_manyptr_ty_key.flags.size == .one);
            dest_manyptr_ty_key.child = dest_elem_ty.toIntern();
            dest_manyptr_ty_key.flags.size = .many;
            break :ptr try sema.coerceCompatiblePtrs(block, try pt.ptrTypeSema(dest_manyptr_ty_key), new_dest_ptr, dest_src);
        } else new_dest_ptr;

        const new_src_ptr_ty = sema.typeOf(new_src_ptr);
        const raw_src_ptr = if (new_src_ptr_ty.isSlice(zcu))
            try sema.analyzeSlicePtr(block, src_src, new_src_ptr, new_src_ptr_ty)
        else if (new_src_ptr_ty.ptrSize(zcu) == .one) ptr: {
            var src_manyptr_ty_key = zcu.intern_pool.indexToKey(new_src_ptr_ty.toIntern()).ptr_type;
            assert(src_manyptr_ty_key.flags.size == .one);
            src_manyptr_ty_key.child = src_elem_ty.toIntern();
            src_manyptr_ty_key.flags.size = .many;
            break :ptr try sema.coerceCompatiblePtrs(block, try pt.ptrTypeSema(src_manyptr_ty_key), new_src_ptr, src_src);
        } else new_src_ptr;

        // ok1: dest >= src + len
        // ok2: src >= dest + len
        const src_plus_len = try sema.analyzePtrArithmetic(block, src, raw_src_ptr, len, .ptr_add, src_src, src);
        const dest_plus_len = try sema.analyzePtrArithmetic(block, src, raw_dest_ptr, len, .ptr_add, dest_src, src);
        const ok1 = try block.addBinOp(.cmp_gte, raw_dest_ptr, src_plus_len);
        const ok2 = try block.addBinOp(.cmp_gte, new_src_ptr, dest_plus_len);
        const ok = try block.addBinOp(.bool_or, ok1, ok2);
        try sema.addSafetyCheck(block, src, ok, .memcpy_alias);
    }

    _ = try block.addInst(.{
        .tag = air_tag,
        .data = .{ .bin_op = .{
            .lhs = new_dest_ptr,
            .rhs = new_src_ptr,
        } },
    });
}

fn zirMemset(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!void {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const gpa = sema.gpa;
    const ip = &zcu.intern_pool;
    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].pl_node;
    const extra = sema.code.extraData(Zir.Inst.Bin, inst_data.payload_index).data;
    const src = block.nodeOffset(inst_data.src_node);
    const dest_src = block.builtinCallArgSrc(inst_data.src_node, 0);
    const value_src = block.builtinCallArgSrc(inst_data.src_node, 1);
    const dest_ptr = try sema.resolveInst(extra.lhs);
    const uncoerced_elem = try sema.resolveInst(extra.rhs);
    const dest_ptr_ty = sema.typeOf(dest_ptr);
    try checkMemOperand(sema, block, dest_src, dest_ptr_ty);

    if (dest_ptr_ty.isConstPtr(zcu)) {
        return sema.fail(block, dest_src, "cannot memset constant pointer", .{});
    }

    const dest_elem_ty: Type = dest_elem_ty: {
        const ptr_info = dest_ptr_ty.ptrInfo(zcu);
        switch (ptr_info.flags.size) {
            .slice => break :dest_elem_ty .fromInterned(ptr_info.child),
            .one => {
                if (Type.fromInterned(ptr_info.child).zigTypeTag(zcu) == .array) {
                    break :dest_elem_ty Type.fromInterned(ptr_info.child).childType(zcu);
                }
            },
            .many, .c => {},
        }
        return sema.failWithOwnedErrorMsg(block, msg: {
            const msg = try sema.errMsg(src, "unknown @memset length", .{});
            errdefer msg.destroy(sema.gpa);
            try sema.errNote(dest_src, msg, "destination type '{}' provides no length", .{
                dest_ptr_ty.fmt(pt),
            });
            break :msg msg;
        });
    };

    const elem = try sema.coerce(block, dest_elem_ty, uncoerced_elem, value_src);

    const runtime_src = rs: {
        const ptr_val = try sema.resolveDefinedValue(block, dest_src, dest_ptr) orelse break :rs dest_src;
        const len_air_ref = try sema.fieldVal(block, src, dest_ptr, try ip.getOrPutString(gpa, pt.tid, "len", .no_embedded_nulls), dest_src);
        const len_val = (try sema.resolveDefinedValue(block, dest_src, len_air_ref)) orelse break :rs dest_src;
        const len_u64 = try len_val.toUnsignedIntSema(pt);
        const len = try sema.usizeCast(block, dest_src, len_u64);
        if (len == 0) {
            // This AIR instruction guarantees length > 0 if it is comptime-known.
            return;
        }

        if (!sema.isComptimeMutablePtr(ptr_val)) break :rs dest_src;
        const elem_val = try sema.resolveValue(elem) orelse break :rs value_src;
        const array_ty = try pt.arrayType(.{
            .child = dest_elem_ty.toIntern(),
            .len = len_u64,
        });
        const array_val = Value.fromInterned(try pt.intern(.{ .aggregate = .{
            .ty = array_ty.toIntern(),
            .storage = .{ .repeated_elem = elem_val.toIntern() },
        } }));
        const array_ptr_ty = ty: {
            var info = dest_ptr_ty.ptrInfo(zcu);
            info.flags.size = .one;
            info.child = array_ty.toIntern();
            break :ty try pt.ptrType(info);
        };
        const raw_ptr_val = if (dest_ptr_ty.isSlice(zcu)) ptr_val.slicePtr(zcu) else ptr_val;
        const array_ptr_val = try pt.getCoerced(raw_ptr_val, array_ptr_ty);
        return sema.storePtrVal(block, src, array_ptr_val, array_val, array_ty);
    };

    try sema.requireRuntimeBlock(block, src, runtime_src);
    try sema.validateRuntimeValue(block, dest_src, dest_ptr);
    try sema.validateRuntimeValue(block, value_src, elem);

    _ = try block.addInst(.{
        .tag = if (block.wantSafety()) .memset_safe else .memset,
        .data = .{ .bin_op = .{
            .lhs = dest_ptr,
            .rhs = elem,
        } },
    });
}

fn zirBuiltinAsyncCall(sema: *Sema, block: *Block, extended: Zir.Inst.Extended.InstData) CompileError!Air.Inst.Ref {
    const extra = sema.code.extraData(Zir.Inst.UnNode, extended.operand).data;
    const src = block.nodeOffset(extra.node);
    return sema.failWithUseOfAsync(block, src);
}

fn zirResume(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].un_node;
    const src = block.nodeOffset(inst_data.src_node);
    return sema.failWithUseOfAsync(block, src);
}

fn zirAwait(
    sema: *Sema,
    block: *Block,
    inst: Zir.Inst.Index,
) CompileError!Air.Inst.Ref {
    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].un_node;
    const src = block.nodeOffset(inst_data.src_node);

    return sema.failWithUseOfAsync(block, src);
}

fn zirAwaitNosuspend(
    sema: *Sema,
    block: *Block,
    extended: Zir.Inst.Extended.InstData,
) CompileError!Air.Inst.Ref {
    const extra = sema.code.extraData(Zir.Inst.UnNode, extended.operand).data;
    const src = block.nodeOffset(extra.node);

    return sema.failWithUseOfAsync(block, src);
}

fn zirFuncFancy(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
    const tracy = trace(@src());
    defer tracy.end();

    const pt = sema.pt;
    const zcu = pt.zcu;
    const ip = &zcu.intern_pool;
    const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].pl_node;
    const extra = sema.code.extraData(Zir.Inst.FuncFancy, inst_data.payload_index);
    const target = zcu.getTarget();

    const cc_src = block.src(.{ .node_offset_fn_type_cc = inst_data.src_node });
    const ret_src = block.src(.{ .node_offset_fn_type_ret_ty = inst_data.src_node });
    const has_body = extra.data.body_len != 0;

    var extra_index: usize = extra.end;

    const cc: std.builtin.CallingConvention = if (extra.data.bits.has_cc_body) blk: {
        const body_len = sema.code.extra[extra_index];
        extra_index += 1;
        const body = sema.code.bodySlice(extra_index, body_len);
        extra_index += body.len;

        const cc_ty = try sema.getBuiltinType(cc_src, .CallingConvention);
        const val = try sema.resolveGenericBody(block, cc_src, body, inst, cc_ty, .{ .simple = .@"callconv" });
        break :blk try sema.analyzeValueAsCallconv(block, cc_src, val);
    } else if (extra.data.bits.has_cc_ref) blk: {
        const cc_ref: Zir.Inst.Ref = @enumFromInt(sema.code.extra[extra_index]);
        extra_index += 1;
        const cc_ty = try sema.getBuiltinType(cc_src, .CallingConvention);
        const uncoerced_cc = try sema.resolveInst(cc_ref);
        const coerced_cc = try sema.coerce(block, cc_ty, uncoerced_cc, cc_src);
        const cc_val = try sema.resolveConstDefinedValue(block, cc_src, coerced_cc, .{ .simple = .@"callconv" });
        break :blk try sema.analyzeValueAsCallconv(block, cc_src, cc_val);
    } else cc: {
        if (has_body) {
            const func_decl_nav = sema.owner.unwrap().nav_val;
            const func_decl_inst = ip.getNav(func_decl_nav).analysis.?.zir_index.resolve(&zcu.intern_pool) orelse return error.AnalysisFail;
            const zir_decl = sema.code.getDeclaration(func_decl_inst);
            if (zir_decl.linkage == .@"export") {
                break :cc target.cCallingConvention() orelse {
                    // This target has no default C calling convention. We sometimes trigger a similar
                    // error by trying to evaluate `std.builtin.CallingConvention.c`, so for consistency,
                    // let's eval that now and just get the transitive error. (It's guaranteed to error
                    // because it does the exact `cCallingConvention` call we just did.)
                    const cc_type = try sema.getBuiltinType(cc_src, .CallingConvention);
                    _ = try sema.namespaceLookupVal(
                        block,
                        LazySrcLoc.unneeded,
                        cc_type.getNamespaceIndex(zcu),
                        try ip.getOrPutString(sema.gpa, pt.tid, "c", .no_embedded_nulls),
                    );
                    // The above should have errored.
                    @panic("std.builtin is corrupt");
                };
            }
        }
        break :cc .auto;
    };

    const ret_ty: Type = if (extra.data.bits.has_ret_ty_body) blk: {
        const body_len = sema.code.extra[extra_index];
        extra_index += 1;
        const body = sema.code.bodySlice(extra_index, body_len);
        extra_index += body.len;
        if (extra.data.bits.ret_ty_is_generic) break :blk .generic_poison;

        const val = try sema.resolveGenericBody(block, ret_src, body, inst, .type, .{ .simple = .function_ret_ty });
        const ty = val.toType();
        break :blk ty;
    } else if (extra.data.bits.has_ret_ty_ref) blk: {
        const ret_ty_ref: Zir.Inst.Ref = @enumFromInt(sema.code.extra[extra_index]);
        extra_index += 1;
        if (extra.data.bits.ret_ty_is_generic) break :blk .generic_poison;

        const ret_ty_air_ref = try sema.resolveInst(ret_ty_ref);
        const ret_ty_val = try sema.resolveConstDefinedValue(block, ret_src, ret_ty_air_ref, .{ .simple = .function_ret_ty });
        break :blk ret_ty_val.toType();
    } else .void;

    const noalias_bits: u32 = if (extra.data.bits.has_any_noalias) blk: {
        const x = sema.code.extra[extra_index];
        extra_index += 1;
        break :blk x;
    } else 0;

    var src_locs: Zir.Inst.Func.SrcLocs = undefined;
    if (has_body) {
        extra_index += extra.data.body_len;
        src_locs = sema.code.extraData(Zir.Inst.Func.SrcLocs, extra_index).data;
    }

    const is_var_args = extra.data.bits.is_var_args;
    const is_inferred_error = extra.data.bits.is_inferred_error;
    const is_noinline = extra.data.bits.is_noinline;

    return sema.funcCommon(
        block,
        inst_data.src_node,
        inst,
        cc,
        ret_ty,
        is_var_args,
        is_inferred_error,
        has_body,
        src_locs,
        noalias_bits,
        is_noinline,
    );
}

fn zirCUndef(
    sema: *Sema,
    block: *Block,
    extended: Zir.Inst.Extended.InstData,
) CompileError!Air.Inst.Ref {
    const extra = sema.code.extraData(Zir.Inst.UnNode, extended.operand).data;
    const src = block.builtinCallArgSrc(extra.node, 0);

    const name = try sema.resolveConstString(block, src, extra.operand, .{ .simple = .operand_cUndef_macro_name });
    try block.c_import_buf.?.writer().print("#undef {s}\n", .{name});
    return .void_value;
}

fn zirCInclude(
    sema: *Sema,
    block: *Block,
    extended: Zir.Inst.Extended.InstData,
) CompileError!Air.Inst.Ref {
    const extra = sema.code.extraData(Zir.Inst.UnNode, extended.operand).data;
    const src = block.builtinCallArgSrc(extra.node, 0);

    const name = try sema.resolveConstString(block, src, extra.operand, .{ .simple = .operand_cInclude_file_name });
    try block.c_import_buf.?.writer().print("#include <{s}>\n", .{name});
    return .void_value;
}

fn zirCDefine(
    sema: *Sema,
    block: *Block,
    extended: Zir.Inst.Extended.InstData,
) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const extra = sema.code.extraData(Zir.Inst.BinNode, extended.operand).data;
    const name_src = block.builtinCallArgSrc(extra.node, 0);
    const val_src = block.builtinCallArgSrc(extra.node, 1);

    const name = try sema.resolveConstString(block, name_src, extra.lhs, .{ .simple = .operand_cDefine_macro_name });
    const rhs = try sema.resolveInst(extra.rhs);
    if (sema.typeOf(rhs).zigTypeTag(zcu) != .void) {
        const value = try sema.resolveConstString(block, val_src, extra.rhs, .{ .simple = .operand_cDefine_macro_value });
        try block.c_import_buf.?.writer().print("#define {s} {s}\n", .{ name, value });
    } else {
        try block.c_import_buf.?.writer().print("#define {s}\n", .{name});
    }
    return .void_value;
}

fn zirWasmMemorySize(
    sema: *Sema,
    block: *Block,
    extended: Zir.Inst.Extended.InstData,
) CompileError!Air.Inst.Ref {
    const extra = sema.code.extraData(Zir.Inst.UnNode, extended.operand).data;
    const index_src = block.builtinCallArgSrc(extra.node, 0);
    const builtin_src = block.nodeOffset(extra.node);
    const target = sema.pt.zcu.getTarget();
    if (!target.cpu.arch.isWasm()) {
        return sema.fail(block, builtin_src, "builtin @wasmMemorySize is available when targeting WebAssembly; targeted CPU architecture is {s}", .{@tagName(target.cpu.arch)});
    }

    const index: u32 = @intCast(try sema.resolveInt(block, index_src, extra.operand, .u32, .{ .simple = .wasm_memory_index }));
    try sema.requireRuntimeBlock(block, builtin_src, null);
    return block.addInst(.{
        .tag = .wasm_memory_size,
        .data = .{ .pl_op = .{
            .operand = .none,
            .payload = index,
        } },
    });
}

fn zirWasmMemoryGrow(
    sema: *Sema,
    block: *Block,
    extended: Zir.Inst.Extended.InstData,
) CompileError!Air.Inst.Ref {
    const extra = sema.code.extraData(Zir.Inst.BinNode, extended.operand).data;
    const builtin_src = block.nodeOffset(extra.node);
    const index_src = block.builtinCallArgSrc(extra.node, 0);
    const delta_src = block.builtinCallArgSrc(extra.node, 1);
    const target = sema.pt.zcu.getTarget();
    if (!target.cpu.arch.isWasm()) {
        return sema.fail(block, builtin_src, "builtin @wasmMemoryGrow is available when targeting WebAssembly; targeted CPU architecture is {s}", .{@tagName(target.cpu.arch)});
    }

    const index: u32 = @intCast(try sema.resolveInt(block, index_src, extra.lhs, .u32, .{ .simple = .wasm_memory_index }));
    const delta = try sema.coerce(block, .usize, try sema.resolveInst(extra.rhs), delta_src);

    try sema.requireRuntimeBlock(block, builtin_src, null);
    return block.addInst(.{
        .tag = .wasm_memory_grow,
        .data = .{ .pl_op = .{
            .operand = delta,
            .payload = index,
        } },
    });
}

fn resolvePrefetchOptions(
    sema: *Sema,
    block: *Block,
    src: LazySrcLoc,
    zir_ref: Zir.Inst.Ref,
) CompileError!std.builtin.PrefetchOptions {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const gpa = sema.gpa;
    const ip = &zcu.intern_pool;
    const options_ty = try sema.getBuiltinType(src, .PrefetchOptions);
    const options = try sema.coerce(block, options_ty, try sema.resolveInst(zir_ref), src);

    const rw_src = block.src(.{ .init_field_rw = src.offset.node_offset_builtin_call_arg.builtin_call_node });
    const locality_src = block.src(.{ .init_field_locality = src.offset.node_offset_builtin_call_arg.builtin_call_node });
    const cache_src = block.src(.{ .init_field_cache = src.offset.node_offset_builtin_call_arg.builtin_call_node });

    const rw = try sema.fieldVal(block, src, options, try ip.getOrPutString(gpa, pt.tid, "rw", .no_embedded_nulls), rw_src);
    const rw_val = try sema.resolveConstDefinedValue(block, rw_src, rw, .{ .simple = .prefetch_options });

    const locality = try sema.fieldVal(block, src, options, try ip.getOrPutString(gpa, pt.tid, "locality", .no_embedded_nulls), locality_src);
    const locality_val = try sema.resolveConstDefinedValue(block, locality_src, locality, .{ .simple = .prefetch_options });

    const cache = try sema.fieldVal(block, src, options, try ip.getOrPutString(gpa, pt.tid, "cache", .no_embedded_nulls), cache_src);
    const cache_val = try sema.resolveConstDefinedValue(block, cache_src, cache, .{ .simple = .prefetch_options });

    return std.builtin.PrefetchOptions{
        .rw = try sema.interpretBuiltinType(block, rw_src, rw_val, std.builtin.PrefetchOptions.Rw),
        .locality = @intCast(try locality_val.toUnsignedIntSema(pt)),
        .cache = try sema.interpretBuiltinType(block, cache_src, cache_val, std.builtin.PrefetchOptions.Cache),
    };
}

fn zirPrefetch(
    sema: *Sema,
    block: *Block,
    extended: Zir.Inst.Extended.InstData,
) CompileError!Air.Inst.Ref {
    const extra = sema.code.extraData(Zir.Inst.BinNode, extended.operand).data;
    const ptr_src = block.builtinCallArgSrc(extra.node, 0);
    const opts_src = block.builtinCallArgSrc(extra.node, 1);
    const ptr = try sema.resolveInst(extra.lhs);
    try sema.checkPtrOperand(block, ptr_src, sema.typeOf(ptr));

    const options = try sema.resolvePrefetchOptions(block, opts_src, extra.rhs);

    if (!block.isComptime()) {
        _ = try block.addInst(.{
            .tag = .prefetch,
            .data = .{ .prefetch = .{
                .ptr = ptr,
                .rw = options.rw,
                .locality = options.locality,
                .cache = options.cache,
            } },
        });
    }

    return .void_value;
}

fn resolveExternOptions(
    sema: *Sema,
    block: *Block,
    src: LazySrcLoc,
    zir_ref: Zir.Inst.Ref,
) CompileError!struct {
    name: InternPool.NullTerminatedString,
    library_name: InternPool.OptionalNullTerminatedString = .none,
    linkage: std.builtin.GlobalLinkage = .strong,
    is_thread_local: bool = false,
    is_dll_import: bool = false,
} {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const gpa = sema.gpa;
    const ip = &zcu.intern_pool;
    const options_inst = try sema.resolveInst(zir_ref);
    const extern_options_ty = try sema.getBuiltinType(src, .ExternOptions);
    const options = try sema.coerce(block, extern_options_ty, options_inst, src);

    const name_src = block.src(.{ .init_field_name = src.offset.node_offset_builtin_call_arg.builtin_call_node });
    const library_src = block.src(.{ .init_field_library = src.offset.node_offset_builtin_call_arg.builtin_call_node });
    const linkage_src = block.src(.{ .init_field_linkage = src.offset.node_offset_builtin_call_arg.builtin_call_node });
    const thread_local_src = block.src(.{ .init_field_thread_local = src.offset.node_offset_builtin_call_arg.builtin_call_node });
    const dll_import_src = block.src(.{ .init_field_dll_import = src.offset.node_offset_builtin_call_arg.builtin_call_node });

    const name_ref = try sema.fieldVal(block, src, options, try ip.getOrPutString(gpa, pt.tid, "name", .no_embedded_nulls), name_src);
    const name = try sema.toConstString(block, name_src, name_ref, .{ .simple = .extern_options });

    const library_name_inst = try sema.fieldVal(block, src, options, try ip.getOrPutString(gpa, pt.tid, "library_name", .no_embedded_nulls), library_src);
    const library_name_val = try sema.resolveConstDefinedValue(block, library_src, library_name_inst, .{ .simple = .extern_options });

    const linkage_ref = try sema.fieldVal(block, src, options, try ip.getOrPutString(gpa, pt.tid, "linkage", .no_embedded_nulls), linkage_src);
    const linkage_val = try sema.resolveConstDefinedValue(block, linkage_src, linkage_ref, .{ .simple = .extern_options });
    const linkage = try sema.interpretBuiltinType(block, linkage_src, linkage_val, std.builtin.GlobalLinkage);

    const is_thread_local = try sema.fieldVal(block, src, options, try ip.getOrPutString(gpa, pt.tid, "is_thread_local", .no_embedded_nulls), thread_local_src);
    const is_thread_local_val = try sema.resolveConstDefinedValue(block, thread_local_src, is_thread_local, .{ .simple = .extern_options });

    const library_name = if (library_name_val.optionalValue(zcu)) |library_name_payload| library_name: {
        const library_name = try sema.toConstString(block, library_src, Air.internedToRef(library_name_payload.toIntern()), .{ .simple = .extern_options });
        if (library_name.len == 0) {
            return sema.fail(block, library_src, "library name cannot be empty", .{});
        }
        try sema.handleExternLibName(block, library_src, library_name);
        break :library_name library_name;
    } else null;

    const is_dll_import_ref = try sema.fieldVal(block, src, options, try ip.getOrPutString(gpa, pt.tid, "is_dll_import", .no_embedded_nulls), dll_import_src);
    const is_dll_import_val = try sema.resolveConstDefinedValue(block, dll_import_src, is_dll_import_ref, .{ .simple = .extern_options });

    if (name.len == 0) {
        return sema.fail(block, name_src, "extern symbol name cannot be empty", .{});
    }

    if (linkage != .weak and linkage != .strong) {
        return sema.fail(block, linkage_src, "extern symbol must use strong or weak linkage", .{});
    }

    return .{
        .name = try ip.getOrPutString(gpa, pt.tid, name, .no_embedded_nulls),
        .library_name = try ip.getOrPutStringOpt(gpa, pt.tid, library_name, .no_embedded_nulls),
        .linkage = linkage,
        .is_thread_local = is_thread_local_val.toBool(),
        .is_dll_import = is_dll_import_val.toBool(),
    };
}

fn zirBuiltinExtern(
    sema: *Sema,
    block: *Block,
    extended: Zir.Inst.Extended.InstData,
) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const ip = &zcu.intern_pool;
    const extra = sema.code.extraData(Zir.Inst.BinNode, extended.operand).data;
    const src = block.nodeOffset(extra.node);
    const ty_src = block.builtinCallArgSrc(extra.node, 0);
    const options_src = block.builtinCallArgSrc(extra.node, 1);

    var ty = try sema.resolveType(block, ty_src, extra.lhs);
    if (!ty.isPtrAtRuntime(zcu)) {
        return sema.fail(block, ty_src, "expected (optional) pointer", .{});
    }
    if (!try sema.validateExternType(ty, .other)) {
        const msg = msg: {
            const msg = try sema.errMsg(ty_src, "extern symbol cannot have type '{}'", .{ty.fmt(pt)});
            errdefer msg.destroy(sema.gpa);
            try sema.explainWhyTypeIsNotExtern(msg, ty_src, ty, .other);
            break :msg msg;
        };
        return sema.failWithOwnedErrorMsg(block, msg);
    }

    const options = try sema.resolveExternOptions(block, options_src, extra.rhs);

    // TODO: error for threadlocal functions, non-const functions, etc

    if (options.linkage == .weak and !ty.ptrAllowsZero(zcu)) {
        ty = try pt.optionalType(ty.toIntern());
    }
    const ptr_info = ty.ptrInfo(zcu);

    const extern_val = try pt.getExtern(.{
        .name = options.name,
        .ty = ptr_info.child,
        .lib_name = options.library_name,
        .is_const = ptr_info.flags.is_const,
        .is_threadlocal = options.is_thread_local,
        .is_weak_linkage = options.linkage == .weak,
        .is_dll_import = options.is_dll_import,
        .alignment = ptr_info.flags.alignment,
        .@"addrspace" = ptr_info.flags.address_space,
        // This instruction is just for source locations.
        // `builtin_extern` doesn't provide enough information, and isn't currently tracked.
        // So, for now, just use our containing `declaration`.
        .zir_index = switch (sema.owner.unwrap()) {
            .@"comptime" => |cu| ip.getComptimeUnit(cu).zir_index,
            .type => |owner_ty| Type.fromInterned(owner_ty).typeDeclInst(zcu).?,
            .memoized_state => unreachable,
            .nav_ty, .nav_val => |nav| ip.getNav(nav).analysis.?.zir_index,
            .func => |func| zir_index: {
                const func_info = zcu.funcInfo(func);
                const owner_func_info = if (func_info.generic_owner != .none) owner: {
                    break :owner zcu.funcInfo(func_info.generic_owner);
                } else func_info;
                break :zir_index ip.getNav(owner_func_info.owner_nav).analysis.?.zir_index;
            },
        },
        .owner_nav = undefined, // ignored by `getExtern`
    });

    const uncasted_ptr = try sema.analyzeNavRef(block, src, ip.indexToKey(extern_val).@"extern".owner_nav);
    // We want to cast to `ty`, but that isn't necessarily an allowed coercion.
    if (try sema.resolveValue(uncasted_ptr)) |uncasted_ptr_val| {
        const casted_ptr_val = try pt.getCoerced(uncasted_ptr_val, ty);
        return Air.internedToRef(casted_ptr_val.toIntern());
    } else {
        return block.addBitCast(ty, uncasted_ptr);
    }
}

fn zirWorkItem(
    sema: *Sema,
    block: *Block,
    extended: Zir.Inst.Extended.InstData,
    zir_tag: Zir.Inst.Extended,
) CompileError!Air.Inst.Ref {
    const extra = sema.code.extraData(Zir.Inst.UnNode, extended.operand).data;
    const dimension_src = block.builtinCallArgSrc(extra.node, 0);
    const builtin_src = block.nodeOffset(extra.node);
    const target = sema.pt.zcu.getTarget();

    switch (target.cpu.arch) {
        // TODO: Allow for other GPU targets.
        .amdgcn, .spirv, .spirv64, .spirv32, .nvptx, .nvptx64 => {},
        else => {
            return sema.fail(block, builtin_src, "builtin only available on GPU targets; targeted architecture is {s}", .{@tagName(target.cpu.arch)});
        },
    }

    const dimension: u32 = @intCast(try sema.resolveInt(block, dimension_src, extra.operand, .u32, .{ .simple = .work_group_dim_index }));
    try sema.requireRuntimeBlock(block, builtin_src, null);

    return block.addInst(.{
        .tag = switch (zir_tag) {
            .work_item_id => .work_item_id,
            .work_group_size => .work_group_size,
            .work_group_id => .work_group_id,
            else => unreachable,
        },
        .data = .{ .pl_op = .{
            .operand = .none,
            .payload = dimension,
        } },
    });
}

fn zirInComptime(
    sema: *Sema,
    block: *Block,
) CompileError!Air.Inst.Ref {
    _ = sema;
    return if (block.isComptime()) .bool_true else .bool_false;
}

fn zirBuiltinValue(sema: *Sema, block: *Block, extended: Zir.Inst.Extended.InstData) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const gpa = zcu.gpa;
    const ip = &zcu.intern_pool;

    const src_node: std.zig.Ast.Node.Offset = @enumFromInt(@as(i32, @bitCast(extended.operand)));
    const src = block.nodeOffset(src_node);
    const value: Zir.Inst.BuiltinValue = @enumFromInt(extended.small);

    const ty = switch (value) {
        // zig fmt: off
        .atomic_order       => try sema.getBuiltinType(src, .AtomicOrder),
        .atomic_rmw_op      => try sema.getBuiltinType(src, .AtomicRmwOp),
        .calling_convention => try sema.getBuiltinType(src, .CallingConvention),
        .address_space      => try sema.getBuiltinType(src, .AddressSpace),
        .float_mode         => try sema.getBuiltinType(src, .FloatMode),
        .reduce_op          => try sema.getBuiltinType(src, .ReduceOp),
        .call_modifier      => try sema.getBuiltinType(src, .CallModifier),
        .prefetch_options   => try sema.getBuiltinType(src, .PrefetchOptions),
        .export_options     => try sema.getBuiltinType(src, .ExportOptions),
        .extern_options     => try sema.getBuiltinType(src, .ExternOptions),
        .type_info          => try sema.getBuiltinType(src, .Type),
        .branch_hint        => try sema.getBuiltinType(src, .BranchHint),
        // zig fmt: on

        // Values are handled here.
        .calling_convention_c => {
            const callconv_ty = try sema.getBuiltinType(src, .CallingConvention);
            return try sema.namespaceLookupVal(
                block,
                src,
                callconv_ty.getNamespaceIndex(zcu),
                try ip.getOrPutString(gpa, pt.tid, "c", .no_embedded_nulls),
            ) orelse @panic("std.builtin is corrupt");
        },
        .calling_convention_inline => {
            comptime assert(@typeInfo(std.builtin.CallingConvention.Tag).@"enum".tag_type == u8);
            const callconv_ty = try sema.getBuiltinType(src, .CallingConvention);
            const callconv_tag_ty = callconv_ty.unionTagType(zcu) orelse @panic("std.builtin is corrupt");
            const inline_tag_val = try pt.enumValue(
                callconv_tag_ty,
                (try pt.intValue(
                    .u8,
                    @intFromEnum(std.builtin.CallingConvention.@"inline"),
                )).toIntern(),
            );
            return sema.coerce(block, callconv_ty, Air.internedToRef(inline_tag_val.toIntern()), src);
        },
    };
    return Air.internedToRef(ty.toIntern());
}

fn zirInplaceArithResultTy(sema: *Sema, extended: Zir.Inst.Extended.InstData) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;

    const lhs = try sema.resolveInst(@enumFromInt(extended.operand));
    const lhs_ty = sema.typeOf(lhs);

    const op: Zir.Inst.InplaceOp = @enumFromInt(extended.small);
    const ty: Type = switch (op) {
        .add_eq => ty: {
            const ptr_size = lhs_ty.ptrSizeOrNull(zcu) orelse break :ty lhs_ty;
            switch (ptr_size) {
                .one, .slice => break :ty lhs_ty, // invalid, let it error
                .many, .c => break :ty .usize, // `[*]T + usize`
            }
        },
        .sub_eq => ty: {
            const ptr_size = lhs_ty.ptrSizeOrNull(zcu) orelse break :ty lhs_ty;
            switch (ptr_size) {
                .one, .slice => break :ty lhs_ty, // invalid, let it error
                .many, .c => break :ty .generic_poison, // could be `[*]T - [*]T` or `[*]T - usize`
            }
        },
    };
    return Air.internedToRef(ty.toIntern());
}

fn zirBranchHint(sema: *Sema, block: *Block, extended: Zir.Inst.Extended.InstData) CompileError!void {
    const extra = sema.code.extraData(Zir.Inst.UnNode, extended.operand).data;
    const uncoerced_hint = try sema.resolveInst(extra.operand);
    const operand_src = block.builtinCallArgSrc(extra.node, 0);

    const hint_ty = try sema.getBuiltinType(operand_src, .BranchHint);
    const coerced_hint = try sema.coerce(block, hint_ty, uncoerced_hint, operand_src);
    const hint_val = try sema.resolveConstDefinedValue(block, operand_src, coerced_hint, .{ .simple = .operand_branchHint });

    // We only apply the first hint in a branch.
    // This allows user-provided hints to override implicit cold hints.
    if (sema.branch_hint == null) {
        sema.branch_hint = try sema.interpretBuiltinType(block, operand_src, hint_val, std.builtin.BranchHint);
    }
}

fn requireRuntimeBlock(sema: *Sema, block: *Block, src: LazySrcLoc, runtime_src: ?LazySrcLoc) !void {
    if (block.isComptime()) {
        const msg, const fail_block = msg: {
            const msg = try sema.errMsg(src, "unable to evaluate comptime expression", .{});
            errdefer msg.destroy(sema.gpa);

            if (runtime_src) |some| {
                try sema.errNote(some, msg, "operation is runtime due to this operand", .{});
            }

            const fail_block = try block.explainWhyBlockIsComptime(msg);

            break :msg .{ msg, fail_block };
        };
        return sema.failWithOwnedErrorMsg(fail_block, msg);
    }
}

/// Emit a compile error if type cannot be used for a runtime variable.
pub fn validateVarType(
    sema: *Sema,
    block: *Block,
    src: LazySrcLoc,
    var_ty: Type,
    is_extern: bool,
) CompileError!void {
    const pt = sema.pt;
    const zcu = pt.zcu;
    if (is_extern) {
        if (!try sema.validateExternType(var_ty, .other)) {
            const msg = msg: {
                const msg = try sema.errMsg(src, "extern variable cannot have type '{}'", .{var_ty.fmt(pt)});
                errdefer msg.destroy(sema.gpa);
                try sema.explainWhyTypeIsNotExtern(msg, src, var_ty, .other);
                break :msg msg;
            };
            return sema.failWithOwnedErrorMsg(block, msg);
        }
    } else {
        if (var_ty.zigTypeTag(zcu) == .@"opaque") {
            return sema.fail(
                block,
                src,
                "non-extern variable with opaque type '{}'",
                .{var_ty.fmt(pt)},
            );
        }
    }

    if (!try var_ty.comptimeOnlySema(pt)) return;

    const msg = msg: {
        const msg = try sema.errMsg(src, "variable of type '{}' must be const or comptime", .{var_ty.fmt(pt)});
        errdefer msg.destroy(sema.gpa);

        try sema.explainWhyTypeIsComptime(msg, src, var_ty);
        if (var_ty.zigTypeTag(zcu) == .comptime_int or var_ty.zigTypeTag(zcu) == .comptime_float) {
            try sema.errNote(src, msg, "to modify this variable at runtime, it must be given an explicit fixed-size number type", .{});
        }

        break :msg msg;
    };
    return sema.failWithOwnedErrorMsg(block, msg);
}

const TypeSet = std.AutoHashMapUnmanaged(InternPool.Index, void);

fn explainWhyTypeIsComptime(
    sema: *Sema,
    msg: *Zcu.ErrorMsg,
    src_loc: LazySrcLoc,
    ty: Type,
) CompileError!void {
    var type_set = TypeSet{};
    defer type_set.deinit(sema.gpa);

    try ty.resolveFully(sema.pt);
    return sema.explainWhyTypeIsComptimeInner(msg, src_loc, ty, &type_set);
}

fn explainWhyTypeIsComptimeInner(
    sema: *Sema,
    msg: *Zcu.ErrorMsg,
    src_loc: LazySrcLoc,
    ty: Type,
    type_set: *TypeSet,
) CompileError!void {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const ip = &zcu.intern_pool;
    switch (ty.zigTypeTag(zcu)) {
        .bool,
        .int,
        .float,
        .error_set,
        .@"enum",
        .frame,
        .@"anyframe",
        .void,
        => return,

        .@"fn" => {
            try sema.errNote(src_loc, msg, "use '*const {}' for a function pointer type", .{ty.fmt(pt)});
        },

        .type => {
            try sema.errNote(src_loc, msg, "types are not available at runtime", .{});
        },

        .comptime_float,
        .comptime_int,
        .enum_literal,
        .noreturn,
        .undefined,
        .null,
        => return,

        .@"opaque" => {
            try sema.errNote(src_loc, msg, "opaque type '{}' has undefined size", .{ty.fmt(pt)});
        },

        .array, .vector => {
            try sema.explainWhyTypeIsComptimeInner(msg, src_loc, ty.childType(zcu), type_set);
        },
        .pointer => {
            const elem_ty = ty.elemType2(zcu);
            if (elem_ty.zigTypeTag(zcu) == .@"fn") {
                const fn_info = zcu.typeToFunc(elem_ty).?;
                if (fn_info.is_generic) {
                    try sema.errNote(src_loc, msg, "function is generic", .{});
                }
                switch (fn_info.cc) {
                    .@"inline" => try sema.errNote(src_loc, msg, "function has inline calling convention", .{}),
                    else => {},
                }
                if (Type.fromInterned(fn_info.return_type).comptimeOnly(zcu)) {
                    try sema.errNote(src_loc, msg, "function has a comptime-only return type", .{});
                }
                return;
            }
            try sema.explainWhyTypeIsComptimeInner(msg, src_loc, ty.childType(zcu), type_set);
        },

        .optional => {
            try sema.explainWhyTypeIsComptimeInner(msg, src_loc, ty.optionalChild(zcu), type_set);
        },
        .error_union => {
            try sema.explainWhyTypeIsComptimeInner(msg, src_loc, ty.errorUnionPayload(zcu), type_set);
        },

        .@"struct" => {
            if ((try type_set.getOrPut(sema.gpa, ty.toIntern())).found_existing) return;

            if (zcu.typeToStruct(ty)) |struct_type| {
                for (0..struct_type.field_types.len) |i| {
                    const field_ty: Type = .fromInterned(struct_type.field_types.get(ip)[i]);
                    const field_src: LazySrcLoc = .{
                        .base_node_inst = struct_type.zir_index,
                        .offset = .{ .container_field_type = @intCast(i) },
                    };

                    if (try field_ty.comptimeOnlySema(pt)) {
                        try sema.errNote(field_src, msg, "struct requires comptime because of this field", .{});
                        try sema.explainWhyTypeIsComptimeInner(msg, field_src, field_ty, type_set);
                    }
                }
            }
            // TODO tuples
        },

        .@"union" => {
            if ((try type_set.getOrPut(sema.gpa, ty.toIntern())).found_existing) return;

            if (zcu.typeToUnion(ty)) |union_obj| {
                for (0..union_obj.field_types.len) |i| {
                    const field_ty: Type = .fromInterned(union_obj.field_types.get(ip)[i]);
                    const field_src: LazySrcLoc = .{
                        .base_node_inst = union_obj.zir_index,
                        .offset = .{ .container_field_type = @intCast(i) },
                    };

                    if (try field_ty.comptimeOnlySema(pt)) {
                        try sema.errNote(field_src, msg, "union requires comptime because of this field", .{});
                        try sema.explainWhyTypeIsComptimeInner(msg, field_src, field_ty, type_set);
                    }
                }
            }
        },
    }
}

const ExternPosition = enum {
    ret_ty,
    param_ty,
    union_field,
    struct_field,
    element,
    other,
};

/// Returns true if `ty` is allowed in extern types.
/// Does *NOT* require `ty` to be resolved in any way.
/// Calls `resolveLayout` for packed containers.
fn validateExternType(
    sema: *Sema,
    ty: Type,
    position: ExternPosition,
) !bool {
    const pt = sema.pt;
    const zcu = pt.zcu;
    switch (ty.zigTypeTag(zcu)) {
        .type,
        .comptime_float,
        .comptime_int,
        .enum_literal,
        .undefined,
        .null,
        .error_union,
        .error_set,
        .frame,
        => return false,
        .void => return position == .union_field or position == .ret_ty or position == .struct_field or position == .element,
        .noreturn => return position == .ret_ty,
        .@"opaque",
        .bool,
        .float,
        .@"anyframe",
        => return true,
        .pointer => {
            if (ty.childType(zcu).zigTypeTag(zcu) == .@"fn") {
                return ty.isConstPtr(zcu) and try sema.validateExternType(ty.childType(zcu), .other);
            }
            return !(ty.isSlice(zcu) or try ty.comptimeOnlySema(pt));
        },
        .int => switch (ty.intInfo(zcu).bits) {
            0, 8, 16, 32, 64, 128 => return true,
            else => return false,
        },
        .@"fn" => {
            if (position != .other) return false;
            // For now we want to authorize PTX kernel to use zig objects, even if we end up exposing the ABI.
            // The goal is to experiment with more integrated CPU/GPU code.
            if (ty.fnCallingConvention(zcu) == .nvptx_kernel) {
                return true;
            }
            return !target_util.fnCallConvAllowsZigTypes(ty.fnCallingConvention(zcu));
        },
        .@"enum" => {
            return sema.validateExternType(ty.intTagType(zcu), position);
        },
        .@"struct", .@"union" => switch (ty.containerLayout(zcu)) {
            .@"extern" => return true,
            .@"packed" => {
                const bit_size = try ty.bitSizeSema(pt);
                switch (bit_size) {
                    0, 8, 16, 32, 64, 128 => return true,
                    else => return false,
                }
            },
            .auto => return !(try ty.hasRuntimeBitsSema(pt)),
        },
        .array => {
            if (position == .ret_ty or position == .param_ty) return false;
            return sema.validateExternType(ty.elemType2(zcu), .element);
        },
        .vector => return sema.validateExternType(ty.elemType2(zcu), .element),
        .optional => return ty.isPtrLikeOptional(zcu),
    }
}

fn explainWhyTypeIsNotExtern(
    sema: *Sema,
    msg: *Zcu.ErrorMsg,
    src_loc: LazySrcLoc,
    ty: Type,
    position: ExternPosition,
) CompileError!void {
    const pt = sema.pt;
    const zcu = pt.zcu;
    switch (ty.zigTypeTag(zcu)) {
        .@"opaque",
        .bool,
        .float,
        .@"anyframe",
        => return,

        .type,
        .comptime_float,
        .comptime_int,
        .enum_literal,
        .undefined,
        .null,
        .error_union,
        .error_set,
        .frame,
        => return,

        .pointer => {
            if (ty.isSlice(zcu)) {
                try sema.errNote(src_loc, msg, "slices have no guaranteed in-memory representation", .{});
            } else {
                const pointee_ty = ty.childType(zcu);
                if (!ty.isConstPtr(zcu) and pointee_ty.zigTypeTag(zcu) == .@"fn") {
                    try sema.errNote(src_loc, msg, "pointer to extern function must be 'const'", .{});
                } else if (try ty.comptimeOnlySema(pt)) {
                    try sema.errNote(src_loc, msg, "pointer to comptime-only type '{}'", .{pointee_ty.fmt(pt)});
                    try sema.explainWhyTypeIsComptime(msg, src_loc, ty);
                }
                try sema.explainWhyTypeIsNotExtern(msg, src_loc, pointee_ty, .other);
            }
        },
        .void => try sema.errNote(src_loc, msg, "'void' is a zero bit type; for C 'void' use 'anyopaque'", .{}),
        .noreturn => try sema.errNote(src_loc, msg, "'noreturn' is only allowed as a return type", .{}),
        .int => if (!std.math.isPowerOfTwo(ty.intInfo(zcu).bits)) {
            try sema.errNote(src_loc, msg, "only integers with 0 or power of two bits are extern compatible", .{});
        } else {
            try sema.errNote(src_loc, msg, "only integers with 0, 8, 16, 32, 64 and 128 bits are extern compatible", .{});
        },
        .@"fn" => {
            if (position != .other) {
                try sema.errNote(src_loc, msg, "type has no guaranteed in-memory representation", .{});
                try sema.errNote(src_loc, msg, "use '*const ' to make a function pointer type", .{});
                return;
            }
            switch (ty.fnCallingConvention(zcu)) {
                .auto => try sema.errNote(src_loc, msg, "extern function must specify calling convention", .{}),
                .@"async" => try sema.errNote(src_loc, msg, "async function cannot be extern", .{}),
                .@"inline" => try sema.errNote(src_loc, msg, "inline function cannot be extern", .{}),
                else => return,
            }
        },
        .@"enum" => {
            const tag_ty = ty.intTagType(zcu);
            try sema.errNote(src_loc, msg, "enum tag type '{}' is not extern compatible", .{tag_ty.fmt(pt)});
            try sema.explainWhyTypeIsNotExtern(msg, src_loc, tag_ty, position);
        },
        .@"struct" => try sema.errNote(src_loc, msg, "only extern structs and ABI sized packed structs are extern compatible", .{}),
        .@"union" => try sema.errNote(src_loc, msg, "only extern unions and ABI sized packed unions are extern compatible", .{}),
        .array => {
            if (position == .ret_ty) {
                return sema.errNote(src_loc, msg, "arrays are not allowed as a return type", .{});
            } else if (position == .param_ty) {
                return sema.errNote(src_loc, msg, "arrays are not allowed as a parameter type", .{});
            }
            try sema.explainWhyTypeIsNotExtern(msg, src_loc, ty.elemType2(zcu), .element);
        },
        .vector => try sema.explainWhyTypeIsNotExtern(msg, src_loc, ty.elemType2(zcu), .element),
        .optional => try sema.errNote(src_loc, msg, "only pointer like optionals are extern compatible", .{}),
    }
}

/// Returns true if `ty` is allowed in packed types.
/// Does not require `ty` to be resolved in any way, but may resolve whether it is comptime-only.
fn validatePackedType(sema: *Sema, ty: Type) !bool {
    const pt = sema.pt;
    const zcu = pt.zcu;
    return switch (ty.zigTypeTag(zcu)) {
        .type,
        .comptime_float,
        .comptime_int,
        .enum_literal,
        .undefined,
        .null,
        .error_union,
        .error_set,
        .frame,
        .noreturn,
        .@"opaque",
        .@"anyframe",
        .@"fn",
        .array,
        => false,
        .optional => return ty.isPtrLikeOptional(zcu),
        .void,
        .bool,
        .float,
        .int,
        .vector,
        => true,
        .@"enum" => switch (zcu.intern_pool.loadEnumType(ty.toIntern()).tag_mode) {
            .auto => false,
            .explicit, .nonexhaustive => true,
        },
        .pointer => !ty.isSlice(zcu) and !try ty.comptimeOnlySema(pt),
        .@"struct", .@"union" => ty.containerLayout(zcu) == .@"packed",
    };
}

fn explainWhyTypeIsNotPacked(
    sema: *Sema,
    msg: *Zcu.ErrorMsg,
    src_loc: LazySrcLoc,
    ty: Type,
) CompileError!void {
    const pt = sema.pt;
    const zcu = pt.zcu;
    switch (ty.zigTypeTag(zcu)) {
        .void,
        .bool,
        .float,
        .int,
        .vector,
        .@"enum",
        => return,
        .type,
        .comptime_float,
        .comptime_int,
        .enum_literal,
        .undefined,
        .null,
        .frame,
        .noreturn,
        .@"opaque",
        .error_union,
        .error_set,
        .@"anyframe",
        .optional,
        .array,
        => try sema.errNote(src_loc, msg, "type has no guaranteed in-memory representation", .{}),
        .pointer => if (ty.isSlice(zcu)) {
            try sema.errNote(src_loc, msg, "slices have no guaranteed in-memory representation", .{});
        } else {
            try sema.errNote(src_loc, msg, "comptime-only pointer has no guaranteed in-memory representation", .{});
            try sema.explainWhyTypeIsComptime(msg, src_loc, ty);
        },
        .@"fn" => {
            try sema.errNote(src_loc, msg, "type has no guaranteed in-memory representation", .{});
            try sema.errNote(src_loc, msg, "use '*const ' to make a function pointer type", .{});
        },
        .@"struct" => try sema.errNote(src_loc, msg, "only packed structs layout are allowed in packed types", .{}),
        .@"union" => try sema.errNote(src_loc, msg, "only packed unions layout are allowed in packed types", .{}),
    }
}

/// Backends depend on panic decls being available when lowering safety-checked
/// instructions. This function ensures the panic function will be available to
/// be called during that time.
fn preparePanicId(sema: *Sema, src: LazySrcLoc, panic_id: Zcu.SimplePanicId) !InternPool.Index {
    const zcu = sema.pt.zcu;
    try sema.ensureMemoizedStateResolved(src, .panic);
    const panic_func = zcu.builtin_decl_values.get(panic_id.toBuiltin());
    try zcu.ensureFuncBodyAnalysisQueued(panic_func);
    switch (sema.owner.unwrap()) {
        .@"comptime", .nav_ty, .nav_val, .type, .memoized_state => {},
        .func => |owner_func| zcu.intern_pool.funcSetHasErrorTrace(owner_func, true),
    }
    return panic_func;
}

fn addSafetyCheck(
    sema: *Sema,
    parent_block: *Block,
    src: LazySrcLoc,
    ok: Air.Inst.Ref,
    panic_id: Zcu.SimplePanicId,
) !void {
    const gpa = sema.gpa;
    assert(!parent_block.isComptime());

    var fail_block: Block = .{
        .parent = parent_block,
        .sema = sema,
        .namespace = parent_block.namespace,
        .instructions = .{},
        .inlining = parent_block.inlining,
        .comptime_reason = null,
        .src_base_inst = parent_block.src_base_inst,
        .type_name_ctx = parent_block.type_name_ctx,
    };

    defer fail_block.instructions.deinit(gpa);

    try sema.safetyPanic(&fail_block, src, panic_id);
    try sema.addSafetyCheckExtra(parent_block, ok, &fail_block);
}

fn addSafetyCheckExtra(
    sema: *Sema,
    parent_block: *Block,
    ok: Air.Inst.Ref,
    fail_block: *Block,
) !void {
    const gpa = sema.gpa;

    try parent_block.instructions.ensureUnusedCapacity(gpa, 1);

    try sema.air_extra.ensureUnusedCapacity(gpa, @typeInfo(Air.Block).@"struct".fields.len +
        1 + // The main block only needs space for the cond_br.
        @typeInfo(Air.CondBr).@"struct".fields.len +
        1 + // The ok branch of the cond_br only needs space for the br.
        fail_block.instructions.items.len);

    try sema.air_instructions.ensureUnusedCapacity(gpa, 3);
    const block_inst: Air.Inst.Index = @enumFromInt(sema.air_instructions.len);
    const cond_br_inst: Air.Inst.Index = @enumFromInt(@intFromEnum(block_inst) + 1);
    const br_inst: Air.Inst.Index = @enumFromInt(@intFromEnum(cond_br_inst) + 1);
    sema.air_instructions.appendAssumeCapacity(.{
        .tag = .block,
        .data = .{ .ty_pl = .{
            .ty = .void_type,
            .payload = sema.addExtraAssumeCapacity(Air.Block{
                .body_len = 1,
            }),
        } },
    });
    sema.air_extra.appendAssumeCapacity(@intFromEnum(cond_br_inst));

    sema.air_instructions.appendAssumeCapacity(.{
        .tag = .cond_br,
        .data = .{
            .pl_op = .{
                .operand = ok,
                .payload = sema.addExtraAssumeCapacity(Air.CondBr{
                    .then_body_len = 1,
                    .else_body_len = @intCast(fail_block.instructions.items.len),
                    .branch_hints = .{
                        // Safety check failure branch is cold.
                        .true = .likely,
                        .false = .cold,
                        // Code coverage not wanted for panic branches.
                        .then_cov = .none,
                        .else_cov = .none,
                    },
                }),
            },
        },
    });
    sema.air_extra.appendAssumeCapacity(@intFromEnum(br_inst));
    sema.air_extra.appendSliceAssumeCapacity(@ptrCast(fail_block.instructions.items));

    sema.air_instructions.appendAssumeCapacity(.{
        .tag = .br,
        .data = .{ .br = .{
            .block_inst = block_inst,
            .operand = .void_value,
        } },
    });

    parent_block.instructions.appendAssumeCapacity(block_inst);
}

fn addSafetyCheckUnwrapError(
    sema: *Sema,
    parent_block: *Block,
    src: LazySrcLoc,
    operand: Air.Inst.Ref,
    unwrap_err_tag: Air.Inst.Tag,
    is_non_err_tag: Air.Inst.Tag,
) !void {
    assert(!parent_block.isComptime());
    const ok = try parent_block.addUnOp(is_non_err_tag, operand);
    const gpa = sema.gpa;

    var fail_block: Block = .{
        .parent = parent_block,
        .sema = sema,
        .namespace = parent_block.namespace,
        .instructions = .{},
        .inlining = parent_block.inlining,
        .comptime_reason = null,
        .src_base_inst = parent_block.src_base_inst,
        .type_name_ctx = parent_block.type_name_ctx,
    };

    defer fail_block.instructions.deinit(gpa);

    const err = try fail_block.addTyOp(unwrap_err_tag, .anyerror, operand);
    try safetyPanicUnwrapError(sema, &fail_block, src, err);

    try sema.addSafetyCheckExtra(parent_block, ok, &fail_block);
}

fn safetyPanicUnwrapError(sema: *Sema, block: *Block, src: LazySrcLoc, err: Air.Inst.Ref) !void {
    const pt = sema.pt;
    const zcu = pt.zcu;
    if (!zcu.backendSupportsFeature(.panic_fn)) {
        _ = try block.addNoOp(.trap);
    } else {
        const panic_fn = try getBuiltin(sema, src, .@"panic.unwrapError");
        try sema.callBuiltin(block, src, Air.internedToRef(panic_fn), .auto, &.{err}, .@"safety check");
    }
}

fn addSafetyCheckIndexOob(
    sema: *Sema,
    parent_block: *Block,
    src: LazySrcLoc,
    index: Air.Inst.Ref,
    len: Air.Inst.Ref,
    cmp_op: Air.Inst.Tag,
) !void {
    assert(!parent_block.isComptime());
    const ok = try parent_block.addBinOp(cmp_op, index, len);
    return addSafetyCheckCall(sema, parent_block, src, ok, .@"panic.outOfBounds", &.{ index, len });
}

fn addSafetyCheckInactiveUnionField(
    sema: *Sema,
    parent_block: *Block,
    src: LazySrcLoc,
    active_tag: Air.Inst.Ref,
    wanted_tag: Air.Inst.Ref,
) !void {
    assert(!parent_block.isComptime());
    const ok = try parent_block.addBinOp(.cmp_eq, active_tag, wanted_tag);
    return addSafetyCheckCall(sema, parent_block, src, ok, .@"panic.inactiveUnionField", &.{ active_tag, wanted_tag });
}

fn addSafetyCheckSentinelMismatch(
    sema: *Sema,
    parent_block: *Block,
    src: LazySrcLoc,
    maybe_sentinel: ?Value,
    sentinel_ty: Type,
    ptr: Air.Inst.Ref,
    sentinel_index: Air.Inst.Ref,
) !void {
    assert(!parent_block.isComptime());
    const pt = sema.pt;
    const zcu = pt.zcu;
    const expected_sentinel_val = maybe_sentinel orelse return;
    const expected_sentinel = Air.internedToRef(expected_sentinel_val.toIntern());

    const ptr_ty = sema.typeOf(ptr);
    const actual_sentinel = if (ptr_ty.isSlice(zcu))
        try parent_block.addBinOp(.slice_elem_val, ptr, sentinel_index)
    else blk: {
        const elem_ptr_ty = try ptr_ty.elemPtrType(null, pt);
        const sentinel_ptr = try parent_block.addPtrElemPtr(ptr, sentinel_index, elem_ptr_ty);
        break :blk try parent_block.addTyOp(.load, sentinel_ty, sentinel_ptr);
    };

    const ok = if (sentinel_ty.zigTypeTag(zcu) == .vector) ok: {
        const eql = try parent_block.addCmpVector(expected_sentinel, actual_sentinel, .eq);
        break :ok try parent_block.addReduce(eql, .And);
    } else ok: {
        assert(sentinel_ty.isSelfComparable(zcu, true));
        break :ok try parent_block.addBinOp(.cmp_eq, expected_sentinel, actual_sentinel);
    };

    return addSafetyCheckCall(sema, parent_block, src, ok, .@"panic.sentinelMismatch", &.{
        expected_sentinel, actual_sentinel,
    });
}

fn addSafetyCheckCall(
    sema: *Sema,
    parent_block: *Block,
    src: LazySrcLoc,
    ok: Air.Inst.Ref,
    comptime func_decl: Zcu.BuiltinDecl,
    args: []const Air.Inst.Ref,
) !void {
    assert(!parent_block.isComptime());
    const gpa = sema.gpa;
    const pt = sema.pt;
    const zcu = pt.zcu;

    var fail_block: Block = .{
        .parent = parent_block,
        .sema = sema,
        .namespace = parent_block.namespace,
        .instructions = .{},
        .inlining = parent_block.inlining,
        .comptime_reason = null,
        .src_base_inst = parent_block.src_base_inst,
        .type_name_ctx = parent_block.type_name_ctx,
    };

    defer fail_block.instructions.deinit(gpa);

    if (!zcu.backendSupportsFeature(.panic_fn)) {
        _ = try fail_block.addNoOp(.trap);
    } else {
        const panic_fn = try getBuiltin(sema, src, func_decl);
        try sema.callBuiltin(&fail_block, src, Air.internedToRef(panic_fn), .auto, args, .@"safety check");
    }

    try sema.addSafetyCheckExtra(parent_block, ok, &fail_block);
}

/// This does not set `sema.branch_hint`.
fn safetyPanic(sema: *Sema, block: *Block, src: LazySrcLoc, panic_id: Zcu.SimplePanicId) CompileError!void {
    if (!sema.pt.zcu.backendSupportsFeature(.panic_fn)) {
        _ = try block.addNoOp(.trap);
    } else {
        const panic_fn = try sema.preparePanicId(src, panic_id);
        try sema.callBuiltin(block, src, Air.internedToRef(panic_fn), .auto, &.{}, .@"safety check");
    }
}

fn emitBackwardBranch(sema: *Sema, block: *Block, src: LazySrcLoc) !void {
    sema.branch_count += 1;
    if (sema.branch_count > sema.branch_quota) {
        const msg = try sema.errMsg(
            src,
            "evaluation exceeded {d} backwards branches",
            .{sema.branch_quota},
        );
        try sema.errNote(
            src,
            msg,
            "use @setEvalBranchQuota() to raise the branch limit from {d}",
            .{sema.branch_quota},
        );
        return sema.failWithOwnedErrorMsg(block, msg);
    }
}

fn fieldVal(
    sema: *Sema,
    block: *Block,
    src: LazySrcLoc,
    object: Air.Inst.Ref,
    field_name: InternPool.NullTerminatedString,
    field_name_src: LazySrcLoc,
) CompileError!Air.Inst.Ref {
    // When editing this function, note that there is corresponding logic to be edited
    // in `fieldPtr`. This function takes a value and returns a value.

    const pt = sema.pt;
    const zcu = pt.zcu;
    const ip = &zcu.intern_pool;
    const object_src = src; // TODO better source location
    const object_ty = sema.typeOf(object);

    // Zig allows dereferencing a single pointer during field lookup. Note that
    // we don't actually need to generate the dereference some field lookups, like the
    // length of arrays and other comptime operations.
    const is_pointer_to = object_ty.isSinglePointer(zcu);

    const inner_ty = if (is_pointer_to)
        object_ty.childType(zcu)
    else
        object_ty;

    switch (inner_ty.zigTypeTag(zcu)) {
        .array => {
            if (field_name.eqlSlice("len", ip)) {
                return Air.internedToRef((try pt.intValue(.usize, inner_ty.arrayLen(zcu))).toIntern());
            } else if (field_name.eqlSlice("ptr", ip) and is_pointer_to) {
                const ptr_info = object_ty.ptrInfo(zcu);
                const result_ty = try pt.ptrTypeSema(.{
                    .child = Type.fromInterned(ptr_info.child).childType(zcu).toIntern(),
                    .sentinel = if (inner_ty.sentinel(zcu)) |s| s.toIntern() else .none,
                    .flags = .{
                        .size = .many,
                        .alignment = ptr_info.flags.alignment,
                        .is_const = ptr_info.flags.is_const,
                        .is_volatile = ptr_info.flags.is_volatile,
                        .is_allowzero = ptr_info.flags.is_allowzero,
                        .address_space = ptr_info.flags.address_space,
                        .vector_index = ptr_info.flags.vector_index,
                    },
                    .packed_offset = ptr_info.packed_offset,
                });
                return sema.coerce(block, result_ty, object, src);
            } else {
                return sema.fail(
                    block,
                    field_name_src,
                    "no member named '{}' in '{}'",
                    .{ field_name.fmt(ip), object_ty.fmt(pt) },
                );
            }
        },
        .pointer => {
            const ptr_info = inner_ty.ptrInfo(zcu);
            if (ptr_info.flags.size == .slice) {
                if (field_name.eqlSlice("ptr", ip)) {
                    const slice = if (is_pointer_to)
                        try sema.analyzeLoad(block, src, object, object_src)
                    else
                        object;
                    return sema.analyzeSlicePtr(block, object_src, slice, inner_ty);
                } else if (field_name.eqlSlice("len", ip)) {
                    const slice = if (is_pointer_to)
                        try sema.analyzeLoad(block, src, object, object_src)
                    else
                        object;
                    return sema.analyzeSliceLen(block, src, slice);
                } else {
                    return sema.fail(
                        block,
                        field_name_src,
                        "no member named '{}' in '{}'",
                        .{ field_name.fmt(ip), object_ty.fmt(pt) },
                    );
                }
            }
        },
        .type => {
            const dereffed_type = if (is_pointer_to)
                try sema.analyzeLoad(block, src, object, object_src)
            else
                object;

            const val = (try sema.resolveDefinedValue(block, object_src, dereffed_type)).?;
            const child_type = val.toType();

            switch (child_type.zigTypeTag(zcu)) {
                .error_set => {
                    switch (ip.indexToKey(child_type.toIntern())) {
                        .error_set_type => |error_set_type| blk: {
                            if (error_set_type.nameIndex(ip, field_name) != null) break :blk;
                            return sema.fail(block, src, "no error named '{}' in '{}'", .{
                                field_name.fmt(ip), child_type.fmt(pt),
                            });
                        },
                        .inferred_error_set_type => {
                            return sema.fail(block, src, "TODO handle inferred error sets here", .{});
                        },
                        .simple_type => |t| {
                            assert(t == .anyerror);
                            _ = try pt.getErrorValue(field_name);
                        },
                        else => unreachable,
                    }

                    const error_set_type = if (!child_type.isAnyError(zcu))
                        child_type
                    else
                        try pt.singleErrorSetType(field_name);
                    return Air.internedToRef((try pt.intern(.{ .err = .{
                        .ty = error_set_type.toIntern(),
                        .name = field_name,
                    } })));
                },
                .@"union" => {
                    if (try sema.namespaceLookupVal(block, src, child_type.getNamespaceIndex(zcu), field_name)) |inst| {
                        return inst;
                    }
                    try child_type.resolveFields(pt);
                    if (child_type.unionTagType(zcu)) |enum_ty| {
                        if (enum_ty.enumFieldIndex(field_name, zcu)) |field_index_usize| {
                            const field_index: u32 = @intCast(field_index_usize);
                            return Air.internedToRef((try pt.enumValueFieldIndex(enum_ty, field_index)).toIntern());
                        }
                    }
                    return sema.failWithBadMemberAccess(block, child_type, field_name_src, field_name);
                },
                .@"enum" => {
                    if (try sema.namespaceLookupVal(block, src, child_type.getNamespaceIndex(zcu), field_name)) |inst| {
                        return inst;
                    }
                    const field_index_usize = child_type.enumFieldIndex(field_name, zcu) orelse
                        return sema.failWithBadMemberAccess(block, child_type, field_name_src, field_name);
                    const field_index: u32 = @intCast(field_index_usize);
                    const enum_val = try pt.enumValueFieldIndex(child_type, field_index);
                    return Air.internedToRef(enum_val.toIntern());
                },
                .@"struct", .@"opaque" => {
                    if (!child_type.isTuple(zcu) and child_type.toIntern() != .anyopaque_type) {
                        if (try sema.namespaceLookupVal(block, src, child_type.getNamespaceIndex(zcu), field_name)) |inst| {
                            return inst;
                        }
                    }
                    return sema.failWithBadMemberAccess(block, child_type, src, field_name);
                },
                else => return sema.failWithOwnedErrorMsg(block, msg: {
                    const msg = try sema.errMsg(src, "type '{}' has no members", .{child_type.fmt(pt)});
                    errdefer msg.destroy(sema.gpa);
                    if (child_type.isSlice(zcu)) try sema.errNote(src, msg, "slice values have 'len' and 'ptr' members", .{});
                    if (child_type.zigTypeTag(zcu) == .array) try sema.errNote(src, msg, "array values have 'len' member", .{});
                    break :msg msg;
                }),
            }
        },
        .@"struct" => if (is_pointer_to) {
            // Avoid loading the entire struct by fetching a pointer and loading that
            const field_ptr = try sema.structFieldPtr(block, src, object, field_name, field_name_src, inner_ty, false);
            return sema.analyzeLoad(block, src, field_ptr, object_src);
        } else {
            return sema.structFieldVal(block, object, field_name, field_name_src, inner_ty);
        },
        .@"union" => if (is_pointer_to) {
            // Avoid loading the entire union by fetching a pointer and loading that
            const field_ptr = try sema.unionFieldPtr(block, src, object, field_name, field_name_src, inner_ty, false);
            return sema.analyzeLoad(block, src, field_ptr, object_src);
        } else {
            return sema.unionFieldVal(block, src, object, field_name, field_name_src, inner_ty);
        },
        else => {},
    }
    return sema.failWithInvalidFieldAccess(block, src, object_ty, field_name);
}

fn fieldPtr(
    sema: *Sema,
    block: *Block,
    src: LazySrcLoc,
    object_ptr: Air.Inst.Ref,
    field_name: InternPool.NullTerminatedString,
    field_name_src: LazySrcLoc,
    initializing: bool,
) CompileError!Air.Inst.Ref {
    // When editing this function, note that there is corresponding logic to be edited
    // in `fieldVal`. This function takes a pointer and returns a pointer.

    const pt = sema.pt;
    const zcu = pt.zcu;
    const ip = &zcu.intern_pool;
    const object_ptr_src = src; // TODO better source location
    const object_ptr_ty = sema.typeOf(object_ptr);
    const object_ty = switch (object_ptr_ty.zigTypeTag(zcu)) {
        .pointer => object_ptr_ty.childType(zcu),
        else => return sema.fail(block, object_ptr_src, "expected pointer, found '{}'", .{object_ptr_ty.fmt(pt)}),
    };

    // Zig allows dereferencing a single pointer during field lookup. Note that
    // we don't actually need to generate the dereference some field lookups, like the
    // length of arrays and other comptime operations.
    const is_pointer_to = object_ty.isSinglePointer(zcu);

    const inner_ty = if (is_pointer_to)
        object_ty.childType(zcu)
    else
        object_ty;

    switch (inner_ty.zigTypeTag(zcu)) {
        .array => {
            if (field_name.eqlSlice("len", ip)) {
                const int_val = try pt.intValue(.usize, inner_ty.arrayLen(zcu));
                return uavRef(sema, int_val.toIntern());
            } else if (field_name.eqlSlice("ptr", ip) and is_pointer_to) {
                const ptr_info = object_ty.ptrInfo(zcu);
                const new_ptr_ty = try pt.ptrTypeSema(.{
                    .child = Type.fromInterned(ptr_info.child).childType(zcu).toIntern(),
                    .sentinel = if (object_ty.sentinel(zcu)) |s| s.toIntern() else .none,
                    .flags = .{
                        .size = .many,
                        .alignment = ptr_info.flags.alignment,
                        .is_const = ptr_info.flags.is_const,
                        .is_volatile = ptr_info.flags.is_volatile,
                        .is_allowzero = ptr_info.flags.is_allowzero,
                        .address_space = ptr_info.flags.address_space,
                        .vector_index = ptr_info.flags.vector_index,
                    },
                    .packed_offset = ptr_info.packed_offset,
                });
                const ptr_ptr_info = object_ptr_ty.ptrInfo(zcu);
                const result_ty = try pt.ptrTypeSema(.{
                    .child = new_ptr_ty.toIntern(),
                    .sentinel = if (object_ptr_ty.sentinel(zcu)) |s| s.toIntern() else .none,
                    .flags = .{
                        .alignment = ptr_ptr_info.flags.alignment,
                        .is_const = ptr_ptr_info.flags.is_const,
                        .is_volatile = ptr_ptr_info.flags.is_volatile,
                        .is_allowzero = ptr_ptr_info.flags.is_allowzero,
                        .address_space = ptr_ptr_info.flags.address_space,
                        .vector_index = ptr_ptr_info.flags.vector_index,
                    },
                    .packed_offset = ptr_ptr_info.packed_offset,
                });
                return sema.bitCast(block, result_ty, object_ptr, src, null);
            } else {
                return sema.fail(
                    block,
                    field_name_src,
                    "no member named '{}' in '{}'",
                    .{ field_name.fmt(ip), object_ty.fmt(pt) },
                );
            }
        },
        .pointer => if (inner_ty.isSlice(zcu)) {
            const inner_ptr = if (is_pointer_to)
                try sema.analyzeLoad(block, src, object_ptr, object_ptr_src)
            else
                object_ptr;

            const attr_ptr_ty = if (is_pointer_to) object_ty else object_ptr_ty;

            if (field_name.eqlSlice("ptr", ip)) {
                const slice_ptr_ty = inner_ty.slicePtrFieldType(zcu);

                const result_ty = try pt.ptrTypeSema(.{
                    .child = slice_ptr_ty.toIntern(),
                    .flags = .{
                        .is_const = !attr_ptr_ty.ptrIsMutable(zcu),
                        .is_volatile = attr_ptr_ty.isVolatilePtr(zcu),
                        .address_space = attr_ptr_ty.ptrAddressSpace(zcu),
                    },
                });

                if (try sema.resolveDefinedValue(block, object_ptr_src, inner_ptr)) |val| {
                    return Air.internedToRef((try val.ptrField(Value.slice_ptr_index, pt)).toIntern());
                }
                try sema.requireRuntimeBlock(block, src, null);

                const field_ptr = try block.addTyOp(.ptr_slice_ptr_ptr, result_ty, inner_ptr);
                try sema.checkKnownAllocPtr(block, inner_ptr, field_ptr);
                return field_ptr;
            } else if (field_name.eqlSlice("len", ip)) {
                const result_ty = try pt.ptrTypeSema(.{
                    .child = .usize_type,
                    .flags = .{
                        .is_const = !attr_ptr_ty.ptrIsMutable(zcu),
                        .is_volatile = attr_ptr_ty.isVolatilePtr(zcu),
                        .address_space = attr_ptr_ty.ptrAddressSpace(zcu),
                    },
                });

                if (try sema.resolveDefinedValue(block, object_ptr_src, inner_ptr)) |val| {
                    return Air.internedToRef((try val.ptrField(Value.slice_len_index, pt)).toIntern());
                }
                try sema.requireRuntimeBlock(block, src, null);

                const field_ptr = try block.addTyOp(.ptr_slice_len_ptr, result_ty, inner_ptr);
                try sema.checkKnownAllocPtr(block, inner_ptr, field_ptr);
                return field_ptr;
            } else {
                return sema.fail(
                    block,
                    field_name_src,
                    "no member named '{}' in '{}'",
                    .{ field_name.fmt(ip), object_ty.fmt(pt) },
                );
            }
        },
        .type => {
            _ = try sema.resolveConstDefinedValue(block, LazySrcLoc.unneeded, object_ptr, undefined);
            const result = try sema.analyzeLoad(block, src, object_ptr, object_ptr_src);
            const inner = if (is_pointer_to)
                try sema.analyzeLoad(block, src, result, object_ptr_src)
            else
                result;

            const val = (sema.resolveDefinedValue(block, src, inner) catch unreachable).?;
            const child_type = val.toType();

            switch (child_type.zigTypeTag(zcu)) {
                .error_set => {
                    switch (ip.indexToKey(child_type.toIntern())) {
                        .error_set_type => |error_set_type| blk: {
                            if (error_set_type.nameIndex(ip, field_name) != null) {
                                break :blk;
                            }
                            return sema.fail(block, src, "no error named '{}' in '{}'", .{
                                field_name.fmt(ip), child_type.fmt(pt),
                            });
                        },
                        .inferred_error_set_type => {
                            return sema.fail(block, src, "TODO handle inferred error sets here", .{});
                        },
                        .simple_type => |t| {
                            assert(t == .anyerror);
                            _ = try pt.getErrorValue(field_name);
                        },
                        else => unreachable,
                    }

                    const error_set_type = if (!child_type.isAnyError(zcu))
                        child_type
                    else
                        try pt.singleErrorSetType(field_name);
                    return uavRef(sema, try pt.intern(.{ .err = .{
                        .ty = error_set_type.toIntern(),
                        .name = field_name,
                    } }));
                },
                .@"union" => {
                    if (try sema.namespaceLookupRef(block, src, child_type.getNamespaceIndex(zcu), field_name)) |inst| {
                        return inst;
                    }
                    try child_type.resolveFields(pt);
                    if (child_type.unionTagType(zcu)) |enum_ty| {
                        if (enum_ty.enumFieldIndex(field_name, zcu)) |field_index| {
                            const field_index_u32: u32 = @intCast(field_index);
                            const idx_val = try pt.enumValueFieldIndex(enum_ty, field_index_u32);
                            return uavRef(sema, idx_val.toIntern());
                        }
                    }
                    return sema.failWithBadMemberAccess(block, child_type, field_name_src, field_name);
                },
                .@"enum" => {
                    if (try sema.namespaceLookupRef(block, src, child_type.getNamespaceIndex(zcu), field_name)) |inst| {
                        return inst;
                    }
                    const field_index = child_type.enumFieldIndex(field_name, zcu) orelse {
                        return sema.failWithBadMemberAccess(block, child_type, field_name_src, field_name);
                    };
                    const field_index_u32: u32 = @intCast(field_index);
                    const idx_val = try pt.enumValueFieldIndex(child_type, field_index_u32);
                    return uavRef(sema, idx_val.toIntern());
                },
                .@"struct", .@"opaque" => {
                    if (try sema.namespaceLookupRef(block, src, child_type.getNamespaceIndex(zcu), field_name)) |inst| {
                        return inst;
                    }
                    return sema.failWithBadMemberAccess(block, child_type, field_name_src, field_name);
                },
                else => return sema.fail(block, src, "type '{}' has no members", .{child_type.fmt(pt)}),
            }
        },
        .@"struct" => {
            const inner_ptr = if (is_pointer_to)
                try sema.analyzeLoad(block, src, object_ptr, object_ptr_src)
            else
                object_ptr;
            const field_ptr = try sema.structFieldPtr(block, src, inner_ptr, field_name, field_name_src, inner_ty, initializing);
            try sema.checkKnownAllocPtr(block, inner_ptr, field_ptr);
            return field_ptr;
        },
        .@"union" => {
            const inner_ptr = if (is_pointer_to)
                try sema.analyzeLoad(block, src, object_ptr, object_ptr_src)
            else
                object_ptr;
            const field_ptr = try sema.unionFieldPtr(block, src, inner_ptr, field_name, field_name_src, inner_ty, initializing);
            try sema.checkKnownAllocPtr(block, inner_ptr, field_ptr);
            return field_ptr;
        },
        else => {},
    }
    return sema.failWithInvalidFieldAccess(block, src, object_ty, field_name);
}

const ResolvedFieldCallee = union(enum) {
    /// The LHS of the call was an actual field with this value.
    direct: Air.Inst.Ref,
    /// This is a method call, with the function and first argument given.
    method: struct {
        func_inst: Air.Inst.Ref,
        arg0_inst: Air.Inst.Ref,
    },
};

fn fieldCallBind(
    sema: *Sema,
    block: *Block,
    src: LazySrcLoc,
    raw_ptr: Air.Inst.Ref,
    field_name: InternPool.NullTerminatedString,
    field_name_src: LazySrcLoc,
) CompileError!ResolvedFieldCallee {
    // When editing this function, note that there is corresponding logic to be edited
    // in `fieldVal`. This function takes a pointer and returns a pointer.

    const pt = sema.pt;
    const zcu = pt.zcu;
    const ip = &zcu.intern_pool;
    const raw_ptr_src = src; // TODO better source location
    const raw_ptr_ty = sema.typeOf(raw_ptr);
    const inner_ty = if (raw_ptr_ty.zigTypeTag(zcu) == .pointer and (raw_ptr_ty.ptrSize(zcu) == .one or raw_ptr_ty.ptrSize(zcu) == .c))
        raw_ptr_ty.childType(zcu)
    else
        return sema.fail(block, raw_ptr_src, "expected single pointer, found '{}'", .{raw_ptr_ty.fmt(pt)});

    // Optionally dereference a second pointer to get the concrete type.
    const is_double_ptr = inner_ty.zigTypeTag(zcu) == .pointer and inner_ty.ptrSize(zcu) == .one;
    const concrete_ty = if (is_double_ptr) inner_ty.childType(zcu) else inner_ty;
    const ptr_ty = if (is_double_ptr) inner_ty else raw_ptr_ty;
    const object_ptr = if (is_double_ptr)
        try sema.analyzeLoad(block, src, raw_ptr, src)
    else
        raw_ptr;

    find_field: {
        switch (concrete_ty.zigTypeTag(zcu)) {
            .@"struct" => {
                try concrete_ty.resolveFields(pt);
                if (zcu.typeToStruct(concrete_ty)) |struct_type| {
                    const field_index = struct_type.nameIndex(ip, field_name) orelse
                        break :find_field;
                    const field_ty: Type = .fromInterned(struct_type.field_types.get(ip)[field_index]);

                    return sema.finishFieldCallBind(block, src, ptr_ty, field_ty, field_index, object_ptr);
                } else if (concrete_ty.isTuple(zcu)) {
                    if (field_name.eqlSlice("len", ip)) {
                        return .{ .direct = try pt.intRef(.usize, concrete_ty.structFieldCount(zcu)) };
                    }
                    if (field_name.toUnsigned(ip)) |field_index| {
                        if (field_index >= concrete_ty.structFieldCount(zcu)) break :find_field;
                        return sema.finishFieldCallBind(block, src, ptr_ty, concrete_ty.fieldType(field_index, zcu), field_index, object_ptr);
                    }
                } else {
                    const max = concrete_ty.structFieldCount(zcu);
                    for (0..max) |i_usize| {
                        const i: u32 = @intCast(i_usize);
                        if (field_name == concrete_ty.structFieldName(i, zcu).unwrap().?) {
                            return sema.finishFieldCallBind(block, src, ptr_ty, concrete_ty.fieldType(i, zcu), i, object_ptr);
                        }
                    }
                }
            },
            .@"union" => {
                try concrete_ty.resolveFields(pt);
                const union_obj = zcu.typeToUnion(concrete_ty).?;
                _ = union_obj.loadTagType(ip).nameIndex(ip, field_name) orelse break :find_field;
                const field_ptr = try unionFieldPtr(sema, block, src, object_ptr, field_name, field_name_src, concrete_ty, false);
                return .{ .direct = try sema.analyzeLoad(block, src, field_ptr, src) };
            },
            .type => {
                const namespace = try sema.analyzeLoad(block, src, object_ptr, src);
                return .{ .direct = try sema.fieldVal(block, src, namespace, field_name, field_name_src) };
            },
            else => {},
        }
    }

    // If we get here, we need to look for a decl in the struct type instead.
    const found_nav = found_nav: {
        const namespace = concrete_ty.getNamespace(zcu).unwrap() orelse
            break :found_nav null;
        const nav_index = try sema.namespaceLookup(block, src, namespace, field_name) orelse
            break :found_nav null;

        const decl_val = try sema.analyzeNavVal(block, src, nav_index);
        const decl_type = sema.typeOf(decl_val);
        if (zcu.typeToFunc(decl_type)) |func_type| f: {
            if (func_type.param_types.len == 0) break :f;

            const first_param_type: Type = .fromInterned(func_type.param_types.get(ip)[0]);
            if (first_param_type.isGenericPoison() or
                (first_param_type.zigTypeTag(zcu) == .pointer and
                    (first_param_type.ptrSize(zcu) == .one or
                        first_param_type.ptrSize(zcu) == .c) and
                    first_param_type.childType(zcu).eql(concrete_ty, zcu)))
            {
                // Note that if the param type is generic poison, we know that it must
                // specifically be `anytype` since it's the first parameter, meaning we
                // can safely assume it can be a pointer.
                // TODO: bound fn calls on rvalues should probably
                // generate a by-value argument somehow.
                return .{ .method = .{
                    .func_inst = decl_val,
                    .arg0_inst = object_ptr,
                } };
            } else if (first_param_type.eql(concrete_ty, zcu)) {
                const deref = try sema.analyzeLoad(block, src, object_ptr, src);
                return .{ .method = .{
                    .func_inst = decl_val,
                    .arg0_inst = deref,
                } };
            } else if (first_param_type.zigTypeTag(zcu) == .optional) {
                const child = first_param_type.optionalChild(zcu);
                if (child.eql(concrete_ty, zcu)) {
                    const deref = try sema.analyzeLoad(block, src, object_ptr, src);
                    return .{ .method = .{
                        .func_inst = decl_val,
                        .arg0_inst = deref,
                    } };
                } else if (child.zigTypeTag(zcu) == .pointer and
                    child.ptrSize(zcu) == .one and
                    child.childType(zcu).eql(concrete_ty, zcu))
                {
                    return .{ .method = .{
                        .func_inst = decl_val,
                        .arg0_inst = object_ptr,
                    } };
                }
            } else if (first_param_type.zigTypeTag(zcu) == .error_union and
                first_param_type.errorUnionPayload(zcu).eql(concrete_ty, zcu))
            {
                const deref = try sema.analyzeLoad(block, src, object_ptr, src);
                return .{ .method = .{
                    .func_inst = decl_val,
                    .arg0_inst = deref,
                } };
            }
        }
        break :found_nav nav_index;
    };

    const msg = msg: {
        const msg = try sema.errMsg(src, "no field or member function named '{}' in '{}'", .{
            field_name.fmt(ip),
            concrete_ty.fmt(pt),
        });
        errdefer msg.destroy(sema.gpa);
        try sema.addDeclaredHereNote(msg, concrete_ty);
        if (found_nav) |nav_index| {
            try sema.errNote(
                zcu.navSrcLoc(nav_index),
                msg,
                "'{}' is not a member function",
                .{field_name.fmt(ip)},
            );
        }
        if (concrete_ty.zigTypeTag(zcu) == .error_union) {
            try sema.errNote(src, msg, "consider using 'try', 'catch', or 'if'", .{});
        }
        if (is_double_ptr) {
            try sema.errNote(src, msg, "method invocation only supports up to one level of implicit pointer dereferencing", .{});
            try sema.errNote(src, msg, "use '.*' to dereference pointer", .{});
        }
        break :msg msg;
    };
    return sema.failWithOwnedErrorMsg(block, msg);
}

fn finishFieldCallBind(
    sema: *Sema,
    block: *Block,
    src: LazySrcLoc,
    ptr_ty: Type,
    field_ty: Type,
    field_index: u32,
    object_ptr: Air.Inst.Ref,
) CompileError!ResolvedFieldCallee {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const ptr_field_ty = try pt.ptrTypeSema(.{
        .child = field_ty.toIntern(),
        .flags = .{
            .is_const = !ptr_ty.ptrIsMutable(zcu),
            .address_space = ptr_ty.ptrAddressSpace(zcu),
        },
    });

    const container_ty = ptr_ty.childType(zcu);
    if (container_ty.zigTypeTag(zcu) == .@"struct") {
        if (container_ty.structFieldIsComptime(field_index, zcu)) {
            try container_ty.resolveStructFieldInits(pt);
            const default_val = (try container_ty.structFieldValueComptime(pt, field_index)).?;
            return .{ .direct = Air.internedToRef(default_val.toIntern()) };
        }
    }

    if (try sema.resolveDefinedValue(block, src, object_ptr)) |struct_ptr_val| {
        const ptr_val = try struct_ptr_val.ptrField(field_index, pt);
        const pointer = Air.internedToRef(ptr_val.toIntern());
        return .{ .direct = try sema.analyzeLoad(block, src, pointer, src) };
    }

    try sema.requireRuntimeBlock(block, src, null);
    const ptr_inst = try block.addStructFieldPtr(object_ptr, field_index, ptr_field_ty);
    return .{ .direct = try sema.analyzeLoad(block, src, ptr_inst, src) };
}

fn namespaceLookup(
    sema: *Sema,
    block: *Block,
    src: LazySrcLoc,
    namespace: InternPool.NamespaceIndex,
    decl_name: InternPool.NullTerminatedString,
) CompileError!?InternPool.Nav.Index {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const gpa = sema.gpa;
    if (try sema.lookupInNamespace(block, src, namespace, decl_name, true)) |lookup| {
        if (!lookup.accessible) {
            return sema.failWithOwnedErrorMsg(block, msg: {
                const msg = try sema.errMsg(src, "'{}' is not marked 'pub'", .{
                    decl_name.fmt(&zcu.intern_pool),
                });
                errdefer msg.destroy(gpa);
                try sema.errNote(zcu.navSrcLoc(lookup.nav), msg, "declared here", .{});
                break :msg msg;
            });
        }
        return lookup.nav;
    }
    return null;
}

fn namespaceLookupRef(
    sema: *Sema,
    block: *Block,
    src: LazySrcLoc,
    namespace: InternPool.NamespaceIndex,
    decl_name: InternPool.NullTerminatedString,
) CompileError!?Air.Inst.Ref {
    const nav = try sema.namespaceLookup(block, src, namespace, decl_name) orelse return null;
    return try sema.analyzeNavRef(block, src, nav);
}

fn namespaceLookupVal(
    sema: *Sema,
    block: *Block,
    src: LazySrcLoc,
    namespace: InternPool.NamespaceIndex,
    decl_name: InternPool.NullTerminatedString,
) CompileError!?Air.Inst.Ref {
    const nav = try sema.namespaceLookup(block, src, namespace, decl_name) orelse return null;
    return try sema.analyzeNavVal(block, src, nav);
}

fn structFieldPtr(
    sema: *Sema,
    block: *Block,
    src: LazySrcLoc,
    struct_ptr: Air.Inst.Ref,
    field_name: InternPool.NullTerminatedString,
    field_name_src: LazySrcLoc,
    struct_ty: Type,
    initializing: bool,
) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const ip = &zcu.intern_pool;
    assert(struct_ty.zigTypeTag(zcu) == .@"struct");

    try struct_ty.resolveFields(pt);
    try struct_ty.resolveLayout(pt);

    if (struct_ty.isTuple(zcu)) {
        if (field_name.eqlSlice("len", ip)) {
            const len_inst = try pt.intRef(.usize, struct_ty.structFieldCount(zcu));
            return sema.analyzeRef(block, src, len_inst);
        }
        const field_index = try sema.tupleFieldIndex(block, struct_ty, field_name, field_name_src);
        return sema.tupleFieldPtr(block, src, struct_ptr, field_name_src, field_index, initializing);
    }

    const struct_type = zcu.typeToStruct(struct_ty).?;

    const field_index = struct_type.nameIndex(ip, field_name) orelse
        return sema.failWithBadStructFieldAccess(block, struct_ty, struct_type, field_name_src, field_name);

    return sema.structFieldPtrByIndex(block, src, struct_ptr, field_index, struct_ty);
}

fn structFieldPtrByIndex(
    sema: *Sema,
    block: *Block,
    src: LazySrcLoc,
    struct_ptr: Air.Inst.Ref,
    field_index: u32,
    struct_ty: Type,
) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const ip = &zcu.intern_pool;

    const struct_type = zcu.typeToStruct(struct_ty).?;
    const field_is_comptime = struct_type.fieldIsComptime(ip, field_index);

    // Comptime fields are handled later
    if (!field_is_comptime) {
        if (try sema.resolveDefinedValue(block, src, struct_ptr)) |struct_ptr_val| {
            const val = try struct_ptr_val.ptrField(field_index, pt);
            return Air.internedToRef(val.toIntern());
        }
    }

    const field_ty = struct_type.field_types.get(ip)[field_index];
    const struct_ptr_ty = sema.typeOf(struct_ptr);
    const struct_ptr_ty_info = struct_ptr_ty.ptrInfo(zcu);

    var ptr_ty_data: InternPool.Key.PtrType = .{
        .child = field_ty,
        .flags = .{
            .is_const = struct_ptr_ty_info.flags.is_const,
            .is_volatile = struct_ptr_ty_info.flags.is_volatile,
            .address_space = struct_ptr_ty_info.flags.address_space,
        },
    };

    const parent_align = if (struct_ptr_ty_info.flags.alignment != .none)
        struct_ptr_ty_info.flags.alignment
    else
        try Type.fromInterned(struct_ptr_ty_info.child).abiAlignmentSema(pt);

    if (struct_type.layout == .@"packed") {
        assert(!field_is_comptime);
        switch (struct_ty.packedStructFieldPtrInfo(struct_ptr_ty, field_index, pt)) {
            .bit_ptr => |packed_offset| {
                ptr_ty_data.flags.alignment = parent_align;
                ptr_ty_data.packed_offset = packed_offset;
            },
            .byte_ptr => |ptr_info| {
                ptr_ty_data.flags.alignment = ptr_info.alignment;
            },
        }
    } else if (struct_type.layout == .@"extern") {
        assert(!field_is_comptime);
        // For extern structs, field alignment might be bigger than type's
        // natural alignment. Eg, in `extern struct { x: u32, y: u16 }` the
        // second field is aligned as u32.
        const field_offset = struct_ty.structFieldOffset(field_index, zcu);
        ptr_ty_data.flags.alignment = if (parent_align == .none)
            .none
        else
            @enumFromInt(@min(@intFromEnum(parent_align), @ctz(field_offset)));
    } else {
        // Our alignment is capped at the field alignment.
        const field_align = try Type.fromInterned(field_ty).structFieldAlignmentSema(
            struct_type.fieldAlign(ip, field_index),
            struct_type.layout,
            pt,
        );
        ptr_ty_data.flags.alignment = if (struct_ptr_ty_info.flags.alignment == .none)
            field_align
        else
            field_align.min(parent_align);
    }

    const ptr_field_ty = try pt.ptrTypeSema(ptr_ty_data);

    if (field_is_comptime) {
        try struct_ty.resolveStructFieldInits(pt);
        const val = try pt.intern(.{ .ptr = .{
            .ty = ptr_field_ty.toIntern(),
            .base_addr = .{ .comptime_field = struct_type.field_inits.get(ip)[field_index] },
            .byte_offset = 0,
        } });
        return Air.internedToRef(val);
    }

    return block.addStructFieldPtr(struct_ptr, field_index, ptr_field_ty);
}

fn structFieldVal(
    sema: *Sema,
    block: *Block,
    struct_byval: Air.Inst.Ref,
    field_name: InternPool.NullTerminatedString,
    field_name_src: LazySrcLoc,
    struct_ty: Type,
) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const ip = &zcu.intern_pool;
    assert(struct_ty.zigTypeTag(zcu) == .@"struct");

    try struct_ty.resolveFields(pt);

    switch (ip.indexToKey(struct_ty.toIntern())) {
        .struct_type => {
            const struct_type = ip.loadStructType(struct_ty.toIntern());

            const field_index = struct_type.nameIndex(ip, field_name) orelse
                return sema.failWithBadStructFieldAccess(block, struct_ty, struct_type, field_name_src, field_name);
            if (struct_type.fieldIsComptime(ip, field_index)) {
                try struct_ty.resolveStructFieldInits(pt);
                return Air.internedToRef(struct_type.field_inits.get(ip)[field_index]);
            }

            const field_ty: Type = .fromInterned(struct_type.field_types.get(ip)[field_index]);
            if (try sema.typeHasOnePossibleValue(field_ty)) |field_val|
                return Air.internedToRef(field_val.toIntern());

            if (try sema.resolveValue(struct_byval)) |struct_val| {
                if (struct_val.isUndef(zcu)) return pt.undefRef(field_ty);
                if ((try sema.typeHasOnePossibleValue(field_ty))) |opv| {
                    return Air.internedToRef(opv.toIntern());
                }
                return Air.internedToRef((try struct_val.fieldValue(pt, field_index)).toIntern());
            }

            try field_ty.resolveLayout(pt);
            return block.addStructFieldVal(struct_byval, field_index, field_ty);
        },
        .tuple_type => {
            return sema.tupleFieldVal(block, struct_byval, field_name, field_name_src, struct_ty);
        },
        else => unreachable,
    }
}

fn tupleFieldVal(
    sema: *Sema,
    block: *Block,
    tuple_byval: Air.Inst.Ref,
    field_name: InternPool.NullTerminatedString,
    field_name_src: LazySrcLoc,
    tuple_ty: Type,
) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    if (field_name.eqlSlice("len", &zcu.intern_pool)) {
        return pt.intRef(.usize, tuple_ty.structFieldCount(zcu));
    }
    const field_index = try sema.tupleFieldIndex(block, tuple_ty, field_name, field_name_src);
    return sema.tupleFieldValByIndex(block, tuple_byval, field_index, tuple_ty);
}

/// Asserts that `field_name` is not "len".
fn tupleFieldIndex(
    sema: *Sema,
    block: *Block,
    tuple_ty: Type,
    field_name: InternPool.NullTerminatedString,
    field_name_src: LazySrcLoc,
) CompileError!u32 {
    const pt = sema.pt;
    const ip = &pt.zcu.intern_pool;
    assert(!field_name.eqlSlice("len", ip));
    if (field_name.toUnsigned(ip)) |field_index| {
        if (field_index < tuple_ty.structFieldCount(pt.zcu)) return field_index;
        return sema.fail(block, field_name_src, "index '{}' out of bounds of tuple '{}'", .{
            field_name.fmt(ip), tuple_ty.fmt(pt),
        });
    }

    return sema.fail(block, field_name_src, "no field named '{}' in tuple '{}'", .{
        field_name.fmt(ip), tuple_ty.fmt(pt),
    });
}

fn tupleFieldValByIndex(
    sema: *Sema,
    block: *Block,
    tuple_byval: Air.Inst.Ref,
    field_index: u32,
    tuple_ty: Type,
) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const field_ty = tuple_ty.fieldType(field_index, zcu);

    if (tuple_ty.structFieldIsComptime(field_index, zcu))
        try tuple_ty.resolveStructFieldInits(pt);
    if (try tuple_ty.structFieldValueComptime(pt, field_index)) |default_value| {
        return Air.internedToRef(default_value.toIntern());
    }

    if (try sema.resolveValue(tuple_byval)) |tuple_val| {
        if ((try sema.typeHasOnePossibleValue(field_ty))) |opv| {
            return Air.internedToRef(opv.toIntern());
        }
        return switch (zcu.intern_pool.indexToKey(tuple_val.toIntern())) {
            .undef => pt.undefRef(field_ty),
            .aggregate => |aggregate| Air.internedToRef(switch (aggregate.storage) {
                .bytes => |bytes| try pt.intValue(.u8, bytes.at(field_index, &zcu.intern_pool)),
                .elems => |elems| Value.fromInterned(elems[field_index]),
                .repeated_elem => |elem| Value.fromInterned(elem),
            }.toIntern()),
            else => unreachable,
        };
    }

    try field_ty.resolveLayout(pt);
    return block.addStructFieldVal(tuple_byval, field_index, field_ty);
}

fn unionFieldPtr(
    sema: *Sema,
    block: *Block,
    src: LazySrcLoc,
    union_ptr: Air.Inst.Ref,
    field_name: InternPool.NullTerminatedString,
    field_name_src: LazySrcLoc,
    union_ty: Type,
    initializing: bool,
) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const ip = &zcu.intern_pool;

    assert(union_ty.zigTypeTag(zcu) == .@"union");

    const union_ptr_ty = sema.typeOf(union_ptr);
    const union_ptr_info = union_ptr_ty.ptrInfo(zcu);
    try union_ty.resolveFields(pt);
    const union_obj = zcu.typeToUnion(union_ty).?;
    const field_index = try sema.unionFieldIndex(block, union_ty, field_name, field_name_src);
    const field_ty: Type = .fromInterned(union_obj.field_types.get(ip)[field_index]);
    const ptr_field_ty = try pt.ptrTypeSema(.{
        .child = field_ty.toIntern(),
        .flags = .{
            .is_const = union_ptr_info.flags.is_const,
            .is_volatile = union_ptr_info.flags.is_volatile,
            .address_space = union_ptr_info.flags.address_space,
            .alignment = if (union_obj.flagsUnordered(ip).layout == .auto) blk: {
                const union_align = if (union_ptr_info.flags.alignment != .none)
                    union_ptr_info.flags.alignment
                else
                    try union_ty.abiAlignmentSema(pt);
                const field_align = try union_ty.fieldAlignmentSema(field_index, pt);
                break :blk union_align.min(field_align);
            } else union_ptr_info.flags.alignment,
        },
        .packed_offset = union_ptr_info.packed_offset,
    });
    const enum_field_index: u32 = @intCast(Type.fromInterned(union_obj.enum_tag_ty).enumFieldIndex(field_name, zcu).?);

    if (initializing and field_ty.zigTypeTag(zcu) == .noreturn) {
        const msg = msg: {
            const msg = try sema.errMsg(src, "cannot initialize 'noreturn' field of union", .{});
            errdefer msg.destroy(sema.gpa);

            try sema.addFieldErrNote(union_ty, field_index, msg, "field '{}' declared here", .{
                field_name.fmt(ip),
            });
            try sema.addDeclaredHereNote(msg, union_ty);
            break :msg msg;
        };
        return sema.failWithOwnedErrorMsg(block, msg);
    }

    if (try sema.resolveDefinedValue(block, src, union_ptr)) |union_ptr_val| ct: {
        switch (union_obj.flagsUnordered(ip).layout) {
            .auto => if (initializing) {
                if (!sema.isComptimeMutablePtr(union_ptr_val)) {
                    // The initialization is a runtime operation.
                    break :ct;
                }
                // Store to the union to initialize the tag.
                const field_tag = try pt.enumValueFieldIndex(.fromInterned(union_obj.enum_tag_ty), enum_field_index);
                const payload_ty: Type = .fromInterned(union_obj.field_types.get(ip)[field_index]);
                const new_union_val = try pt.unionValue(union_ty, field_tag, try pt.undefValue(payload_ty));
                try sema.storePtrVal(block, src, union_ptr_val, new_union_val, union_ty);
            } else {
                const union_val = (try sema.pointerDeref(block, src, union_ptr_val, union_ptr_ty)) orelse
                    break :ct;
                if (union_val.isUndef(zcu)) {
                    return sema.failWithUseOfUndef(block, src);
                }
                const un = ip.indexToKey(union_val.toIntern()).un;
                const field_tag = try pt.enumValueFieldIndex(.fromInterned(union_obj.enum_tag_ty), enum_field_index);
                const tag_matches = un.tag == field_tag.toIntern();
                if (!tag_matches) {
                    const msg = msg: {
                        const active_index = Type.fromInterned(union_obj.enum_tag_ty).enumTagFieldIndex(Value.fromInterned(un.tag), zcu).?;
                        const active_field_name = Type.fromInterned(union_obj.enum_tag_ty).enumFieldName(active_index, zcu);
                        const msg = try sema.errMsg(src, "access of union field '{}' while field '{}' is active", .{
                            field_name.fmt(ip),
                            active_field_name.fmt(ip),
                        });
                        errdefer msg.destroy(sema.gpa);
                        try sema.addDeclaredHereNote(msg, union_ty);
                        break :msg msg;
                    };
                    return sema.failWithOwnedErrorMsg(block, msg);
                }
            },
            .@"packed", .@"extern" => {},
        }
        const field_ptr_val = try union_ptr_val.ptrField(field_index, pt);
        return Air.internedToRef(field_ptr_val.toIntern());
    }

    if (!initializing and union_obj.flagsUnordered(ip).layout == .auto and block.wantSafety() and
        union_ty.unionTagTypeSafety(zcu) != null and union_obj.field_types.len > 1)
    {
        const wanted_tag_val = try pt.enumValueFieldIndex(.fromInterned(union_obj.enum_tag_ty), enum_field_index);
        const wanted_tag = Air.internedToRef(wanted_tag_val.toIntern());
        // TODO would it be better if get_union_tag supported pointers to unions?
        const union_val = try block.addTyOp(.load, union_ty, union_ptr);
        const active_tag = try block.addTyOp(.get_union_tag, .fromInterned(union_obj.enum_tag_ty), union_val);
        try sema.addSafetyCheckInactiveUnionField(block, src, active_tag, wanted_tag);
    }
    if (field_ty.zigTypeTag(zcu) == .noreturn) {
        _ = try block.addNoOp(.unreach);
        return .unreachable_value;
    }
    return block.addStructFieldPtr(union_ptr, field_index, ptr_field_ty);
}

fn unionFieldVal(
    sema: *Sema,
    block: *Block,
    src: LazySrcLoc,
    union_byval: Air.Inst.Ref,
    field_name: InternPool.NullTerminatedString,
    field_name_src: LazySrcLoc,
    union_ty: Type,
) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const ip = &zcu.intern_pool;
    assert(union_ty.zigTypeTag(zcu) == .@"union");

    try union_ty.resolveFields(pt);
    const union_obj = zcu.typeToUnion(union_ty).?;
    const field_index = try sema.unionFieldIndex(block, union_ty, field_name, field_name_src);
    const field_ty: Type = .fromInterned(union_obj.field_types.get(ip)[field_index]);
    const enum_field_index: u32 = @intCast(Type.fromInterned(union_obj.enum_tag_ty).enumFieldIndex(field_name, zcu).?);

    if (try sema.resolveValue(union_byval)) |union_val| {
        if (union_val.isUndef(zcu)) return pt.undefRef(field_ty);

        const un = ip.indexToKey(union_val.toIntern()).un;
        const field_tag = try pt.enumValueFieldIndex(.fromInterned(union_obj.enum_tag_ty), enum_field_index);
        const tag_matches = un.tag == field_tag.toIntern();
        switch (union_obj.flagsUnordered(ip).layout) {
            .auto => {
                if (tag_matches) {
                    return Air.internedToRef(un.val);
                } else {
                    const msg = msg: {
                        const active_index = Type.fromInterned(union_obj.enum_tag_ty).enumTagFieldIndex(Value.fromInterned(un.tag), zcu).?;
                        const active_field_name = Type.fromInterned(union_obj.enum_tag_ty).enumFieldName(active_index, zcu);
                        const msg = try sema.errMsg(src, "access of union field '{}' while field '{}' is active", .{
                            field_name.fmt(ip), active_field_name.fmt(ip),
                        });
                        errdefer msg.destroy(sema.gpa);
                        try sema.addDeclaredHereNote(msg, union_ty);
                        break :msg msg;
                    };
                    return sema.failWithOwnedErrorMsg(block, msg);
                }
            },
            .@"extern" => if (tag_matches) {
                // Fast path - no need to use bitcast logic.
                return Air.internedToRef(un.val);
            } else if (try sema.bitCastVal(union_val, field_ty, 0, 0, 0)) |field_val| {
                return Air.internedToRef(field_val.toIntern());
            },
            .@"packed" => if (tag_matches) {
                // Fast path - no need to use bitcast logic.
                return Air.internedToRef(un.val);
            } else if (try sema.bitCastVal(union_val, field_ty, 0, try union_ty.bitSizeSema(pt), 0)) |field_val| {
                return Air.internedToRef(field_val.toIntern());
            },
        }
    }

    if (union_obj.flagsUnordered(ip).layout == .auto and block.wantSafety() and
        union_ty.unionTagTypeSafety(zcu) != null and union_obj.field_types.len > 1)
    {
        const wanted_tag_val = try pt.enumValueFieldIndex(.fromInterned(union_obj.enum_tag_ty), enum_field_index);
        const wanted_tag = Air.internedToRef(wanted_tag_val.toIntern());
        const active_tag = try block.addTyOp(.get_union_tag, .fromInterned(union_obj.enum_tag_ty), union_byval);
        try sema.addSafetyCheckInactiveUnionField(block, src, active_tag, wanted_tag);
    }
    if (field_ty.zigTypeTag(zcu) == .noreturn) {
        _ = try block.addNoOp(.unreach);
        return .unreachable_value;
    }
    try field_ty.resolveLayout(pt);
    return block.addStructFieldVal(union_byval, field_index, field_ty);
}

fn elemPtr(
    sema: *Sema,
    block: *Block,
    src: LazySrcLoc,
    indexable_ptr: Air.Inst.Ref,
    elem_index: Air.Inst.Ref,
    elem_index_src: LazySrcLoc,
    init: bool,
    oob_safety: bool,
) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const indexable_ptr_src = src; // TODO better source location
    const indexable_ptr_ty = sema.typeOf(indexable_ptr);

    const indexable_ty = switch (indexable_ptr_ty.zigTypeTag(zcu)) {
        .pointer => indexable_ptr_ty.childType(zcu),
        else => return sema.fail(block, indexable_ptr_src, "expected pointer, found '{}'", .{indexable_ptr_ty.fmt(pt)}),
    };
    try sema.checkIndexable(block, src, indexable_ty);

    const elem_ptr = switch (indexable_ty.zigTypeTag(zcu)) {
        .array, .vector => try sema.elemPtrArray(block, src, indexable_ptr_src, indexable_ptr, elem_index_src, elem_index, init, oob_safety),
        .@"struct" => blk: {
            // Tuple field access.
            const index_val = try sema.resolveConstDefinedValue(block, elem_index_src, elem_index, .{ .simple = .tuple_field_index });
            const index: u32 = @intCast(try index_val.toUnsignedIntSema(pt));
            break :blk try sema.tupleFieldPtr(block, src, indexable_ptr, elem_index_src, index, init);
        },
        else => {
            const indexable = try sema.analyzeLoad(block, indexable_ptr_src, indexable_ptr, indexable_ptr_src);
            return elemPtrOneLayerOnly(sema, block, src, indexable, elem_index, elem_index_src, init, oob_safety);
        },
    };

    try sema.checkKnownAllocPtr(block, indexable_ptr, elem_ptr);
    return elem_ptr;
}

/// Asserts that the type of indexable is pointer.
fn elemPtrOneLayerOnly(
    sema: *Sema,
    block: *Block,
    src: LazySrcLoc,
    indexable: Air.Inst.Ref,
    elem_index: Air.Inst.Ref,
    elem_index_src: LazySrcLoc,
    init: bool,
    oob_safety: bool,
) CompileError!Air.Inst.Ref {
    const indexable_src = src; // TODO better source location
    const indexable_ty = sema.typeOf(indexable);
    const pt = sema.pt;
    const zcu = pt.zcu;

    try sema.checkIndexable(block, src, indexable_ty);

    switch (indexable_ty.ptrSize(zcu)) {
        .slice => return sema.elemPtrSlice(block, src, indexable_src, indexable, elem_index_src, elem_index, oob_safety),
        .many, .c => {
            const maybe_ptr_val = try sema.resolveDefinedValue(block, indexable_src, indexable);
            const maybe_index_val = try sema.resolveDefinedValue(block, elem_index_src, elem_index);
            ct: {
                const ptr_val = maybe_ptr_val orelse break :ct;
                const index_val = maybe_index_val orelse break :ct;
                const index: usize = @intCast(try index_val.toUnsignedIntSema(pt));
                const elem_ptr = try ptr_val.ptrElem(index, pt);
                return Air.internedToRef(elem_ptr.toIntern());
            }

            try sema.checkLogicalPtrOperation(block, src, indexable_ty);
            const result_ty = try indexable_ty.elemPtrType(null, pt);

            try sema.validateRuntimeElemAccess(block, elem_index_src, result_ty, indexable_ty, indexable_src);
            try sema.validateRuntimeValue(block, indexable_src, indexable);

            if (!try result_ty.childType(zcu).hasRuntimeBitsIgnoreComptimeSema(pt)) {
                // zero-bit child type; just bitcast the pointer
                return block.addBitCast(result_ty, indexable);
            }

            return block.addPtrElemPtr(indexable, elem_index, result_ty);
        },
        .one => {
            const child_ty = indexable_ty.childType(zcu);
            const elem_ptr = switch (child_ty.zigTypeTag(zcu)) {
                .array, .vector => try sema.elemPtrArray(block, src, indexable_src, indexable, elem_index_src, elem_index, init, oob_safety),
                .@"struct" => blk: {
                    assert(child_ty.isTuple(zcu));
                    const index_val = try sema.resolveConstDefinedValue(block, elem_index_src, elem_index, .{ .simple = .tuple_field_index });
                    const index: u32 = @intCast(try index_val.toUnsignedIntSema(pt));
                    break :blk try sema.tupleFieldPtr(block, indexable_src, indexable, elem_index_src, index, false);
                },
                else => unreachable, // Guaranteed by checkIndexable
            };
            try sema.checkKnownAllocPtr(block, indexable, elem_ptr);
            return elem_ptr;
        },
    }
}

fn elemVal(
    sema: *Sema,
    block: *Block,
    src: LazySrcLoc,
    indexable: Air.Inst.Ref,
    elem_index_uncasted: Air.Inst.Ref,
    elem_index_src: LazySrcLoc,
    oob_safety: bool,
) CompileError!Air.Inst.Ref {
    const indexable_src = src; // TODO better source location
    const indexable_ty = sema.typeOf(indexable);
    const pt = sema.pt;
    const zcu = pt.zcu;

    try sema.checkIndexable(block, src, indexable_ty);

    // TODO in case of a vector of pointers, we need to detect whether the element
    // index is a scalar or vector instead of unconditionally casting to usize.
    const elem_index = try sema.coerce(block, .usize, elem_index_uncasted, elem_index_src);

    switch (indexable_ty.zigTypeTag(zcu)) {
        .pointer => switch (indexable_ty.ptrSize(zcu)) {
            .slice => return sema.elemValSlice(block, src, indexable_src, indexable, elem_index_src, elem_index, oob_safety),
            .many, .c => {
                const maybe_indexable_val = try sema.resolveDefinedValue(block, indexable_src, indexable);
                const maybe_index_val = try sema.resolveDefinedValue(block, elem_index_src, elem_index);

                ct: {
                    const indexable_val = maybe_indexable_val orelse break :ct;
                    const index_val = maybe_index_val orelse break :ct;
                    const index: usize = @intCast(try index_val.toUnsignedIntSema(pt));
                    const elem_ty = indexable_ty.elemType2(zcu);
                    const many_ptr_ty = try pt.manyConstPtrType(elem_ty);
                    const many_ptr_val = try pt.getCoerced(indexable_val, many_ptr_ty);
                    const elem_ptr_ty = try pt.singleConstPtrType(elem_ty);
                    const elem_ptr_val = try many_ptr_val.ptrElem(index, pt);
                    const elem_val = try sema.pointerDeref(block, indexable_src, elem_ptr_val, elem_ptr_ty) orelse break :ct;
                    return Air.internedToRef((try pt.getCoerced(elem_val, elem_ty)).toIntern());
                }

                try sema.checkLogicalPtrOperation(block, src, indexable_ty);
                return block.addBinOp(.ptr_elem_val, indexable, elem_index);
            },
            .one => {
                arr_sent: {
                    const inner_ty = indexable_ty.childType(zcu);
                    if (inner_ty.zigTypeTag(zcu) != .array) break :arr_sent;
                    const sentinel = inner_ty.sentinel(zcu) orelse break :arr_sent;
                    const index_val = try sema.resolveDefinedValue(block, elem_index_src, elem_index) orelse break :arr_sent;
                    const index = try sema.usizeCast(block, src, try index_val.toUnsignedIntSema(pt));
                    if (index != inner_ty.arrayLen(zcu)) break :arr_sent;
                    return Air.internedToRef(sentinel.toIntern());
                }
                const elem_ptr = try sema.elemPtr(block, indexable_src, indexable, elem_index, elem_index_src, false, oob_safety);
                return sema.analyzeLoad(block, indexable_src, elem_ptr, elem_index_src);
            },
        },
        .array => return sema.elemValArray(block, src, indexable_src, indexable, elem_index_src, elem_index, oob_safety),
        .vector => {
            // TODO: If the index is a vector, the result should be a vector.
            return sema.elemValArray(block, src, indexable_src, indexable, elem_index_src, elem_index, oob_safety);
        },
        .@"struct" => {
            // Tuple field access.
            const index_val = try sema.resolveConstDefinedValue(block, elem_index_src, elem_index, .{ .simple = .tuple_field_index });
            const index: u32 = @intCast(try index_val.toUnsignedIntSema(pt));
            return sema.tupleField(block, indexable_src, indexable, elem_index_src, index);
        },
        else => unreachable,
    }
}

fn validateRuntimeElemAccess(
    sema: *Sema,
    block: *Block,
    elem_index_src: LazySrcLoc,
    elem_ty: Type,
    parent_ty: Type,
    parent_src: LazySrcLoc,
) CompileError!void {
    const pt = sema.pt;
    const zcu = pt.zcu;

    if (try elem_ty.comptimeOnlySema(sema.pt)) {
        const msg = msg: {
            const msg = try sema.errMsg(
                elem_index_src,
                "values of type '{}' must be comptime-known, but index value is runtime-known",
                .{parent_ty.fmt(sema.pt)},
            );
            errdefer msg.destroy(sema.gpa);

            try sema.explainWhyTypeIsComptime(msg, parent_src, parent_ty);

            break :msg msg;
        };
        return sema.failWithOwnedErrorMsg(block, msg);
    }

    if (zcu.intern_pool.indexToKey(parent_ty.toIntern()) == .ptr_type) {
        const target = zcu.getTarget();
        const as = parent_ty.ptrAddressSpace(zcu);
        if (target_util.arePointersLogical(target, as)) {
            return sema.fail(block, elem_index_src, "cannot access element of logical pointer '{}'", .{parent_ty.fmt(pt)});
        }
    }
}

fn tupleFieldPtr(
    sema: *Sema,
    block: *Block,
    tuple_ptr_src: LazySrcLoc,
    tuple_ptr: Air.Inst.Ref,
    field_index_src: LazySrcLoc,
    field_index: u32,
    init: bool,
) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const tuple_ptr_ty = sema.typeOf(tuple_ptr);
    const tuple_ptr_info = tuple_ptr_ty.ptrInfo(zcu);
    const tuple_ty: Type = .fromInterned(tuple_ptr_info.child);
    try tuple_ty.resolveFields(pt);
    const field_count = tuple_ty.structFieldCount(zcu);

    if (field_count == 0) {
        return sema.fail(block, tuple_ptr_src, "indexing into empty tuple is not allowed", .{});
    }

    if (field_index >= field_count) {
        return sema.fail(block, field_index_src, "index {d} outside tuple of length {d}", .{
            field_index, field_count,
        });
    }

    const field_ty = tuple_ty.fieldType(field_index, zcu);
    const ptr_field_ty = try pt.ptrTypeSema(.{
        .child = field_ty.toIntern(),
        .flags = .{
            .is_const = tuple_ptr_info.flags.is_const,
            .is_volatile = tuple_ptr_info.flags.is_volatile,
            .address_space = tuple_ptr_info.flags.address_space,
            .alignment = a: {
                if (tuple_ptr_info.flags.alignment == .none) break :a .none;
                // The tuple pointer isn't naturally aligned, so the field pointer might be underaligned.
                const tuple_align = tuple_ptr_info.flags.alignment;
                const field_align = try field_ty.abiAlignmentSema(pt);
                break :a tuple_align.min(field_align);
            },
        },
    });

    if (tuple_ty.structFieldIsComptime(field_index, zcu))
        try tuple_ty.resolveStructFieldInits(pt);

    if (try tuple_ty.structFieldValueComptime(pt, field_index)) |default_val| {
        return Air.internedToRef((try pt.intern(.{ .ptr = .{
            .ty = ptr_field_ty.toIntern(),
            .base_addr = .{ .comptime_field = default_val.toIntern() },
            .byte_offset = 0,
        } })));
    }

    if (try sema.resolveValue(tuple_ptr)) |tuple_ptr_val| {
        const field_ptr_val = try tuple_ptr_val.ptrField(field_index, pt);
        return Air.internedToRef(field_ptr_val.toIntern());
    }

    if (!init) {
        try sema.validateRuntimeElemAccess(block, field_index_src, field_ty, tuple_ty, tuple_ptr_src);
    }

    return block.addStructFieldPtr(tuple_ptr, field_index, ptr_field_ty);
}

fn tupleField(
    sema: *Sema,
    block: *Block,
    tuple_src: LazySrcLoc,
    tuple: Air.Inst.Ref,
    field_index_src: LazySrcLoc,
    field_index: u32,
) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const tuple_ty = sema.typeOf(tuple);
    try tuple_ty.resolveFields(pt);
    const field_count = tuple_ty.structFieldCount(zcu);

    if (field_count == 0) {
        return sema.fail(block, tuple_src, "indexing into empty tuple is not allowed", .{});
    }

    if (field_index >= field_count) {
        return sema.fail(block, field_index_src, "index {d} outside tuple of length {d}", .{
            field_index, field_count,
        });
    }

    const field_ty = tuple_ty.fieldType(field_index, zcu);

    if (tuple_ty.structFieldIsComptime(field_index, zcu))
        try tuple_ty.resolveStructFieldInits(pt);
    if (try tuple_ty.structFieldValueComptime(pt, field_index)) |default_value| {
        return Air.internedToRef(default_value.toIntern()); // comptime field
    }

    if (try sema.resolveValue(tuple)) |tuple_val| {
        if (tuple_val.isUndef(zcu)) return pt.undefRef(field_ty);
        return Air.internedToRef((try tuple_val.fieldValue(pt, field_index)).toIntern());
    }

    try sema.validateRuntimeElemAccess(block, field_index_src, field_ty, tuple_ty, tuple_src);

    try field_ty.resolveLayout(pt);
    return block.addStructFieldVal(tuple, field_index, field_ty);
}

fn elemValArray(
    sema: *Sema,
    block: *Block,
    src: LazySrcLoc,
    array_src: LazySrcLoc,
    array: Air.Inst.Ref,
    elem_index_src: LazySrcLoc,
    elem_index: Air.Inst.Ref,
    oob_safety: bool,
) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const array_ty = sema.typeOf(array);
    const array_sent = array_ty.sentinel(zcu);
    const array_len = array_ty.arrayLen(zcu);
    const array_len_s = array_len + @intFromBool(array_sent != null);
    const elem_ty = array_ty.childType(zcu);

    if (array_len_s == 0) {
        return sema.fail(block, array_src, "indexing into empty array is not allowed", .{});
    }

    const maybe_undef_array_val = try sema.resolveValue(array);
    // index must be defined since it can access out of bounds
    const maybe_index_val = try sema.resolveDefinedValue(block, elem_index_src, elem_index);

    if (maybe_index_val) |index_val| {
        const index: usize = @intCast(try index_val.toUnsignedIntSema(pt));
        if (array_sent) |s| {
            if (index == array_len) {
                return Air.internedToRef(s.toIntern());
            }
        }
        if (index >= array_len_s) {
            const sentinel_label: []const u8 = if (array_sent != null) " +1 (sentinel)" else "";
            return sema.fail(block, elem_index_src, "index {d} outside array of length {d}{s}", .{ index, array_len, sentinel_label });
        }
    }
    if (maybe_undef_array_val) |array_val| {
        if (array_val.isUndef(zcu)) {
            return pt.undefRef(elem_ty);
        }
        if (maybe_index_val) |index_val| {
            const index: usize = @intCast(try index_val.toUnsignedIntSema(pt));
            const elem_val = try array_val.elemValue(pt, index);
            return Air.internedToRef(elem_val.toIntern());
        }
    }

    try sema.validateRuntimeElemAccess(block, elem_index_src, elem_ty, array_ty, array_src);
    try sema.validateRuntimeValue(block, array_src, array);

    if (oob_safety and block.wantSafety()) {
        // Runtime check is only needed if unable to comptime check.
        if (maybe_index_val == null) {
            const len_inst = try pt.intRef(.usize, array_len);
            const cmp_op: Air.Inst.Tag = if (array_sent != null) .cmp_lte else .cmp_lt;
            try sema.addSafetyCheckIndexOob(block, src, elem_index, len_inst, cmp_op);
        }
    }

    if (try sema.typeHasOnePossibleValue(elem_ty)) |elem_val|
        return Air.internedToRef(elem_val.toIntern());

    return block.addBinOp(.array_elem_val, array, elem_index);
}

fn elemPtrArray(
    sema: *Sema,
    block: *Block,
    src: LazySrcLoc,
    array_ptr_src: LazySrcLoc,
    array_ptr: Air.Inst.Ref,
    elem_index_src: LazySrcLoc,
    elem_index: Air.Inst.Ref,
    init: bool,
    oob_safety: bool,
) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const array_ptr_ty = sema.typeOf(array_ptr);
    const array_ty = array_ptr_ty.childType(zcu);
    const array_sent = array_ty.sentinel(zcu) != null;
    const array_len = array_ty.arrayLen(zcu);
    const array_len_s = array_len + @intFromBool(array_sent);

    if (array_len_s == 0) {
        return sema.fail(block, array_ptr_src, "indexing into empty array is not allowed", .{});
    }

    const maybe_undef_array_ptr_val = try sema.resolveValue(array_ptr);
    // The index must not be undefined since it can be out of bounds.
    const offset: ?usize = if (try sema.resolveDefinedValue(block, elem_index_src, elem_index)) |index_val| o: {
        const index = try sema.usizeCast(block, elem_index_src, try index_val.toUnsignedIntSema(pt));
        if (index >= array_len_s) {
            const sentinel_label: []const u8 = if (array_sent) " +1 (sentinel)" else "";
            return sema.fail(block, elem_index_src, "index {d} outside array of length {d}{s}", .{ index, array_len, sentinel_label });
        }
        break :o index;
    } else null;

    const elem_ptr_ty = try array_ptr_ty.elemPtrType(offset, pt);

    if (maybe_undef_array_ptr_val) |array_ptr_val| {
        if (array_ptr_val.isUndef(zcu)) {
            return pt.undefRef(elem_ptr_ty);
        }
        if (offset) |index| {
            const elem_ptr = try array_ptr_val.ptrElem(index, pt);
            return Air.internedToRef(elem_ptr.toIntern());
        }
    }

    if (!init) {
        try sema.validateRuntimeElemAccess(block, elem_index_src, array_ty.elemType2(zcu), array_ty, array_ptr_src);
        try sema.validateRuntimeValue(block, array_ptr_src, array_ptr);
    }

    // Runtime check is only needed if unable to comptime check.
    if (oob_safety and block.wantSafety() and offset == null) {
        const len_inst = try pt.intRef(.usize, array_len);
        const cmp_op: Air.Inst.Tag = if (array_sent) .cmp_lte else .cmp_lt;
        try sema.addSafetyCheckIndexOob(block, src, elem_index, len_inst, cmp_op);
    }

    return block.addPtrElemPtr(array_ptr, elem_index, elem_ptr_ty);
}

fn elemValSlice(
    sema: *Sema,
    block: *Block,
    src: LazySrcLoc,
    slice_src: LazySrcLoc,
    slice: Air.Inst.Ref,
    elem_index_src: LazySrcLoc,
    elem_index: Air.Inst.Ref,
    oob_safety: bool,
) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const slice_ty = sema.typeOf(slice);
    const slice_sent = slice_ty.sentinel(zcu) != null;
    const elem_ty = slice_ty.elemType2(zcu);
    var runtime_src = slice_src;

    // slice must be defined since it can dereferenced as null
    const maybe_slice_val = try sema.resolveDefinedValue(block, slice_src, slice);
    // index must be defined since it can index out of bounds
    const maybe_index_val = try sema.resolveDefinedValue(block, elem_index_src, elem_index);

    if (maybe_slice_val) |slice_val| {
        runtime_src = elem_index_src;
        const slice_len = try slice_val.sliceLen(pt);
        const slice_len_s = slice_len + @intFromBool(slice_sent);
        if (slice_len_s == 0) {
            return sema.fail(block, slice_src, "indexing into empty slice is not allowed", .{});
        }
        if (maybe_index_val) |index_val| {
            const index: usize = @intCast(try index_val.toUnsignedIntSema(pt));
            if (index >= slice_len_s) {
                const sentinel_label: []const u8 = if (slice_sent) " +1 (sentinel)" else "";
                return sema.fail(block, elem_index_src, "index {d} outside slice of length {d}{s}", .{ index, slice_len, sentinel_label });
            }
            const elem_ptr_ty = try slice_ty.elemPtrType(index, pt);
            const elem_ptr_val = try slice_val.ptrElem(index, pt);
            if (try sema.pointerDeref(block, slice_src, elem_ptr_val, elem_ptr_ty)) |elem_val| {
                return Air.internedToRef(elem_val.toIntern());
            }
            runtime_src = slice_src;
        }
    }

    try sema.validateRuntimeElemAccess(block, elem_index_src, elem_ty, slice_ty, slice_src);
    try sema.validateRuntimeValue(block, slice_src, slice);

    if (oob_safety and block.wantSafety()) {
        const len_inst = if (maybe_slice_val) |slice_val|
            try pt.intRef(.usize, try slice_val.sliceLen(pt))
        else
            try block.addTyOp(.slice_len, .usize, slice);
        const cmp_op: Air.Inst.Tag = if (slice_sent) .cmp_lte else .cmp_lt;
        try sema.addSafetyCheckIndexOob(block, src, elem_index, len_inst, cmp_op);
    }
    return block.addBinOp(.slice_elem_val, slice, elem_index);
}

fn elemPtrSlice(
    sema: *Sema,
    block: *Block,
    src: LazySrcLoc,
    slice_src: LazySrcLoc,
    slice: Air.Inst.Ref,
    elem_index_src: LazySrcLoc,
    elem_index: Air.Inst.Ref,
    oob_safety: bool,
) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const slice_ty = sema.typeOf(slice);
    const slice_sent = slice_ty.sentinel(zcu) != null;

    const maybe_undef_slice_val = try sema.resolveValue(slice);
    // The index must not be undefined since it can be out of bounds.
    const offset: ?usize = if (try sema.resolveDefinedValue(block, elem_index_src, elem_index)) |index_val| o: {
        const index = try sema.usizeCast(block, elem_index_src, try index_val.toUnsignedIntSema(pt));
        break :o index;
    } else null;

    const elem_ptr_ty = try slice_ty.elemPtrType(offset, pt);

    if (maybe_undef_slice_val) |slice_val| {
        if (slice_val.isUndef(zcu)) {
            return pt.undefRef(elem_ptr_ty);
        }
        const slice_len = try slice_val.sliceLen(pt);
        const slice_len_s = slice_len + @intFromBool(slice_sent);
        if (slice_len_s == 0) {
            return sema.fail(block, slice_src, "indexing into empty slice is not allowed", .{});
        }
        if (offset) |index| {
            if (index >= slice_len_s) {
                const sentinel_label: []const u8 = if (slice_sent) " +1 (sentinel)" else "";
                return sema.fail(block, elem_index_src, "index {d} outside slice of length {d}{s}", .{ index, slice_len, sentinel_label });
            }
            const elem_ptr_val = try slice_val.ptrElem(index, pt);
            return Air.internedToRef(elem_ptr_val.toIntern());
        }
    }

    try sema.validateRuntimeElemAccess(block, elem_index_src, elem_ptr_ty, slice_ty, slice_src);
    try sema.validateRuntimeValue(block, slice_src, slice);

    if (oob_safety and block.wantSafety()) {
        const len_inst = len: {
            if (maybe_undef_slice_val) |slice_val|
                if (!slice_val.isUndef(zcu))
                    break :len try pt.intRef(.usize, try slice_val.sliceLen(pt));
            break :len try block.addTyOp(.slice_len, .usize, slice);
        };
        const cmp_op: Air.Inst.Tag = if (slice_sent) .cmp_lte else .cmp_lt;
        try sema.addSafetyCheckIndexOob(block, src, elem_index, len_inst, cmp_op);
    }
    if (!try slice_ty.childType(zcu).hasRuntimeBitsIgnoreComptimeSema(pt)) {
        // zero-bit child type; just extract the pointer and bitcast it
        const slice_ptr = try block.addTyOp(.slice_ptr, slice_ty.slicePtrFieldType(zcu), slice);
        return block.addBitCast(elem_ptr_ty, slice_ptr);
    }
    return block.addSliceElemPtr(slice, elem_index, elem_ptr_ty);
}

pub fn coerce(
    sema: *Sema,
    block: *Block,
    dest_ty_unresolved: Type,
    inst: Air.Inst.Ref,
    inst_src: LazySrcLoc,
) CompileError!Air.Inst.Ref {
    return sema.coerceExtra(block, dest_ty_unresolved, inst, inst_src, .{}) catch |err| switch (err) {
        error.NotCoercible => unreachable,
        else => |e| return e,
    };
}

const CoersionError = CompileError || error{
    /// When coerce is called recursively, this error should be returned instead of using `fail`
    /// to ensure correct types in compile errors.
    NotCoercible,
};

const CoerceOpts = struct {
    /// Should coerceExtra emit error messages.
    report_err: bool = true,
    /// Ignored if `report_err == false`.
    is_ret: bool = false,
    /// Should coercion to comptime_int emit an error message.
    no_cast_to_comptime_int: bool = false,

    param_src: struct {
        func_inst: Air.Inst.Ref = .none,
        param_i: u32 = undefined,

        fn get(info: @This(), sema: *Sema) !?LazySrcLoc {
            if (info.func_inst == .none) return null;
            const func_inst = try sema.funcDeclSrcInst(info.func_inst) orelse return null;
            return .{
                .base_node_inst = func_inst,
                .offset = .{ .fn_proto_param_type = .{
                    .fn_proto_node_offset = .zero,
                    .param_index = info.param_i,
                } },
            };
        }
    } = .{ .func_inst = .none, .param_i = undefined },
};

fn coerceExtra(
    sema: *Sema,
    block: *Block,
    dest_ty: Type,
    inst: Air.Inst.Ref,
    inst_src: LazySrcLoc,
    opts: CoerceOpts,
) CoersionError!Air.Inst.Ref {
    if (dest_ty.isGenericPoison()) return inst;
    const pt = sema.pt;
    const zcu = pt.zcu;
    const ip = &zcu.intern_pool;
    const dest_ty_src = inst_src; // TODO better source location
    try dest_ty.resolveFields(pt);
    const inst_ty = sema.typeOf(inst);
    try inst_ty.resolveFields(pt);
    const target = zcu.getTarget();
    // If the types are the same, we can return the operand.
    if (dest_ty.eql(inst_ty, zcu))
        return inst;

    const maybe_inst_val = try sema.resolveValue(inst);

    var in_memory_result = try sema.coerceInMemoryAllowed(block, dest_ty, inst_ty, false, target, dest_ty_src, inst_src, maybe_inst_val);
    if (in_memory_result == .ok) {
        if (maybe_inst_val) |val| {
            return sema.coerceInMemory(val, dest_ty);
        }
        try sema.requireRuntimeBlock(block, inst_src, null);
        const new_val = try block.addBitCast(dest_ty, inst);
        try sema.checkKnownAllocPtr(block, inst, new_val);
        return new_val;
    }

    switch (dest_ty.zigTypeTag(zcu)) {
        .optional => optional: {
            if (maybe_inst_val) |val| {
                // undefined sets the optional bit also to undefined.
                if (val.toIntern() == .undef) {
                    return pt.undefRef(dest_ty);
                }

                // null to ?T
                if (val.toIntern() == .null_value) {
                    return Air.internedToRef((try pt.intern(.{ .opt = .{
                        .ty = dest_ty.toIntern(),
                        .val = .none,
                    } })));
                }
            }

            // cast from ?*T and ?[*]T to ?*anyopaque
            // but don't do it if the source type is a double pointer
            if (dest_ty.isPtrLikeOptional(zcu) and
                dest_ty.elemType2(zcu).toIntern() == .anyopaque_type and
                inst_ty.isPtrAtRuntime(zcu))
            anyopaque_check: {
                if (!sema.checkPtrAttributes(dest_ty, inst_ty, &in_memory_result)) break :optional;
                const elem_ty = inst_ty.elemType2(zcu);
                if (elem_ty.zigTypeTag(zcu) == .pointer or elem_ty.isPtrLikeOptional(zcu)) {
                    in_memory_result = .{ .double_ptr_to_anyopaque = .{
                        .actual = inst_ty,
                        .wanted = dest_ty,
                    } };
                    break :optional;
                }
                // Let the logic below handle wrapping the optional now that
                // it has been checked to correctly coerce.
                if (!inst_ty.isPtrLikeOptional(zcu)) break :anyopaque_check;
                return sema.coerceCompatiblePtrs(block, dest_ty, inst, inst_src);
            }

            // T to ?T
            const child_type = dest_ty.optionalChild(zcu);
            const intermediate = sema.coerceExtra(block, child_type, inst, inst_src, .{ .report_err = false }) catch |err| switch (err) {
                error.NotCoercible => {
                    if (in_memory_result == .no_match) {
                        // Try to give more useful notes
                        in_memory_result = try sema.coerceInMemoryAllowed(block, child_type, inst_ty, false, target, dest_ty_src, inst_src, maybe_inst_val);
                    }
                    break :optional;
                },
                else => |e| return e,
            };
            return try sema.wrapOptional(block, dest_ty, intermediate, inst_src);
        },
        .pointer => pointer: {
            const dest_info = dest_ty.ptrInfo(zcu);

            // Function body to function pointer.
            if (inst_ty.zigTypeTag(zcu) == .@"fn") {
                const fn_val = try sema.resolveConstDefinedValue(block, LazySrcLoc.unneeded, inst, undefined);
                const fn_nav = switch (zcu.intern_pool.indexToKey(fn_val.toIntern())) {
                    .func => |f| f.owner_nav,
                    .@"extern" => |e| e.owner_nav,
                    else => unreachable,
                };
                const inst_as_ptr = try sema.analyzeNavRef(block, inst_src, fn_nav);
                return sema.coerce(block, dest_ty, inst_as_ptr, inst_src);
            }

            // *T to *[1]T
            single_item: {
                if (dest_info.flags.size != .one) break :single_item;
                if (!inst_ty.isSinglePointer(zcu)) break :single_item;
                if (!sema.checkPtrAttributes(dest_ty, inst_ty, &in_memory_result)) break :pointer;
                const ptr_elem_ty = inst_ty.childType(zcu);
                const array_ty: Type = .fromInterned(dest_info.child);
                if (array_ty.zigTypeTag(zcu) != .array) break :single_item;
                const array_elem_ty = array_ty.childType(zcu);
                if (array_ty.arrayLen(zcu) != 1) break :single_item;
                const dest_is_mut = !dest_info.flags.is_const;
                switch (try sema.coerceInMemoryAllowed(block, array_elem_ty, ptr_elem_ty, dest_is_mut, target, dest_ty_src, inst_src, maybe_inst_val)) {
                    .ok => {},
                    else => break :single_item,
                }
                return sema.coerceCompatiblePtrs(block, dest_ty, inst, inst_src);
            }

            // Coercions where the source is a single pointer to an array.
            src_array_ptr: {
                if (!inst_ty.isSinglePointer(zcu)) break :src_array_ptr;
                if (dest_info.flags.size == .one) break :src_array_ptr; // `*[n]T` -> `*T` isn't valid
                if (!sema.checkPtrAttributes(dest_ty, inst_ty, &in_memory_result)) break :pointer;
                const array_ty = inst_ty.childType(zcu);
                if (array_ty.zigTypeTag(zcu) != .array) break :src_array_ptr;
                const array_elem_type = array_ty.childType(zcu);
                const dest_is_mut = !dest_info.flags.is_const;

                const dst_elem_type: Type = .fromInterned(dest_info.child);
                const elem_res = try sema.coerceInMemoryAllowed(block, dst_elem_type, array_elem_type, dest_is_mut, target, dest_ty_src, inst_src, maybe_inst_val);
                switch (elem_res) {
                    .ok => {},
                    else => {
                        in_memory_result = .{ .ptr_child = .{
                            .child = try elem_res.dupe(sema.arena),
                            .actual = array_elem_type,
                            .wanted = dst_elem_type,
                        } };
                        break :src_array_ptr;
                    },
                }

                if (dest_info.sentinel != .none) {
                    if (array_ty.sentinel(zcu)) |inst_sent| {
                        if (Air.internedToRef(dest_info.sentinel) !=
                            try sema.coerceInMemory(inst_sent, dst_elem_type))
                        {
                            in_memory_result = .{ .ptr_sentinel = .{
                                .actual = inst_sent,
                                .wanted = Value.fromInterned(dest_info.sentinel),
                                .ty = dst_elem_type,
                            } };
                            break :src_array_ptr;
                        }
                    } else {
                        in_memory_result = .{ .ptr_sentinel = .{
                            .actual = Value.@"unreachable",
                            .wanted = Value.fromInterned(dest_info.sentinel),
                            .ty = dst_elem_type,
                        } };
                        break :src_array_ptr;
                    }
                }

                switch (dest_info.flags.size) {
                    .slice => {
                        // *[N]T to []T
                        return sema.coerceArrayPtrToSlice(block, dest_ty, inst, inst_src);
                    },
                    .c => {
                        // *[N]T to [*c]T
                        return sema.coerceCompatiblePtrs(block, dest_ty, inst, inst_src);
                    },
                    .many => {
                        // *[N]T to [*]T
                        return sema.coerceCompatiblePtrs(block, dest_ty, inst, inst_src);
                    },
                    .one => unreachable, // early exit at top of block
                }
            }

            // coercion from C pointer
            if (inst_ty.isCPtr(zcu)) src_c_ptr: {
                if (dest_info.flags.size == .slice) break :src_c_ptr;
                if (!sema.checkPtrAttributes(dest_ty, inst_ty, &in_memory_result)) break :src_c_ptr;
                // In this case we must add a safety check because the C pointer
                // could be null.
                const src_elem_ty = inst_ty.childType(zcu);
                const dest_is_mut = !dest_info.flags.is_const;
                const dst_elem_type: Type = .fromInterned(dest_info.child);
                switch (try sema.coerceInMemoryAllowed(block, dst_elem_type, src_elem_ty, dest_is_mut, target, dest_ty_src, inst_src, maybe_inst_val)) {
                    .ok => {},
                    else => break :src_c_ptr,
                }
                return sema.coerceCompatiblePtrs(block, dest_ty, inst, inst_src);
            }

            // cast from *T and [*]T to *anyopaque
            // but don't do it if the source type is a double pointer
            if (dest_info.child == .anyopaque_type and inst_ty.zigTypeTag(zcu) == .pointer) to_anyopaque: {
                if (!sema.checkPtrAttributes(dest_ty, inst_ty, &in_memory_result)) break :pointer;
                const elem_ty = inst_ty.elemType2(zcu);
                if (elem_ty.zigTypeTag(zcu) == .pointer or elem_ty.isPtrLikeOptional(zcu)) {
                    in_memory_result = .{ .double_ptr_to_anyopaque = .{
                        .actual = inst_ty,
                        .wanted = dest_ty,
                    } };
                    break :pointer;
                }
                if (dest_ty.isSlice(zcu)) break :to_anyopaque;
                if (inst_ty.isSlice(zcu)) {
                    in_memory_result = .{ .slice_to_anyopaque = .{
                        .actual = inst_ty,
                        .wanted = dest_ty,
                    } };
                    break :pointer;
                }
                return sema.coerceCompatiblePtrs(block, dest_ty, inst, inst_src);
            }

            switch (dest_info.flags.size) {
                // coercion to C pointer
                .c => switch (inst_ty.zigTypeTag(zcu)) {
                    .null => return Air.internedToRef(try pt.intern(.{ .ptr = .{
                        .ty = dest_ty.toIntern(),
                        .base_addr = .int,
                        .byte_offset = 0,
                    } })),
                    .comptime_int => {
                        const addr = sema.coerceExtra(block, .usize, inst, inst_src, .{ .report_err = false }) catch |err| switch (err) {
                            error.NotCoercible => break :pointer,
                            else => |e| return e,
                        };
                        return try sema.coerceCompatiblePtrs(block, dest_ty, addr, inst_src);
                    },
                    .int => {
                        const ptr_size_ty: Type = switch (inst_ty.intInfo(zcu).signedness) {
                            .signed => .isize,
                            .unsigned => .usize,
                        };
                        const addr = sema.coerceExtra(block, ptr_size_ty, inst, inst_src, .{ .report_err = false }) catch |err| switch (err) {
                            error.NotCoercible => {
                                // Try to give more useful notes
                                in_memory_result = try sema.coerceInMemoryAllowed(block, ptr_size_ty, inst_ty, false, target, dest_ty_src, inst_src, maybe_inst_val);
                                break :pointer;
                            },
                            else => |e| return e,
                        };
                        return try sema.coerceCompatiblePtrs(block, dest_ty, addr, inst_src);
                    },
                    .pointer => p: {
                        if (!sema.checkPtrAttributes(dest_ty, inst_ty, &in_memory_result)) break :p;
                        const inst_info = inst_ty.ptrInfo(zcu);
                        switch (try sema.coerceInMemoryAllowed(
                            block,
                            .fromInterned(dest_info.child),
                            .fromInterned(inst_info.child),
                            !dest_info.flags.is_const,
                            target,
                            dest_ty_src,
                            inst_src,
                            maybe_inst_val,
                        )) {
                            .ok => {},
                            else => break :p,
                        }
                        if (inst_info.flags.size == .slice) {
                            assert(dest_info.sentinel == .none);
                            if (inst_info.sentinel == .none or
                                inst_info.sentinel != (try pt.intValue(.fromInterned(inst_info.child), 0)).toIntern())
                                break :p;

                            const slice_ptr = try sema.analyzeSlicePtr(block, inst_src, inst, inst_ty);
                            return sema.coerceCompatiblePtrs(block, dest_ty, slice_ptr, inst_src);
                        }
                        return sema.coerceCompatiblePtrs(block, dest_ty, inst, inst_src);
                    },
                    else => {},
                },
                .one => {},
                .slice => to_slice: {
                    if (inst_ty.zigTypeTag(zcu) == .array) {
                        return sema.fail(
                            block,
                            inst_src,
                            "array literal requires address-of operator (&) to coerce to slice type '{}'",
                            .{dest_ty.fmt(pt)},
                        );
                    }

                    if (!inst_ty.isSinglePointer(zcu)) break :to_slice;
                    const inst_child_ty = inst_ty.childType(zcu);
                    if (!inst_child_ty.isTuple(zcu)) break :to_slice;

                    // empty tuple to zero-length slice
                    // note that this allows coercing to a mutable slice.
                    if (inst_child_ty.structFieldCount(zcu) == 0) {
                        const align_val = try dest_ty.ptrAlignmentSema(pt);
                        return Air.internedToRef(try pt.intern(.{ .slice = .{
                            .ty = dest_ty.toIntern(),
                            .ptr = try pt.intern(.{ .ptr = .{
                                .ty = dest_ty.slicePtrFieldType(zcu).toIntern(),
                                .base_addr = .int,
                                .byte_offset = align_val.toByteUnits().?,
                            } }),
                            .len = .zero_usize,
                        } }));
                    }

                    // pointer to tuple to slice
                    if (!dest_info.flags.is_const) {
                        const err_msg = err_msg: {
                            const err_msg = try sema.errMsg(inst_src, "cannot cast pointer to tuple to '{}'", .{dest_ty.fmt(pt)});
                            errdefer err_msg.destroy(sema.gpa);
                            try sema.errNote(dest_ty_src, err_msg, "pointers to tuples can only coerce to constant pointers", .{});
                            break :err_msg err_msg;
                        };
                        return sema.failWithOwnedErrorMsg(block, err_msg);
                    }
                    return sema.coerceTupleToSlicePtrs(block, dest_ty, dest_ty_src, inst, inst_src);
                },
                .many => p: {
                    if (!inst_ty.isSlice(zcu)) break :p;
                    if (!sema.checkPtrAttributes(dest_ty, inst_ty, &in_memory_result)) break :p;
                    const inst_info = inst_ty.ptrInfo(zcu);

                    switch (try sema.coerceInMemoryAllowed(
                        block,
                        .fromInterned(dest_info.child),
                        .fromInterned(inst_info.child),
                        !dest_info.flags.is_const,
                        target,
                        dest_ty_src,
                        inst_src,
                        maybe_inst_val,
                    )) {
                        .ok => {},
                        else => break :p,
                    }

                    if (dest_info.sentinel == .none or inst_info.sentinel == .none or
                        Air.internedToRef(dest_info.sentinel) !=
                            try sema.coerceInMemory(Value.fromInterned(inst_info.sentinel), .fromInterned(dest_info.child)))
                        break :p;

                    const slice_ptr = try sema.analyzeSlicePtr(block, inst_src, inst, inst_ty);
                    return sema.coerceCompatiblePtrs(block, dest_ty, slice_ptr, inst_src);
                },
            }
        },
        .int, .comptime_int => switch (inst_ty.zigTypeTag(zcu)) {
            .float, .comptime_float => float: {
                const val = maybe_inst_val orelse {
                    if (dest_ty.zigTypeTag(zcu) == .comptime_int) {
                        if (!opts.report_err) return error.NotCoercible;
                        return sema.failWithNeededComptime(block, inst_src, .{ .simple = .casted_to_comptime_int });
                    }
                    break :float;
                };
                const result_val = try sema.intFromFloat(block, inst_src, val, inst_ty, dest_ty, .exact);
                return Air.internedToRef(result_val.toIntern());
            },
            .int, .comptime_int => {
                if (maybe_inst_val) |val| {
                    // comptime-known integer to other number
                    if (!(try sema.intFitsInType(val, dest_ty, null))) {
                        if (!opts.report_err) return error.NotCoercible;
                        return sema.fail(block, inst_src, "type '{}' cannot represent integer value '{}'", .{ dest_ty.fmt(pt), val.fmtValueSema(pt, sema) });
                    }
                    return switch (zcu.intern_pool.indexToKey(val.toIntern())) {
                        .undef => try pt.undefRef(dest_ty),
                        .int => |int| Air.internedToRef(
                            try zcu.intern_pool.getCoercedInts(zcu.gpa, pt.tid, int, dest_ty.toIntern()),
                        ),
                        else => unreachable,
                    };
                }
                if (dest_ty.zigTypeTag(zcu) == .comptime_int) {
                    if (!opts.report_err) return error.NotCoercible;
                    if (opts.no_cast_to_comptime_int) return inst;
                    return sema.failWithNeededComptime(block, inst_src, .{ .simple = .casted_to_comptime_int });
                }

                // integer widening
                const dst_info = dest_ty.intInfo(zcu);
                const src_info = inst_ty.intInfo(zcu);
                if ((src_info.signedness == dst_info.signedness and dst_info.bits >= src_info.bits) or
                    // small enough unsigned ints can get casted to large enough signed ints
                    (dst_info.signedness == .signed and dst_info.bits > src_info.bits))
                {
                    try sema.requireRuntimeBlock(block, inst_src, null);
                    return block.addTyOp(.intcast, dest_ty, inst);
                }
            },
            else => {},
        },
        .float, .comptime_float => switch (inst_ty.zigTypeTag(zcu)) {
            .comptime_float => {
                const val = try sema.resolveConstDefinedValue(block, LazySrcLoc.unneeded, inst, undefined);
                const result_val = try val.floatCast(dest_ty, pt);
                return Air.internedToRef(result_val.toIntern());
            },
            .float => {
                if (maybe_inst_val) |val| {
                    const result_val = try val.floatCast(dest_ty, pt);
                    if (!val.eql(try result_val.floatCast(inst_ty, pt), inst_ty, zcu)) {
                        return sema.fail(
                            block,
                            inst_src,
                            "type '{}' cannot represent float value '{}'",
                            .{ dest_ty.fmt(pt), val.fmtValueSema(pt, sema) },
                        );
                    }
                    return Air.internedToRef(result_val.toIntern());
                } else if (dest_ty.zigTypeTag(zcu) == .comptime_float) {
                    if (!opts.report_err) return error.NotCoercible;
                    return sema.failWithNeededComptime(block, inst_src, .{ .simple = .casted_to_comptime_float });
                }

                // float widening
                const src_bits = inst_ty.floatBits(target);
                const dst_bits = dest_ty.floatBits(target);
                if (dst_bits >= src_bits) {
                    try sema.requireRuntimeBlock(block, inst_src, null);
                    return block.addTyOp(.fpext, dest_ty, inst);
                }
            },
            .int, .comptime_int => int: {
                const val = maybe_inst_val orelse {
                    if (dest_ty.zigTypeTag(zcu) == .comptime_float) {
                        if (!opts.report_err) return error.NotCoercible;
                        return sema.failWithNeededComptime(block, inst_src, .{ .simple = .casted_to_comptime_float });
                    }
                    break :int;
                };
                const result_val = try val.floatFromIntAdvanced(sema.arena, inst_ty, dest_ty, pt, .sema);
                // TODO implement this compile error
                //const int_again_val = try result_val.intFromFloat(sema.arena, inst_ty);
                //if (!int_again_val.eql(val, inst_ty, zcu)) {
                //    return sema.fail(
                //        block,
                //        inst_src,
                //        "type '{}' cannot represent integer value '{}'",
                //        .{ dest_ty.fmt(pt), val },
                //    );
                //}
                return Air.internedToRef(result_val.toIntern());
            },
            else => {},
        },
        .@"enum" => switch (inst_ty.zigTypeTag(zcu)) {
            .enum_literal => {
                // enum literal to enum
                const val = try sema.resolveConstDefinedValue(block, LazySrcLoc.unneeded, inst, undefined);
                const string = zcu.intern_pool.indexToKey(val.toIntern()).enum_literal;
                const field_index = dest_ty.enumFieldIndex(string, zcu) orelse {
                    return sema.fail(block, inst_src, "no field named '{}' in enum '{}'", .{
                        string.fmt(&zcu.intern_pool), dest_ty.fmt(pt),
                    });
                };
                return Air.internedToRef((try pt.enumValueFieldIndex(dest_ty, @intCast(field_index))).toIntern());
            },
            .@"union" => blk: {
                // union to its own tag type
                const union_tag_ty = inst_ty.unionTagType(zcu) orelse break :blk;
                if (union_tag_ty.eql(dest_ty, zcu)) {
                    return sema.unionToTag(block, dest_ty, inst, inst_src);
                }
            },
            else => {},
        },
        .error_union => switch (inst_ty.zigTypeTag(zcu)) {
            .error_union => eu: {
                if (maybe_inst_val) |inst_val| {
                    switch (inst_val.toIntern()) {
                        .undef => return pt.undefRef(dest_ty),
                        else => switch (zcu.intern_pool.indexToKey(inst_val.toIntern())) {
                            .error_union => |error_union| switch (error_union.val) {
                                .err_name => |err_name| {
                                    const error_set_ty = inst_ty.errorUnionSet(zcu);
                                    const error_set_val = Air.internedToRef((try pt.intern(.{ .err = .{
                                        .ty = error_set_ty.toIntern(),
                                        .name = err_name,
                                    } })));
                                    return sema.wrapErrorUnionSet(block, dest_ty, error_set_val, inst_src);
                                },
                                .payload => |payload| {
                                    const payload_val = Air.internedToRef(payload);
                                    return sema.wrapErrorUnionPayload(block, dest_ty, payload_val, inst_src) catch |err| switch (err) {
                                        error.NotCoercible => break :eu,
                                        else => |e| return e,
                                    };
                                },
                            },
                            else => unreachable,
                        },
                    }
                }
            },
            .error_set => {
                // E to E!T
                return sema.wrapErrorUnionSet(block, dest_ty, inst, inst_src);
            },
            else => eu: {
                // T to E!T
                return sema.wrapErrorUnionPayload(block, dest_ty, inst, inst_src) catch |err| switch (err) {
                    error.NotCoercible => {
                        if (in_memory_result == .no_match) {
                            const payload_type = dest_ty.errorUnionPayload(zcu);
                            // Try to give more useful notes
                            in_memory_result = try sema.coerceInMemoryAllowed(block, payload_type, inst_ty, false, target, dest_ty_src, inst_src, maybe_inst_val);
                        }
                        break :eu;
                    },
                    else => |e| return e,
                };
            },
        },
        .@"union" => switch (inst_ty.zigTypeTag(zcu)) {
            .@"enum", .enum_literal => return sema.coerceEnumToUnion(block, dest_ty, dest_ty_src, inst, inst_src),
            else => {},
        },
        .array => switch (inst_ty.zigTypeTag(zcu)) {
            .array => array_to_array: {
                // Array coercions are allowed only if the child is IMC and the sentinel is unchanged or removed.
                if (.ok != try sema.coerceInMemoryAllowed(
                    block,
                    dest_ty.childType(zcu),
                    inst_ty.childType(zcu),
                    false,
                    target,
                    dest_ty_src,
                    inst_src,
                    maybe_inst_val,
                )) {
                    break :array_to_array;
                }

                if (dest_ty.sentinel(zcu)) |dest_sent| {
                    const src_sent = inst_ty.sentinel(zcu) orelse break :array_to_array;
                    if (dest_sent.toIntern() != (try pt.getCoerced(src_sent, dest_ty.childType(zcu))).toIntern()) {
                        break :array_to_array;
                    }
                }

                return sema.coerceArrayLike(block, dest_ty, dest_ty_src, inst, inst_src);
            },
            .vector => return sema.coerceArrayLike(block, dest_ty, dest_ty_src, inst, inst_src),
            .@"struct" => {
                if (inst_ty.isTuple(zcu)) {
                    return sema.coerceTupleToArray(block, dest_ty, dest_ty_src, inst, inst_src);
                }
            },
            else => {},
        },
        .vector => switch (inst_ty.zigTypeTag(zcu)) {
            .array, .vector => return sema.coerceArrayLike(block, dest_ty, dest_ty_src, inst, inst_src),
            .@"struct" => {
                if (inst_ty.isTuple(zcu)) {
                    return sema.coerceTupleToArray(block, dest_ty, dest_ty_src, inst, inst_src);
                }
            },
            else => {},
        },
        .@"struct" => blk: {
            if (dest_ty.isTuple(zcu) and inst_ty.isTuple(zcu)) {
                return sema.coerceTupleToTuple(block, dest_ty, inst, inst_src) catch |err| switch (err) {
                    error.NotCoercible => break :blk,
                    else => |e| return e,
                };
            }
        },
        else => {},
    }

    const can_coerce_to = switch (dest_ty.zigTypeTag(zcu)) {
        .noreturn, .@"opaque" => false,
        else => true,
    };

    if (can_coerce_to) {
        // undefined to anything. We do this after the big switch above so that
        // special logic has a chance to run first, such as `*[N]T` to `[]T` which
        // should initialize the length field of the slice.
        if (maybe_inst_val) |val| if (val.toIntern() == .undef) return pt.undefRef(dest_ty);
    }

    if (!opts.report_err) return error.NotCoercible;

    if (opts.is_ret and dest_ty.zigTypeTag(zcu) == .noreturn) {
        const msg = msg: {
            const msg = try sema.errMsg(inst_src, "function declared 'noreturn' returns", .{});
            errdefer msg.destroy(sema.gpa);

            const ret_ty_src: LazySrcLoc = .{
                .base_node_inst = ip.getNav(zcu.funcInfo(sema.func_index).owner_nav).srcInst(ip),
                .offset = .{ .node_offset_fn_type_ret_ty = .zero },
            };
            try sema.errNote(ret_ty_src, msg, "'noreturn' declared here", .{});
            break :msg msg;
        };
        return sema.failWithOwnedErrorMsg(block, msg);
    }

    const msg = msg: {
        const msg = try sema.errMsg(inst_src, "expected type '{}', found '{}'", .{ dest_ty.fmt(pt), inst_ty.fmt(pt) });
        errdefer msg.destroy(sema.gpa);

        if (!can_coerce_to) {
            try sema.errNote(inst_src, msg, "cannot coerce to '{}'", .{dest_ty.fmt(pt)});
        }

        // E!T to T
        if (inst_ty.zigTypeTag(zcu) == .error_union and
            (try sema.coerceInMemoryAllowed(block, inst_ty.errorUnionPayload(zcu), dest_ty, false, target, dest_ty_src, inst_src, maybe_inst_val)) == .ok)
        {
            try sema.errNote(inst_src, msg, "cannot convert error union to payload type", .{});
            try sema.errNote(inst_src, msg, "consider using 'try', 'catch', or 'if'", .{});
        }

        // ?T to T
        if (inst_ty.zigTypeTag(zcu) == .optional and
            (try sema.coerceInMemoryAllowed(block, inst_ty.optionalChild(zcu), dest_ty, false, target, dest_ty_src, inst_src, maybe_inst_val)) == .ok)
        {
            try sema.errNote(inst_src, msg, "cannot convert optional to payload type", .{});
            try sema.errNote(inst_src, msg, "consider using '.?', 'orelse', or 'if'", .{});
        }

        try in_memory_result.report(sema, inst_src, msg);

        // Add notes about function return type
        if (opts.is_ret and
            !zcu.test_functions.contains(zcu.funcInfo(sema.func_index).owner_nav))
        {
            const ret_ty_src: LazySrcLoc = .{
                .base_node_inst = ip.getNav(zcu.funcInfo(sema.func_index).owner_nav).srcInst(ip),
                .offset = .{ .node_offset_fn_type_ret_ty = .zero },
            };
            if (inst_ty.isError(zcu) and !dest_ty.isError(zcu)) {
                try sema.errNote(ret_ty_src, msg, "function cannot return an error", .{});
            } else {
                try sema.errNote(ret_ty_src, msg, "function return type declared here", .{});
            }
        }

        if (try opts.param_src.get(sema)) |param_src| {
            try sema.errNote(param_src, msg, "parameter type declared here", .{});
        }

        // TODO maybe add "cannot store an error in type '{}'" note

        break :msg msg;
    };
    return sema.failWithOwnedErrorMsg(block, msg);
}

fn coerceInMemory(
    sema: *Sema,
    val: Value,
    dst_ty: Type,
) CompileError!Air.Inst.Ref {
    return Air.internedToRef((try sema.pt.getCoerced(val, dst_ty)).toIntern());
}

const InMemoryCoercionResult = union(enum) {
    ok,
    no_match: Pair,
    int_not_coercible: Int,
    comptime_int_not_coercible: TypeValuePair,
    error_union_payload: PairAndChild,
    array_len: IntPair,
    array_sentinel: Sentinel,
    array_elem: PairAndChild,
    vector_len: IntPair,
    vector_elem: PairAndChild,
    optional_shape: Pair,
    optional_child: PairAndChild,
    from_anyerror,
    missing_error: []const InternPool.NullTerminatedString,
    /// true if wanted is var args
    fn_var_args: bool,
    /// true if wanted is generic
    fn_generic: bool,
    fn_param_count: IntPair,
    fn_param_noalias: IntPair,
    fn_param_comptime: ComptimeParam,
    fn_param: Param,
    fn_cc: CC,
    fn_return_type: PairAndChild,
    ptr_child: PairAndChild,
    ptr_addrspace: AddressSpace,
    ptr_sentinel: Sentinel,
    ptr_size: Size,
    ptr_const: Pair,
    ptr_volatile: Pair,
    ptr_allowzero: Pair,
    ptr_bit_range: BitRange,
    ptr_alignment: AlignPair,
    double_ptr_to_anyopaque: Pair,
    slice_to_anyopaque: Pair,

    const Pair = struct {
        actual: Type,
        wanted: Type,
    };

    const TypeValuePair = struct {
        actual: Value,
        wanted: Type,
    };

    const PairAndChild = struct {
        child: *InMemoryCoercionResult,
        actual: Type,
        wanted: Type,
    };

    const Param = struct {
        child: *InMemoryCoercionResult,
        actual: Type,
        wanted: Type,
        index: u64,
    };

    const ComptimeParam = struct {
        index: u64,
        wanted: bool,
    };

    const Sentinel = struct {
        // unreachable_value indicates no sentinel
        actual: Value,
        wanted: Value,
        ty: Type,
    };

    const Int = struct {
        actual_signedness: std.builtin.Signedness,
        wanted_signedness: std.builtin.Signedness,
        actual_bits: u16,
        wanted_bits: u16,
    };

    const IntPair = struct {
        actual: u64,
        wanted: u64,
    };

    const AlignPair = struct {
        actual: Alignment,
        wanted: Alignment,
    };

    const Size = struct {
        actual: std.builtin.Type.Pointer.Size,
        wanted: std.builtin.Type.Pointer.Size,
    };

    const AddressSpace = struct {
        actual: std.builtin.AddressSpace,
        wanted: std.builtin.AddressSpace,
    };

    const CC = struct {
        actual: std.builtin.CallingConvention,
        wanted: std.builtin.CallingConvention,
    };

    const BitRange = struct {
        actual_host: u16,
        wanted_host: u16,
        actual_offset: u16,
        wanted_offset: u16,
    };

    fn dupe(child: *const InMemoryCoercionResult, arena: Allocator) !*InMemoryCoercionResult {
        const res = try arena.create(InMemoryCoercionResult);
        res.* = child.*;
        return res;
    }

    fn report(res: *const InMemoryCoercionResult, sema: *Sema, src: LazySrcLoc, msg: *Zcu.ErrorMsg) !void {
        const pt = sema.pt;
        var cur = res;
        while (true) switch (cur.*) {
            .ok => unreachable,
            .no_match => |types| {
                try sema.addDeclaredHereNote(msg, types.wanted);
                try sema.addDeclaredHereNote(msg, types.actual);
                break;
            },
            .int_not_coercible => |int| {
                try sema.errNote(src, msg, "{s} {d}-bit int cannot represent all possible {s} {d}-bit values", .{
                    @tagName(int.wanted_signedness), int.wanted_bits, @tagName(int.actual_signedness), int.actual_bits,
                });
                break;
            },
            .comptime_int_not_coercible => |int| {
                try sema.errNote(src, msg, "type '{}' cannot represent value '{}'", .{
                    int.wanted.fmt(pt), int.actual.fmtValueSema(pt, sema),
                });
                break;
            },
            .error_union_payload => |pair| {
                try sema.errNote(src, msg, "error union payload '{}' cannot cast into error union payload '{}'", .{
                    pair.actual.fmt(pt), pair.wanted.fmt(pt),
                });
                cur = pair.child;
            },
            .array_len => |lens| {
                try sema.errNote(src, msg, "array of length {d} cannot cast into an array of length {d}", .{
                    lens.actual, lens.wanted,
                });
                break;
            },
            .array_sentinel => |sentinel| {
                if (sentinel.actual.toIntern() != .unreachable_value) {
                    try sema.errNote(src, msg, "array sentinel '{}' cannot cast into array sentinel '{}'", .{
                        sentinel.actual.fmtValueSema(pt, sema), sentinel.wanted.fmtValueSema(pt, sema),
                    });
                } else {
                    try sema.errNote(src, msg, "destination array requires '{}' sentinel", .{
                        sentinel.wanted.fmtValueSema(pt, sema),
                    });
                }
                break;
            },
            .array_elem => |pair| {
                try sema.errNote(src, msg, "array element type '{}' cannot cast into array element type '{}'", .{
                    pair.actual.fmt(pt), pair.wanted.fmt(pt),
                });
                cur = pair.child;
            },
            .vector_len => |lens| {
                try sema.errNote(src, msg, "vector of length {d} cannot cast into a vector of length {d}", .{
                    lens.actual, lens.wanted,
                });
                break;
            },
            .vector_elem => |pair| {
                try sema.errNote(src, msg, "vector element type '{}' cannot cast into vector element type '{}'", .{
                    pair.actual.fmt(pt), pair.wanted.fmt(pt),
                });
                cur = pair.child;
            },
            .optional_shape => |pair| {
                try sema.errNote(src, msg, "optional type child '{}' cannot cast into optional type child '{}'", .{
                    pair.actual.optionalChild(pt.zcu).fmt(pt), pair.wanted.optionalChild(pt.zcu).fmt(pt),
                });
                break;
            },
            .optional_child => |pair| {
                try sema.errNote(src, msg, "optional type child '{}' cannot cast into optional type child '{}'", .{
                    pair.actual.fmt(pt), pair.wanted.fmt(pt),
                });
                cur = pair.child;
            },
            .from_anyerror => {
                try sema.errNote(src, msg, "global error set cannot cast into a smaller set", .{});
                break;
            },
            .missing_error => |missing_errors| {
                for (missing_errors) |err| {
                    try sema.errNote(src, msg, "'error.{}' not a member of destination error set", .{err.fmt(&pt.zcu.intern_pool)});
                }
                break;
            },
            .fn_var_args => |wanted_var_args| {
                if (wanted_var_args) {
                    try sema.errNote(src, msg, "non-variadic function cannot cast into a variadic function", .{});
                } else {
                    try sema.errNote(src, msg, "variadic function cannot cast into a non-variadic function", .{});
                }
                break;
            },
            .fn_generic => |wanted_generic| {
                if (wanted_generic) {
                    try sema.errNote(src, msg, "non-generic function cannot cast into a generic function", .{});
                } else {
                    try sema.errNote(src, msg, "generic function cannot cast into a non-generic function", .{});
                }
                break;
            },
            .fn_param_count => |lens| {
                try sema.errNote(src, msg, "function with {d} parameters cannot cast into a function with {d} parameters", .{
                    lens.actual, lens.wanted,
                });
                break;
            },
            .fn_param_noalias => |param| {
                var index: u6 = 0;
                var actual_noalias = false;
                while (true) : (index += 1) {
                    const actual: u1 = @truncate(param.actual >> index);
                    const wanted: u1 = @truncate(param.wanted >> index);
                    if (actual != wanted) {
                        actual_noalias = actual == 1;
                        break;
                    }
                }
                if (!actual_noalias) {
                    try sema.errNote(src, msg, "regular parameter {d} cannot cast into a noalias parameter", .{index});
                } else {
                    try sema.errNote(src, msg, "noalias parameter {d} cannot cast into a regular parameter", .{index});
                }
                break;
            },
            .fn_param_comptime => |param| {
                if (param.wanted) {
                    try sema.errNote(src, msg, "non-comptime parameter {d} cannot cast into a comptime parameter", .{param.index});
                } else {
                    try sema.errNote(src, msg, "comptime parameter {d} cannot cast into a non-comptime parameter", .{param.index});
                }
                break;
            },
            .fn_param => |param| {
                try sema.errNote(src, msg, "parameter {d} '{}' cannot cast into '{}'", .{
                    param.index, param.actual.fmt(pt), param.wanted.fmt(pt),
                });
                cur = param.child;
            },
            .fn_cc => |cc| {
                try sema.errNote(src, msg, "calling convention '{s}' cannot cast into calling convention '{s}'", .{ @tagName(cc.actual), @tagName(cc.wanted) });
                break;
            },
            .fn_return_type => |pair| {
                try sema.errNote(src, msg, "return type '{}' cannot cast into return type '{}'", .{
                    pair.actual.fmt(pt), pair.wanted.fmt(pt),
                });
                cur = pair.child;
            },
            .ptr_child => |pair| {
                try sema.errNote(src, msg, "pointer type child '{}' cannot cast into pointer type child '{}'", .{
                    pair.actual.fmt(pt), pair.wanted.fmt(pt),
                });
                cur = pair.child;
            },
            .ptr_addrspace => |@"addrspace"| {
                try sema.errNote(src, msg, "address space '{s}' cannot cast into address space '{s}'", .{ @tagName(@"addrspace".actual), @tagName(@"addrspace".wanted) });
                break;
            },
            .ptr_sentinel => |sentinel| {
                if (sentinel.actual.toIntern() != .unreachable_value) {
                    try sema.errNote(src, msg, "pointer sentinel '{}' cannot cast into pointer sentinel '{}'", .{
                        sentinel.actual.fmtValueSema(pt, sema), sentinel.wanted.fmtValueSema(pt, sema),
                    });
                } else {
                    try sema.errNote(src, msg, "destination pointer requires '{}' sentinel", .{
                        sentinel.wanted.fmtValueSema(pt, sema),
                    });
                }
                break;
            },
            .ptr_size => |size| {
                try sema.errNote(src, msg, "a {s} cannot cast into a {s}", .{ pointerSizeString(size.actual), pointerSizeString(size.wanted) });
                break;
            },
            .ptr_allowzero => |pair| {
                const wanted_allow_zero = pair.wanted.ptrAllowsZero(pt.zcu);
                const actual_allow_zero = pair.actual.ptrAllowsZero(pt.zcu);
                if (actual_allow_zero and !wanted_allow_zero) {
                    try sema.errNote(src, msg, "'{}' could have null values which are illegal in type '{}'", .{
                        pair.actual.fmt(pt), pair.wanted.fmt(pt),
                    });
                } else {
                    try sema.errNote(src, msg, "mutable '{}' would allow illegal null values stored to type '{}'", .{
                        pair.wanted.fmt(pt), pair.actual.fmt(pt),
                    });
                }
                break;
            },
            .ptr_const => |pair| {
                const wanted_const = pair.wanted.isConstPtr(pt.zcu);
                const actual_const = pair.actual.isConstPtr(pt.zcu);
                if (actual_const and !wanted_const) {
                    try sema.errNote(src, msg, "cast discards const qualifier", .{});
                } else {
                    try sema.errNote(src, msg, "mutable '{}' would allow illegal const pointers stored to type '{}'", .{
                        pair.wanted.fmt(pt), pair.actual.fmt(pt),
                    });
                }
                break;
            },
            .ptr_volatile => |pair| {
                const wanted_volatile = pair.wanted.isVolatilePtr(pt.zcu);
                const actual_volatile = pair.actual.isVolatilePtr(pt.zcu);
                if (actual_volatile and !wanted_volatile) {
                    try sema.errNote(src, msg, "cast discards volatile qualifier", .{});
                } else {
                    try sema.errNote(src, msg, "mutable '{}' would allow illegal volatile pointers stored to type '{}'", .{
                        pair.wanted.fmt(pt), pair.actual.fmt(pt),
                    });
                }
                break;
            },
            .ptr_bit_range => |bit_range| {
                if (bit_range.actual_host != bit_range.wanted_host) {
                    try sema.errNote(src, msg, "pointer host size '{}' cannot cast into pointer host size '{}'", .{
                        bit_range.actual_host, bit_range.wanted_host,
                    });
                }
                if (bit_range.actual_offset != bit_range.wanted_offset) {
                    try sema.errNote(src, msg, "pointer bit offset '{}' cannot cast into pointer bit offset '{}'", .{
                        bit_range.actual_offset, bit_range.wanted_offset,
                    });
                }
                break;
            },
            .ptr_alignment => |pair| {
                try sema.errNote(src, msg, "pointer alignment '{d}' cannot cast into pointer alignment '{d}'", .{
                    pair.actual.toByteUnits() orelse 0, pair.wanted.toByteUnits() orelse 0,
                });
                break;
            },
            .double_ptr_to_anyopaque => |pair| {
                try sema.errNote(src, msg, "cannot implicitly cast double pointer '{}' to anyopaque pointer '{}'", .{
                    pair.actual.fmt(pt), pair.wanted.fmt(pt),
                });
                break;
            },
            .slice_to_anyopaque => |pair| {
                try sema.errNote(src, msg, "cannot implicitly cast slice '{}' to anyopaque pointer '{}'", .{
                    pair.actual.fmt(pt), pair.wanted.fmt(pt),
                });
                try sema.errNote(src, msg, "consider using '.ptr'", .{});
                break;
            },
        };
    }
};

fn pointerSizeString(size: std.builtin.Type.Pointer.Size) []const u8 {
    return switch (size) {
        .one => "single pointer",
        .many => "many pointer",
        .c => "C pointer",
        .slice => "slice",
    };
}

/// If types `A` and `B` have identical representations in runtime memory, they are considered
/// "in-memory coercible". This is a subset of normal coercions. Not only can `A` coerce to `B`, but
/// also, coercions can happen through pointers. For instance, `*const A` can coerce to `*const B`.
///
/// If this function is called, the coercion must be applied, or a compile error emitted if `.ok`
/// is not returned. This is because this function may modify inferred error sets to make a
/// coercion possible, even if `.ok` is not returned.
pub fn coerceInMemoryAllowed(
    sema: *Sema,
    block: *Block,
    dest_ty: Type,
    src_ty: Type,
    /// If `true`, this query comes from an attempted coercion of the form `*Src` -> `*Dest`, where
    /// both pointers are mutable. If this coercion is allowed, one could store to the `*Dest` and
    /// load from the `*Src` to effectively perform an in-memory coercion from `Dest` to `Src`.
    /// Therefore, when `dest_is_mut`, the in-memory coercion must be valid in *both directions*.
    dest_is_mut: bool,
    target: std.Target,
    dest_src: LazySrcLoc,
    src_src: LazySrcLoc,
    src_val: ?Value,
) CompileError!InMemoryCoercionResult {
    const pt = sema.pt;
    const zcu = pt.zcu;

    if (dest_ty.eql(src_ty, zcu))
        return .ok;

    const dest_tag = dest_ty.zigTypeTag(zcu);
    const src_tag = src_ty.zigTypeTag(zcu);

    // Differently-named integers with the same number of bits.
    if (dest_tag == .int and src_tag == .int) {
        const dest_info = dest_ty.intInfo(zcu);
        const src_info = src_ty.intInfo(zcu);

        if (dest_info.signedness == src_info.signedness and
            dest_info.bits == src_info.bits)
        {
            return .ok;
        }

        if ((src_info.signedness == dest_info.signedness and dest_info.bits < src_info.bits) or
            // small enough unsigned ints can get casted to large enough signed ints
            (dest_info.signedness == .signed and src_info.signedness == .unsigned and dest_info.bits <= src_info.bits) or
            (dest_info.signedness == .unsigned and src_info.signedness == .signed))
        {
            return InMemoryCoercionResult{ .int_not_coercible = .{
                .actual_signedness = src_info.signedness,
                .wanted_signedness = dest_info.signedness,
                .actual_bits = src_info.bits,
                .wanted_bits = dest_info.bits,
            } };
        }
    }

    // Comptime int to regular int.
    if (dest_tag == .int and src_tag == .comptime_int) {
        if (src_val) |val| {
            if (!(try sema.intFitsInType(val, dest_ty, null))) {
                return .{ .comptime_int_not_coercible = .{ .wanted = dest_ty, .actual = val } };
            }
        }
    }

    // Differently-named floats with the same number of bits.
    if (dest_tag == .float and src_tag == .float) {
        const dest_bits = dest_ty.floatBits(target);
        const src_bits = src_ty.floatBits(target);
        if (dest_bits == src_bits) {
            return .ok;
        }
    }

    // Pointers / Pointer-like Optionals
    const maybe_dest_ptr_ty = try sema.typePtrOrOptionalPtrTy(dest_ty);
    const maybe_src_ptr_ty = try sema.typePtrOrOptionalPtrTy(src_ty);
    if (maybe_dest_ptr_ty) |dest_ptr_ty| {
        if (maybe_src_ptr_ty) |src_ptr_ty| {
            return try sema.coerceInMemoryAllowedPtrs(block, dest_ty, src_ty, dest_ptr_ty, src_ptr_ty, dest_is_mut, target, dest_src, src_src);
        }
    }

    // Slices
    if (dest_ty.isSlice(zcu) and src_ty.isSlice(zcu)) {
        return try sema.coerceInMemoryAllowedPtrs(block, dest_ty, src_ty, dest_ty, src_ty, dest_is_mut, target, dest_src, src_src);
    }

    // Functions
    if (dest_tag == .@"fn" and src_tag == .@"fn") {
        return try sema.coerceInMemoryAllowedFns(block, dest_ty, src_ty, dest_is_mut, target, dest_src, src_src);
    }

    // Error Unions
    if (dest_tag == .error_union and src_tag == .error_union) {
        const dest_payload = dest_ty.errorUnionPayload(zcu);
        const src_payload = src_ty.errorUnionPayload(zcu);
        const child = try sema.coerceInMemoryAllowed(block, dest_payload, src_payload, dest_is_mut, target, dest_src, src_src, null);
        if (child != .ok) {
            return .{ .error_union_payload = .{
                .child = try child.dupe(sema.arena),
                .actual = src_payload,
                .wanted = dest_payload,
            } };
        }
        return try sema.coerceInMemoryAllowed(block, dest_ty.errorUnionSet(zcu), src_ty.errorUnionSet(zcu), dest_is_mut, target, dest_src, src_src, null);
    }

    // Error Sets
    if (dest_tag == .error_set and src_tag == .error_set) {
        const res1 = try sema.coerceInMemoryAllowedErrorSets(block, dest_ty, src_ty, dest_src, src_src);
        if (!dest_is_mut or res1 != .ok) return res1;
        // src -> dest is okay, but `dest_is_mut`, so it needs to be allowed in the other direction.
        const res2 = try sema.coerceInMemoryAllowedErrorSets(block, src_ty, dest_ty, src_src, dest_src);
        return res2;
    }

    // Arrays
    if (dest_tag == .array and src_tag == .array) {
        const dest_info = dest_ty.arrayInfo(zcu);
        const src_info = src_ty.arrayInfo(zcu);
        if (dest_info.len != src_info.len) {
            return .{ .array_len = .{
                .actual = src_info.len,
                .wanted = dest_info.len,
            } };
        }

        const child = try sema.coerceInMemoryAllowed(block, dest_info.elem_type, src_info.elem_type, dest_is_mut, target, dest_src, src_src, null);
        switch (child) {
            .ok => {},
            .no_match => return child,
            else => {
                return .{ .array_elem = .{
                    .child = try child.dupe(sema.arena),
                    .actual = src_info.elem_type,
                    .wanted = dest_info.elem_type,
                } };
            },
        }
        const ok_sent = (dest_info.sentinel == null and src_info.sentinel == null) or
            (src_info.sentinel != null and
                dest_info.sentinel != null and
                dest_info.sentinel.?.eql(
                    try pt.getCoerced(src_info.sentinel.?, dest_info.elem_type),
                    dest_info.elem_type,
                    zcu,
                ));
        if (!ok_sent) {
            return .{ .array_sentinel = .{
                .actual = src_info.sentinel orelse Value.@"unreachable",
                .wanted = dest_info.sentinel orelse Value.@"unreachable",
                .ty = dest_info.elem_type,
            } };
        }
        return .ok;
    }

    // Vectors
    if (dest_tag == .vector and src_tag == .vector) {
        const dest_len = dest_ty.vectorLen(zcu);
        const src_len = src_ty.vectorLen(zcu);
        if (dest_len != src_len) {
            return .{ .vector_len = .{
                .actual = src_len,
                .wanted = dest_len,
            } };
        }

        const dest_elem_ty = dest_ty.scalarType(zcu);
        const src_elem_ty = src_ty.scalarType(zcu);
        const child = try sema.coerceInMemoryAllowed(block, dest_elem_ty, src_elem_ty, dest_is_mut, target, dest_src, src_src, null);
        if (child != .ok) {
            return .{ .vector_elem = .{
                .child = try child.dupe(sema.arena),
                .actual = src_elem_ty,
                .wanted = dest_elem_ty,
            } };
        }

        return .ok;
    }

    // Arrays <-> Vectors
    if ((dest_tag == .vector and src_tag == .array) or
        (dest_tag == .array and src_tag == .vector))
    {
        const dest_len = dest_ty.arrayLen(zcu);
        const src_len = src_ty.arrayLen(zcu);
        if (dest_len != src_len) {
            return .{ .array_len = .{
                .actual = src_len,
                .wanted = dest_len,
            } };
        }

        const dest_elem_ty = dest_ty.childType(zcu);
        const src_elem_ty = src_ty.childType(zcu);
        const child = try sema.coerceInMemoryAllowed(block, dest_elem_ty, src_elem_ty, dest_is_mut, target, dest_src, src_src, null);
        if (child != .ok) {
            return .{ .array_elem = .{
                .child = try child.dupe(sema.arena),
                .actual = src_elem_ty,
                .wanted = dest_elem_ty,
            } };
        }

        if (dest_tag == .array) {
            const dest_info = dest_ty.arrayInfo(zcu);
            if (dest_info.sentinel != null) {
                return .{ .array_sentinel = .{
                    .actual = Value.@"unreachable",
                    .wanted = dest_info.sentinel.?,
                    .ty = dest_info.elem_type,
                } };
            }
        }

        // The memory layout of @Vector(N, iM) is the same as the integer type i(N*M),
        // that is to say, the padding bits are not in the same place as the array [N]iM.
        // If there's no padding, the bitcast is possible.
        const elem_bit_size = dest_elem_ty.bitSize(zcu);
        const elem_abi_byte_size = dest_elem_ty.abiSize(zcu);
        if (elem_abi_byte_size * 8 == elem_bit_size)
            return .ok;
    }

    // Optionals
    if (dest_tag == .optional and src_tag == .optional) {
        if ((maybe_dest_ptr_ty != null) != (maybe_src_ptr_ty != null)) {
            return .{ .optional_shape = .{
                .actual = src_ty,
                .wanted = dest_ty,
            } };
        }
        const dest_child_type = dest_ty.optionalChild(zcu);
        const src_child_type = src_ty.optionalChild(zcu);

        const child = try sema.coerceInMemoryAllowed(block, dest_child_type, src_child_type, dest_is_mut, target, dest_src, src_src, null);
        if (child != .ok) {
            return .{ .optional_child = .{
                .child = try child.dupe(sema.arena),
                .actual = src_child_type,
                .wanted = dest_child_type,
            } };
        }

        return .ok;
    }

    // Tuples (with in-memory-coercible fields)
    if (dest_ty.isTuple(zcu) and src_ty.isTuple(zcu)) tuple: {
        if (dest_ty.structFieldCount(zcu) != src_ty.structFieldCount(zcu)) break :tuple;
        const field_count = dest_ty.structFieldCount(zcu);
        for (0..field_count) |field_idx| {
            if (dest_ty.structFieldIsComptime(field_idx, zcu) != src_ty.structFieldIsComptime(field_idx, zcu)) break :tuple;
            if (dest_ty.fieldAlignment(field_idx, zcu) != src_ty.fieldAlignment(field_idx, zcu)) break :tuple;
            const dest_field_ty = dest_ty.fieldType(field_idx, zcu);
            const src_field_ty = src_ty.fieldType(field_idx, zcu);
            const field = try sema.coerceInMemoryAllowed(block, dest_field_ty, src_field_ty, dest_is_mut, target, dest_src, src_src, null);
            if (field != .ok) break :tuple;
        }
        return .ok;
    }

    return .{ .no_match = .{
        .actual = dest_ty,
        .wanted = src_ty,
    } };
}

fn coerceInMemoryAllowedErrorSets(
    sema: *Sema,
    block: *Block,
    dest_ty: Type,
    src_ty: Type,
    dest_src: LazySrcLoc,
    src_src: LazySrcLoc,
) !InMemoryCoercionResult {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const gpa = sema.gpa;
    const ip = &zcu.intern_pool;

    // Coercion to `anyerror`. Note that this check can return false negatives
    // in case the error sets did not get resolved.
    if (dest_ty.isAnyError(zcu)) {
        return .ok;
    }

    if (dest_ty.toIntern() == .adhoc_inferred_error_set_type) {
        // We are trying to coerce an error set to the current function's
        // inferred error set.
        const dst_ies = sema.fn_ret_ty_ies.?;
        try dst_ies.addErrorSet(src_ty, ip, sema.arena);
        return .ok;
    }

    if (ip.isInferredErrorSetType(dest_ty.toIntern())) {
        const dst_ies_func_index = ip.iesFuncIndex(dest_ty.toIntern());
        if (sema.fn_ret_ty_ies) |dst_ies| {
            if (dst_ies.func == dst_ies_func_index) {
                // We are trying to coerce an error set to the current function's
                // inferred error set.
                try dst_ies.addErrorSet(src_ty, ip, sema.arena);
                return .ok;
            }
        }
        switch (try sema.resolveInferredErrorSet(block, dest_src, dest_ty.toIntern())) {
            // isAnyError might have changed from a false negative to a true
            // positive after resolution.
            .anyerror_type => return .ok,
            else => {},
        }
    }

    var missing_error_buf = std.ArrayList(InternPool.NullTerminatedString).init(gpa);
    defer missing_error_buf.deinit();

    switch (src_ty.toIntern()) {
        .anyerror_type => switch (ip.indexToKey(dest_ty.toIntern())) {
            .simple_type => unreachable, // filtered out above
            .error_set_type, .inferred_error_set_type => return .from_anyerror,
            else => unreachable,
        },

        else => switch (ip.indexToKey(src_ty.toIntern())) {
            .inferred_error_set_type => {
                const resolved_src_ty = try sema.resolveInferredErrorSet(block, src_src, src_ty.toIntern());
                // src anyerror status might have changed after the resolution.
                if (resolved_src_ty == .anyerror_type) {
                    // dest_ty.isAnyError(zcu) == true is already checked for at this point.
                    return .from_anyerror;
                }

                for (ip.indexToKey(resolved_src_ty).error_set_type.names.get(ip)) |key| {
                    if (!Type.errorSetHasFieldIp(ip, dest_ty.toIntern(), key)) {
                        try missing_error_buf.append(key);
                    }
                }

                if (missing_error_buf.items.len != 0) {
                    return InMemoryCoercionResult{
                        .missing_error = try sema.arena.dupe(InternPool.NullTerminatedString, missing_error_buf.items),
                    };
                }

                return .ok;
            },
            .error_set_type => |error_set_type| {
                for (error_set_type.names.get(ip)) |name| {
                    if (!Type.errorSetHasFieldIp(ip, dest_ty.toIntern(), name)) {
                        try missing_error_buf.append(name);
                    }
                }

                if (missing_error_buf.items.len != 0) {
                    return InMemoryCoercionResult{
                        .missing_error = try sema.arena.dupe(InternPool.NullTerminatedString, missing_error_buf.items),
                    };
                }

                return .ok;
            },
            else => unreachable,
        },
    }
}

fn coerceInMemoryAllowedFns(
    sema: *Sema,
    block: *Block,
    dest_ty: Type,
    src_ty: Type,
    /// If set, the coercion must be valid in both directions.
    dest_is_mut: bool,
    target: std.Target,
    dest_src: LazySrcLoc,
    src_src: LazySrcLoc,
) !InMemoryCoercionResult {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const ip = &zcu.intern_pool;

    const dest_info = zcu.typeToFunc(dest_ty).?;
    const src_info = zcu.typeToFunc(src_ty).?;

    {
        if (dest_info.is_var_args != src_info.is_var_args) {
            return InMemoryCoercionResult{ .fn_var_args = dest_info.is_var_args };
        }

        if (dest_info.is_generic != src_info.is_generic) {
            return InMemoryCoercionResult{ .fn_generic = dest_info.is_generic };
        }

        const callconv_ok = callconvCoerceAllowed(target, src_info.cc, dest_info.cc) and
            (!dest_is_mut or callconvCoerceAllowed(target, dest_info.cc, src_info.cc));

        if (!callconv_ok) {
            return .{ .fn_cc = .{
                .actual = src_info.cc,
                .wanted = dest_info.cc,
            } };
        }

        if (!switch (src_info.return_type) {
            .generic_poison_type => true,
            .noreturn_type => !dest_is_mut,
            else => false,
        }) {
            const rt = try sema.coerceInMemoryAllowed(
                block,
                .fromInterned(dest_info.return_type),
                .fromInterned(src_info.return_type),
                dest_is_mut,
                target,
                dest_src,
                src_src,
                null,
            );
            if (rt != .ok) return .{ .fn_return_type = .{
                .child = try rt.dupe(sema.arena),
                .actual = .fromInterned(src_info.return_type),
                .wanted = .fromInterned(dest_info.return_type),
            } };
        }
    }

    const params_len = params_len: {
        if (dest_info.param_types.len != src_info.param_types.len) {
            return .{ .fn_param_count = .{
                .actual = src_info.param_types.len,
                .wanted = dest_info.param_types.len,
            } };
        }

        if (dest_info.noalias_bits != src_info.noalias_bits) {
            return .{ .fn_param_noalias = .{
                .actual = src_info.noalias_bits,
                .wanted = dest_info.noalias_bits,
            } };
        }

        break :params_len dest_info.param_types.len;
    };

    for (0..params_len) |param_i| {
        const dest_param_ty: Type = .fromInterned(dest_info.param_types.get(ip)[param_i]);
        const src_param_ty: Type = .fromInterned(src_info.param_types.get(ip)[param_i]);

        comptime_param: {
            const src_is_comptime = src_info.paramIsComptime(@intCast(param_i));
            const dest_is_comptime = dest_info.paramIsComptime(@intCast(param_i));
            if (src_is_comptime == dest_is_comptime) break :comptime_param;
            if (!dest_is_mut and src_is_comptime and !dest_is_comptime and try dest_param_ty.comptimeOnlySema(pt)) {
                // A parameter which is marked `comptime` can drop that annotation if the type is comptime-only.
                // The function remains generic, and the parameter is going to be comptime-resolved either way,
                // so this just affects whether or not the argument is comptime-evaluated at the call site.
                break :comptime_param;
            }
            return .{ .fn_param_comptime = .{
                .index = param_i,
                .wanted = dest_is_comptime,
            } };
        }

        if (!src_param_ty.isGenericPoison() and !dest_param_ty.isGenericPoison()) {
            // Note: Cast direction is reversed here.
            const param = try sema.coerceInMemoryAllowed(block, src_param_ty, dest_param_ty, dest_is_mut, target, dest_src, src_src, null);
            if (param != .ok) {
                return .{ .fn_param = .{
                    .child = try param.dupe(sema.arena),
                    .actual = src_param_ty,
                    .wanted = dest_param_ty,
                    .index = param_i,
                } };
            }
        }
    }

    return .ok;
}

fn callconvCoerceAllowed(
    target: std.Target,
    src_cc: std.builtin.CallingConvention,
    dest_cc: std.builtin.CallingConvention,
) bool {
    const Tag = std.builtin.CallingConvention.Tag;
    if (@as(Tag, src_cc) != @as(Tag, dest_cc)) return false;

    switch (src_cc) {
        inline else => |src_data, tag| {
            const dest_data = @field(dest_cc, @tagName(tag));
            if (@TypeOf(src_data) != void) {
                const default_stack_align = target.stackAlignment();
                const src_stack_align = src_data.incoming_stack_alignment orelse default_stack_align;
                const dest_stack_align = src_data.incoming_stack_alignment orelse default_stack_align;
                if (dest_stack_align < src_stack_align) return false;
            }
            switch (@TypeOf(src_data)) {
                void, std.builtin.CallingConvention.CommonOptions => {},
                std.builtin.CallingConvention.X86RegparmOptions => {
                    if (src_data.register_params != dest_data.register_params) return false;
                },
                std.builtin.CallingConvention.ArmInterruptOptions => {
                    if (src_data.type != dest_data.type) return false;
                },
                std.builtin.CallingConvention.MipsInterruptOptions => {
                    if (src_data.mode != dest_data.mode) return false;
                },
                std.builtin.CallingConvention.RiscvInterruptOptions => {
                    if (src_data.mode != dest_data.mode) return false;
                },
                else => comptime unreachable,
            }
        },
    }
    return true;
}

fn coerceInMemoryAllowedPtrs(
    sema: *Sema,
    block: *Block,
    dest_ty: Type,
    src_ty: Type,
    dest_ptr_ty: Type,
    src_ptr_ty: Type,
    /// If set, the coercion must be valid in both directions.
    dest_is_mut: bool,
    target: std.Target,
    dest_src: LazySrcLoc,
    src_src: LazySrcLoc,
) !InMemoryCoercionResult {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const dest_info = dest_ptr_ty.ptrInfo(zcu);
    const src_info = src_ptr_ty.ptrInfo(zcu);

    const ok_ptr_size = src_info.flags.size == dest_info.flags.size or
        src_info.flags.size == .c or dest_info.flags.size == .c;
    if (!ok_ptr_size) {
        return InMemoryCoercionResult{ .ptr_size = .{
            .actual = src_info.flags.size,
            .wanted = dest_info.flags.size,
        } };
    }

    const ok_const = src_info.flags.is_const == dest_info.flags.is_const or
        (!dest_is_mut and dest_info.flags.is_const);

    if (!ok_const) return .{ .ptr_const = .{
        .actual = src_ty,
        .wanted = dest_ty,
    } };

    const ok_volatile = src_info.flags.is_volatile == dest_info.flags.is_volatile or
        (!dest_is_mut and dest_info.flags.is_volatile);

    if (!ok_volatile) return .{ .ptr_volatile = .{
        .actual = src_ty,
        .wanted = dest_ty,
    } };

    const dest_allowzero = dest_ty.ptrAllowsZero(zcu);
    const src_allowzero = src_ty.ptrAllowsZero(zcu);
    const ok_allowzero = src_allowzero == dest_allowzero or
        (!dest_is_mut and dest_allowzero);

    if (!ok_allowzero) return .{ .ptr_allowzero = .{
        .actual = src_ty,
        .wanted = dest_ty,
    } };

    if (dest_info.flags.address_space != src_info.flags.address_space) {
        return .{ .ptr_addrspace = .{
            .actual = src_info.flags.address_space,
            .wanted = dest_info.flags.address_space,
        } };
    }

    const dest_child: Type = .fromInterned(dest_info.child);
    const src_child: Type = .fromInterned(src_info.child);
    const child = try sema.coerceInMemoryAllowed(
        block,
        dest_child,
        src_child,
        // We must also include `dest_is_mut`.
        // Otherwise, this code is valid:
        //
        // const b: B = ...;
        // var pa: *const A = undefined;
        // const ppa: **const A = &pa;
        // const ppb: **const B = ppa; // <-- this is what that allows
        // ppb.* = &b;
        // const a: A = pa.*;
        //
        // ...effectively performing an in-memory coercion from B to A.
        dest_is_mut or !dest_info.flags.is_const,
        target,
        dest_src,
        src_src,
        null,
    );
    if (child != .ok and !dest_is_mut) allow: {
        // As a special case, we also allow coercing `*[n:s]T` to `*[n]T`, akin to dropping the sentinel from a slice.
        // `*[n:s]T` cannot coerce in memory to `*[n]T` since they have different sizes.
        if (src_child.zigTypeTag(zcu) == .array and dest_child.zigTypeTag(zcu) == .array and
            src_child.arrayLen(zcu) == dest_child.arrayLen(zcu) and
            src_child.sentinel(zcu) != null and dest_child.sentinel(zcu) == null and
            .ok == try sema.coerceInMemoryAllowed(block, dest_child.childType(zcu), src_child.childType(zcu), !dest_info.flags.is_const, target, dest_src, src_src, null))
        {
            break :allow;
        }
        return .{ .ptr_child = .{
            .child = try child.dupe(sema.arena),
            .actual = .fromInterned(src_info.child),
            .wanted = .fromInterned(dest_info.child),
        } };
    }

    if (src_info.packed_offset.host_size != dest_info.packed_offset.host_size or
        src_info.packed_offset.bit_offset != dest_info.packed_offset.bit_offset)
    {
        return .{ .ptr_bit_range = .{
            .actual_host = src_info.packed_offset.host_size,
            .wanted_host = dest_info.packed_offset.host_size,
            .actual_offset = src_info.packed_offset.bit_offset,
            .wanted_offset = dest_info.packed_offset.bit_offset,
        } };
    }

    const sentinel_ok = ok: {
        const ss = src_info.sentinel;
        const ds = dest_info.sentinel;
        if (ss == .none and ds == .none) break :ok true;
        if (ss != .none and ds != .none) {
            if (ds == try zcu.intern_pool.getCoerced(sema.gpa, pt.tid, ss, dest_info.child)) break :ok true;
        }
        if (src_info.flags.size == .c) break :ok true;
        if (!dest_is_mut and dest_info.sentinel == .none) break :ok true;
        break :ok false;
    };

    if (!sentinel_ok) {
        return .{ .ptr_sentinel = .{
            .actual = switch (src_info.sentinel) {
                .none => Value.@"unreachable",
                else => Value.fromInterned(src_info.sentinel),
            },
            .wanted = switch (dest_info.sentinel) {
                .none => Value.@"unreachable",
                else => Value.fromInterned(dest_info.sentinel),
            },
            .ty = .fromInterned(dest_info.child),
        } };
    }

    // If both pointers have alignment 0, it means they both want ABI alignment.
    // In this case, if they share the same child type, no need to resolve
    // pointee type alignment. Otherwise both pointee types must have their alignment
    // resolved and we compare the alignment numerically.
    if (src_info.flags.alignment != .none or dest_info.flags.alignment != .none or
        dest_info.child != src_info.child)
    {
        const src_align = if (src_info.flags.alignment != .none)
            src_info.flags.alignment
        else
            try Type.fromInterned(src_info.child).abiAlignmentSema(pt);

        const dest_align = if (dest_info.flags.alignment != .none)
            dest_info.flags.alignment
        else
            try Type.fromInterned(dest_info.child).abiAlignmentSema(pt);

        if (dest_align.compare(if (dest_is_mut) .neq else .gt, src_align)) {
            return InMemoryCoercionResult{ .ptr_alignment = .{
                .actual = src_align,
                .wanted = dest_align,
            } };
        }
    }

    return .ok;
}

fn coerceVarArgParam(
    sema: *Sema,
    block: *Block,
    inst: Air.Inst.Ref,
    inst_src: LazySrcLoc,
) !Air.Inst.Ref {
    if (block.is_typeof) return inst;

    const pt = sema.pt;
    const zcu = pt.zcu;
    const uncasted_ty = sema.typeOf(inst);
    const coerced = switch (uncasted_ty.zigTypeTag(zcu)) {
        // TODO consider casting to c_int/f64 if they fit
        .comptime_int, .comptime_float => return sema.fail(
            block,
            inst_src,
            "integer and float literals passed to variadic function must be casted to a fixed-size number type",
            .{},
        ),
        .@"fn" => fn_ptr: {
            const fn_val = try sema.resolveConstDefinedValue(block, LazySrcLoc.unneeded, inst, undefined);
            const fn_nav = zcu.funcInfo(fn_val.toIntern()).owner_nav;
            break :fn_ptr try sema.analyzeNavRef(block, inst_src, fn_nav);
        },
        .array => return sema.fail(block, inst_src, "arrays must be passed by reference to variadic function", .{}),
        .float => float: {
            const target = zcu.getTarget();
            const double_bits = target.cTypeBitSize(.double);
            const inst_bits = uncasted_ty.floatBits(target);
            if (inst_bits >= double_bits) break :float inst;
            switch (double_bits) {
                32 => break :float try sema.coerce(block, .f32, inst, inst_src),
                64 => break :float try sema.coerce(block, .f64, inst, inst_src),
                else => unreachable,
            }
        },
        else => if (uncasted_ty.isAbiInt(zcu)) int: {
            if (!try sema.validateExternType(uncasted_ty, .param_ty)) break :int inst;
            const target = zcu.getTarget();
            const uncasted_info = uncasted_ty.intInfo(zcu);
            if (uncasted_info.bits <= target.cTypeBitSize(switch (uncasted_info.signedness) {
                .signed => .int,
                .unsigned => .uint,
            })) break :int try sema.coerce(block, switch (uncasted_info.signedness) {
                .signed => .c_int,
                .unsigned => .c_uint,
            }, inst, inst_src);
            if (uncasted_info.bits <= target.cTypeBitSize(switch (uncasted_info.signedness) {
                .signed => .long,
                .unsigned => .ulong,
            })) break :int try sema.coerce(block, switch (uncasted_info.signedness) {
                .signed => .c_long,
                .unsigned => .c_ulong,
            }, inst, inst_src);
            if (uncasted_info.bits <= target.cTypeBitSize(switch (uncasted_info.signedness) {
                .signed => .longlong,
                .unsigned => .ulonglong,
            })) break :int try sema.coerce(block, switch (uncasted_info.signedness) {
                .signed => .c_longlong,
                .unsigned => .c_ulonglong,
            }, inst, inst_src);
            break :int inst;
        } else inst,
    };

    const coerced_ty = sema.typeOf(coerced);
    if (!try sema.validateExternType(coerced_ty, .param_ty)) {
        const msg = msg: {
            const msg = try sema.errMsg(inst_src, "cannot pass '{}' to variadic function", .{coerced_ty.fmt(pt)});
            errdefer msg.destroy(sema.gpa);

            try sema.explainWhyTypeIsNotExtern(msg, inst_src, coerced_ty, .param_ty);

            try sema.addDeclaredHereNote(msg, coerced_ty);
            break :msg msg;
        };
        return sema.failWithOwnedErrorMsg(block, msg);
    }
    return coerced;
}

// TODO migrate callsites to use storePtr2 instead.
fn storePtr(
    sema: *Sema,
    block: *Block,
    src: LazySrcLoc,
    ptr: Air.Inst.Ref,
    uncasted_operand: Air.Inst.Ref,
) CompileError!void {
    const air_tag: Air.Inst.Tag = if (block.wantSafety()) .store_safe else .store;
    return sema.storePtr2(block, src, ptr, src, uncasted_operand, src, air_tag);
}

fn storePtr2(
    sema: *Sema,
    block: *Block,
    src: LazySrcLoc,
    ptr: Air.Inst.Ref,
    ptr_src: LazySrcLoc,
    uncasted_operand: Air.Inst.Ref,
    operand_src: LazySrcLoc,
    air_tag: Air.Inst.Tag,
) CompileError!void {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const ptr_ty = sema.typeOf(ptr);
    if (ptr_ty.isConstPtr(zcu))
        return sema.fail(block, ptr_src, "cannot assign to constant", .{});

    const elem_ty = ptr_ty.childType(zcu);

    // To generate better code for tuples, we detect a tuple operand here, and
    // analyze field loads and stores directly. This avoids an extra allocation + memcpy
    // which would occur if we used `coerce`.
    // However, we avoid this mechanism if the destination element type is a tuple,
    // because the regular store will be better for this case.
    // If the destination type is a struct we don't want this mechanism to trigger, because
    // this code does not handle tuple-to-struct coercion which requires dealing with missing
    // fields.
    const operand_ty = sema.typeOf(uncasted_operand);
    if (operand_ty.isTuple(zcu) and elem_ty.zigTypeTag(zcu) == .array) {
        const field_count = operand_ty.structFieldCount(zcu);
        var i: u32 = 0;
        while (i < field_count) : (i += 1) {
            const elem_src = operand_src; // TODO better source location
            const elem = try sema.tupleField(block, operand_src, uncasted_operand, elem_src, i);
            const elem_index = try pt.intRef(.usize, i);
            const elem_ptr = try sema.elemPtr(block, ptr_src, ptr, elem_index, elem_src, false, true);
            try sema.storePtr2(block, src, elem_ptr, elem_src, elem, elem_src, .store);
        }
        return;
    }

    // TODO do the same thing for anon structs as for tuples above.
    // However, beware of the need to handle missing/extra fields.

    const is_ret = air_tag == .ret_ptr;

    // Detect if we are storing an array operand to a bitcasted vector pointer.
    // If so, we instead reach through the bitcasted pointer to the vector pointer,
    // bitcast the array operand to a vector, and then lower this as a store of
    // a vector value to a vector pointer. This generally results in better code,
    // as well as working around an LLVM bug:
    // https://github.com/ziglang/zig/issues/11154
    if (sema.obtainBitCastedVectorPtr(ptr)) |vector_ptr| {
        const vector_ty = sema.typeOf(vector_ptr).childType(zcu);
        const vector = sema.coerceExtra(block, vector_ty, uncasted_operand, operand_src, .{ .is_ret = is_ret }) catch |err| switch (err) {
            error.NotCoercible => unreachable,
            else => |e| return e,
        };
        try sema.storePtr2(block, src, vector_ptr, ptr_src, vector, operand_src, .store);
        return;
    }

    const operand = sema.coerceExtra(block, elem_ty, uncasted_operand, operand_src, .{ .is_ret = is_ret }) catch |err| switch (err) {
        error.NotCoercible => unreachable,
        else => |e| return e,
    };
    const maybe_operand_val = try sema.resolveValue(operand);

    const runtime_src = rs: {
        const ptr_val = try sema.resolveDefinedValue(block, ptr_src, ptr) orelse break :rs ptr_src;
        if (!sema.isComptimeMutablePtr(ptr_val)) break :rs ptr_src;
        const operand_val = maybe_operand_val orelse return sema.fail(block, ptr_src, "cannot store runtime value in compile time variable", .{});
        return sema.storePtrVal(block, src, ptr_val, operand_val, elem_ty);
    };

    // We're performing the store at runtime; as such, we need to make sure the pointee type
    // is not comptime-only. We can hit this case with a `@ptrFromInt` pointer.
    if (try elem_ty.comptimeOnlySema(pt)) {
        return sema.failWithOwnedErrorMsg(block, msg: {
            const msg = try sema.errMsg(src, "cannot store comptime-only type '{}' at runtime", .{elem_ty.fmt(pt)});
            errdefer msg.destroy(sema.gpa);
            try sema.errNote(ptr_src, msg, "operation is runtime due to this pointer", .{});
            break :msg msg;
        });
    }

    // We do this after the possible comptime store above, for the case of field_ptr stores
    // to unions because we want the comptime tag to be set, even if the field type is void.
    if ((try sema.typeHasOnePossibleValue(elem_ty)) != null) {
        return;
    }

    try sema.requireRuntimeBlock(block, src, runtime_src);

    if (ptr_ty.ptrInfo(zcu).flags.vector_index == .runtime) {
        const ptr_inst = ptr.toIndex().?;
        const air_tags = sema.air_instructions.items(.tag);
        if (air_tags[@intFromEnum(ptr_inst)] == .ptr_elem_ptr) {
            const ty_pl = sema.air_instructions.items(.data)[@intFromEnum(ptr_inst)].ty_pl;
            const bin_op = sema.getTmpAir().extraData(Air.Bin, ty_pl.payload).data;
            _ = try block.addInst(.{
                .tag = .vector_store_elem,
                .data = .{ .vector_store_elem = .{
                    .vector_ptr = bin_op.lhs,
                    .payload = try block.sema.addExtra(Air.Bin{
                        .lhs = bin_op.rhs,
                        .rhs = operand,
                    }),
                } },
            });
            return;
        }
        return sema.fail(block, ptr_src, "unable to determine vector element index of type '{}'", .{
            ptr_ty.fmt(pt),
        });
    }

    const store_inst = if (is_ret)
        try block.addBinOp(.store, ptr, operand)
    else
        try block.addBinOp(air_tag, ptr, operand);

    try sema.checkComptimeKnownStore(block, store_inst, operand_src);

    return;
}

/// Given an AIR store instruction, checks whether we are performing a
/// comptime-known store to a local alloc, and updates `maybe_comptime_allocs`
/// accordingly.
/// Handles calling `validateRuntimeValue` if the store is runtime for any reason.
fn checkComptimeKnownStore(sema: *Sema, block: *Block, store_inst_ref: Air.Inst.Ref, store_src: LazySrcLoc) !void {
    const store_inst = store_inst_ref.toIndex().?;
    const inst_data = sema.air_instructions.items(.data)[@intFromEnum(store_inst)].bin_op;
    const ptr = inst_data.lhs.toIndex() orelse return;
    const operand = inst_data.rhs;

    known: {
        const maybe_base_alloc = sema.base_allocs.get(ptr) orelse break :known;
        const maybe_comptime_alloc = sema.maybe_comptime_allocs.getPtr(maybe_base_alloc) orelse break :known;

        if ((try sema.resolveValue(operand)) != null and
            block.runtime_index == maybe_comptime_alloc.runtime_index)
        {
            try maybe_comptime_alloc.stores.append(sema.arena, .{
                .inst = store_inst,
                .src = store_src,
            });
            return;
        }

        // We're newly discovering that this alloc is runtime-known.
        try sema.markMaybeComptimeAllocRuntime(block, maybe_base_alloc);
    }

    try sema.validateRuntimeValue(block, store_src, operand);
}

/// Given an AIR instruction transforming a pointer (struct_field_ptr,
/// ptr_elem_ptr, bitcast, etc), checks whether the base pointer refers to a
/// local alloc, and updates `base_allocs` accordingly.
fn checkKnownAllocPtr(sema: *Sema, block: *Block, base_ptr: Air.Inst.Ref, new_ptr: Air.Inst.Ref) !void {
    const base_ptr_inst = base_ptr.toIndex() orelse return;
    const new_ptr_inst = new_ptr.toIndex() orelse return;
    const alloc_inst = sema.base_allocs.get(base_ptr_inst) orelse return;
    try sema.base_allocs.put(sema.gpa, new_ptr_inst, alloc_inst);

    switch (sema.air_instructions.items(.tag)[@intFromEnum(new_ptr_inst)]) {
        .optional_payload_ptr_set, .errunion_payload_ptr_set => {
            const maybe_comptime_alloc = sema.maybe_comptime_allocs.getPtr(alloc_inst) orelse return;

            // This is functionally a store, since it writes the optional payload bit.
            // Thus, if it is behind a runtime condition, we must mark the alloc as runtime appropriately.
            if (block.runtime_index != maybe_comptime_alloc.runtime_index) {
                return sema.markMaybeComptimeAllocRuntime(block, alloc_inst);
            }

            try maybe_comptime_alloc.stores.append(sema.arena, .{
                .inst = new_ptr_inst,
                .src = LazySrcLoc.unneeded,
            });
        },
        .ptr_elem_ptr => {
            const tmp_air = sema.getTmpAir();
            const pl_idx = tmp_air.instructions.items(.data)[@intFromEnum(new_ptr_inst)].ty_pl.payload;
            const bin = tmp_air.extraData(Air.Bin, pl_idx).data;
            const index_ref = bin.rhs;

            // If the index value is runtime-known, this pointer is also runtime-known, so
            // we must in turn make the alloc value runtime-known.
            if (null == try sema.resolveValue(index_ref)) {
                try sema.markMaybeComptimeAllocRuntime(block, alloc_inst);
            }
        },
        else => {},
    }
}

fn markMaybeComptimeAllocRuntime(sema: *Sema, block: *Block, alloc_inst: Air.Inst.Index) CompileError!void {
    const maybe_comptime_alloc = (sema.maybe_comptime_allocs.fetchRemove(alloc_inst) orelse return).value;
    // Since the alloc has been determined to be runtime, we must check that
    // all other stores to it are permitted to be runtime values.
    const slice = maybe_comptime_alloc.stores.slice();
    for (slice.items(.inst), slice.items(.src)) |other_inst, other_src| {
        if (other_src.offset == .unneeded) {
            switch (sema.air_instructions.items(.tag)[@intFromEnum(other_inst)]) {
                .set_union_tag, .optional_payload_ptr_set, .errunion_payload_ptr_set => continue,
                else => unreachable, // assertion failure
            }
        }
        const other_data = sema.air_instructions.items(.data)[@intFromEnum(other_inst)].bin_op;
        const other_operand = other_data.rhs;
        try sema.validateRuntimeValue(block, other_src, other_operand);
    }
}

/// Traverse an arbitrary number of bitcasted pointers and return the underyling vector
/// pointer. Only if the final element type matches the vector element type, and the
/// lengths match.
fn obtainBitCastedVectorPtr(sema: *Sema, ptr: Air.Inst.Ref) ?Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const array_ty = sema.typeOf(ptr).childType(zcu);
    if (array_ty.zigTypeTag(zcu) != .array) return null;
    var ptr_ref = ptr;
    var ptr_inst = ptr_ref.toIndex() orelse return null;
    const air_datas = sema.air_instructions.items(.data);
    const air_tags = sema.air_instructions.items(.tag);
    const vector_ty = while (air_tags[@intFromEnum(ptr_inst)] == .bitcast) {
        ptr_ref = air_datas[@intFromEnum(ptr_inst)].ty_op.operand;
        if (!sema.isKnownZigType(ptr_ref, .pointer)) return null;
        const child_ty = sema.typeOf(ptr_ref).childType(zcu);
        if (child_ty.zigTypeTag(zcu) == .vector) break child_ty;
        ptr_inst = ptr_ref.toIndex() orelse return null;
    } else return null;

    // We have a pointer-to-array and a pointer-to-vector. If the elements and
    // lengths match, return the result.
    if (array_ty.childType(zcu).eql(vector_ty.childType(zcu), zcu) and
        array_ty.arrayLen(zcu) == vector_ty.vectorLen(zcu))
    {
        return ptr_ref;
    } else {
        return null;
    }
}

/// Call when you have Value objects rather than Air instructions, and you want to
/// assert the store must be done at comptime.
fn storePtrVal(
    sema: *Sema,
    block: *Block,
    src: LazySrcLoc,
    ptr_val: Value,
    operand_val: Value,
    operand_ty: Type,
) !void {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const ip = &zcu.intern_pool;
    // TODO: audit use sites to eliminate this coercion
    const coerced_operand_val = try pt.getCoerced(operand_val, operand_ty);
    // TODO: audit use sites to eliminate this coercion
    const ptr_ty = try pt.ptrType(info: {
        var info = ptr_val.typeOf(zcu).ptrInfo(zcu);
        info.child = operand_ty.toIntern();
        break :info info;
    });
    const coerced_ptr_val = try pt.getCoerced(ptr_val, ptr_ty);

    switch (try sema.storeComptimePtr(block, src, coerced_ptr_val, coerced_operand_val)) {
        .success => {},
        .runtime_store => unreachable, // use sites check this
        // TODO use failWithInvalidComptimeFieldStore
        .comptime_field_mismatch => return sema.fail(
            block,
            src,
            "value stored in comptime field does not match the default value of the field",
            .{},
        ),
        .undef => return sema.failWithUseOfUndef(block, src),
        .err_payload => |err_name| return sema.fail(block, src, "attempt to unwrap error: {}", .{err_name.fmt(ip)}),
        .null_payload => return sema.fail(block, src, "attempt to use null value", .{}),
        .inactive_union_field => return sema.fail(block, src, "access of inactive union field", .{}),
        .needed_well_defined => |ty| return sema.fail(
            block,
            src,
            "comptime dereference requires '{}' to have a well-defined layout",
            .{ty.fmt(pt)},
        ),
        .out_of_bounds => |ty| return sema.fail(
            block,
            src,
            "dereference of '{}' exceeds bounds of containing decl of type '{}'",
            .{ ptr_ty.fmt(pt), ty.fmt(pt) },
        ),
        .exceeds_host_size => return sema.fail(block, src, "bit-pointer target exceeds host size", .{}),
    }
}

fn bitCast(
    sema: *Sema,
    block: *Block,
    dest_ty: Type,
    inst: Air.Inst.Ref,
    inst_src: LazySrcLoc,
    operand_src: ?LazySrcLoc,
) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    try dest_ty.resolveLayout(pt);

    const old_ty = sema.typeOf(inst);
    try old_ty.resolveLayout(pt);

    const dest_bits = dest_ty.bitSize(zcu);
    const old_bits = old_ty.bitSize(zcu);

    if (old_bits != dest_bits) {
        return sema.fail(block, inst_src, "@bitCast size mismatch: destination type '{}' has {d} bits but source type '{}' has {d} bits", .{
            dest_ty.fmt(pt),
            dest_bits,
            old_ty.fmt(pt),
            old_bits,
        });
    }

    if (try sema.resolveValue(inst)) |val| {
        if (val.isUndef(zcu))
            return pt.undefRef(dest_ty);
        if (old_ty.zigTypeTag(zcu) == .error_set and dest_ty.zigTypeTag(zcu) == .error_set) {
            // Special case: we sometimes call `bitCast` on error set values, but they
            // don't have a well-defined layout, so we can't use `bitCastVal` on them.
            return Air.internedToRef((try pt.getCoerced(val, dest_ty)).toIntern());
        }
        if (try sema.bitCastVal(val, dest_ty, 0, 0, 0)) |result_val| {
            return Air.internedToRef(result_val.toIntern());
        }
    }
    try sema.requireRuntimeBlock(block, inst_src, operand_src);
    try sema.validateRuntimeValue(block, inst_src, inst);
    return block.addBitCast(dest_ty, inst);
}

fn coerceArrayPtrToSlice(
    sema: *Sema,
    block: *Block,
    dest_ty: Type,
    inst: Air.Inst.Ref,
    inst_src: LazySrcLoc,
) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    if (try sema.resolveValue(inst)) |val| {
        const ptr_array_ty = sema.typeOf(inst);
        const array_ty = ptr_array_ty.childType(zcu);
        const slice_ptr_ty = dest_ty.slicePtrFieldType(zcu);
        const slice_ptr = try pt.getCoerced(val, slice_ptr_ty);
        const slice_val = try pt.intern(.{ .slice = .{
            .ty = dest_ty.toIntern(),
            .ptr = slice_ptr.toIntern(),
            .len = (try pt.intValue(.usize, array_ty.arrayLen(zcu))).toIntern(),
        } });
        return Air.internedToRef(slice_val);
    }
    try sema.requireRuntimeBlock(block, inst_src, null);
    return block.addTyOp(.array_to_slice, dest_ty, inst);
}

fn checkPtrAttributes(sema: *Sema, dest_ty: Type, inst_ty: Type, in_memory_result: *InMemoryCoercionResult) bool {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const dest_info = dest_ty.ptrInfo(zcu);
    const inst_info = inst_ty.ptrInfo(zcu);
    const len0 = (Type.fromInterned(inst_info.child).zigTypeTag(zcu) == .array and (Type.fromInterned(inst_info.child).arrayLenIncludingSentinel(zcu) == 0 or
        (Type.fromInterned(inst_info.child).arrayLen(zcu) == 0 and dest_info.sentinel == .none and dest_info.flags.size != .c and dest_info.flags.size != .many))) or
        (Type.fromInterned(inst_info.child).isTuple(zcu) and Type.fromInterned(inst_info.child).structFieldCount(zcu) == 0);

    const ok_const = (!inst_info.flags.is_const or dest_info.flags.is_const) or len0;
    const ok_volatile = !inst_info.flags.is_volatile or dest_info.flags.is_volatile;
    if (!ok_const) {
        in_memory_result.* = .{ .ptr_const = .{
            .actual = inst_ty,
            .wanted = dest_ty,
        } };
        return false;
    }
    if (!ok_volatile) {
        in_memory_result.* = .{ .ptr_volatile = .{
            .actual = inst_ty,
            .wanted = dest_ty,
        } };
        return false;
    }

    if (dest_info.flags.address_space != inst_info.flags.address_space) {
        in_memory_result.* = .{ .ptr_addrspace = .{
            .actual = inst_info.flags.address_space,
            .wanted = dest_info.flags.address_space,
        } };
        return false;
    }
    if (inst_info.flags.alignment == .none and dest_info.flags.alignment == .none) return true;
    if (len0) return true;

    const inst_align = if (inst_info.flags.alignment != .none)
        inst_info.flags.alignment
    else
        Type.fromInterned(inst_info.child).abiAlignment(zcu);

    const dest_align = if (dest_info.flags.alignment != .none)
        dest_info.flags.alignment
    else
        Type.fromInterned(dest_info.child).abiAlignment(zcu);

    if (dest_align.compare(.gt, inst_align)) {
        in_memory_result.* = .{ .ptr_alignment = .{
            .actual = inst_align,
            .wanted = dest_align,
        } };
        return false;
    }
    return true;
}

fn coerceCompatiblePtrs(
    sema: *Sema,
    block: *Block,
    dest_ty: Type,
    inst: Air.Inst.Ref,
    inst_src: LazySrcLoc,
) !Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const inst_ty = sema.typeOf(inst);
    if (try sema.resolveValue(inst)) |val| {
        if (!val.isUndef(zcu) and val.isNull(zcu) and !dest_ty.isAllowzeroPtr(zcu)) {
            return sema.fail(block, inst_src, "null pointer casted to type '{}'", .{dest_ty.fmt(pt)});
        }
        // The comptime Value representation is compatible with both types.
        return Air.internedToRef(
            (try pt.getCoerced(val, dest_ty)).toIntern(),
        );
    }
    try sema.requireRuntimeBlock(block, inst_src, null);
    const inst_allows_zero = inst_ty.zigTypeTag(zcu) != .pointer or inst_ty.ptrAllowsZero(zcu);
    if (block.wantSafety() and inst_allows_zero and !dest_ty.ptrAllowsZero(zcu) and
        (try dest_ty.elemType2(zcu).hasRuntimeBitsSema(pt) or dest_ty.elemType2(zcu).zigTypeTag(zcu) == .@"fn"))
    {
        try sema.checkLogicalPtrOperation(block, inst_src, inst_ty);
        const actual_ptr = if (inst_ty.isSlice(zcu))
            try sema.analyzeSlicePtr(block, inst_src, inst, inst_ty)
        else
            inst;
        const ptr_int = try block.addBitCast(.usize, actual_ptr);
        const is_non_zero = try block.addBinOp(.cmp_neq, ptr_int, .zero_usize);
        const ok = if (inst_ty.isSlice(zcu)) ok: {
            const len = try sema.analyzeSliceLen(block, inst_src, inst);
            const len_zero = try block.addBinOp(.cmp_eq, len, .zero_usize);
            break :ok try block.addBinOp(.bool_or, len_zero, is_non_zero);
        } else is_non_zero;
        try sema.addSafetyCheck(block, inst_src, ok, .cast_to_null);
    }
    const new_ptr = try sema.bitCast(block, dest_ty, inst, inst_src, null);
    try sema.checkKnownAllocPtr(block, inst, new_ptr);
    return new_ptr;
}

fn coerceEnumToUnion(
    sema: *Sema,
    block: *Block,
    union_ty: Type,
    union_ty_src: LazySrcLoc,
    inst: Air.Inst.Ref,
    inst_src: LazySrcLoc,
) !Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const ip = &zcu.intern_pool;
    const inst_ty = sema.typeOf(inst);

    const tag_ty = union_ty.unionTagType(zcu) orelse {
        const msg = msg: {
            const msg = try sema.errMsg(inst_src, "expected type '{}', found '{}'", .{
                union_ty.fmt(pt), inst_ty.fmt(pt),
            });
            errdefer msg.destroy(sema.gpa);
            try sema.errNote(union_ty_src, msg, "cannot coerce enum to untagged union", .{});
            try sema.addDeclaredHereNote(msg, union_ty);
            break :msg msg;
        };
        return sema.failWithOwnedErrorMsg(block, msg);
    };

    const enum_tag = try sema.coerce(block, tag_ty, inst, inst_src);
    if (try sema.resolveDefinedValue(block, inst_src, enum_tag)) |val| {
        const field_index = union_ty.unionTagFieldIndex(val, pt.zcu) orelse {
            return sema.fail(block, inst_src, "union '{}' has no tag with value '{}'", .{
                union_ty.fmt(pt), val.fmtValueSema(pt, sema),
            });
        };

        const union_obj = zcu.typeToUnion(union_ty).?;
        const field_ty: Type = .fromInterned(union_obj.field_types.get(ip)[field_index]);
        try field_ty.resolveFields(pt);
        if (field_ty.zigTypeTag(zcu) == .noreturn) {
            const msg = msg: {
                const msg = try sema.errMsg(inst_src, "cannot initialize 'noreturn' field of union", .{});
                errdefer msg.destroy(sema.gpa);

                const field_name = union_obj.loadTagType(ip).names.get(ip)[field_index];
                try sema.addFieldErrNote(union_ty, field_index, msg, "field '{}' declared here", .{
                    field_name.fmt(ip),
                });
                try sema.addDeclaredHereNote(msg, union_ty);
                break :msg msg;
            };
            return sema.failWithOwnedErrorMsg(block, msg);
        }
        const opv = (try sema.typeHasOnePossibleValue(field_ty)) orelse {
            const msg = msg: {
                const field_name = union_obj.loadTagType(ip).names.get(ip)[field_index];
                const msg = try sema.errMsg(inst_src, "coercion from enum '{}' to union '{}' must initialize '{}' field '{}'", .{
                    inst_ty.fmt(pt),  union_ty.fmt(pt),
                    field_ty.fmt(pt), field_name.fmt(ip),
                });
                errdefer msg.destroy(sema.gpa);

                try sema.addFieldErrNote(union_ty, field_index, msg, "field '{}' declared here", .{
                    field_name.fmt(ip),
                });
                try sema.addDeclaredHereNote(msg, union_ty);
                break :msg msg;
            };
            return sema.failWithOwnedErrorMsg(block, msg);
        };

        return Air.internedToRef((try pt.unionValue(union_ty, val, opv)).toIntern());
    }

    try sema.requireRuntimeBlock(block, inst_src, null);

    if (tag_ty.isNonexhaustiveEnum(zcu)) {
        const msg = msg: {
            const msg = try sema.errMsg(inst_src, "runtime coercion to union '{}' from non-exhaustive enum", .{
                union_ty.fmt(pt),
            });
            errdefer msg.destroy(sema.gpa);
            try sema.addDeclaredHereNote(msg, tag_ty);
            break :msg msg;
        };
        return sema.failWithOwnedErrorMsg(block, msg);
    }

    const union_obj = zcu.typeToUnion(union_ty).?;
    {
        var msg: ?*Zcu.ErrorMsg = null;
        errdefer if (msg) |some| some.destroy(sema.gpa);

        for (union_obj.field_types.get(ip), 0..) |field_ty, field_index| {
            if (Type.fromInterned(field_ty).zigTypeTag(zcu) == .noreturn) {
                const err_msg = msg orelse try sema.errMsg(
                    inst_src,
                    "runtime coercion from enum '{}' to union '{}' which has a 'noreturn' field",
                    .{ tag_ty.fmt(pt), union_ty.fmt(pt) },
                );
                msg = err_msg;

                try sema.addFieldErrNote(union_ty, field_index, err_msg, "'noreturn' field here", .{});
            }
        }
        if (msg) |some| {
            msg = null;
            try sema.addDeclaredHereNote(some, union_ty);
            return sema.failWithOwnedErrorMsg(block, some);
        }
    }

    // If the union has all fields 0 bits, the union value is just the enum value.
    if (union_ty.unionHasAllZeroBitFieldTypes(zcu)) {
        return block.addBitCast(union_ty, enum_tag);
    }

    const msg = msg: {
        const msg = try sema.errMsg(
            inst_src,
            "runtime coercion from enum '{}' to union '{}' which has non-void fields",
            .{ tag_ty.fmt(pt), union_ty.fmt(pt) },
        );
        errdefer msg.destroy(sema.gpa);

        for (0..union_obj.field_types.len) |field_index| {
            const field_name = union_obj.loadTagType(ip).names.get(ip)[field_index];
            const field_ty: Type = .fromInterned(union_obj.field_types.get(ip)[field_index]);
            if (!(try field_ty.hasRuntimeBitsSema(pt))) continue;
            try sema.addFieldErrNote(union_ty, field_index, msg, "field '{}' has type '{}'", .{
                field_name.fmt(ip),
                field_ty.fmt(pt),
            });
        }
        try sema.addDeclaredHereNote(msg, union_ty);
        break :msg msg;
    };
    return sema.failWithOwnedErrorMsg(block, msg);
}

/// If the lengths match, coerces element-wise.
fn coerceArrayLike(
    sema: *Sema,
    block: *Block,
    dest_ty: Type,
    dest_ty_src: LazySrcLoc,
    inst: Air.Inst.Ref,
    inst_src: LazySrcLoc,
) !Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const inst_ty = sema.typeOf(inst);
    const target = zcu.getTarget();

    // try coercion of the whole array
    const in_memory_result = try sema.coerceInMemoryAllowed(block, dest_ty, inst_ty, false, target, dest_ty_src, inst_src, null);
    if (in_memory_result == .ok) {
        if (try sema.resolveValue(inst)) |inst_val| {
            // These types share the same comptime value representation.
            return sema.coerceInMemory(inst_val, dest_ty);
        }
        try sema.requireRuntimeBlock(block, inst_src, null);
        return block.addBitCast(dest_ty, inst);
    }

    // otherwise, try element by element
    const inst_len = inst_ty.arrayLen(zcu);
    const dest_len = try sema.usizeCast(block, dest_ty_src, dest_ty.arrayLen(zcu));
    if (dest_len != inst_len) {
        const msg = msg: {
            const msg = try sema.errMsg(inst_src, "expected type '{}', found '{}'", .{
                dest_ty.fmt(pt), inst_ty.fmt(pt),
            });
            errdefer msg.destroy(sema.gpa);
            try sema.errNote(dest_ty_src, msg, "destination has length {d}", .{dest_len});
            try sema.errNote(inst_src, msg, "source has length {d}", .{inst_len});
            break :msg msg;
        };
        return sema.failWithOwnedErrorMsg(block, msg);
    }

    const dest_elem_ty = dest_ty.childType(zcu);
    if (dest_ty.isVector(zcu) and inst_ty.isVector(zcu) and (try sema.resolveValue(inst)) == null) {
        const inst_elem_ty = inst_ty.childType(zcu);
        switch (dest_elem_ty.zigTypeTag(zcu)) {
            .int => if (inst_elem_ty.isInt(zcu)) {
                // integer widening
                const dst_info = dest_elem_ty.intInfo(zcu);
                const src_info = inst_elem_ty.intInfo(zcu);
                if ((src_info.signedness == dst_info.signedness and dst_info.bits >= src_info.bits) or
                    // small enough unsigned ints can get casted to large enough signed ints
                    (dst_info.signedness == .signed and dst_info.bits > src_info.bits))
                {
                    try sema.requireRuntimeBlock(block, inst_src, null);
                    return block.addTyOp(.intcast, dest_ty, inst);
                }
            },
            .float => if (inst_elem_ty.isRuntimeFloat()) {
                // float widening
                const src_bits = inst_elem_ty.floatBits(target);
                const dst_bits = dest_elem_ty.floatBits(target);
                if (dst_bits >= src_bits) {
                    try sema.requireRuntimeBlock(block, inst_src, null);
                    return block.addTyOp(.fpext, dest_ty, inst);
                }
            },
            else => {},
        }
    }

    const element_vals = try sema.arena.alloc(InternPool.Index, dest_len);
    const element_refs = try sema.arena.alloc(Air.Inst.Ref, dest_len);
    var runtime_src: ?LazySrcLoc = null;

    for (element_vals, element_refs, 0..) |*val, *ref, i| {
        const index_ref = Air.internedToRef((try pt.intValue(.usize, i)).toIntern());
        const src = inst_src; // TODO better source location
        const elem_src = inst_src; // TODO better source location
        const elem_ref = try sema.elemValArray(block, src, inst_src, inst, elem_src, index_ref, true);
        const coerced = try sema.coerce(block, dest_elem_ty, elem_ref, elem_src);
        ref.* = coerced;
        if (runtime_src == null) {
            if (try sema.resolveValue(coerced)) |elem_val| {
                val.* = elem_val.toIntern();
            } else {
                runtime_src = elem_src;
            }
        }
    }

    if (runtime_src) |rs| {
        try sema.requireRuntimeBlock(block, inst_src, rs);
        return block.addAggregateInit(dest_ty, element_refs);
    }

    return Air.internedToRef((try pt.intern(.{ .aggregate = .{
        .ty = dest_ty.toIntern(),
        .storage = .{ .elems = element_vals },
    } })));
}

/// If the lengths match, coerces element-wise.
fn coerceTupleToArray(
    sema: *Sema,
    block: *Block,
    dest_ty: Type,
    dest_ty_src: LazySrcLoc,
    inst: Air.Inst.Ref,
    inst_src: LazySrcLoc,
) !Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const inst_ty = sema.typeOf(inst);
    const inst_len = inst_ty.arrayLen(zcu);
    const dest_len = dest_ty.arrayLen(zcu);

    if (dest_len != inst_len) {
        const msg = msg: {
            const msg = try sema.errMsg(inst_src, "expected type '{}', found '{}'", .{
                dest_ty.fmt(pt), inst_ty.fmt(pt),
            });
            errdefer msg.destroy(sema.gpa);
            try sema.errNote(dest_ty_src, msg, "destination has length {d}", .{dest_len});
            try sema.errNote(inst_src, msg, "source has length {d}", .{inst_len});
            break :msg msg;
        };
        return sema.failWithOwnedErrorMsg(block, msg);
    }

    const dest_elems = try sema.usizeCast(block, dest_ty_src, dest_len);
    const element_vals = try sema.arena.alloc(InternPool.Index, dest_elems);
    const element_refs = try sema.arena.alloc(Air.Inst.Ref, dest_elems);
    const dest_elem_ty = dest_ty.childType(zcu);

    var runtime_src: ?LazySrcLoc = null;
    for (element_vals, element_refs, 0..) |*val, *ref, i_usize| {
        const i: u32 = @intCast(i_usize);
        if (i_usize == inst_len) {
            const sentinel_val = dest_ty.sentinel(zcu).?;
            val.* = sentinel_val.toIntern();
            ref.* = Air.internedToRef(sentinel_val.toIntern());
            break;
        }
        const elem_src = inst_src; // TODO better source location
        const elem_ref = try sema.tupleField(block, inst_src, inst, elem_src, i);
        const coerced = try sema.coerce(block, dest_elem_ty, elem_ref, elem_src);
        ref.* = coerced;
        if (runtime_src == null) {
            if (try sema.resolveValue(coerced)) |elem_val| {
                val.* = elem_val.toIntern();
            } else {
                runtime_src = elem_src;
            }
        }
    }

    if (runtime_src) |rs| {
        try sema.requireRuntimeBlock(block, inst_src, rs);
        return block.addAggregateInit(dest_ty, element_refs);
    }

    return Air.internedToRef((try pt.intern(.{ .aggregate = .{
        .ty = dest_ty.toIntern(),
        .storage = .{ .elems = element_vals },
    } })));
}

/// If the lengths match, coerces element-wise.
fn coerceTupleToSlicePtrs(
    sema: *Sema,
    block: *Block,
    slice_ty: Type,
    slice_ty_src: LazySrcLoc,
    ptr_tuple: Air.Inst.Ref,
    tuple_src: LazySrcLoc,
) !Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const tuple_ty = sema.typeOf(ptr_tuple).childType(zcu);
    const tuple = try sema.analyzeLoad(block, tuple_src, ptr_tuple, tuple_src);
    const slice_info = slice_ty.ptrInfo(zcu);
    const array_ty = try pt.arrayType(.{
        .len = tuple_ty.structFieldCount(zcu),
        .sentinel = slice_info.sentinel,
        .child = slice_info.child,
    });
    const array_inst = try sema.coerceTupleToArray(block, array_ty, slice_ty_src, tuple, tuple_src);
    if (slice_info.flags.alignment != .none) {
        return sema.fail(block, slice_ty_src, "TODO: override the alignment of the array decl we create here", .{});
    }
    const ptr_array = try sema.analyzeRef(block, slice_ty_src, array_inst);
    return sema.coerceArrayPtrToSlice(block, slice_ty, ptr_array, slice_ty_src);
}

/// If the lengths match, coerces element-wise.
fn coerceTupleToArrayPtrs(
    sema: *Sema,
    block: *Block,
    ptr_array_ty: Type,
    array_ty_src: LazySrcLoc,
    ptr_tuple: Air.Inst.Ref,
    tuple_src: LazySrcLoc,
) !Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const tuple = try sema.analyzeLoad(block, tuple_src, ptr_tuple, tuple_src);
    const ptr_info = ptr_array_ty.ptrInfo(zcu);
    const array_ty: Type = .fromInterned(ptr_info.child);
    const array_inst = try sema.coerceTupleToArray(block, array_ty, array_ty_src, tuple, tuple_src);
    if (ptr_info.flags.alignment != .none) {
        return sema.fail(block, array_ty_src, "TODO: override the alignment of the array decl we create here", .{});
    }
    const ptr_array = try sema.analyzeRef(block, array_ty_src, array_inst);
    return ptr_array;
}

fn coerceTupleToTuple(
    sema: *Sema,
    block: *Block,
    tuple_ty: Type,
    inst: Air.Inst.Ref,
    inst_src: LazySrcLoc,
) !Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const ip = &zcu.intern_pool;
    const dest_field_count = switch (ip.indexToKey(tuple_ty.toIntern())) {
        .tuple_type => |tuple_type| tuple_type.types.len,
        else => unreachable,
    };
    const field_vals = try sema.arena.alloc(InternPool.Index, dest_field_count);
    const field_refs = try sema.arena.alloc(Air.Inst.Ref, field_vals.len);
    @memset(field_refs, .none);

    const inst_ty = sema.typeOf(inst);
    const src_field_count = switch (ip.indexToKey(inst_ty.toIntern())) {
        .tuple_type => |tuple_type| tuple_type.types.len,
        else => unreachable,
    };
    if (src_field_count > dest_field_count) return error.NotCoercible;

    var runtime_src: ?LazySrcLoc = null;
    for (0..dest_field_count) |field_index_usize| {
        const field_i: u32 = @intCast(field_index_usize);
        const field_src = inst_src; // TODO better source location

        const field_ty = switch (ip.indexToKey(tuple_ty.toIntern())) {
            .tuple_type => |tuple_type| tuple_type.types.get(ip)[field_index_usize],
            .struct_type => ip.loadStructType(tuple_ty.toIntern()).field_types.get(ip)[field_index_usize],
            else => unreachable,
        };
        const default_val = switch (ip.indexToKey(tuple_ty.toIntern())) {
            .tuple_type => |tuple_type| tuple_type.values.get(ip)[field_index_usize],
            .struct_type => ip.loadStructType(tuple_ty.toIntern()).fieldInit(ip, field_index_usize),
            else => unreachable,
        };

        const field_index: u32 = @intCast(field_index_usize);

        const elem_ref = try sema.tupleField(block, inst_src, inst, field_src, field_i);
        const coerced = try sema.coerce(block, .fromInterned(field_ty), elem_ref, field_src);
        field_refs[field_index] = coerced;
        if (default_val != .none) {
            const init_val = (try sema.resolveValue(coerced)) orelse {
                return sema.failWithNeededComptime(block, field_src, .{ .simple = .stored_to_comptime_field });
            };

            if (!init_val.eql(Value.fromInterned(default_val), .fromInterned(field_ty), pt.zcu)) {
                return sema.failWithInvalidComptimeFieldStore(block, field_src, inst_ty, field_i);
            }
        }
        if (runtime_src == null) {
            if (try sema.resolveValue(coerced)) |field_val| {
                field_vals[field_index] = field_val.toIntern();
            } else {
                runtime_src = field_src;
            }
        }
    }

    // Populate default field values and report errors for missing fields.
    var root_msg: ?*Zcu.ErrorMsg = null;
    errdefer if (root_msg) |msg| msg.destroy(sema.gpa);

    for (field_refs, 0..) |*field_ref, i_usize| {
        const i: u32 = @intCast(i_usize);
        if (field_ref.* != .none) continue;

        const default_val = switch (ip.indexToKey(tuple_ty.toIntern())) {
            .tuple_type => |tuple_type| tuple_type.values.get(ip)[i],
            .struct_type => ip.loadStructType(tuple_ty.toIntern()).fieldInit(ip, i),
            else => unreachable,
        };

        const field_src = inst_src; // TODO better source location
        if (default_val == .none) {
            const template = "missing tuple field: {d}";
            if (root_msg) |msg| {
                try sema.errNote(field_src, msg, template, .{i});
            } else {
                root_msg = try sema.errMsg(field_src, template, .{i});
            }
            continue;
        }
        if (runtime_src == null) {
            field_vals[i] = default_val;
        } else {
            field_ref.* = Air.internedToRef(default_val);
        }
    }

    if (root_msg) |msg| {
        try sema.addDeclaredHereNote(msg, tuple_ty);
        root_msg = null;
        return sema.failWithOwnedErrorMsg(block, msg);
    }

    if (runtime_src) |rs| {
        try sema.requireRuntimeBlock(block, inst_src, rs);
        return block.addAggregateInit(tuple_ty, field_refs);
    }

    return Air.internedToRef((try pt.intern(.{ .aggregate = .{
        .ty = tuple_ty.toIntern(),
        .storage = .{ .elems = field_vals },
    } })));
}

fn analyzeNavVal(
    sema: *Sema,
    block: *Block,
    src: LazySrcLoc,
    nav_index: InternPool.Nav.Index,
) CompileError!Air.Inst.Ref {
    const ref = try sema.analyzeNavRefInner(block, src, nav_index, false);
    return sema.analyzeLoad(block, src, ref, src);
}

fn addReferenceEntry(
    sema: *Sema,
    opt_block: ?*Block,
    src: LazySrcLoc,
    referenced_unit: AnalUnit,
) !void {
    const zcu = sema.pt.zcu;
    if (!zcu.comp.incremental and zcu.comp.reference_trace == 0) return;
    const gop = try sema.references.getOrPut(sema.gpa, referenced_unit);
    if (gop.found_existing) return;
    try zcu.addUnitReference(sema.owner, referenced_unit, src, inline_frame: {
        const block = opt_block orelse break :inline_frame .none;
        const inlining = block.inlining orelse break :inline_frame .none;
        const frame = try inlining.refFrame(zcu);
        break :inline_frame frame.toOptional();
    });
}

pub fn addTypeReferenceEntry(
    sema: *Sema,
    src: LazySrcLoc,
    referenced_type: InternPool.Index,
) !void {
    const zcu = sema.pt.zcu;
    if (!zcu.comp.incremental and zcu.comp.reference_trace == 0) return;
    const gop = try sema.type_references.getOrPut(sema.gpa, referenced_type);
    if (gop.found_existing) return;
    try zcu.addTypeReference(sema.owner, referenced_type, src);
}

fn ensureMemoizedStateResolved(sema: *Sema, src: LazySrcLoc, stage: InternPool.MemoizedStateStage) SemaError!void {
    const pt = sema.pt;

    const unit: AnalUnit = .wrap(.{ .memoized_state = stage });
    try sema.addReferenceEntry(null, src, unit);
    try sema.declareDependency(.{ .memoized_state = stage });

    if (pt.zcu.analysis_in_progress.contains(unit)) {
        return sema.failWithOwnedErrorMsg(null, try sema.errMsg(src, "dependency loop detected", .{}));
    }
    try pt.ensureMemoizedStateUpToDate(stage);
}

pub fn ensureNavResolved(sema: *Sema, block: *Block, src: LazySrcLoc, nav_index: InternPool.Nav.Index, kind: enum { type, fully }) CompileError!void {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const ip = &zcu.intern_pool;

    const nav = ip.getNav(nav_index);
    if (nav.analysis == null) {
        assert(nav.status == .fully_resolved);
        return;
    }

    try sema.declareDependency(switch (kind) {
        .type => .{ .nav_ty = nav_index },
        .fully => .{ .nav_val = nav_index },
    });

    // Note that even if `nav.status == .resolved`, we must still trigger `ensureNavValUpToDate`
    // to make sure the value is up-to-date on incremental updates.

    const anal_unit: AnalUnit = .wrap(switch (kind) {
        .type => .{ .nav_ty = nav_index },
        .fully => .{ .nav_val = nav_index },
    });
    try sema.addReferenceEntry(block, src, anal_unit);

    if (zcu.analysis_in_progress.contains(anal_unit)) {
        return sema.failWithOwnedErrorMsg(null, try sema.errMsg(.{
            .base_node_inst = nav.analysis.?.zir_index,
            .offset = LazySrcLoc.Offset.nodeOffset(.zero),
        }, "dependency loop detected", .{}));
    }

    switch (kind) {
        .type => {
            try zcu.ensureNavValAnalysisQueued(nav_index);
            return pt.ensureNavTypeUpToDate(nav_index);
        },
        .fully => return pt.ensureNavValUpToDate(nav_index),
    }
}

fn optRefValue(sema: *Sema, opt_val: ?Value) !Value {
    const pt = sema.pt;
    const ptr_anyopaque_ty = try pt.singleConstPtrType(.anyopaque);
    return Value.fromInterned(try pt.intern(.{ .opt = .{
        .ty = (try pt.optionalType(ptr_anyopaque_ty.toIntern())).toIntern(),
        .val = if (opt_val) |val| (try pt.getCoerced(
            Value.fromInterned(try pt.refValue(val.toIntern())),
            ptr_anyopaque_ty,
        )).toIntern() else .none,
    } }));
}

fn analyzeNavRef(sema: *Sema, block: *Block, src: LazySrcLoc, nav_index: InternPool.Nav.Index) CompileError!Air.Inst.Ref {
    return sema.analyzeNavRefInner(block, src, nav_index, true);
}

/// Analyze a reference to the `Nav` at the given index. Ensures the underlying `Nav` is analyzed.
/// If this pointer will be used directly, `is_ref` must be `true`.
/// If this pointer will be immediately loaded (i.e. a `decl_val` instruction), `is_ref` must be `false`.
fn analyzeNavRefInner(sema: *Sema, block: *Block, src: LazySrcLoc, orig_nav_index: InternPool.Nav.Index, is_ref: bool) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const ip = &zcu.intern_pool;

    try sema.ensureNavResolved(block, src, orig_nav_index, if (is_ref) .type else .fully);

    const nav_index = nav: {
        if (ip.getNav(orig_nav_index).isExternOrFn(ip)) {
            // Getting a pointer to this `Nav` might mean we actually get a pointer to something else!
            // We need to resolve the value to know for sure.
            if (is_ref) try sema.ensureNavResolved(block, src, orig_nav_index, .fully);
            switch (ip.indexToKey(ip.getNav(orig_nav_index).status.fully_resolved.val)) {
                .func => |f| break :nav f.owner_nav,
                .@"extern" => |e| break :nav e.owner_nav,
                else => {},
            }
        }
        break :nav orig_nav_index;
    };

    const nav_status = ip.getNav(nav_index).status;

    const is_tlv_or_dllimport = switch (nav_status) {
        .unresolved => unreachable,
        // dllimports go straight to `fully_resolved`; the only option is threadlocal
        .type_resolved => |r| r.is_threadlocal,
        .fully_resolved => |r| switch (ip.indexToKey(r.val)) {
            .@"extern" => |e| e.is_threadlocal or e.is_dll_import,
            .variable => |v| v.is_threadlocal,
            else => false,
        },
    };

    const ty, const alignment, const @"addrspace", const is_const = switch (nav_status) {
        .unresolved => unreachable,
        .type_resolved => |r| .{ r.type, r.alignment, r.@"addrspace", r.is_const },
        .fully_resolved => |r| .{ ip.typeOf(r.val), r.alignment, r.@"addrspace", zcu.navValIsConst(r.val) },
    };
    const ptr_ty = try pt.ptrTypeSema(.{
        .child = ty,
        .flags = .{
            .alignment = alignment,
            .is_const = is_const,
            .address_space = @"addrspace",
        },
    });

    if (is_tlv_or_dllimport) {
        // This pointer is runtime-known; we need to emit an AIR instruction to create it.
        return block.addInst(.{
            .tag = .tlv_dllimport_ptr,
            .data = .{ .ty_nav = .{
                .ty = ptr_ty.toIntern(),
                .nav = nav_index,
            } },
        });
    }

    if (is_ref) {
        try sema.maybeQueueFuncBodyAnalysis(block, src, nav_index);
    }

    return Air.internedToRef((try pt.intern(.{ .ptr = .{
        .ty = ptr_ty.toIntern(),
        .base_addr = .{ .nav = nav_index },
        .byte_offset = 0,
    } })));
}

fn maybeQueueFuncBodyAnalysis(sema: *Sema, block: *Block, src: LazySrcLoc, nav_index: InternPool.Nav.Index) !void {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const ip = &zcu.intern_pool;

    // To avoid forcing too much resolution, let's first resolve the type, and check if it's a function.
    // If it is, we can resolve the *value*, and queue analysis as needed.

    try sema.ensureNavResolved(block, src, nav_index, .type);
    const nav_ty: Type = .fromInterned(ip.getNav(nav_index).typeOf(ip));
    if (nav_ty.zigTypeTag(zcu) != .@"fn") return;
    if (!try nav_ty.fnHasRuntimeBitsSema(pt)) return;

    try sema.ensureNavResolved(block, src, nav_index, .fully);
    const nav_val = zcu.navValue(nav_index);
    if (!ip.isFuncBody(nav_val.toIntern())) return;

    try sema.addReferenceEntry(block, src, AnalUnit.wrap(.{ .func = nav_val.toIntern() }));
    try zcu.ensureFuncBodyAnalysisQueued(nav_val.toIntern());
}

fn analyzeRef(
    sema: *Sema,
    block: *Block,
    src: LazySrcLoc,
    operand: Air.Inst.Ref,
) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const operand_ty = sema.typeOf(operand);

    if (try sema.resolveValue(operand)) |val| {
        switch (zcu.intern_pool.indexToKey(val.toIntern())) {
            .@"extern" => |e| return sema.analyzeNavRef(block, src, e.owner_nav),
            .func => |f| return sema.analyzeNavRef(block, src, f.owner_nav),
            else => return uavRef(sema, val.toIntern()),
        }
    }

    // No `requireRuntimeBlock`; it's okay to `ref` to a runtime value in a comptime context,
    // it's just that we can only use the *type* of the result, since the value is runtime-known.

    const address_space = target_util.defaultAddressSpace(zcu.getTarget(), .local);
    const ptr_type = try pt.ptrTypeSema(.{
        .child = operand_ty.toIntern(),
        .flags = .{
            .is_const = true,
            .address_space = address_space,
        },
    });
    const mut_ptr_type = try pt.ptrTypeSema(.{
        .child = operand_ty.toIntern(),
        .flags = .{ .address_space = address_space },
    });
    const alloc = try block.addTy(.alloc, mut_ptr_type);

    // In a comptime context, the store would fail, since the operand is runtime-known. But that's
    // okay; we don't actually need this store to succeed, since we're creating a runtime value in a
    // comptime scope, so the value can never be used aside from to get its type.
    if (!block.isComptime()) {
        try sema.storePtr(block, src, alloc, operand);
    }

    // Cast to the constant pointer type. We do this directly rather than going via `coerce` to
    // avoid errors in the `block.isComptime()` case.
    return block.addBitCast(ptr_type, alloc);
}

fn analyzeLoad(
    sema: *Sema,
    block: *Block,
    src: LazySrcLoc,
    ptr: Air.Inst.Ref,
    ptr_src: LazySrcLoc,
) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const ptr_ty = sema.typeOf(ptr);
    const elem_ty = switch (ptr_ty.zigTypeTag(zcu)) {
        .pointer => ptr_ty.childType(zcu),
        else => return sema.fail(block, ptr_src, "expected pointer, found '{}'", .{ptr_ty.fmt(pt)}),
    };
    if (elem_ty.zigTypeTag(zcu) == .@"opaque") {
        return sema.fail(block, ptr_src, "cannot load opaque type '{}'", .{elem_ty.fmt(pt)});
    }

    if (try sema.typeHasOnePossibleValue(elem_ty)) |opv| {
        return Air.internedToRef(opv.toIntern());
    }

    if (try sema.resolveDefinedValue(block, ptr_src, ptr)) |ptr_val| {
        if (try sema.pointerDeref(block, src, ptr_val, ptr_ty)) |elem_val| {
            return Air.internedToRef(elem_val.toIntern());
        }
    }

    if (ptr_ty.ptrInfo(zcu).flags.vector_index == .runtime) {
        const ptr_inst = ptr.toIndex().?;
        const air_tags = sema.air_instructions.items(.tag);
        if (air_tags[@intFromEnum(ptr_inst)] == .ptr_elem_ptr) {
            const ty_pl = sema.air_instructions.items(.data)[@intFromEnum(ptr_inst)].ty_pl;
            const bin_op = sema.getTmpAir().extraData(Air.Bin, ty_pl.payload).data;
            return block.addBinOp(.ptr_elem_val, bin_op.lhs, bin_op.rhs);
        }
        return sema.fail(block, ptr_src, "unable to determine vector element index of type '{}'", .{
            ptr_ty.fmt(pt),
        });
    }

    return block.addTyOp(.load, elem_ty, ptr);
}

fn analyzeSlicePtr(
    sema: *Sema,
    block: *Block,
    slice_src: LazySrcLoc,
    slice: Air.Inst.Ref,
    slice_ty: Type,
) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const result_ty = slice_ty.slicePtrFieldType(zcu);
    if (try sema.resolveValue(slice)) |val| {
        if (val.isUndef(zcu)) return pt.undefRef(result_ty);
        return Air.internedToRef(val.slicePtr(zcu).toIntern());
    }
    try sema.requireRuntimeBlock(block, slice_src, null);
    return block.addTyOp(.slice_ptr, result_ty, slice);
}

fn analyzeOptionalSlicePtr(
    sema: *Sema,
    block: *Block,
    opt_slice_src: LazySrcLoc,
    opt_slice: Air.Inst.Ref,
    opt_slice_ty: Type,
) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const slice_ty = opt_slice_ty.optionalChild(zcu);
    const result_ty = slice_ty.slicePtrFieldType(zcu);

    if (try sema.resolveValue(opt_slice)) |opt_val| {
        if (opt_val.isUndef(zcu)) return pt.undefRef(result_ty);
        const slice_ptr: InternPool.Index = if (opt_val.optionalValue(zcu)) |val|
            val.slicePtr(zcu).toIntern()
        else
            .null_value;

        return Air.internedToRef(slice_ptr);
    }

    try sema.requireRuntimeBlock(block, opt_slice_src, null);

    const slice = try block.addTyOp(.optional_payload, slice_ty, opt_slice);
    return block.addTyOp(.slice_ptr, result_ty, slice);
}

fn analyzeSliceLen(
    sema: *Sema,
    block: *Block,
    src: LazySrcLoc,
    slice_inst: Air.Inst.Ref,
) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    if (try sema.resolveValue(slice_inst)) |slice_val| {
        if (slice_val.isUndef(zcu)) {
            return .undef_usize;
        }
        return pt.intRef(.usize, try slice_val.sliceLen(pt));
    }
    try sema.requireRuntimeBlock(block, src, null);
    return block.addTyOp(.slice_len, .usize, slice_inst);
}

fn analyzeIsNull(
    sema: *Sema,
    block: *Block,
    operand: Air.Inst.Ref,
    invert_logic: bool,
) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const result_ty: Type = .bool;
    if (try sema.resolveValue(operand)) |opt_val| {
        if (opt_val.isUndef(zcu)) {
            return pt.undefRef(result_ty);
        }
        const is_null = opt_val.isNull(zcu);
        const bool_value = if (invert_logic) !is_null else is_null;
        return if (bool_value) .bool_true else .bool_false;
    }

    if (sema.typeOf(operand).isNullFromType(zcu)) |is_null| {
        const result = is_null != invert_logic;
        return if (result) .bool_true else .bool_false;
    }
    const air_tag: Air.Inst.Tag = if (invert_logic) .is_non_null else .is_null;
    return block.addUnOp(air_tag, operand);
}

fn analyzePtrIsNonErrComptimeOnly(
    sema: *Sema,
    block: *Block,
    src: LazySrcLoc,
    operand: Air.Inst.Ref,
) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const ptr_ty = sema.typeOf(operand);
    assert(ptr_ty.zigTypeTag(zcu) == .pointer);
    const child_ty = ptr_ty.childType(zcu);

    const child_tag = child_ty.zigTypeTag(zcu);
    if (child_tag != .error_set and child_tag != .error_union) return .bool_true;
    if (child_tag == .error_set) return .bool_false;
    assert(child_tag == .error_union);

    _ = block;
    _ = src;

    return .none;
}

fn analyzeIsNonErrComptimeOnly(
    sema: *Sema,
    block: *Block,
    src: LazySrcLoc,
    operand: Air.Inst.Ref,
) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const ip = &zcu.intern_pool;
    const operand_ty = sema.typeOf(operand);
    const ot = operand_ty.zigTypeTag(zcu);
    if (ot != .error_set and ot != .error_union) return .bool_true;
    if (ot == .error_set) return .bool_false;
    assert(ot == .error_union);

    const payload_ty = operand_ty.errorUnionPayload(zcu);
    if (payload_ty.zigTypeTag(zcu) == .noreturn) {
        return .bool_false;
    }

    if (operand.toIndex()) |operand_inst| {
        switch (sema.air_instructions.items(.tag)[@intFromEnum(operand_inst)]) {
            .wrap_errunion_payload => return .bool_true,
            .wrap_errunion_err => return .bool_false,
            else => {},
        }
    } else if (operand == .undef) {
        return .undef_bool;
    } else if (@intFromEnum(operand) < InternPool.static_len) {
        // None of the ref tags can be errors.
        return .bool_true;
    }

    const maybe_operand_val = try sema.resolveValue(operand);

    // exception if the error union error set is known to be empty,
    // we allow the comparison but always make it comptime-known.
    const set_ty = ip.errorUnionSet(operand_ty.toIntern());
    switch (set_ty) {
        .anyerror_type => {},
        .adhoc_inferred_error_set_type => if (sema.fn_ret_ty_ies) |ies| blk: {
            // If the error set is empty, we must return a comptime true or false.
            // However we want to avoid unnecessarily resolving an inferred error set
            // in case it is already non-empty.
            switch (ies.resolved) {
                .anyerror_type => break :blk,
                .none => {},
                else => |i| if (ip.indexToKey(i).error_set_type.names.len != 0) break :blk,
            }

            if (maybe_operand_val != null) break :blk;

            // Try to avoid resolving inferred error set if possible.
            if (ies.errors.count() != 0) return .none;
            switch (ies.resolved) {
                .anyerror_type => return .none,
                .none => {},
                else => switch (ip.indexToKey(ies.resolved).error_set_type.names.len) {
                    0 => return .bool_true,
                    else => return .none,
                },
            }
            // We do not have a comptime answer because this inferred error
            // set is not resolved, and an instruction later in this function
            // body may or may not cause an error to be added to this set.
            return .none;
        },
        else => switch (ip.indexToKey(set_ty)) {
            .error_set_type => |error_set_type| {
                if (error_set_type.names.len == 0) return .bool_true;
            },
            .inferred_error_set_type => |func_index| blk: {
                // If the error set is empty, we must return a comptime true or false.
                // However we want to avoid unnecessarily resolving an inferred error set
                // in case it is already non-empty.
                try zcu.maybeUnresolveIes(func_index);
                switch (ip.funcIesResolvedUnordered(func_index)) {
                    .anyerror_type => break :blk,
                    .none => {},
                    else => |i| if (ip.indexToKey(i).error_set_type.names.len != 0) break :blk,
                }
                if (maybe_operand_val != null) break :blk;
                if (sema.fn_ret_ty_ies) |ies| {
                    if (ies.func == func_index) {
                        // Try to avoid resolving inferred error set if possible.
                        if (ies.errors.count() != 0) return .none;
                        switch (ies.resolved) {
                            .anyerror_type => return .none,
                            .none => {},
                            else => switch (ip.indexToKey(ies.resolved).error_set_type.names.len) {
                                0 => return .bool_true,
                                else => return .none,
                            },
                        }
                        // We do not have a comptime answer because this inferred error
                        // set is not resolved, and an instruction later in this function
                        // body may or may not cause an error to be added to this set.
                        return .none;
                    }
                }
                const resolved_ty = try sema.resolveInferredErrorSet(block, src, set_ty);
                if (resolved_ty == .anyerror_type)
                    break :blk;
                if (ip.indexToKey(resolved_ty).error_set_type.names.len == 0)
                    return .bool_true;
            },
            else => unreachable,
        },
    }

    if (maybe_operand_val) |err_union| {
        return if (err_union.isUndef(zcu)) .undef_bool else if (err_union.getErrorName(zcu) == .none) .bool_true else .bool_false;
    }
    return .none;
}

fn analyzeIsNonErr(
    sema: *Sema,
    block: *Block,
    src: LazySrcLoc,
    operand: Air.Inst.Ref,
) CompileError!Air.Inst.Ref {
    const result = try sema.analyzeIsNonErrComptimeOnly(block, src, operand);
    if (result == .none) {
        try sema.requireRuntimeBlock(block, src, null);
        return block.addUnOp(.is_non_err, operand);
    } else {
        return result;
    }
}

fn analyzePtrIsNonErr(
    sema: *Sema,
    block: *Block,
    src: LazySrcLoc,
    operand: Air.Inst.Ref,
) CompileError!Air.Inst.Ref {
    const result = try sema.analyzePtrIsNonErrComptimeOnly(block, src, operand);
    if (result == .none) {
        try sema.requireRuntimeBlock(block, src, null);
        return block.addUnOp(.is_non_err_ptr, operand);
    } else {
        return result;
    }
}

fn analyzeSlice(
    sema: *Sema,
    block: *Block,
    src: LazySrcLoc,
    ptr_ptr: Air.Inst.Ref,
    uncasted_start: Air.Inst.Ref,
    uncasted_end_opt: Air.Inst.Ref,
    sentinel_opt: Air.Inst.Ref,
    sentinel_src: LazySrcLoc,
    ptr_src: LazySrcLoc,
    start_src: LazySrcLoc,
    end_src: LazySrcLoc,
    by_length: bool,
) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    // Slice expressions can operate on a variable whose type is an array. This requires
    // the slice operand to be a pointer. In the case of a non-array, it will be a double pointer.
    const ptr_ptr_ty = sema.typeOf(ptr_ptr);
    const ptr_ptr_child_ty = switch (ptr_ptr_ty.zigTypeTag(zcu)) {
        .pointer => ptr_ptr_ty.childType(zcu),
        else => return sema.fail(block, ptr_src, "expected pointer, found '{}'", .{ptr_ptr_ty.fmt(pt)}),
    };

    var array_ty = ptr_ptr_child_ty;
    var slice_ty = ptr_ptr_ty;
    var ptr_or_slice = ptr_ptr;
    var elem_ty: Type = undefined;
    var ptr_sentinel: ?Value = null;
    switch (ptr_ptr_child_ty.zigTypeTag(zcu)) {
        .array => {
            ptr_sentinel = ptr_ptr_child_ty.sentinel(zcu);
            elem_ty = ptr_ptr_child_ty.childType(zcu);
        },
        .pointer => switch (ptr_ptr_child_ty.ptrSize(zcu)) {
            .one => {
                const double_child_ty = ptr_ptr_child_ty.childType(zcu);
                ptr_or_slice = try sema.analyzeLoad(block, src, ptr_ptr, ptr_src);
                if (double_child_ty.zigTypeTag(zcu) == .array) {
                    ptr_sentinel = double_child_ty.sentinel(zcu);
                    slice_ty = ptr_ptr_child_ty;
                    array_ty = double_child_ty;
                    elem_ty = double_child_ty.childType(zcu);
                } else {
                    if (uncasted_end_opt == .none) {
                        return sema.fail(block, src, "slice of single-item pointer must be bounded", .{});
                    }
                    const start_value = try sema.resolveConstDefinedValue(
                        block,
                        start_src,
                        uncasted_start,
                        .{ .simple = .slice_single_item_ptr_bounds },
                    );

                    const end_value = try sema.resolveConstDefinedValue(
                        block,
                        end_src,
                        uncasted_end_opt,
                        .{ .simple = .slice_single_item_ptr_bounds },
                    );

                    const bounds_error_message = "slice of single-item pointer must have bounds [0..0], [0..1], or [1..1]";
                    if (try sema.compareScalar(start_value, .neq, end_value, .comptime_int)) {
                        if (try sema.compareScalar(start_value, .neq, Value.zero_comptime_int, .comptime_int)) {
                            const msg = msg: {
                                const msg = try sema.errMsg(start_src, bounds_error_message, .{});
                                errdefer msg.destroy(sema.gpa);
                                try sema.errNote(
                                    start_src,
                                    msg,
                                    "expected '{}', found '{}'",
                                    .{
                                        Value.zero_comptime_int.fmtValueSema(pt, sema),
                                        start_value.fmtValueSema(pt, sema),
                                    },
                                );
                                break :msg msg;
                            };
                            return sema.failWithOwnedErrorMsg(block, msg);
                        } else if (try sema.compareScalar(end_value, .neq, Value.one_comptime_int, .comptime_int)) {
                            const msg = msg: {
                                const msg = try sema.errMsg(end_src, bounds_error_message, .{});
                                errdefer msg.destroy(sema.gpa);
                                try sema.errNote(
                                    end_src,
                                    msg,
                                    "expected '{}', found '{}'",
                                    .{
                                        Value.one_comptime_int.fmtValueSema(pt, sema),
                                        end_value.fmtValueSema(pt, sema),
                                    },
                                );
                                break :msg msg;
                            };
                            return sema.failWithOwnedErrorMsg(block, msg);
                        }
                    } else {
                        if (try sema.compareScalar(end_value, .gt, Value.one_comptime_int, .comptime_int)) {
                            return sema.fail(
                                block,
                                end_src,
                                "end index {} out of bounds for slice of single-item pointer",
                                .{end_value.fmtValueSema(pt, sema)},
                            );
                        }
                    }

                    array_ty = try pt.arrayType(.{
                        .len = 1,
                        .child = double_child_ty.toIntern(),
                    });
                    const ptr_info = ptr_ptr_child_ty.ptrInfo(zcu);
                    slice_ty = try pt.ptrType(.{
                        .child = array_ty.toIntern(),
                        .flags = .{
                            .alignment = ptr_info.flags.alignment,
                            .is_const = ptr_info.flags.is_const,
                            .is_allowzero = ptr_info.flags.is_allowzero,
                            .is_volatile = ptr_info.flags.is_volatile,
                            .address_space = ptr_info.flags.address_space,
                        },
                    });
                    elem_ty = double_child_ty;
                }
            },
            .many, .c => {
                ptr_sentinel = ptr_ptr_child_ty.sentinel(zcu);
                ptr_or_slice = try sema.analyzeLoad(block, src, ptr_ptr, ptr_src);
                slice_ty = ptr_ptr_child_ty;
                array_ty = ptr_ptr_child_ty;
                elem_ty = ptr_ptr_child_ty.childType(zcu);

                if (ptr_ptr_child_ty.ptrSize(zcu) == .c) {
                    if (try sema.resolveDefinedValue(block, ptr_src, ptr_or_slice)) |ptr_val| {
                        if (ptr_val.isNull(zcu)) {
                            return sema.fail(block, src, "slice of null pointer", .{});
                        }
                    }
                }
            },
            .slice => {
                ptr_sentinel = ptr_ptr_child_ty.sentinel(zcu);
                ptr_or_slice = try sema.analyzeLoad(block, src, ptr_ptr, ptr_src);
                slice_ty = ptr_ptr_child_ty;
                array_ty = ptr_ptr_child_ty;
                elem_ty = ptr_ptr_child_ty.childType(zcu);
            },
        },
        else => return sema.fail(block, src, "slice of non-array type '{}'", .{ptr_ptr_child_ty.fmt(pt)}),
    }

    const ptr = if (slice_ty.isSlice(zcu))
        try sema.analyzeSlicePtr(block, ptr_src, ptr_or_slice, slice_ty)
    else if (array_ty.zigTypeTag(zcu) == .array) ptr: {
        var manyptr_ty_key = zcu.intern_pool.indexToKey(slice_ty.toIntern()).ptr_type;
        assert(manyptr_ty_key.child == array_ty.toIntern());
        assert(manyptr_ty_key.flags.size == .one);
        manyptr_ty_key.child = elem_ty.toIntern();
        manyptr_ty_key.flags.size = .many;
        break :ptr try sema.coerceCompatiblePtrs(block, try pt.ptrTypeSema(manyptr_ty_key), ptr_or_slice, ptr_src);
    } else ptr_or_slice;

    const start = try sema.coerce(block, .usize, uncasted_start, start_src);
    const new_ptr = try sema.analyzePtrArithmetic(block, src, ptr, start, .ptr_add, ptr_src, start_src);
    const new_ptr_ty = sema.typeOf(new_ptr);

    // true if and only if the end index of the slice, implicitly or explicitly, equals
    // the length of the underlying object being sliced. we might learn the length of the
    // underlying object because it is an array (which has the length in the type), or
    // we might learn of the length because it is a comptime-known slice value.
    var end_is_len = uncasted_end_opt == .none;
    const end = e: {
        if (array_ty.zigTypeTag(zcu) == .array) {
            const len_val = try pt.intValue(.usize, array_ty.arrayLen(zcu));

            if (!end_is_len) {
                const end = if (by_length) end: {
                    const len = try sema.coerce(block, .usize, uncasted_end_opt, end_src);
                    const uncasted_end = try sema.analyzeArithmetic(block, .add, start, len, src, start_src, end_src, false);
                    break :end try sema.coerce(block, .usize, uncasted_end, end_src);
                } else try sema.coerce(block, .usize, uncasted_end_opt, end_src);
                if (try sema.resolveDefinedValue(block, end_src, end)) |end_val| {
                    const len_s_val = try pt.intValue(
                        .usize,
                        array_ty.arrayLenIncludingSentinel(zcu),
                    );
                    if (!(try sema.compareAll(end_val, .lte, len_s_val, .usize))) {
                        const sentinel_label: []const u8 = if (array_ty.sentinel(zcu) != null)
                            " +1 (sentinel)"
                        else
                            "";

                        return sema.fail(
                            block,
                            end_src,
                            "end index {} out of bounds for array of length {}{s}",
                            .{
                                end_val.fmtValueSema(pt, sema),
                                len_val.fmtValueSema(pt, sema),
                                sentinel_label,
                            },
                        );
                    }

                    // end_is_len is only true if we are NOT using the sentinel
                    // length. For sentinel-length, we don't want the type to
                    // contain the sentinel.
                    if (end_val.eql(len_val, .usize, zcu)) {
                        end_is_len = true;
                    }
                }
                break :e end;
            }

            break :e Air.internedToRef(len_val.toIntern());
        } else if (slice_ty.isSlice(zcu)) {
            if (!end_is_len) {
                const end = if (by_length) end: {
                    const len = try sema.coerce(block, .usize, uncasted_end_opt, end_src);
                    const uncasted_end = try sema.analyzeArithmetic(block, .add, start, len, src, start_src, end_src, false);
                    break :end try sema.coerce(block, .usize, uncasted_end, end_src);
                } else try sema.coerce(block, .usize, uncasted_end_opt, end_src);
                if (try sema.resolveDefinedValue(block, end_src, end)) |end_val| {
                    if (try sema.resolveValue(ptr_or_slice)) |slice_val| {
                        if (slice_val.isUndef(zcu)) {
                            return sema.fail(block, src, "slice of undefined", .{});
                        }
                        const has_sentinel = slice_ty.sentinel(zcu) != null;
                        const slice_len = try slice_val.sliceLen(pt);
                        const len_plus_sent = slice_len + @intFromBool(has_sentinel);
                        const slice_len_val_with_sentinel = try pt.intValue(.usize, len_plus_sent);
                        if (!(try sema.compareAll(end_val, .lte, slice_len_val_with_sentinel, .usize))) {
                            const sentinel_label: []const u8 = if (has_sentinel)
                                " +1 (sentinel)"
                            else
                                "";

                            return sema.fail(
                                block,
                                end_src,
                                "end index {} out of bounds for slice of length {d}{s}",
                                .{
                                    end_val.fmtValueSema(pt, sema),
                                    try slice_val.sliceLen(pt),
                                    sentinel_label,
                                },
                            );
                        }

                        // If the slice has a sentinel, we consider end_is_len
                        // is only true if it equals the length WITHOUT the
                        // sentinel, so we don't add a sentinel type.
                        const slice_len_val = try pt.intValue(.usize, slice_len);
                        if (end_val.eql(slice_len_val, .usize, zcu)) {
                            end_is_len = true;
                        }
                    }
                }
                break :e end;
            }
            break :e try sema.analyzeSliceLen(block, src, ptr_or_slice);
        }
        if (!end_is_len) {
            if (by_length) {
                const len = try sema.coerce(block, .usize, uncasted_end_opt, end_src);
                const uncasted_end = try sema.analyzeArithmetic(block, .add, start, len, src, start_src, end_src, false);
                break :e try sema.coerce(block, .usize, uncasted_end, end_src);
            } else break :e try sema.coerce(block, .usize, uncasted_end_opt, end_src);
        }
        return sema.analyzePtrArithmetic(block, src, ptr, start, .ptr_add, ptr_src, start_src);
    };

    const sentinel = s: {
        if (sentinel_opt != .none) {
            const casted = try sema.coerce(block, elem_ty, sentinel_opt, sentinel_src);
            try checkSentinelType(sema, block, sentinel_src, elem_ty);
            break :s try sema.resolveConstDefinedValue(block, sentinel_src, casted, .{ .simple = .slice_sentinel });
        }
        // If we are slicing to the end of something that is sentinel-terminated
        // then the resulting slice type is also sentinel-terminated.
        if (end_is_len) {
            if (ptr_sentinel) |sent| {
                break :s sent;
            }
        }
        break :s null;
    };
    const slice_sentinel = if (sentinel_opt != .none) sentinel else null;

    var checked_start_lte_end = by_length;
    var runtime_src: ?LazySrcLoc = null;

    // requirement: start <= end
    if (try sema.resolveDefinedValue(block, end_src, end)) |end_val| {
        if (try sema.resolveDefinedValue(block, start_src, start)) |start_val| {
            if (!by_length and !(try sema.compareAll(start_val, .lte, end_val, .usize))) {
                return sema.fail(
                    block,
                    start_src,
                    "start index {} is larger than end index {}",
                    .{
                        start_val.fmtValueSema(pt, sema),
                        end_val.fmtValueSema(pt, sema),
                    },
                );
            }
            checked_start_lte_end = true;
            if (try sema.resolveValue(new_ptr)) |ptr_val| sentinel_check: {
                const expected_sentinel = sentinel orelse break :sentinel_check;
                const start_int = start_val.toUnsignedInt(zcu);
                const end_int = end_val.toUnsignedInt(zcu);
                const sentinel_index = try sema.usizeCast(block, end_src, end_int - start_int);

                const many_ptr_ty = try pt.manyConstPtrType(elem_ty);
                const many_ptr_val = try pt.getCoerced(ptr_val, many_ptr_ty);
                const elem_ptr = try many_ptr_val.ptrElem(sentinel_index, pt);
                const res = try sema.pointerDerefExtra(block, src, elem_ptr);
                const actual_sentinel = switch (res) {
                    .runtime_load => break :sentinel_check,
                    .val => |v| v,
                    .needed_well_defined => |ty| return sema.fail(
                        block,
                        src,
                        "comptime dereference requires '{}' to have a well-defined layout",
                        .{ty.fmt(pt)},
                    ),
                    .out_of_bounds => |ty| return sema.fail(
                        block,
                        end_src,
                        "slice end index {d} exceeds bounds of containing decl of type '{}'",
                        .{ end_int, ty.fmt(pt) },
                    ),
                };

                if (!actual_sentinel.eql(expected_sentinel, elem_ty, zcu)) {
                    const msg = msg: {
                        const msg = try sema.errMsg(src, "value in memory does not match slice sentinel", .{});
                        errdefer msg.destroy(sema.gpa);
                        try sema.errNote(src, msg, "expected '{}', found '{}'", .{
                            expected_sentinel.fmtValueSema(pt, sema),
                            actual_sentinel.fmtValueSema(pt, sema),
                        });

                        break :msg msg;
                    };
                    return sema.failWithOwnedErrorMsg(block, msg);
                }
            } else {
                runtime_src = ptr_src;
            }
        } else {
            runtime_src = start_src;
        }
    } else {
        runtime_src = end_src;
    }

    if (!checked_start_lte_end and block.wantSafety() and !block.isComptime()) {
        // requirement: start <= end
        assert(!block.isComptime());
        try sema.requireRuntimeBlock(block, src, runtime_src.?);
        const ok = try block.addBinOp(.cmp_lte, start, end);
        try sema.addSafetyCheckCall(block, src, ok, .@"panic.startGreaterThanEnd", &.{ start, end });
    }
    const new_len = if (by_length)
        try sema.coerce(block, .usize, uncasted_end_opt, end_src)
    else
        try sema.analyzeArithmetic(block, .sub, end, start, src, end_src, start_src, false);
    const opt_new_len_val = try sema.resolveDefinedValue(block, src, new_len);

    const new_ptr_ty_info = new_ptr_ty.ptrInfo(zcu);
    const new_allowzero = new_ptr_ty_info.flags.is_allowzero and sema.typeOf(ptr).ptrSize(zcu) != .c;

    if (opt_new_len_val) |new_len_val| {
        const new_len_int = try new_len_val.toUnsignedIntSema(pt);

        const return_ty = try pt.ptrTypeSema(.{
            .child = (try pt.arrayType(.{
                .len = new_len_int,
                .sentinel = if (sentinel) |s| s.toIntern() else .none,
                .child = elem_ty.toIntern(),
            })).toIntern(),
            .flags = .{
                .alignment = new_ptr_ty_info.flags.alignment,
                .is_const = new_ptr_ty_info.flags.is_const,
                .is_allowzero = new_allowzero,
                .is_volatile = new_ptr_ty_info.flags.is_volatile,
                .address_space = new_ptr_ty_info.flags.address_space,
            },
        });

        const opt_new_ptr_val = try sema.resolveValue(new_ptr);
        const new_ptr_val = opt_new_ptr_val orelse {
            const result = try block.addBitCast(return_ty, new_ptr);
            if (block.wantSafety()) {
                // requirement: slicing C ptr is non-null
                if (ptr_ptr_child_ty.isCPtr(zcu)) {
                    const is_non_null = try sema.analyzeIsNull(block, ptr, true);
                    try sema.addSafetyCheck(block, src, is_non_null, .unwrap_null);
                }

                bounds_check: {
                    const actual_len = if (array_ty.zigTypeTag(zcu) == .array)
                        try pt.intRef(.usize, array_ty.arrayLenIncludingSentinel(zcu))
                    else if (slice_ty.isSlice(zcu)) l: {
                        const slice_len_inst = try block.addTyOp(.slice_len, .usize, ptr_or_slice);
                        break :l if (slice_ty.sentinel(zcu) == null)
                            slice_len_inst
                        else
                            try sema.analyzeArithmetic(block, .add, slice_len_inst, .one, src, end_src, end_src, true);
                    } else break :bounds_check;

                    const actual_end = if (slice_sentinel != null)
                        try sema.analyzeArithmetic(block, .add, end, .one, src, end_src, end_src, true)
                    else
                        end;

                    try sema.addSafetyCheckIndexOob(block, src, actual_end, actual_len, .cmp_lte);
                }

                // requirement: result[new_len] == slice_sentinel
                try sema.addSafetyCheckSentinelMismatch(block, src, slice_sentinel, elem_ty, result, new_len);
            }
            return result;
        };

        if (!new_ptr_val.isUndef(zcu)) {
            return Air.internedToRef((try pt.getCoerced(new_ptr_val, return_ty)).toIntern());
        }

        // Special case: @as([]i32, undefined)[x..x]
        if (new_len_int == 0) {
            return pt.undefRef(return_ty);
        }

        return sema.fail(block, src, "non-zero length slice of undefined pointer", .{});
    }

    const return_ty = try pt.ptrTypeSema(.{
        .child = elem_ty.toIntern(),
        .sentinel = if (sentinel) |s| s.toIntern() else .none,
        .flags = .{
            .size = .slice,
            .alignment = new_ptr_ty_info.flags.alignment,
            .is_const = new_ptr_ty_info.flags.is_const,
            .is_volatile = new_ptr_ty_info.flags.is_volatile,
            .is_allowzero = new_allowzero,
            .address_space = new_ptr_ty_info.flags.address_space,
        },
    });

    try sema.requireRuntimeBlock(block, src, runtime_src.?);
    if (block.wantSafety()) {
        // requirement: slicing C ptr is non-null
        if (ptr_ptr_child_ty.isCPtr(zcu)) {
            const is_non_null = try sema.analyzeIsNull(block, ptr, true);
            try sema.addSafetyCheck(block, src, is_non_null, .unwrap_null);
        }

        // requirement: end <= len
        const opt_len_inst = if (array_ty.zigTypeTag(zcu) == .array)
            try pt.intRef(.usize, array_ty.arrayLenIncludingSentinel(zcu))
        else if (slice_ty.isSlice(zcu)) blk: {
            if (try sema.resolveDefinedValue(block, src, ptr_or_slice)) |slice_val| {
                // we don't need to add one for sentinels because the
                // underlying value data includes the sentinel
                break :blk try pt.intRef(.usize, try slice_val.sliceLen(pt));
            }

            const slice_len_inst = try block.addTyOp(.slice_len, .usize, ptr_or_slice);
            if (slice_ty.sentinel(zcu) == null) break :blk slice_len_inst;

            // we have to add one because slice lengths don't include the sentinel
            break :blk try sema.analyzeArithmetic(block, .add, slice_len_inst, .one, src, end_src, end_src, true);
        } else null;
        if (opt_len_inst) |len_inst| {
            const actual_end = if (slice_sentinel != null)
                try sema.analyzeArithmetic(block, .add, end, .one, src, end_src, end_src, true)
            else
                end;
            try sema.addSafetyCheckIndexOob(block, src, actual_end, len_inst, .cmp_lte);
        }

        // requirement: start <= end
        try sema.addSafetyCheckIndexOob(block, src, start, end, .cmp_lte);
    }
    const result = try block.addInst(.{
        .tag = .slice,
        .data = .{ .ty_pl = .{
            .ty = Air.internedToRef(return_ty.toIntern()),
            .payload = try sema.addExtra(Air.Bin{
                .lhs = new_ptr,
                .rhs = new_len,
            }),
        } },
    });
    if (block.wantSafety()) {
        // requirement: result[new_len] == slice_sentinel
        try sema.addSafetyCheckSentinelMismatch(block, src, slice_sentinel, elem_ty, result, new_len);
    }
    return result;
}

/// Asserts that lhs and rhs types are both numeric.
fn cmpNumeric(
    sema: *Sema,
    block: *Block,
    src: LazySrcLoc,
    uncasted_lhs: Air.Inst.Ref,
    uncasted_rhs: Air.Inst.Ref,
    op: std.math.CompareOperator,
    lhs_src: LazySrcLoc,
    rhs_src: LazySrcLoc,
) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const lhs_ty = sema.typeOf(uncasted_lhs);
    const rhs_ty = sema.typeOf(uncasted_rhs);

    assert(lhs_ty.isNumeric(zcu));
    assert(rhs_ty.isNumeric(zcu));

    const lhs_ty_tag = lhs_ty.zigTypeTag(zcu);
    const rhs_ty_tag = rhs_ty.zigTypeTag(zcu);
    const target = zcu.getTarget();

    // One exception to heterogeneous comparison: comptime_float needs to
    // coerce to fixed-width float.

    const lhs = if (lhs_ty_tag == .comptime_float and rhs_ty_tag == .float)
        try sema.coerce(block, rhs_ty, uncasted_lhs, lhs_src)
    else
        uncasted_lhs;

    const rhs = if (lhs_ty_tag == .float and rhs_ty_tag == .comptime_float)
        try sema.coerce(block, lhs_ty, uncasted_rhs, rhs_src)
    else
        uncasted_rhs;

    const maybe_lhs_val = try sema.resolveValue(lhs);
    const maybe_rhs_val = try sema.resolveValue(rhs);

    // If the LHS is const, check if there is a guaranteed result which does not depend on ths RHS value.
    if (maybe_lhs_val) |lhs_val| {
        // Result based on comparison exceeding type bounds
        if (!lhs_val.isUndef(zcu) and (lhs_ty_tag == .int or lhs_ty_tag == .comptime_int) and rhs_ty.isInt(zcu)) {
            if (try sema.compareIntsOnlyPossibleResult(lhs_val, op, rhs_ty)) |res| {
                return if (res) .bool_true else .bool_false;
            }
        }
        // Result based on NaN comparison
        if (lhs_val.isNan(zcu)) {
            return if (op == .neq) .bool_true else .bool_false;
        }
        // Result based on inf comparison to int
        if (lhs_val.isInf(zcu) and rhs_ty_tag == .int) return switch (op) {
            .neq => .bool_true,
            .eq => .bool_false,
            .gt, .gte => if (lhs_val.isNegativeInf(zcu)) .bool_false else .bool_true,
            .lt, .lte => if (lhs_val.isNegativeInf(zcu)) .bool_true else .bool_false,
        };
    }

    // If the RHS is const, check if there is a guaranteed result which does not depend on ths LHS value.
    if (maybe_rhs_val) |rhs_val| {
        // Result based on comparison exceeding type bounds
        if (!rhs_val.isUndef(zcu) and (rhs_ty_tag == .int or rhs_ty_tag == .comptime_int) and lhs_ty.isInt(zcu)) {
            if (try sema.compareIntsOnlyPossibleResult(rhs_val, op.reverse(), lhs_ty)) |res| {
                return if (res) .bool_true else .bool_false;
            }
        }
        // Result based on NaN comparison
        if (rhs_val.isNan(zcu)) {
            return if (op == .neq) .bool_true else .bool_false;
        }
        // Result based on inf comparison to int
        if (rhs_val.isInf(zcu) and lhs_ty_tag == .int) return switch (op) {
            .neq => .bool_true,
            .eq => .bool_false,
            .gt, .gte => if (rhs_val.isNegativeInf(zcu)) .bool_true else .bool_false,
            .lt, .lte => if (rhs_val.isNegativeInf(zcu)) .bool_false else .bool_true,
        };
    }

    // Any other comparison depends on both values, so the result is undef if either is undef.
    if (maybe_lhs_val) |v| if (v.isUndef(zcu)) return .undef_bool;
    if (maybe_rhs_val) |v| if (v.isUndef(zcu)) return .undef_bool;

    const runtime_src: LazySrcLoc = if (maybe_lhs_val) |lhs_val| rs: {
        if (maybe_rhs_val) |rhs_val| {
            const res = try Value.compareHeteroSema(lhs_val, op, rhs_val, pt);
            return if (res) .bool_true else .bool_false;
        } else break :rs rhs_src;
    } else lhs_src;

    // TODO handle comparisons against lazy zero values
    // Some values can be compared against zero without being runtime-known or without forcing
    // a full resolution of their value, for example `@sizeOf(@Frame(function))` is known to
    // always be nonzero, and we benefit from not forcing the full evaluation and stack frame layout
    // of this function if we don't need to.
    try sema.requireRuntimeBlock(block, src, runtime_src);

    // For floats, emit a float comparison instruction.
    const lhs_is_float = switch (lhs_ty_tag) {
        .float, .comptime_float => true,
        else => false,
    };
    const rhs_is_float = switch (rhs_ty_tag) {
        .float, .comptime_float => true,
        else => false,
    };

    if (lhs_is_float and rhs_is_float) {
        // Smaller fixed-width floats coerce to larger fixed-width floats.
        // comptime_float coerces to fixed-width float.
        const dest_ty = x: {
            if (lhs_ty_tag == .comptime_float) {
                break :x rhs_ty;
            } else if (rhs_ty_tag == .comptime_float) {
                break :x lhs_ty;
            }
            if (lhs_ty.floatBits(target) >= rhs_ty.floatBits(target)) {
                break :x lhs_ty;
            } else {
                break :x rhs_ty;
            }
        };
        const casted_lhs = try sema.coerce(block, dest_ty, lhs, lhs_src);
        const casted_rhs = try sema.coerce(block, dest_ty, rhs, rhs_src);
        return block.addBinOp(Air.Inst.Tag.fromCmpOp(op, block.float_mode == .optimized), casted_lhs, casted_rhs);
    }

    // For mixed unsigned integer sizes, implicit cast both operands to the larger integer.
    // For mixed signed and unsigned integers, implicit cast both operands to a signed
    // integer with + 1 bit.
    // For mixed floats and integers, extract the integer part from the float, cast that to
    // a signed integer with mantissa bits + 1, and if there was any non-integral part of the float,
    // add/subtract 1.
    const lhs_is_signed = if (maybe_lhs_val) |lhs_val|
        !(try lhs_val.compareAllWithZeroSema(.gte, pt))
    else
        (lhs_ty.isRuntimeFloat() or lhs_ty.isSignedInt(zcu));
    const rhs_is_signed = if (maybe_rhs_val) |rhs_val|
        !(try rhs_val.compareAllWithZeroSema(.gte, pt))
    else
        (rhs_ty.isRuntimeFloat() or rhs_ty.isSignedInt(zcu));
    const dest_int_is_signed = lhs_is_signed or rhs_is_signed;

    var dest_float_type: ?Type = null;

    var lhs_bits: usize = undefined;
    if (maybe_lhs_val) |unresolved_lhs_val| {
        const lhs_val = try sema.resolveLazyValue(unresolved_lhs_val);
        if (!rhs_is_signed) {
            switch (lhs_val.orderAgainstZero(zcu)) {
                .gt => {},
                .eq => switch (op) { // LHS = 0, RHS is unsigned
                    .lte => return .bool_true,
                    .gt => return .bool_false,
                    else => {},
                },
                .lt => switch (op) { // LHS < 0, RHS is unsigned
                    .neq, .lt, .lte => return .bool_true,
                    .eq, .gt, .gte => return .bool_false,
                },
            }
        }
        if (lhs_is_float) {
            if (lhs_val.floatHasFraction(zcu)) {
                switch (op) {
                    .eq => return .bool_false,
                    .neq => return .bool_true,
                    else => {},
                }
            }

            var bigint = try float128IntPartToBigInt(sema.gpa, lhs_val.toFloat(f128, zcu));
            defer bigint.deinit();
            if (lhs_val.floatHasFraction(zcu)) {
                if (lhs_is_signed) {
                    try bigint.addScalar(&bigint, -1);
                } else {
                    try bigint.addScalar(&bigint, 1);
                }
            }
            lhs_bits = bigint.toConst().bitCountTwosComp();
        } else {
            lhs_bits = lhs_val.intBitCountTwosComp(zcu);
        }
        lhs_bits += @intFromBool(!lhs_is_signed and dest_int_is_signed);
    } else if (lhs_is_float) {
        dest_float_type = lhs_ty;
    } else {
        const int_info = lhs_ty.intInfo(zcu);
        lhs_bits = int_info.bits + @intFromBool(int_info.signedness == .unsigned and dest_int_is_signed);
    }

    var rhs_bits: usize = undefined;
    if (maybe_rhs_val) |unresolved_rhs_val| {
        const rhs_val = try sema.resolveLazyValue(unresolved_rhs_val);
        if (!lhs_is_signed) {
            switch (rhs_val.orderAgainstZero(zcu)) {
                .gt => {},
                .eq => switch (op) { // RHS = 0, LHS is unsigned
                    .gte => return .bool_true,
                    .lt => return .bool_false,
                    else => {},
                },
                .lt => switch (op) { // RHS < 0, LHS is unsigned
                    .neq, .gt, .gte => return .bool_true,
                    .eq, .lt, .lte => return .bool_false,
                },
            }
        }
        if (rhs_is_float) {
            if (rhs_val.floatHasFraction(zcu)) {
                switch (op) {
                    .eq => return .bool_false,
                    .neq => return .bool_true,
                    else => {},
                }
            }

            var bigint = try float128IntPartToBigInt(sema.gpa, rhs_val.toFloat(f128, zcu));
            defer bigint.deinit();
            if (rhs_val.floatHasFraction(zcu)) {
                if (rhs_is_signed) {
                    try bigint.addScalar(&bigint, -1);
                } else {
                    try bigint.addScalar(&bigint, 1);
                }
            }
            rhs_bits = bigint.toConst().bitCountTwosComp();
        } else {
            rhs_bits = rhs_val.intBitCountTwosComp(zcu);
        }
        rhs_bits += @intFromBool(!rhs_is_signed and dest_int_is_signed);
    } else if (rhs_is_float) {
        dest_float_type = rhs_ty;
    } else {
        const int_info = rhs_ty.intInfo(zcu);
        rhs_bits = int_info.bits + @intFromBool(int_info.signedness == .unsigned and dest_int_is_signed);
    }

    const dest_ty = if (dest_float_type) |ft| ft else blk: {
        const max_bits = @max(lhs_bits, rhs_bits);
        const casted_bits = std.math.cast(u16, max_bits) orelse return sema.fail(block, src, "{d} exceeds maximum integer bit count", .{max_bits});
        const signedness: std.builtin.Signedness = if (dest_int_is_signed) .signed else .unsigned;
        break :blk try pt.intType(signedness, casted_bits);
    };
    const casted_lhs = try sema.coerce(block, dest_ty, lhs, lhs_src);
    const casted_rhs = try sema.coerce(block, dest_ty, rhs, rhs_src);

    return block.addBinOp(Air.Inst.Tag.fromCmpOp(op, block.float_mode == .optimized), casted_lhs, casted_rhs);
}

/// Asserts that LHS value is an int or comptime int and not undefined, and
/// that RHS type is an int. Given a const LHS and an unknown RHS, attempt to
/// determine whether `op` has a guaranteed result.
/// If it cannot be determined, returns null.
/// Otherwise returns a bool for the guaranteed comparison operation.
fn compareIntsOnlyPossibleResult(
    sema: *Sema,
    lhs_val: Value,
    op: std.math.CompareOperator,
    rhs_ty: Type,
) SemaError!?bool {
    const pt = sema.pt;
    const zcu = pt.zcu;

    const min_rhs = try rhs_ty.minInt(pt, rhs_ty);
    const max_rhs = try rhs_ty.maxInt(pt, rhs_ty);

    if (min_rhs.toIntern() == max_rhs.toIntern()) {
        // RHS is effectively comptime-known.
        return try Value.compareHeteroSema(lhs_val, op, min_rhs, pt);
    }

    const against_min = try lhs_val.orderAdvanced(min_rhs, .sema, zcu, pt.tid);
    const against_max = try lhs_val.orderAdvanced(max_rhs, .sema, zcu, pt.tid);

    switch (op) {
        .eq => {
            if (against_min.compare(.lt)) return false;
            if (against_max.compare(.gt)) return false;
        },
        .neq => {
            if (against_min.compare(.lt)) return true;
            if (against_max.compare(.gt)) return true;
        },
        .lt => {
            if (against_min.compare(.lt)) return true;
            if (against_max.compare(.gte)) return false;
        },
        .gt => {
            if (against_max.compare(.gt)) return true;
            if (against_min.compare(.lte)) return false;
        },
        .lte => {
            if (against_min.compare(.lte)) return true;
            if (against_max.compare(.gt)) return false;
        },
        .gte => {
            if (against_max.compare(.gte)) return true;
            if (against_min.compare(.lt)) return false;
        },
    }

    return null;
}

/// Asserts that lhs and rhs types are both vectors.
fn cmpVector(
    sema: *Sema,
    block: *Block,
    src: LazySrcLoc,
    lhs: Air.Inst.Ref,
    rhs: Air.Inst.Ref,
    op: std.math.CompareOperator,
    lhs_src: LazySrcLoc,
    rhs_src: LazySrcLoc,
) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const lhs_ty = sema.typeOf(lhs);
    const rhs_ty = sema.typeOf(rhs);
    assert(lhs_ty.zigTypeTag(zcu) == .vector);
    assert(rhs_ty.zigTypeTag(zcu) == .vector);
    try sema.checkVectorizableBinaryOperands(block, src, lhs_ty, rhs_ty, lhs_src, rhs_src);

    const resolved_ty = try sema.resolvePeerTypes(block, src, &.{ lhs, rhs }, .{ .override = &.{ lhs_src, rhs_src } });
    const casted_lhs = try sema.coerce(block, resolved_ty, lhs, lhs_src);
    const casted_rhs = try sema.coerce(block, resolved_ty, rhs, rhs_src);

    const result_ty = try pt.vectorType(.{
        .len = lhs_ty.vectorLen(zcu),
        .child = .bool_type,
    });

    const maybe_lhs_val = try sema.resolveValue(casted_lhs);
    const maybe_rhs_val = try sema.resolveValue(casted_rhs);
    if (maybe_lhs_val) |v| if (v.isUndef(zcu)) return pt.undefRef(result_ty);
    if (maybe_rhs_val) |v| if (v.isUndef(zcu)) return pt.undefRef(result_ty);

    const runtime_src: LazySrcLoc = if (maybe_lhs_val) |lhs_val| src: {
        if (maybe_rhs_val) |rhs_val| {
            const cmp_val = try sema.compareVector(lhs_val, op, rhs_val, resolved_ty);
            return Air.internedToRef(cmp_val.toIntern());
        } else break :src rhs_src;
    } else lhs_src;

    try sema.requireRuntimeBlock(block, src, runtime_src);
    return block.addCmpVector(casted_lhs, casted_rhs, op);
}

fn wrapOptional(
    sema: *Sema,
    block: *Block,
    dest_ty: Type,
    inst: Air.Inst.Ref,
    inst_src: LazySrcLoc,
) !Air.Inst.Ref {
    if (try sema.resolveValue(inst)) |val| {
        return Air.internedToRef((try sema.pt.intern(.{ .opt = .{
            .ty = dest_ty.toIntern(),
            .val = val.toIntern(),
        } })));
    }

    try sema.requireRuntimeBlock(block, inst_src, null);
    return block.addTyOp(.wrap_optional, dest_ty, inst);
}

fn wrapErrorUnionPayload(
    sema: *Sema,
    block: *Block,
    dest_ty: Type,
    inst: Air.Inst.Ref,
    inst_src: LazySrcLoc,
) !Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const dest_payload_ty = dest_ty.errorUnionPayload(zcu);
    const coerced = try sema.coerceExtra(block, dest_payload_ty, inst, inst_src, .{ .report_err = false });
    if (try sema.resolveValue(coerced)) |val| {
        return Air.internedToRef((try pt.intern(.{ .error_union = .{
            .ty = dest_ty.toIntern(),
            .val = .{ .payload = val.toIntern() },
        } })));
    }
    try sema.requireRuntimeBlock(block, inst_src, null);
    return block.addTyOp(.wrap_errunion_payload, dest_ty, coerced);
}

fn wrapErrorUnionSet(
    sema: *Sema,
    block: *Block,
    dest_ty: Type,
    inst: Air.Inst.Ref,
    inst_src: LazySrcLoc,
) !Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const ip = &zcu.intern_pool;
    const inst_ty = sema.typeOf(inst);
    const dest_err_set_ty = dest_ty.errorUnionSet(zcu);
    if (try sema.resolveValue(inst)) |val| {
        const expected_name = zcu.intern_pool.indexToKey(val.toIntern()).err.name;
        switch (dest_err_set_ty.toIntern()) {
            .anyerror_type => {},
            .adhoc_inferred_error_set_type => ok: {
                const ies = sema.fn_ret_ty_ies.?;
                switch (ies.resolved) {
                    .anyerror_type => break :ok,
                    .none => if (.ok == try sema.coerceInMemoryAllowedErrorSets(block, dest_err_set_ty, inst_ty, inst_src, inst_src)) {
                        break :ok;
                    },
                    else => |i| if (ip.indexToKey(i).error_set_type.nameIndex(ip, expected_name) != null) {
                        break :ok;
                    },
                }
                return sema.failWithErrorSetCodeMissing(block, inst_src, dest_err_set_ty, inst_ty);
            },
            else => switch (ip.indexToKey(dest_err_set_ty.toIntern())) {
                .error_set_type => |error_set_type| ok: {
                    if (error_set_type.nameIndex(ip, expected_name) != null) break :ok;
                    return sema.failWithErrorSetCodeMissing(block, inst_src, dest_err_set_ty, inst_ty);
                },
                .inferred_error_set_type => |func_index| ok: {
                    // We carefully do this in an order that avoids unnecessarily
                    // resolving the destination error set type.
                    try zcu.maybeUnresolveIes(func_index);
                    switch (ip.funcIesResolvedUnordered(func_index)) {
                        .anyerror_type => break :ok,
                        .none => if (.ok == try sema.coerceInMemoryAllowedErrorSets(block, dest_err_set_ty, inst_ty, inst_src, inst_src)) {
                            break :ok;
                        },
                        else => |i| if (ip.indexToKey(i).error_set_type.nameIndex(ip, expected_name) != null) {
                            break :ok;
                        },
                    }

                    return sema.failWithErrorSetCodeMissing(block, inst_src, dest_err_set_ty, inst_ty);
                },
                else => unreachable,
            },
        }
        return Air.internedToRef((try pt.intern(.{ .error_union = .{
            .ty = dest_ty.toIntern(),
            .val = .{ .err_name = expected_name },
        } })));
    }

    try sema.requireRuntimeBlock(block, inst_src, null);
    const coerced = try sema.coerce(block, dest_err_set_ty, inst, inst_src);
    return block.addTyOp(.wrap_errunion_err, dest_ty, coerced);
}

fn unionToTag(
    sema: *Sema,
    block: *Block,
    enum_ty: Type,
    un: Air.Inst.Ref,
    un_src: LazySrcLoc,
) !Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;
    if ((try sema.typeHasOnePossibleValue(enum_ty))) |opv| {
        return Air.internedToRef(opv.toIntern());
    }
    if (try sema.resolveValue(un)) |un_val| {
        const tag_val = un_val.unionTag(zcu).?;
        if (tag_val.isUndef(zcu))
            return try pt.undefRef(enum_ty);
        return Air.internedToRef(tag_val.toIntern());
    }
    try sema.requireRuntimeBlock(block, un_src, null);
    return block.addTyOp(.get_union_tag, enum_ty, un);
}

const PeerResolveStrategy = enum {
    /// The type is not known.
    /// If refined no further, this is equivalent to `exact`.
    unknown,
    /// The type may be an error set or error union.
    /// If refined no further, it is an error set.
    error_set,
    /// The type must be some error union.
    error_union,
    /// The type may be @TypeOf(null), an optional or a C pointer.
    /// If refined no further, it is @TypeOf(null).
    nullable,
    /// The type must be some optional or a C pointer.
    /// If refined no further, it is an optional.
    optional,
    /// The type must be either an array or a vector.
    /// If refined no further, it is an array.
    array,
    /// The type must be a vector.
    vector,
    /// The type must be a C pointer.
    c_ptr,
    /// The type must be a pointer (C or not).
    /// If refined no further, it is a non-C pointer.
    ptr,
    /// The type must be a function or a pointer to a function.
    /// If refined no further, it is a function.
    func,
    /// The type must be an enum literal, or some specific enum or union. Which one is decided
    /// afterwards based on the types in question.
    enum_or_union,
    /// The type must be some integer or float type.
    /// If refined no further, it is `comptime_int`.
    comptime_int,
    /// The type must be some float type.
    /// If refined no further, it is `comptime_float`.
    comptime_float,
    /// The type must be some float or fixed-width integer type.
    /// If refined no further, it is some fixed-width integer type.
    fixed_int,
    /// The type must be some fixed-width float type.
    fixed_float,
    /// The type must be a tuple.
    tuple,
    /// The peers must all be of the same type.
    exact,

    /// Given two strategies, find a strategy that satisfies both, if one exists. If no such
    /// strategy exists, any strategy may be returned; an error will be emitted when the caller
    /// attempts to use the strategy to resolve the type.
    /// Strategy `a` comes from the peer in `reason_peer`, while strategy `b` comes from the peer at
    /// index `b_peer_idx`. `reason_peer` is updated to reflect the reason for the new strategy.
    fn merge(a: PeerResolveStrategy, b: PeerResolveStrategy, reason_peer: *usize, b_peer_idx: usize) PeerResolveStrategy {
        // Our merging should be order-independent. Thus, even though the union order is arbitrary,
        // by sorting the tags and switching first on the smaller, we have half as many cases to
        // worry about (since we avoid the duplicates).
        const s0_is_a = @intFromEnum(a) <= @intFromEnum(b);
        const s0 = if (s0_is_a) a else b;
        const s1 = if (s0_is_a) b else a;

        const ReasonMethod = enum {
            all_s0,
            all_s1,
            either,
        };

        const reason_method: ReasonMethod, const strat: PeerResolveStrategy = switch (s0) {
            .unknown => .{ .all_s1, s1 },
            .error_set => switch (s1) {
                .error_set => .{ .either, .error_set },
                else => .{ .all_s0, .error_union },
            },
            .error_union => switch (s1) {
                .error_union => .{ .either, .error_union },
                else => .{ .all_s0, .error_union },
            },
            .nullable => switch (s1) {
                .nullable => .{ .either, .nullable },
                .c_ptr => .{ .all_s1, .c_ptr },
                else => .{ .all_s0, .optional },
            },
            .optional => switch (s1) {
                .optional => .{ .either, .optional },
                .c_ptr => .{ .all_s1, .c_ptr },
                else => .{ .all_s0, .optional },
            },
            .array => switch (s1) {
                .array => .{ .either, .array },
                .vector => .{ .all_s1, .vector },
                else => .{ .all_s0, .array },
            },
            .vector => switch (s1) {
                .vector => .{ .either, .vector },
                else => .{ .all_s0, .vector },
            },
            .c_ptr => switch (s1) {
                .c_ptr => .{ .either, .c_ptr },
                else => .{ .all_s0, .c_ptr },
            },
            .ptr => switch (s1) {
                .ptr => .{ .either, .ptr },
                else => .{ .all_s0, .ptr },
            },
            .func => switch (s1) {
                .func => .{ .either, .func },
                else => .{ .all_s1, s1 }, // doesn't override anything later
            },
            .enum_or_union => switch (s1) {
                .enum_or_union => .{ .either, .enum_or_union },
                else => .{ .all_s0, .enum_or_union },
            },
            .comptime_int => switch (s1) {
                .comptime_int => .{ .either, .comptime_int },
                else => .{ .all_s1, s1 }, // doesn't override anything later
            },
            .comptime_float => switch (s1) {
                .comptime_float => .{ .either, .comptime_float },
                else => .{ .all_s1, s1 }, // doesn't override anything later
            },
            .fixed_int => switch (s1) {
                .fixed_int => .{ .either, .fixed_int },
                else => .{ .all_s1, s1 }, // doesn't override anything later
            },
            .fixed_float => switch (s1) {
                .fixed_float => .{ .either, .fixed_float },
                else => .{ .all_s1, s1 }, // doesn't override anything later
            },
            .tuple => switch (s1) {
                .exact => .{ .all_s1, .exact },
                else => .{ .all_s0, .tuple },
            },
            .exact => .{ .all_s0, .exact },
        };

        switch (reason_method) {
            .all_s0 => {
                if (!s0_is_a) {
                    reason_peer.* = b_peer_idx;
                }
            },
            .all_s1 => {
                if (s0_is_a) {
                    reason_peer.* = b_peer_idx;
                }
            },
            .either => {
                // Prefer the earliest peer
                reason_peer.* = @min(reason_peer.*, b_peer_idx);
            },
        }

        return strat;
    }

    fn select(ty: Type, zcu: *Zcu) PeerResolveStrategy {
        return switch (ty.zigTypeTag(zcu)) {
            .type, .void, .bool, .@"opaque", .frame, .@"anyframe" => .exact,
            .noreturn, .undefined => .unknown,
            .null => .nullable,
            .comptime_int => .comptime_int,
            .int => .fixed_int,
            .comptime_float => .comptime_float,
            .float => .fixed_float,
            .pointer => if (ty.ptrInfo(zcu).flags.size == .c) .c_ptr else .ptr,
            .array => .array,
            .vector => .vector,
            .optional => .optional,
            .error_set => .error_set,
            .error_union => .error_union,
            .enum_literal, .@"enum", .@"union" => .enum_or_union,
            .@"struct" => if (ty.isTuple(zcu)) .tuple else .exact,
            .@"fn" => .func,
        };
    }
};

const PeerTypeCandidateSrc = union(enum) {
    /// Do not print out error notes for candidate sources
    none: void,
    /// When we want to know the the src of candidate i, look up at
    /// index i in this slice
    override: []const ?LazySrcLoc,
    /// resolvePeerTypes originates from a @TypeOf(...) call
    typeof_builtin_call_node_offset: std.zig.Ast.Node.Offset,

    pub fn resolve(
        self: PeerTypeCandidateSrc,
        block: *Block,
        candidate_i: usize,
    ) ?LazySrcLoc {
        return switch (self) {
            .none => null,
            .override => |candidate_srcs| if (candidate_i >= candidate_srcs.len)
                null
            else
                candidate_srcs[candidate_i],
            .typeof_builtin_call_node_offset => |node_offset| block.builtinCallArgSrc(node_offset, @intCast(candidate_i)),
        };
    }
};

const PeerResolveResult = union(enum) {
    /// The peer type resolution was successful, and resulted in the given type.
    success: Type,
    /// There was some generic conflict between two peers.
    conflict: struct {
        peer_idx_a: usize,
        peer_idx_b: usize,
    },
    /// There was an error when resolving the type of a struct or tuple field.
    field_error: struct {
        /// The name of the field which caused the failure.
        field_name: InternPool.NullTerminatedString,
        /// The type of this field in each peer.
        field_types: []Type,
        /// The error from resolving the field type. Guaranteed not to be `success`.
        sub_result: *PeerResolveResult,
    },

    fn report(
        result: PeerResolveResult,
        sema: *Sema,
        block: *Block,
        src: LazySrcLoc,
        instructions: []const Air.Inst.Ref,
        candidate_srcs: PeerTypeCandidateSrc,
    ) !*Zcu.ErrorMsg {
        const pt = sema.pt;

        var opt_msg: ?*Zcu.ErrorMsg = null;
        errdefer if (opt_msg) |msg| msg.destroy(sema.gpa);

        // If we mention fields we'll want to include field types, so put peer types in a buffer
        var peer_tys = try sema.arena.alloc(Type, instructions.len);
        for (peer_tys, instructions) |*ty, inst| {
            ty.* = sema.typeOf(inst);
        }

        var cur = result;
        while (true) {
            var conflict_idx: [2]usize = undefined;

            switch (cur) {
                .success => unreachable,
                .conflict => |conflict| {
                    // Fall through to two-peer conflict handling below
                    conflict_idx = .{
                        conflict.peer_idx_a,
                        conflict.peer_idx_b,
                    };
                },
                .field_error => |field_error| {
                    const fmt = "struct field '{}' has conflicting types";
                    const args = .{field_error.field_name.fmt(&pt.zcu.intern_pool)};
                    if (opt_msg) |msg| {
                        try sema.errNote(src, msg, fmt, args);
                    } else {
                        opt_msg = try sema.errMsg(src, fmt, args);
                    }

                    // Continue on to child error
                    cur = field_error.sub_result.*;
                    peer_tys = field_error.field_types;
                    continue;
                },
            }

            // This is the path for reporting a generic conflict between two peers.

            if (conflict_idx[1] < conflict_idx[0]) {
                // b comes first in source, so it's better if it comes first in the error
                std.mem.swap(usize, &conflict_idx[0], &conflict_idx[1]);
            }

            const conflict_tys: [2]Type = .{
                peer_tys[conflict_idx[0]],
                peer_tys[conflict_idx[1]],
            };
            const conflict_srcs: [2]?LazySrcLoc = .{
                candidate_srcs.resolve(block, conflict_idx[0]),
                candidate_srcs.resolve(block, conflict_idx[1]),
            };

            const fmt = "incompatible types: '{}' and '{}'";
            const args = .{
                conflict_tys[0].fmt(pt),
                conflict_tys[1].fmt(pt),
            };
            const msg = if (opt_msg) |msg| msg: {
                try sema.errNote(src, msg, fmt, args);
                break :msg msg;
            } else msg: {
                const msg = try sema.errMsg(src, fmt, args);
                opt_msg = msg;
                break :msg msg;
            };

            if (conflict_srcs[0]) |src_loc| try sema.errNote(src_loc, msg, "type '{}' here", .{conflict_tys[0].fmt(pt)});
            if (conflict_srcs[1]) |src_loc| try sema.errNote(src_loc, msg, "type '{}' here", .{conflict_tys[1].fmt(pt)});

            // No child error
            break;
        }

        return opt_msg.?;
    }
};

fn resolvePeerTypes(
    sema: *Sema,
    block: *Block,
    src: LazySrcLoc,
    instructions: []const Air.Inst.Ref,
    candidate_srcs: PeerTypeCandidateSrc,
) !Type {
    switch (instructions.len) {
        0 => return .noreturn,
        1 => return sema.typeOf(instructions[0]),
        else => {},
    }

    // Fast path: check if everything has the same type to bypass the main PTR logic.
    same_type: {
        const ty = sema.typeOf(instructions[0]);
        for (instructions[1..]) |inst| {
            if (sema.typeOf(inst).toIntern() != ty.toIntern()) {
                break :same_type;
            }
        }
        return ty;
    }

    const peer_tys = try sema.arena.alloc(?Type, instructions.len);
    const peer_vals = try sema.arena.alloc(?Value, instructions.len);

    for (instructions, peer_tys, peer_vals) |inst, *ty, *val| {
        ty.* = sema.typeOf(inst);
        val.* = try sema.resolveValue(inst);
    }

    switch (try sema.resolvePeerTypesInner(block, src, peer_tys, peer_vals)) {
        .success => |ty| return ty,
        else => |result| {
            const msg = try result.report(sema, block, src, instructions, candidate_srcs);
            return sema.failWithOwnedErrorMsg(block, msg);
        },
    }
}

fn resolvePeerTypesInner(
    sema: *Sema,
    block: *Block,
    src: LazySrcLoc,
    peer_tys: []?Type,
    peer_vals: []?Value,
) !PeerResolveResult {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const ip = &zcu.intern_pool;

    var strat_reason: usize = 0;
    var s: PeerResolveStrategy = .unknown;
    for (peer_tys, 0..) |opt_ty, i| {
        const ty = opt_ty orelse continue;
        s = s.merge(PeerResolveStrategy.select(ty, zcu), &strat_reason, i);
    }

    if (s == .unknown) {
        // The whole thing was noreturn or undefined - try to do an exact match
        s = .exact;
    } else {
        // There was something other than noreturn and undefined, so we can ignore those peers
        for (peer_tys) |*ty_ptr| {
            const ty = ty_ptr.* orelse continue;
            switch (ty.zigTypeTag(zcu)) {
                .noreturn, .undefined => ty_ptr.* = null,
                else => {},
            }
        }
    }

    const target = zcu.getTarget();

    switch (s) {
        .unknown => unreachable,

        .error_set => {
            var final_set: ?Type = null;
            for (peer_tys, 0..) |opt_ty, i| {
                const ty = opt_ty orelse continue;
                if (ty.zigTypeTag(zcu) != .error_set) return .{ .conflict = .{
                    .peer_idx_a = strat_reason,
                    .peer_idx_b = i,
                } };
                if (final_set) |cur_set| {
                    final_set = try sema.maybeMergeErrorSets(block, src, cur_set, ty);
                } else {
                    final_set = ty;
                }
            }
            return .{ .success = final_set.? };
        },

        .error_union => {
            var final_set: ?Type = null;
            for (peer_tys, peer_vals) |*ty_ptr, *val_ptr| {
                const ty = ty_ptr.* orelse continue;
                const set_ty = switch (ty.zigTypeTag(zcu)) {
                    .error_set => blk: {
                        ty_ptr.* = null; // no payload to decide on
                        val_ptr.* = null;
                        break :blk ty;
                    },
                    .error_union => blk: {
                        const set_ty = ty.errorUnionSet(zcu);
                        ty_ptr.* = ty.errorUnionPayload(zcu);
                        if (val_ptr.*) |eu_val| switch (ip.indexToKey(eu_val.toIntern())) {
                            .error_union => |eu| switch (eu.val) {
                                .payload => |payload_ip| val_ptr.* = Value.fromInterned(payload_ip),
                                .err_name => val_ptr.* = null,
                            },
                            .undef => val_ptr.* = Value.fromInterned(try pt.intern(.{ .undef = ty_ptr.*.?.toIntern() })),
                            else => unreachable,
                        };
                        break :blk set_ty;
                    },
                    else => continue, // whole type is the payload
                };
                if (final_set) |cur_set| {
                    final_set = try sema.maybeMergeErrorSets(block, src, cur_set, set_ty);
                } else {
                    final_set = set_ty;
                }
            }
            assert(final_set != null);
            const final_payload = switch (try sema.resolvePeerTypesInner(
                block,
                src,
                peer_tys,
                peer_vals,
            )) {
                .success => |ty| ty,
                else => |result| return result,
            };
            return .{ .success = try pt.errorUnionType(final_set.?, final_payload) };
        },

        .nullable => {
            for (peer_tys, 0..) |opt_ty, i| {
                const ty = opt_ty orelse continue;
                if (!ty.eql(.null, zcu)) return .{ .conflict = .{
                    .peer_idx_a = strat_reason,
                    .peer_idx_b = i,
                } };
            }
            return .{ .success = .null };
        },

        .optional => {
            for (peer_tys, peer_vals) |*ty_ptr, *val_ptr| {
                const ty = ty_ptr.* orelse continue;
                switch (ty.zigTypeTag(zcu)) {
                    .null => {
                        ty_ptr.* = null;
                        val_ptr.* = null;
                    },
                    .optional => {
                        ty_ptr.* = ty.optionalChild(zcu);
                        if (val_ptr.*) |opt_val| val_ptr.* = if (!opt_val.isUndef(zcu)) opt_val.optionalValue(zcu) else null;
                    },
                    else => {},
                }
            }
            const child_ty = switch (try sema.resolvePeerTypesInner(
                block,
                src,
                peer_tys,
                peer_vals,
            )) {
                .success => |ty| ty,
                else => |result| return result,
            };
            return .{ .success = try pt.optionalType(child_ty.toIntern()) };
        },

        .array => {
            // Index of the first non-null peer
            var opt_first_idx: ?usize = null;
            // Index of the first array or vector peer (i.e. not a tuple)
            var opt_first_arr_idx: ?usize = null;
            // Set to non-null once we see any peer, even a tuple
            var len: u64 = undefined;
            var sentinel: ?Value = undefined;
            // Only set once we see a non-tuple peer
            var elem_ty: Type = undefined;

            for (peer_tys, 0..) |*ty_ptr, i| {
                const ty = ty_ptr.* orelse continue;

                if (!ty.isArrayOrVector(zcu)) {
                    // We allow tuples of the correct length. We won't validate their elem type, since the elements can be coerced.
                    const arr_like = sema.typeIsArrayLike(ty) orelse return .{ .conflict = .{
                        .peer_idx_a = strat_reason,
                        .peer_idx_b = i,
                    } };

                    if (opt_first_idx) |first_idx| {
                        if (arr_like.len != len) return .{ .conflict = .{
                            .peer_idx_a = first_idx,
                            .peer_idx_b = i,
                        } };
                    } else {
                        opt_first_idx = i;
                        len = arr_like.len;
                    }

                    sentinel = null;

                    continue;
                }

                const first_arr_idx = opt_first_arr_idx orelse {
                    if (opt_first_idx == null) {
                        opt_first_idx = i;
                        len = ty.arrayLen(zcu);
                        sentinel = ty.sentinel(zcu);
                    }
                    opt_first_arr_idx = i;
                    elem_ty = ty.childType(zcu);
                    continue;
                };

                if (ty.arrayLen(zcu) != len) return .{ .conflict = .{
                    .peer_idx_a = first_arr_idx,
                    .peer_idx_b = i,
                } };

                const peer_elem_ty = ty.childType(zcu);
                if (!peer_elem_ty.eql(elem_ty, zcu)) coerce: {
                    const peer_elem_coerces_to_elem =
                        try sema.coerceInMemoryAllowed(block, elem_ty, peer_elem_ty, false, zcu.getTarget(), src, src, null);
                    if (peer_elem_coerces_to_elem == .ok) {
                        break :coerce;
                    }

                    const elem_coerces_to_peer_elem =
                        try sema.coerceInMemoryAllowed(block, peer_elem_ty, elem_ty, false, zcu.getTarget(), src, src, null);
                    if (elem_coerces_to_peer_elem == .ok) {
                        elem_ty = peer_elem_ty;
                        break :coerce;
                    }

                    return .{ .conflict = .{
                        .peer_idx_a = first_arr_idx,
                        .peer_idx_b = i,
                    } };
                }

                if (sentinel) |cur_sent| {
                    if (ty.sentinel(zcu)) |peer_sent| {
                        if (!peer_sent.eql(cur_sent, elem_ty, zcu)) sentinel = null;
                    } else {
                        sentinel = null;
                    }
                }
            }

            // There should always be at least one array or vector peer
            assert(opt_first_arr_idx != null);

            return .{ .success = try pt.arrayType(.{
                .len = len,
                .child = elem_ty.toIntern(),
                .sentinel = if (sentinel) |sent_val| sent_val.toIntern() else .none,
            }) };
        },

        .vector => {
            var len: ?u64 = null;
            var first_idx: usize = undefined;
            for (peer_tys, peer_vals, 0..) |*ty_ptr, *val_ptr, i| {
                const ty = ty_ptr.* orelse continue;

                if (!ty.isArrayOrVector(zcu)) {
                    // Allow tuples of the correct length
                    const arr_like = sema.typeIsArrayLike(ty) orelse return .{ .conflict = .{
                        .peer_idx_a = strat_reason,
                        .peer_idx_b = i,
                    } };

                    if (len) |expect_len| {
                        if (arr_like.len != expect_len) return .{ .conflict = .{
                            .peer_idx_a = first_idx,
                            .peer_idx_b = i,
                        } };
                    } else {
                        len = arr_like.len;
                        first_idx = i;
                    }

                    // Tuples won't participate in the child type resolution. We'll resolve without
                    // them, and if the tuples have a bad type, we'll get a coercion error later.
                    ty_ptr.* = null;
                    val_ptr.* = null;

                    continue;
                }

                if (len) |expect_len| {
                    if (ty.arrayLen(zcu) != expect_len) return .{ .conflict = .{
                        .peer_idx_a = first_idx,
                        .peer_idx_b = i,
                    } };
                } else {
                    len = ty.arrayLen(zcu);
                    first_idx = i;
                }

                ty_ptr.* = ty.childType(zcu);
                val_ptr.* = null; // multiple child vals, so we can't easily use them in PTR
            }

            const child_ty = switch (try sema.resolvePeerTypesInner(
                block,
                src,
                peer_tys,
                peer_vals,
            )) {
                .success => |ty| ty,
                else => |result| return result,
            };

            return .{ .success = try pt.vectorType(.{
                .len = @intCast(len.?),
                .child = child_ty.toIntern(),
            }) };
        },

        .c_ptr => {
            var opt_ptr_info: ?InternPool.Key.PtrType = null;
            var first_idx: usize = undefined;
            for (peer_tys, peer_vals, 0..) |opt_ty, opt_val, i| {
                const ty = opt_ty orelse continue;
                switch (ty.zigTypeTag(zcu)) {
                    .comptime_int => continue, // comptime-known integers can always coerce to C pointers
                    .int => {
                        if (opt_val != null) {
                            // Always allow the coercion for comptime-known ints
                            continue;
                        } else {
                            // Runtime-known, so check if the type is no bigger than a usize
                            const ptr_bits = target.ptrBitWidth();
                            const bits = ty.intInfo(zcu).bits;
                            if (bits <= ptr_bits) continue;
                        }
                    },
                    .null => continue,
                    else => {},
                }

                if (!ty.isPtrAtRuntime(zcu)) return .{ .conflict = .{
                    .peer_idx_a = strat_reason,
                    .peer_idx_b = i,
                } };

                // Goes through optionals
                const peer_info = ty.ptrInfo(zcu);

                var ptr_info = opt_ptr_info orelse {
                    opt_ptr_info = peer_info;
                    opt_ptr_info.?.flags.size = .c;
                    first_idx = i;
                    continue;
                };

                // Try peer -> cur, then cur -> peer
                ptr_info.child = ((try sema.resolvePairInMemoryCoercible(block, src, .fromInterned(ptr_info.child), .fromInterned(peer_info.child))) orelse {
                    return .{ .conflict = .{
                        .peer_idx_a = first_idx,
                        .peer_idx_b = i,
                    } };
                }).toIntern();

                if (ptr_info.sentinel != .none and peer_info.sentinel != .none) {
                    const peer_sent = try ip.getCoerced(sema.gpa, pt.tid, ptr_info.sentinel, ptr_info.child);
                    const ptr_sent = try ip.getCoerced(sema.gpa, pt.tid, peer_info.sentinel, ptr_info.child);
                    if (ptr_sent == peer_sent) {
                        ptr_info.sentinel = ptr_sent;
                    } else {
                        ptr_info.sentinel = .none;
                    }
                } else {
                    ptr_info.sentinel = .none;
                }

                // Note that the align can be always non-zero; Zcu.ptrType will canonicalize it
                ptr_info.flags.alignment = InternPool.Alignment.min(
                    if (ptr_info.flags.alignment != .none)
                        ptr_info.flags.alignment
                    else
                        Type.fromInterned(ptr_info.child).abiAlignment(zcu),

                    if (peer_info.flags.alignment != .none)
                        peer_info.flags.alignment
                    else
                        Type.fromInterned(peer_info.child).abiAlignment(zcu),
                );
                if (ptr_info.flags.address_space != peer_info.flags.address_space) {
                    return .{ .conflict = .{
                        .peer_idx_a = first_idx,
                        .peer_idx_b = i,
                    } };
                }

                if (ptr_info.packed_offset.bit_offset != peer_info.packed_offset.bit_offset or
                    ptr_info.packed_offset.host_size != peer_info.packed_offset.host_size)
                {
                    return .{ .conflict = .{
                        .peer_idx_a = first_idx,
                        .peer_idx_b = i,
                    } };
                }

                ptr_info.flags.is_const = ptr_info.flags.is_const or peer_info.flags.is_const;
                ptr_info.flags.is_volatile = ptr_info.flags.is_volatile or peer_info.flags.is_volatile;

                opt_ptr_info = ptr_info;
            }
            return .{ .success = try pt.ptrTypeSema(opt_ptr_info.?) };
        },

        .ptr => {
            // If we've resolved to a `[]T` but then see a `[*]T`, we can resolve to a `[*]T` only
            // if there were no actual slices. Else, we want the slice index to report a conflict.
            var opt_slice_idx: ?usize = null;

            var any_abi_aligned = false;
            var opt_ptr_info: ?InternPool.Key.PtrType = null;
            var first_idx: usize = undefined;
            var other_idx: usize = undefined; // We sometimes need a second peer index to report a generic error

            for (peer_tys, 0..) |opt_ty, i| {
                const ty = opt_ty orelse continue;
                const peer_info: InternPool.Key.PtrType = switch (ty.zigTypeTag(zcu)) {
                    .pointer => ty.ptrInfo(zcu),
                    .@"fn" => .{
                        .child = ty.toIntern(),
                        .flags = .{
                            .address_space = target_util.defaultAddressSpace(target, .global_constant),
                        },
                    },
                    else => return .{ .conflict = .{
                        .peer_idx_a = strat_reason,
                        .peer_idx_b = i,
                    } },
                };

                switch (peer_info.flags.size) {
                    .one, .many => {},
                    .slice => opt_slice_idx = i,
                    .c => return .{ .conflict = .{
                        .peer_idx_a = strat_reason,
                        .peer_idx_b = i,
                    } },
                }

                var ptr_info = opt_ptr_info orelse {
                    opt_ptr_info = peer_info;
                    first_idx = i;
                    continue;
                };

                other_idx = i;

                // We want to return this in a lot of cases, so alias it here for convenience
                const generic_err: PeerResolveResult = .{ .conflict = .{
                    .peer_idx_a = first_idx,
                    .peer_idx_b = i,
                } };

                // Note that the align can be always non-zero; Type.ptr will canonicalize it
                if (peer_info.flags.alignment == .none) {
                    any_abi_aligned = true;
                } else if (ptr_info.flags.alignment == .none) {
                    any_abi_aligned = true;
                    ptr_info.flags.alignment = peer_info.flags.alignment;
                } else {
                    ptr_info.flags.alignment = ptr_info.flags.alignment.minStrict(peer_info.flags.alignment);
                }

                if (ptr_info.flags.address_space != peer_info.flags.address_space) {
                    return generic_err;
                }

                if (ptr_info.packed_offset.bit_offset != peer_info.packed_offset.bit_offset or
                    ptr_info.packed_offset.host_size != peer_info.packed_offset.host_size)
                {
                    return generic_err;
                }

                ptr_info.flags.is_const = ptr_info.flags.is_const or peer_info.flags.is_const;
                ptr_info.flags.is_volatile = ptr_info.flags.is_volatile or peer_info.flags.is_volatile;
                ptr_info.flags.is_allowzero = ptr_info.flags.is_allowzero or peer_info.flags.is_allowzero;

                const peer_sentinel: InternPool.Index = switch (peer_info.flags.size) {
                    .one => switch (ip.indexToKey(peer_info.child)) {
                        .array_type => |array_type| array_type.sentinel,
                        else => .none,
                    },
                    .many, .slice => peer_info.sentinel,
                    .c => unreachable,
                };

                const cur_sentinel: InternPool.Index = switch (ptr_info.flags.size) {
                    .one => switch (ip.indexToKey(ptr_info.child)) {
                        .array_type => |array_type| array_type.sentinel,
                        else => .none,
                    },
                    .many, .slice => ptr_info.sentinel,
                    .c => unreachable,
                };

                // We abstract array handling slightly so that tuple pointers can work like array pointers
                const peer_pointee_array = sema.typeIsArrayLike(.fromInterned(peer_info.child));
                const cur_pointee_array = sema.typeIsArrayLike(.fromInterned(ptr_info.child));

                // This switch is just responsible for deciding the size and pointee (not including
                // single-pointer array sentinel).
                good: {
                    switch (peer_info.flags.size) {
                        .one => switch (ptr_info.flags.size) {
                            .one => {
                                if (try sema.resolvePairInMemoryCoercible(block, src, .fromInterned(ptr_info.child), .fromInterned(peer_info.child))) |pointee| {
                                    ptr_info.child = pointee.toIntern();
                                    break :good;
                                }

                                const cur_arr = cur_pointee_array orelse return generic_err;
                                const peer_arr = peer_pointee_array orelse return generic_err;

                                if (try sema.resolvePairInMemoryCoercible(block, src, cur_arr.elem_ty, peer_arr.elem_ty)) |elem_ty| {
                                    // *[n:x]T + *[n:y]T = *[n]T
                                    if (cur_arr.len == peer_arr.len) {
                                        ptr_info.child = (try pt.arrayType(.{
                                            .len = cur_arr.len,
                                            .child = elem_ty.toIntern(),
                                        })).toIntern();
                                        break :good;
                                    }
                                    // *[a]T + *[b]T = []T
                                    ptr_info.flags.size = .slice;
                                    ptr_info.child = elem_ty.toIntern();
                                    break :good;
                                }

                                if (peer_arr.elem_ty.toIntern() == .noreturn_type) {
                                    // *struct{} + *[a]T = []T
                                    ptr_info.flags.size = .slice;
                                    ptr_info.child = cur_arr.elem_ty.toIntern();
                                    break :good;
                                }

                                if (cur_arr.elem_ty.toIntern() == .noreturn_type) {
                                    // *[a]T + *struct{} = []T
                                    ptr_info.flags.size = .slice;
                                    ptr_info.child = peer_arr.elem_ty.toIntern();
                                    break :good;
                                }

                                return generic_err;
                            },
                            .many => {
                                // Only works for *[n]T + [*]T -> [*]T
                                const arr = peer_pointee_array orelse return generic_err;
                                if (try sema.resolvePairInMemoryCoercible(block, src, .fromInterned(ptr_info.child), arr.elem_ty)) |pointee| {
                                    ptr_info.child = pointee.toIntern();
                                    break :good;
                                }
                                if (arr.elem_ty.toIntern() == .noreturn_type) {
                                    // *struct{} + [*]T -> [*]T
                                    break :good;
                                }
                                return generic_err;
                            },
                            .slice => {
                                // Only works for *[n]T + []T -> []T
                                const arr = peer_pointee_array orelse return generic_err;
                                if (try sema.resolvePairInMemoryCoercible(block, src, .fromInterned(ptr_info.child), arr.elem_ty)) |pointee| {
                                    ptr_info.child = pointee.toIntern();
                                    break :good;
                                }
                                if (arr.elem_ty.toIntern() == .noreturn_type) {
                                    // *struct{} + []T -> []T
                                    break :good;
                                }
                                return generic_err;
                            },
                            .c => unreachable,
                        },
                        .many => switch (ptr_info.flags.size) {
                            .one => {
                                // Only works for [*]T + *[n]T -> [*]T
                                const arr = cur_pointee_array orelse return generic_err;
                                if (try sema.resolvePairInMemoryCoercible(block, src, arr.elem_ty, .fromInterned(peer_info.child))) |pointee| {
                                    ptr_info.flags.size = .many;
                                    ptr_info.child = pointee.toIntern();
                                    break :good;
                                }
                                if (arr.elem_ty.toIntern() == .noreturn_type) {
                                    // [*]T + *struct{} -> [*]T
                                    ptr_info.flags.size = .many;
                                    ptr_info.child = peer_info.child;
                                    break :good;
                                }
                                return generic_err;
                            },
                            .many => {
                                if (try sema.resolvePairInMemoryCoercible(block, src, .fromInterned(ptr_info.child), .fromInterned(peer_info.child))) |pointee| {
                                    ptr_info.child = pointee.toIntern();
                                    break :good;
                                }
                                return generic_err;
                            },
                            .slice => {
                                // Only works if no peers are actually slices
                                if (opt_slice_idx) |slice_idx| {
                                    return .{ .conflict = .{
                                        .peer_idx_a = slice_idx,
                                        .peer_idx_b = i,
                                    } };
                                }
                                // Okay, then works for [*]T + "[]T" -> [*]T
                                if (try sema.resolvePairInMemoryCoercible(block, src, .fromInterned(ptr_info.child), .fromInterned(peer_info.child))) |pointee| {
                                    ptr_info.flags.size = .many;
                                    ptr_info.child = pointee.toIntern();
                                    break :good;
                                }
                                return generic_err;
                            },
                            .c => unreachable,
                        },
                        .slice => switch (ptr_info.flags.size) {
                            .one => {
                                // Only works for []T + *[n]T -> []T
                                const arr = cur_pointee_array orelse return generic_err;
                                if (try sema.resolvePairInMemoryCoercible(block, src, arr.elem_ty, .fromInterned(peer_info.child))) |pointee| {
                                    ptr_info.flags.size = .slice;
                                    ptr_info.child = pointee.toIntern();
                                    break :good;
                                }
                                if (arr.elem_ty.toIntern() == .noreturn_type) {
                                    // []T + *struct{} -> []T
                                    ptr_info.flags.size = .slice;
                                    ptr_info.child = peer_info.child;
                                    break :good;
                                }
                                return generic_err;
                            },
                            .many => {
                                // Impossible! (current peer is an actual slice)
                                return generic_err;
                            },
                            .slice => {
                                if (try sema.resolvePairInMemoryCoercible(block, src, .fromInterned(ptr_info.child), .fromInterned(peer_info.child))) |pointee| {
                                    ptr_info.child = pointee.toIntern();
                                    break :good;
                                }
                                return generic_err;
                            },
                            .c => unreachable,
                        },
                        .c => unreachable,
                    }
                }

                const sentinel_ty = switch (ptr_info.flags.size) {
                    .one => switch (ip.indexToKey(ptr_info.child)) {
                        .array_type => |array_type| array_type.child,
                        else => ptr_info.child,
                    },
                    .many, .slice, .c => ptr_info.child,
                };

                sentinel: {
                    no_sentinel: {
                        if (peer_sentinel == .none) break :no_sentinel;
                        if (cur_sentinel == .none) break :no_sentinel;
                        const peer_sent_coerced = try ip.getCoerced(sema.gpa, pt.tid, peer_sentinel, sentinel_ty);
                        const cur_sent_coerced = try ip.getCoerced(sema.gpa, pt.tid, cur_sentinel, sentinel_ty);
                        if (peer_sent_coerced != cur_sent_coerced) break :no_sentinel;
                        // Sentinels match
                        if (ptr_info.flags.size == .one) switch (ip.indexToKey(ptr_info.child)) {
                            .array_type => |array_type| ptr_info.child = (try pt.arrayType(.{
                                .len = array_type.len,
                                .child = array_type.child,
                                .sentinel = cur_sent_coerced,
                            })).toIntern(),
                            else => unreachable,
                        } else {
                            ptr_info.sentinel = cur_sent_coerced;
                        }
                        break :sentinel;
                    }
                    // Clear existing sentinel
                    ptr_info.sentinel = .none;
                    if (ptr_info.flags.size == .one) switch (ip.indexToKey(ptr_info.child)) {
                        .array_type => |array_type| ptr_info.child = (try pt.arrayType(.{
                            .len = array_type.len,
                            .child = array_type.child,
                            .sentinel = .none,
                        })).toIntern(),
                        else => {},
                    };
                }

                opt_ptr_info = ptr_info;
            }

            // Before we succeed, check the pointee type. If we tried to apply PTR to (for instance)
            // &.{} and &.{}, we'll currently have a pointer type of `*[0]noreturn` - we wanted to
            // coerce the empty struct to a specific type, but no peer provided one. We need to
            // detect this case and emit an error.
            const pointee = opt_ptr_info.?.child;
            switch (pointee) {
                .noreturn_type => return .{ .conflict = .{
                    .peer_idx_a = first_idx,
                    .peer_idx_b = other_idx,
                } },
                else => switch (ip.indexToKey(pointee)) {
                    .array_type => |array_type| if (array_type.child == .noreturn_type) return .{ .conflict = .{
                        .peer_idx_a = first_idx,
                        .peer_idx_b = other_idx,
                    } },
                    else => {},
                },
            }

            if (any_abi_aligned and opt_ptr_info.?.flags.alignment != .none) {
                opt_ptr_info.?.flags.alignment = opt_ptr_info.?.flags.alignment.minStrict(
                    try Type.fromInterned(pointee).abiAlignmentSema(pt),
                );
            }

            return .{ .success = try pt.ptrTypeSema(opt_ptr_info.?) };
        },

        .func => {
            var opt_cur_ty: ?Type = null;
            var first_idx: usize = undefined;
            for (peer_tys, 0..) |opt_ty, i| {
                const ty = opt_ty orelse continue;
                const cur_ty = opt_cur_ty orelse {
                    opt_cur_ty = ty;
                    first_idx = i;
                    continue;
                };
                if (ty.zigTypeTag(zcu) != .@"fn") return .{ .conflict = .{
                    .peer_idx_a = strat_reason,
                    .peer_idx_b = i,
                } };
                // ty -> cur_ty
                if (.ok == try sema.coerceInMemoryAllowedFns(block, cur_ty, ty, false, target, src, src)) {
                    continue;
                }
                // cur_ty -> ty
                if (.ok == try sema.coerceInMemoryAllowedFns(block, ty, cur_ty, false, target, src, src)) {
                    opt_cur_ty = ty;
                    continue;
                }
                return .{ .conflict = .{
                    .peer_idx_a = first_idx,
                    .peer_idx_b = i,
                } };
            }
            return .{ .success = opt_cur_ty.? };
        },

        .enum_or_union => {
            var opt_cur_ty: ?Type = null;
            // The peer index which gave the current type
            var cur_ty_idx: usize = undefined;

            for (peer_tys, 0..) |opt_ty, i| {
                const ty = opt_ty orelse continue;
                switch (ty.zigTypeTag(zcu)) {
                    .enum_literal, .@"enum", .@"union" => {},
                    else => return .{ .conflict = .{
                        .peer_idx_a = strat_reason,
                        .peer_idx_b = i,
                    } },
                }
                const cur_ty = opt_cur_ty orelse {
                    opt_cur_ty = ty;
                    cur_ty_idx = i;
                    continue;
                };

                // We want to return this in a lot of cases, so alias it here for convenience
                const generic_err: PeerResolveResult = .{ .conflict = .{
                    .peer_idx_a = cur_ty_idx,
                    .peer_idx_b = i,
                } };

                switch (cur_ty.zigTypeTag(zcu)) {
                    .enum_literal => {
                        opt_cur_ty = ty;
                        cur_ty_idx = i;
                    },
                    .@"enum" => switch (ty.zigTypeTag(zcu)) {
                        .enum_literal => {},
                        .@"enum" => {
                            if (!ty.eql(cur_ty, zcu)) return generic_err;
                        },
                        .@"union" => {
                            const tag_ty = ty.unionTagTypeHypothetical(zcu);
                            if (!tag_ty.eql(cur_ty, zcu)) return generic_err;
                            opt_cur_ty = ty;
                            cur_ty_idx = i;
                        },
                        else => unreachable,
                    },
                    .@"union" => switch (ty.zigTypeTag(zcu)) {
                        .enum_literal => {},
                        .@"enum" => {
                            const cur_tag_ty = cur_ty.unionTagTypeHypothetical(zcu);
                            if (!ty.eql(cur_tag_ty, zcu)) return generic_err;
                        },
                        .@"union" => {
                            if (!ty.eql(cur_ty, zcu)) return generic_err;
                        },
                        else => unreachable,
                    },
                    else => unreachable,
                }
            }
            return .{ .success = opt_cur_ty.? };
        },

        .comptime_int => {
            for (peer_tys, 0..) |opt_ty, i| {
                const ty = opt_ty orelse continue;
                switch (ty.zigTypeTag(zcu)) {
                    .comptime_int => {},
                    else => return .{ .conflict = .{
                        .peer_idx_a = strat_reason,
                        .peer_idx_b = i,
                    } },
                }
            }
            return .{ .success = .comptime_int };
        },

        .comptime_float => {
            for (peer_tys, 0..) |opt_ty, i| {
                const ty = opt_ty orelse continue;
                switch (ty.zigTypeTag(zcu)) {
                    .comptime_int, .comptime_float => {},
                    else => return .{ .conflict = .{
                        .peer_idx_a = strat_reason,
                        .peer_idx_b = i,
                    } },
                }
            }
            return .{ .success = .comptime_float };
        },

        .fixed_int => {
            var idx_unsigned: ?usize = null;
            var idx_signed: ?usize = null;

            // TODO: this is for compatibility with legacy behavior. See beneath the loop.
            var any_comptime_known = false;

            for (peer_tys, peer_vals, 0..) |opt_ty, *ptr_opt_val, i| {
                const ty = opt_ty orelse continue;
                const opt_val = ptr_opt_val.*;

                const peer_tag = ty.zigTypeTag(zcu);
                switch (peer_tag) {
                    .comptime_int => {
                        // If the value is undefined, we can't refine to a fixed-width int
                        if (opt_val == null or opt_val.?.isUndef(zcu)) return .{ .conflict = .{
                            .peer_idx_a = strat_reason,
                            .peer_idx_b = i,
                        } };
                        any_comptime_known = true;
                        ptr_opt_val.* = try sema.resolveLazyValue(opt_val.?);
                        continue;
                    },
                    .int => {},
                    else => return .{ .conflict = .{
                        .peer_idx_a = strat_reason,
                        .peer_idx_b = i,
                    } },
                }

                if (opt_val != null) any_comptime_known = true;

                const info = ty.intInfo(zcu);

                const idx_ptr = switch (info.signedness) {
                    .unsigned => &idx_unsigned,
                    .signed => &idx_signed,
                };

                const largest_idx = idx_ptr.* orelse {
                    idx_ptr.* = i;
                    continue;
                };

                const cur_info = peer_tys[largest_idx].?.intInfo(zcu);
                if (info.bits > cur_info.bits) {
                    idx_ptr.* = i;
                }
            }

            if (idx_signed == null) {
                return .{ .success = peer_tys[idx_unsigned.?].? };
            }

            if (idx_unsigned == null) {
                return .{ .success = peer_tys[idx_signed.?].? };
            }

            const unsigned_info = peer_tys[idx_unsigned.?].?.intInfo(zcu);
            const signed_info = peer_tys[idx_signed.?].?.intInfo(zcu);
            if (signed_info.bits > unsigned_info.bits) {
                return .{ .success = peer_tys[idx_signed.?].? };
            }

            // TODO: this is for compatibility with legacy behavior. Before this version of PTR was
            // implemented, the algorithm very often returned false positives, with the expectation
            // that you'd just hit a coercion error later. One of these was that for integers, the
            // largest type would always be returned, even if it couldn't fit everything. This had
            // an unintentional consequence to semantics, which is that if values were known at
            // comptime, they would be coerced down to the smallest type where possible. This
            // behavior is unintuitive and order-dependent, so in my opinion should be eliminated,
            // but for now we'll retain compatibility.
            if (any_comptime_known) {
                if (unsigned_info.bits > signed_info.bits) {
                    return .{ .success = peer_tys[idx_unsigned.?].? };
                }
                const idx = @min(idx_unsigned.?, idx_signed.?);
                return .{ .success = peer_tys[idx].? };
            }

            return .{ .conflict = .{
                .peer_idx_a = idx_unsigned.?,
                .peer_idx_b = idx_signed.?,
            } };
        },

        .fixed_float => {
            var opt_cur_ty: ?Type = null;

            for (peer_tys, peer_vals, 0..) |opt_ty, opt_val, i| {
                const ty = opt_ty orelse continue;
                switch (ty.zigTypeTag(zcu)) {
                    .comptime_float, .comptime_int => {},
                    .int => {
                        if (opt_val == null) return .{ .conflict = .{
                            .peer_idx_a = strat_reason,
                            .peer_idx_b = i,
                        } };
                    },
                    .float => {
                        if (opt_cur_ty) |cur_ty| {
                            if (cur_ty.eql(ty, zcu)) continue;
                            // Recreate the type so we eliminate any c_longdouble
                            const bits = @max(cur_ty.floatBits(target), ty.floatBits(target));
                            opt_cur_ty = switch (bits) {
                                16 => .f16,
                                32 => .f32,
                                64 => .f64,
                                80 => .f80,
                                128 => .f128,
                                else => unreachable,
                            };
                        } else {
                            opt_cur_ty = ty;
                        }
                    },
                    else => return .{ .conflict = .{
                        .peer_idx_a = strat_reason,
                        .peer_idx_b = i,
                    } },
                }
            }

            // Note that fixed_float is only chosen if there is at least one fixed-width float peer,
            // so opt_cur_ty must be non-null.
            return .{ .success = opt_cur_ty.? };
        },

        .tuple => {
            // First, check that every peer has the same approximate structure (field count)

            var opt_first_idx: ?usize = null;
            var is_tuple: bool = undefined;
            var field_count: usize = undefined;

            for (peer_tys, 0..) |opt_ty, i| {
                const ty = opt_ty orelse continue;

                if (!ty.isTuple(zcu)) {
                    return .{ .conflict = .{
                        .peer_idx_a = strat_reason,
                        .peer_idx_b = i,
                    } };
                }

                const first_idx = opt_first_idx orelse {
                    opt_first_idx = i;
                    is_tuple = ty.isTuple(zcu);
                    field_count = ty.structFieldCount(zcu);
                    continue;
                };

                if (ty.structFieldCount(zcu) != field_count) {
                    return .{ .conflict = .{
                        .peer_idx_a = first_idx,
                        .peer_idx_b = i,
                    } };
                }
            }

            assert(opt_first_idx != null);

            // Now, we'll recursively resolve the field types
            const field_types = try sema.arena.alloc(InternPool.Index, field_count);
            // Values for `comptime` fields - `.none` used for non-comptime fields
            const field_vals = try sema.arena.alloc(InternPool.Index, field_count);
            const sub_peer_tys = try sema.arena.alloc(?Type, peer_tys.len);
            const sub_peer_vals = try sema.arena.alloc(?Value, peer_vals.len);

            for (field_types, field_vals, 0..) |*field_ty, *field_val, field_index| {
                // Fill buffers with types and values of the field
                for (peer_tys, peer_vals, sub_peer_tys, sub_peer_vals) |opt_ty, opt_val, *peer_field_ty, *peer_field_val| {
                    const ty = opt_ty orelse {
                        peer_field_ty.* = null;
                        peer_field_val.* = null;
                        continue;
                    };
                    peer_field_ty.* = ty.fieldType(field_index, zcu);
                    peer_field_val.* = if (opt_val) |val| try val.fieldValue(pt, field_index) else null;
                }

                // Resolve field type recursively
                field_ty.* = switch (try sema.resolvePeerTypesInner(block, src, sub_peer_tys, sub_peer_vals)) {
                    .success => |ty| ty.toIntern(),
                    else => |result| {
                        const result_buf = try sema.arena.create(PeerResolveResult);
                        result_buf.* = result;
                        const field_name = try ip.getOrPutStringFmt(sema.gpa, pt.tid, "{d}", .{field_index}, .no_embedded_nulls);

                        // The error info needs the field types, but we can't reuse sub_peer_tys
                        // since the recursive call may have clobbered it.
                        const peer_field_tys = try sema.arena.alloc(Type, peer_tys.len);
                        for (peer_tys, peer_field_tys) |opt_ty, *peer_field_ty| {
                            // Already-resolved types won't be referenced by the error so it's fine
                            // to leave them undefined.
                            const ty = opt_ty orelse continue;
                            peer_field_ty.* = ty.fieldType(field_index, zcu);
                        }

                        return .{ .field_error = .{
                            .field_name = field_name,
                            .field_types = peer_field_tys,
                            .sub_result = result_buf,
                        } };
                    },
                };

                // Decide if this is a comptime field. If it is comptime in all peers, and the
                // coerced comptime values are all the same, we say it is comptime, else not.

                var comptime_val: ?Value = null;
                for (peer_tys) |opt_ty| {
                    const struct_ty = opt_ty orelse continue;
                    try struct_ty.resolveStructFieldInits(pt);

                    const uncoerced_field_val = try struct_ty.structFieldValueComptime(pt, field_index) orelse {
                        comptime_val = null;
                        break;
                    };
                    const uncoerced_field = Air.internedToRef(uncoerced_field_val.toIntern());
                    const coerced_inst = sema.coerceExtra(block, .fromInterned(field_ty.*), uncoerced_field, src, .{ .report_err = false }) catch |err| switch (err) {
                        // It's possible for PTR to give false positives. Just give up on making this a comptime field, we'll get an error later anyway
                        error.NotCoercible => {
                            comptime_val = null;
                            break;
                        },
                        else => |e| return e,
                    };
                    const coerced_val = (try sema.resolveValue(coerced_inst)) orelse continue;
                    const existing = comptime_val orelse {
                        comptime_val = coerced_val;
                        continue;
                    };
                    if (!coerced_val.eql(existing, .fromInterned(field_ty.*), zcu)) {
                        comptime_val = null;
                        break;
                    }
                }

                field_val.* = if (comptime_val) |v| v.toIntern() else .none;
            }

            const final_ty = try ip.getTupleType(zcu.gpa, pt.tid, .{
                .types = field_types,
                .values = field_vals,
            });

            return .{ .success = .fromInterned(final_ty) };
        },

        .exact => {
            var expect_ty: ?Type = null;
            var first_idx: usize = undefined;
            for (peer_tys, 0..) |opt_ty, i| {
                const ty = opt_ty orelse continue;
                if (expect_ty) |expect| {
                    if (!ty.eql(expect, zcu)) return .{ .conflict = .{
                        .peer_idx_a = first_idx,
                        .peer_idx_b = i,
                    } };
                } else {
                    expect_ty = ty;
                    first_idx = i;
                }
            }
            return .{ .success = expect_ty.? };
        },
    }
}

fn maybeMergeErrorSets(sema: *Sema, block: *Block, src: LazySrcLoc, e0: Type, e1: Type) !Type {
    // e0 -> e1
    if (.ok == try sema.coerceInMemoryAllowedErrorSets(block, e1, e0, src, src)) {
        return e1;
    }

    // e1 -> e0
    if (.ok == try sema.coerceInMemoryAllowedErrorSets(block, e0, e1, src, src)) {
        return e0;
    }

    return sema.errorSetMerge(e0, e1);
}

fn resolvePairInMemoryCoercible(sema: *Sema, block: *Block, src: LazySrcLoc, ty_a: Type, ty_b: Type) !?Type {
    const target = sema.pt.zcu.getTarget();

    // ty_b -> ty_a
    if (.ok == try sema.coerceInMemoryAllowed(block, ty_a, ty_b, false, target, src, src, null)) {
        return ty_a;
    }

    // ty_a -> ty_b
    if (.ok == try sema.coerceInMemoryAllowed(block, ty_b, ty_a, false, target, src, src, null)) {
        return ty_b;
    }

    return null;
}

const ArrayLike = struct {
    len: u64,
    /// `noreturn` indicates that this type is `struct{}` so can coerce to anything
    elem_ty: Type,
};
fn typeIsArrayLike(sema: *Sema, ty: Type) ?ArrayLike {
    const pt = sema.pt;
    const zcu = pt.zcu;
    return switch (ty.zigTypeTag(zcu)) {
        .array => .{
            .len = ty.arrayLen(zcu),
            .elem_ty = ty.childType(zcu),
        },
        .@"struct" => {
            const field_count = ty.structFieldCount(zcu);
            if (field_count == 0) return .{
                .len = 0,
                .elem_ty = .noreturn,
            };
            if (!ty.isTuple(zcu)) return null;
            const elem_ty = ty.fieldType(0, zcu);
            for (1..field_count) |i| {
                if (!ty.fieldType(i, zcu).eql(elem_ty, zcu)) {
                    return null;
                }
            }
            return .{
                .len = field_count,
                .elem_ty = elem_ty,
            };
        },
        else => null,
    };
}

pub fn resolveIes(sema: *Sema, block: *Block, src: LazySrcLoc) CompileError!void {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const ip = &zcu.intern_pool;

    if (sema.fn_ret_ty_ies) |ies| {
        try sema.resolveInferredErrorSetPtr(block, src, ies);
        assert(ies.resolved != .none);
        ip.funcIesResolved(sema.func_index).* = ies.resolved;
    }
}

pub fn resolveFnTypes(sema: *Sema, fn_ty: Type, src: LazySrcLoc) CompileError!void {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const ip = &zcu.intern_pool;
    const fn_ty_info = zcu.typeToFunc(fn_ty).?;

    try Type.fromInterned(fn_ty_info.return_type).resolveFully(pt);

    if (zcu.comp.config.any_error_tracing and
        Type.fromInterned(fn_ty_info.return_type).isError(zcu))
    {
        // Ensure the type exists so that backends can assume that.
        _ = try sema.getBuiltinType(src, .StackTrace);
    }

    for (0..fn_ty_info.param_types.len) |i| {
        try Type.fromInterned(fn_ty_info.param_types.get(ip)[i]).resolveFully(pt);
    }
}

fn resolveLazyValue(sema: *Sema, val: Value) CompileError!Value {
    return val.resolveLazy(sema.arena, sema.pt);
}

/// Resolve a struct's alignment only without triggering resolution of its layout.
/// Asserts that the alignment is not yet resolved and the layout is non-packed.
pub fn resolveStructAlignment(
    sema: *Sema,
    ty: InternPool.Index,
    struct_type: InternPool.LoadedStructType,
) SemaError!void {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const ip = &zcu.intern_pool;
    const target = zcu.getTarget();

    assert(sema.owner.unwrap().type == ty);

    assert(struct_type.layout != .@"packed");
    assert(struct_type.flagsUnordered(ip).alignment == .none);

    const ptr_align = Alignment.fromByteUnits(@divExact(target.ptrBitWidth(), 8));

    // We'll guess "pointer-aligned", if the struct has an
    // underaligned pointer field then some allocations
    // might require explicit alignment.
    if (struct_type.assumePointerAlignedIfFieldTypesWip(ip, ptr_align)) return;

    try sema.resolveStructFieldTypes(ty, struct_type);

    // We'll guess "pointer-aligned", if the struct has an
    // underaligned pointer field then some allocations
    // might require explicit alignment.
    if (struct_type.assumePointerAlignedIfWip(ip, ptr_align)) return;
    defer struct_type.clearAlignmentWip(ip);

    var alignment: Alignment = .@"1";

    for (0..struct_type.field_types.len) |i| {
        const field_ty: Type = .fromInterned(struct_type.field_types.get(ip)[i]);
        if (struct_type.fieldIsComptime(ip, i) or try field_ty.comptimeOnlySema(pt))
            continue;
        const field_align = try field_ty.structFieldAlignmentSema(
            struct_type.fieldAlign(ip, i),
            struct_type.layout,
            pt,
        );
        alignment = alignment.maxStrict(field_align);
    }

    struct_type.setAlignment(ip, alignment);
}

pub fn resolveStructLayout(sema: *Sema, ty: Type) SemaError!void {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const ip = &zcu.intern_pool;
    const struct_type = zcu.typeToStruct(ty) orelse return;

    assert(sema.owner.unwrap().type == ty.toIntern());

    if (struct_type.haveLayout(ip))
        return;

    try sema.resolveStructFieldTypes(ty.toIntern(), struct_type);

    if (struct_type.layout == .@"packed") {
        sema.backingIntType(struct_type) catch |err| switch (err) {
            error.OutOfMemory, error.AnalysisFail => |e| return e,
            error.ComptimeBreak, error.ComptimeReturn => unreachable,
        };
        return;
    }

    if (struct_type.setLayoutWip(ip)) {
        const msg = try sema.errMsg(
            ty.srcLoc(zcu),
            "struct '{}' depends on itself",
            .{ty.fmt(pt)},
        );
        return sema.failWithOwnedErrorMsg(null, msg);
    }
    defer struct_type.clearLayoutWip(ip);

    const aligns = try sema.arena.alloc(Alignment, struct_type.field_types.len);
    const sizes = try sema.arena.alloc(u64, struct_type.field_types.len);

    var big_align: Alignment = .@"1";

    for (aligns, sizes, 0..) |*field_align, *field_size, i| {
        const field_ty: Type = .fromInterned(struct_type.field_types.get(ip)[i]);
        if (struct_type.fieldIsComptime(ip, i) or try field_ty.comptimeOnlySema(pt)) {
            struct_type.offsets.get(ip)[i] = 0;
            field_size.* = 0;
            field_align.* = .none;
            continue;
        }

        field_size.* = field_ty.abiSizeSema(pt) catch |err| switch (err) {
            error.AnalysisFail => {
                const msg = sema.err orelse return err;
                try sema.addFieldErrNote(ty, i, msg, "while checking this field", .{});
                return err;
            },
            else => return err,
        };
        field_align.* = try field_ty.structFieldAlignmentSema(
            struct_type.fieldAlign(ip, i),
            struct_type.layout,
            pt,
        );
        big_align = big_align.maxStrict(field_align.*);
    }

    if (struct_type.flagsUnordered(ip).assumed_runtime_bits and !(try ty.hasRuntimeBitsSema(pt))) {
        const msg = try sema.errMsg(
            ty.srcLoc(zcu),
            "struct layout depends on it having runtime bits",
            .{},
        );
        return sema.failWithOwnedErrorMsg(null, msg);
    }

    if (struct_type.flagsUnordered(ip).assumed_pointer_aligned and
        big_align.compareStrict(.neq, Alignment.fromByteUnits(@divExact(zcu.getTarget().ptrBitWidth(), 8))))
    {
        const msg = try sema.errMsg(
            ty.srcLoc(zcu),
            "struct layout depends on being pointer aligned",
            .{},
        );
        return sema.failWithOwnedErrorMsg(null, msg);
    }

    if (struct_type.hasReorderedFields()) {
        const runtime_order = struct_type.runtime_order.get(ip);

        for (runtime_order, 0..) |*ro, i| {
            const field_ty: Type = .fromInterned(struct_type.field_types.get(ip)[i]);
            if (struct_type.fieldIsComptime(ip, i) or try field_ty.comptimeOnlySema(pt)) {
                ro.* = .omitted;
            } else {
                ro.* = @enumFromInt(i);
            }
        }

        const RuntimeOrder = InternPool.LoadedStructType.RuntimeOrder;

        const AlignSortContext = struct {
            aligns: []const Alignment,

            fn lessThan(ctx: @This(), a: RuntimeOrder, b: RuntimeOrder) bool {
                if (a == .omitted) return false;
                if (b == .omitted) return true;
                const a_align = ctx.aligns[@intFromEnum(a)];
                const b_align = ctx.aligns[@intFromEnum(b)];
                return a_align.compare(.gt, b_align);
            }
        };
        if (!zcu.backendSupportsFeature(.field_reordering)) {
            // TODO: we should probably also reorder tuple fields? This is a bit weird because it'll involve
            // mutating the `InternPool` for a non-container type.
            //
            // TODO: implement field reordering support in all the backends!
            //
            // This logic does not reorder fields; it only moves the omitted ones to the end
            // so that logic elsewhere does not need to special-case here.
            var i: usize = 0;
            var off: usize = 0;
            while (i + off < runtime_order.len) {
                if (runtime_order[i + off] == .omitted) {
                    off += 1;
                    continue;
                }
                runtime_order[i] = runtime_order[i + off];
                i += 1;
            }
            @memset(runtime_order[i..], .omitted);
        } else {
            mem.sortUnstable(RuntimeOrder, runtime_order, AlignSortContext{
                .aligns = aligns,
            }, AlignSortContext.lessThan);
        }
    }

    // Calculate size, alignment, and field offsets.
    const offsets = struct_type.offsets.get(ip);
    var it = struct_type.iterateRuntimeOrder(ip);
    var offset: u64 = 0;
    while (it.next()) |i| {
        offsets[i] = @intCast(aligns[i].forward(offset));
        offset = offsets[i] + sizes[i];
    }
    const size = std.math.cast(u32, big_align.forward(offset)) orelse {
        const msg = try sema.errMsg(
            ty.srcLoc(zcu),
            "struct layout requires size {d}, this compiler implementation supports up to {d}",
            .{ big_align.forward(offset), std.math.maxInt(u32) },
        );
        return sema.failWithOwnedErrorMsg(null, msg);
    };
    struct_type.setLayoutResolved(ip, size, big_align);
    _ = try ty.comptimeOnlySema(pt);
}

fn backingIntType(
    sema: *Sema,
    struct_type: InternPool.LoadedStructType,
) CompileError!void {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const gpa = zcu.gpa;
    const ip = &zcu.intern_pool;

    var analysis_arena = std.heap.ArenaAllocator.init(gpa);
    defer analysis_arena.deinit();

    var block: Block = .{
        .parent = null,
        .sema = sema,
        .namespace = struct_type.namespace,
        .instructions = .{},
        .inlining = null,
        .comptime_reason = null, // set below if needed
        .src_base_inst = struct_type.zir_index,
        .type_name_ctx = struct_type.name,
    };
    defer assert(block.instructions.items.len == 0);

    const fields_bit_sum = blk: {
        var accumulator: u64 = 0;
        for (0..struct_type.field_types.len) |i| {
            const field_ty: Type = .fromInterned(struct_type.field_types.get(ip)[i]);
            accumulator += try field_ty.bitSizeSema(pt);
        }
        break :blk accumulator;
    };

    const zir = zcu.namespacePtr(struct_type.namespace).fileScope(zcu).zir.?;
    const zir_index = struct_type.zir_index.resolve(ip) orelse return error.AnalysisFail;
    const extended = zir.instructions.items(.data)[@intFromEnum(zir_index)].extended;
    assert(extended.opcode == .struct_decl);
    const small: Zir.Inst.StructDecl.Small = @bitCast(extended.small);

    if (small.has_backing_int) {
        var extra_index: usize = extended.operand + @typeInfo(Zir.Inst.StructDecl).@"struct".fields.len;
        const captures_len = if (small.has_captures_len) blk: {
            const captures_len = zir.extra[extra_index];
            extra_index += 1;
            break :blk captures_len;
        } else 0;
        extra_index += @intFromBool(small.has_fields_len);
        extra_index += @intFromBool(small.has_decls_len);

        extra_index += captures_len * 2;

        const backing_int_body_len = zir.extra[extra_index];
        extra_index += 1;

        const backing_int_src: LazySrcLoc = .{
            .base_node_inst = struct_type.zir_index,
            .offset = .{ .node_offset_container_tag = .zero },
        };
        block.comptime_reason = .{ .reason = .{
            .src = backing_int_src,
            .r = .{ .simple = .type },
        } };
        const backing_int_ty = blk: {
            if (backing_int_body_len == 0) {
                const backing_int_ref: Zir.Inst.Ref = @enumFromInt(zir.extra[extra_index]);
                break :blk try sema.resolveType(&block, backing_int_src, backing_int_ref);
            } else {
                const body = zir.bodySlice(extra_index, backing_int_body_len);
                const ty_ref = try sema.resolveInlineBody(&block, body, zir_index);
                break :blk try sema.analyzeAsType(&block, backing_int_src, ty_ref);
            }
        };

        try sema.checkBackingIntType(&block, backing_int_src, backing_int_ty, fields_bit_sum);
        struct_type.setBackingIntType(ip, backing_int_ty.toIntern());
    } else {
        if (fields_bit_sum > std.math.maxInt(u16)) {
            return sema.fail(&block, block.nodeOffset(.zero), "size of packed struct '{d}' exceeds maximum bit width of 65535", .{fields_bit_sum});
        }
        const backing_int_ty = try pt.intType(.unsigned, @intCast(fields_bit_sum));
        struct_type.setBackingIntType(ip, backing_int_ty.toIntern());
    }

    try sema.flushExports();
}

fn checkBackingIntType(sema: *Sema, block: *Block, src: LazySrcLoc, backing_int_ty: Type, fields_bit_sum: u64) CompileError!void {
    const pt = sema.pt;
    const zcu = pt.zcu;

    if (!backing_int_ty.isInt(zcu)) {
        return sema.fail(block, src, "expected backing integer type, found '{}'", .{backing_int_ty.fmt(pt)});
    }
    if (backing_int_ty.bitSize(zcu) != fields_bit_sum) {
        return sema.fail(
            block,
            src,
            "backing integer type '{}' has bit size {} but the struct fields have a total bit size of {}",
            .{ backing_int_ty.fmt(pt), backing_int_ty.bitSize(zcu), fields_bit_sum },
        );
    }
}

fn checkIndexable(sema: *Sema, block: *Block, src: LazySrcLoc, ty: Type) !void {
    const pt = sema.pt;
    if (!ty.isIndexable(pt.zcu)) {
        const msg = msg: {
            const msg = try sema.errMsg(src, "type '{}' does not support indexing", .{ty.fmt(pt)});
            errdefer msg.destroy(sema.gpa);
            try sema.errNote(src, msg, "operand must be an array, slice, tuple, or vector", .{});
            break :msg msg;
        };
        return sema.failWithOwnedErrorMsg(block, msg);
    }
}

fn checkMemOperand(sema: *Sema, block: *Block, src: LazySrcLoc, ty: Type) !void {
    const pt = sema.pt;
    const zcu = pt.zcu;
    if (ty.zigTypeTag(zcu) == .pointer) {
        switch (ty.ptrSize(zcu)) {
            .slice, .many, .c => return,
            .one => {
                const elem_ty = ty.childType(zcu);
                if (elem_ty.zigTypeTag(zcu) == .array) return;
                // TODO https://github.com/ziglang/zig/issues/15479
                // if (elem_ty.isTuple()) return;
            },
        }
    }
    const msg = msg: {
        const msg = try sema.errMsg(src, "type '{}' is not an indexable pointer", .{ty.fmt(pt)});
        errdefer msg.destroy(sema.gpa);
        try sema.errNote(src, msg, "operand must be a slice, a many pointer or a pointer to an array", .{});
        break :msg msg;
    };
    return sema.failWithOwnedErrorMsg(block, msg);
}

/// Resolve a unions's alignment only without triggering resolution of its layout.
/// Asserts that the alignment is not yet resolved.
pub fn resolveUnionAlignment(
    sema: *Sema,
    ty: Type,
    union_type: InternPool.LoadedUnionType,
) SemaError!void {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const ip = &zcu.intern_pool;
    const target = zcu.getTarget();

    assert(sema.owner.unwrap().type == ty.toIntern());

    assert(!union_type.haveLayout(ip));

    const ptr_align = Alignment.fromByteUnits(@divExact(target.ptrBitWidth(), 8));

    // We'll guess "pointer-aligned", if the union has an
    // underaligned pointer field then some allocations
    // might require explicit alignment.
    if (union_type.assumePointerAlignedIfFieldTypesWip(ip, ptr_align)) return;

    try sema.resolveUnionFieldTypes(ty, union_type);

    var max_align: Alignment = .@"1";
    for (0..union_type.field_types.len) |field_index| {
        const field_ty: Type = .fromInterned(union_type.field_types.get(ip)[field_index]);
        if (!(try field_ty.hasRuntimeBitsSema(pt))) continue;

        const explicit_align = union_type.fieldAlign(ip, field_index);
        const field_align = if (explicit_align != .none)
            explicit_align
        else
            try field_ty.abiAlignmentSema(sema.pt);

        max_align = max_align.max(field_align);
    }

    union_type.setAlignment(ip, max_align);
}

/// This logic must be kept in sync with `Zcu.getUnionLayout`.
pub fn resolveUnionLayout(sema: *Sema, ty: Type) SemaError!void {
    const pt = sema.pt;
    const ip = &pt.zcu.intern_pool;

    try sema.resolveUnionFieldTypes(ty, ip.loadUnionType(ty.ip_index));

    // Load again, since the tag type might have changed due to resolution.
    const union_type = ip.loadUnionType(ty.ip_index);

    assert(sema.owner.unwrap().type == ty.toIntern());

    const old_flags = union_type.flagsUnordered(ip);
    switch (old_flags.status) {
        .none, .have_field_types => {},
        .field_types_wip, .layout_wip => {
            const msg = try sema.errMsg(
                ty.srcLoc(pt.zcu),
                "union '{}' depends on itself",
                .{ty.fmt(pt)},
            );
            return sema.failWithOwnedErrorMsg(null, msg);
        },
        .have_layout, .fully_resolved_wip, .fully_resolved => return,
    }

    errdefer union_type.setStatusIfLayoutWip(ip, old_flags.status);

    union_type.setStatus(ip, .layout_wip);

    var max_size: u64 = 0;
    var max_align: Alignment = .@"1";
    for (0..union_type.field_types.len) |field_index| {
        const field_ty: Type = .fromInterned(union_type.field_types.get(ip)[field_index]);

        if (try field_ty.comptimeOnlySema(pt) or field_ty.zigTypeTag(pt.zcu) == .noreturn) continue; // TODO: should this affect alignment?

        max_size = @max(max_size, field_ty.abiSizeSema(pt) catch |err| switch (err) {
            error.AnalysisFail => {
                const msg = sema.err orelse return err;
                try sema.addFieldErrNote(ty, field_index, msg, "while checking this field", .{});
                return err;
            },
            else => return err,
        });

        const explicit_align = union_type.fieldAlign(ip, field_index);
        const field_align = if (explicit_align != .none)
            explicit_align
        else
            try field_ty.abiAlignmentSema(pt);

        max_align = max_align.max(field_align);
    }

    const has_runtime_tag = union_type.flagsUnordered(ip).runtime_tag.hasTag() and
        try Type.fromInterned(union_type.enum_tag_ty).hasRuntimeBitsSema(pt);
    const size, const alignment, const padding = if (has_runtime_tag) layout: {
        const enum_tag_type: Type = .fromInterned(union_type.enum_tag_ty);
        const tag_align = try enum_tag_type.abiAlignmentSema(pt);
        const tag_size = try enum_tag_type.abiSizeSema(pt);

        // Put the tag before or after the payload depending on which one's
        // alignment is greater.
        var size: u64 = 0;
        var padding: u32 = 0;
        if (tag_align.order(max_align).compare(.gte)) {
            // {Tag, Payload}
            size += tag_size;
            size = max_align.forward(size);
            size += max_size;
            const prev_size = size;
            size = tag_align.forward(size);
            padding = @intCast(size - prev_size);
        } else {
            // {Payload, Tag}
            size += max_size;
            size = switch (pt.zcu.getTarget().ofmt) {
                .c => max_align,
                else => tag_align,
            }.forward(size);
            size += tag_size;
            const prev_size = size;
            size = max_align.forward(size);
            padding = @intCast(size - prev_size);
        }

        break :layout .{ size, max_align.max(tag_align), padding };
    } else .{ max_align.forward(max_size), max_align, 0 };

    const casted_size = std.math.cast(u32, size) orelse {
        const msg = try sema.errMsg(
            ty.srcLoc(pt.zcu),
            "union layout requires size {d}, this compiler implementation supports up to {d}",
            .{ size, std.math.maxInt(u32) },
        );
        return sema.failWithOwnedErrorMsg(null, msg);
    };
    union_type.setHaveLayout(ip, casted_size, padding, alignment);

    if (union_type.flagsUnordered(ip).assumed_runtime_bits and !(try ty.hasRuntimeBitsSema(pt))) {
        const msg = try sema.errMsg(
            ty.srcLoc(pt.zcu),
            "union layout depends on it having runtime bits",
            .{},
        );
        return sema.failWithOwnedErrorMsg(null, msg);
    }

    if (union_type.flagsUnordered(ip).assumed_pointer_aligned and
        alignment.compareStrict(.neq, Alignment.fromByteUnits(@divExact(pt.zcu.getTarget().ptrBitWidth(), 8))))
    {
        const msg = try sema.errMsg(
            ty.srcLoc(pt.zcu),
            "union layout depends on being pointer aligned",
            .{},
        );
        return sema.failWithOwnedErrorMsg(null, msg);
    }
    _ = try ty.comptimeOnlySema(pt);
}

/// Returns `error.AnalysisFail` if any of the types (recursively) failed to
/// be resolved.
pub fn resolveStructFully(sema: *Sema, ty: Type) SemaError!void {
    try sema.resolveStructLayout(ty);
    try sema.resolveStructFieldInits(ty);

    const pt = sema.pt;
    const zcu = pt.zcu;
    const ip = &zcu.intern_pool;
    const struct_type = zcu.typeToStruct(ty).?;

    assert(sema.owner.unwrap().type == ty.toIntern());

    if (struct_type.setFullyResolved(ip)) return;
    errdefer struct_type.clearFullyResolved(ip);

    // After we have resolve struct layout we have to go over the fields again to
    // make sure pointer fields get their child types resolved as well.
    // See also similar code for unions.

    for (0..struct_type.field_types.len) |i| {
        const field_ty: Type = .fromInterned(struct_type.field_types.get(ip)[i]);
        try field_ty.resolveFully(pt);
    }
}

pub fn resolveUnionFully(sema: *Sema, ty: Type) SemaError!void {
    try sema.resolveUnionLayout(ty);

    const pt = sema.pt;
    const zcu = pt.zcu;
    const ip = &zcu.intern_pool;
    const union_obj = zcu.typeToUnion(ty).?;

    assert(sema.owner.unwrap().type == ty.toIntern());

    switch (union_obj.flagsUnordered(ip).status) {
        .none, .have_field_types, .field_types_wip, .layout_wip, .have_layout => {},
        .fully_resolved_wip, .fully_resolved => return,
    }

    {
        // After we have resolve union layout we have to go over the fields again to
        // make sure pointer fields get their child types resolved as well.
        // See also similar code for structs.
        const prev_status = union_obj.flagsUnordered(ip).status;
        errdefer union_obj.setStatus(ip, prev_status);

        union_obj.setStatus(ip, .fully_resolved_wip);
        for (0..union_obj.field_types.len) |field_index| {
            const field_ty: Type = .fromInterned(union_obj.field_types.get(ip)[field_index]);
            try field_ty.resolveFully(pt);
        }
        union_obj.setStatus(ip, .fully_resolved);
    }

    // And let's not forget comptime-only status.
    _ = try ty.comptimeOnlySema(pt);
}

pub fn resolveStructFieldTypes(
    sema: *Sema,
    ty: InternPool.Index,
    struct_type: InternPool.LoadedStructType,
) SemaError!void {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const ip = &zcu.intern_pool;

    assert(sema.owner.unwrap().type == ty);

    if (struct_type.haveFieldTypes(ip)) return;

    if (struct_type.setFieldTypesWip(ip)) {
        const msg = try sema.errMsg(
            Type.fromInterned(ty).srcLoc(zcu),
            "struct '{}' depends on itself",
            .{Type.fromInterned(ty).fmt(pt)},
        );
        return sema.failWithOwnedErrorMsg(null, msg);
    }
    defer struct_type.clearFieldTypesWip(ip);

    sema.structFields(struct_type) catch |err| switch (err) {
        error.AnalysisFail, error.OutOfMemory => |e| return e,
        error.ComptimeBreak, error.ComptimeReturn => unreachable,
    };
}

pub fn resolveStructFieldInits(sema: *Sema, ty: Type) SemaError!void {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const ip = &zcu.intern_pool;
    const struct_type = zcu.typeToStruct(ty) orelse return;

    assert(sema.owner.unwrap().type == ty.toIntern());

    // Inits can start as resolved
    if (struct_type.haveFieldInits(ip)) return;

    try sema.resolveStructLayout(ty);

    if (struct_type.setInitsWip(ip)) {
        const msg = try sema.errMsg(
            ty.srcLoc(zcu),
            "struct '{}' depends on itself",
            .{ty.fmt(pt)},
        );
        return sema.failWithOwnedErrorMsg(null, msg);
    }
    defer struct_type.clearInitsWip(ip);

    sema.structFieldInits(struct_type) catch |err| switch (err) {
        error.AnalysisFail, error.OutOfMemory => |e| return e,
        error.ComptimeBreak, error.ComptimeReturn => unreachable,
    };
    struct_type.setHaveFieldInits(ip);
}

pub fn resolveUnionFieldTypes(sema: *Sema, ty: Type, union_type: InternPool.LoadedUnionType) SemaError!void {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const ip = &zcu.intern_pool;

    assert(sema.owner.unwrap().type == ty.toIntern());

    switch (union_type.flagsUnordered(ip).status) {
        .none => {},
        .field_types_wip => {
            const msg = try sema.errMsg(
                ty.srcLoc(zcu),
                "union '{}' depends on itself",
                .{ty.fmt(pt)},
            );
            return sema.failWithOwnedErrorMsg(null, msg);
        },
        .have_field_types,
        .have_layout,
        .layout_wip,
        .fully_resolved_wip,
        .fully_resolved,
        => return,
    }

    union_type.setStatus(ip, .field_types_wip);
    errdefer union_type.setStatus(ip, .none);
    sema.unionFields(ty.toIntern(), union_type) catch |err| switch (err) {
        error.AnalysisFail, error.OutOfMemory => |e| return e,
        error.ComptimeBreak, error.ComptimeReturn => unreachable,
    };
    union_type.setStatus(ip, .have_field_types);
}

/// Returns a normal error set corresponding to the fully populated inferred
/// error set.
fn resolveInferredErrorSet(
    sema: *Sema,
    block: *Block,
    src: LazySrcLoc,
    ies_index: InternPool.Index,
) CompileError!InternPool.Index {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const ip = &zcu.intern_pool;
    const func_index = ip.iesFuncIndex(ies_index);
    const func = zcu.funcInfo(func_index);

    try sema.declareDependency(.{ .interned = func_index }); // resolved IES

    try zcu.maybeUnresolveIes(func_index);
    const resolved_ty = func.resolvedErrorSetUnordered(ip);
    if (resolved_ty != .none) return resolved_ty;

    if (zcu.analysis_in_progress.contains(AnalUnit.wrap(.{ .func = func_index }))) {
        return sema.fail(block, src, "unable to resolve inferred error set", .{});
    }

    // In order to ensure that all dependencies are properly added to the set,
    // we need to ensure the function body is analyzed of the inferred error
    // set. However, in the case of comptime/inline function calls with
    // inferred error sets, each call gets an adhoc InferredErrorSet object, which
    // has no corresponding function body.
    const ies_func_info = zcu.typeToFunc(.fromInterned(func.ty)).?;
    // if ies declared by a inline function with generic return type, the return_type should be generic_poison,
    // because inline function does not create a new declaration, and the ies has been filled with analyzeCall,
    // so here we can simply skip this case.
    if (ies_func_info.return_type == .generic_poison_type) {
        assert(ies_func_info.cc == .@"inline");
    } else if (ip.errorUnionSet(ies_func_info.return_type) == ies_index) {
        if (ies_func_info.is_generic) {
            return sema.failWithOwnedErrorMsg(block, msg: {
                const msg = try sema.errMsg(src, "unable to resolve inferred error set of generic function", .{});
                errdefer msg.destroy(sema.gpa);
                try sema.errNote(zcu.navSrcLoc(func.owner_nav), msg, "generic function declared here", .{});
                break :msg msg;
            });
        }
        // In this case we are dealing with the actual InferredErrorSet object that
        // corresponds to the function, not one created to track an inline/comptime call.
        try sema.addReferenceEntry(block, src, AnalUnit.wrap(.{ .func = func_index }));
        try pt.ensureFuncBodyUpToDate(func_index);
    }

    // This will now have been resolved by the logic at the end of `Zcu.analyzeFnBody`
    // which calls `resolveInferredErrorSetPtr`.
    const final_resolved_ty = func.resolvedErrorSetUnordered(ip);
    assert(final_resolved_ty != .none);
    return final_resolved_ty;
}

pub fn resolveInferredErrorSetPtr(
    sema: *Sema,
    block: *Block,
    src: LazySrcLoc,
    ies: *InferredErrorSet,
) CompileError!void {
    const pt = sema.pt;
    const ip = &pt.zcu.intern_pool;

    if (ies.resolved != .none) return;

    const ies_index = ip.errorUnionSet(sema.fn_ret_ty.toIntern());

    for (ies.inferred_error_sets.keys()) |other_ies_index| {
        if (ies_index == other_ies_index) continue;
        switch (try sema.resolveInferredErrorSet(block, src, other_ies_index)) {
            .anyerror_type => {
                ies.resolved = .anyerror_type;
                return;
            },
            else => |error_set_ty_index| {
                const names = ip.indexToKey(error_set_ty_index).error_set_type.names;
                for (names.get(ip)) |name| {
                    try ies.errors.put(sema.arena, name, {});
                }
            },
        }
    }

    const resolved_error_set_ty = try pt.errorSetFromUnsortedNames(ies.errors.keys());
    ies.resolved = resolved_error_set_ty.toIntern();
}

fn resolveAdHocInferredErrorSet(
    sema: *Sema,
    block: *Block,
    src: LazySrcLoc,
    value: InternPool.Index,
) CompileError!InternPool.Index {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const gpa = sema.gpa;
    const ip = &zcu.intern_pool;
    const new_ty = try resolveAdHocInferredErrorSetTy(sema, block, src, ip.typeOf(value));
    if (new_ty == .none) return value;
    return ip.getCoerced(gpa, pt.tid, value, new_ty);
}

fn resolveAdHocInferredErrorSetTy(
    sema: *Sema,
    block: *Block,
    src: LazySrcLoc,
    ty: InternPool.Index,
) CompileError!InternPool.Index {
    const ies = sema.fn_ret_ty_ies orelse return .none;
    const pt = sema.pt;
    const zcu = pt.zcu;
    const ip = &zcu.intern_pool;
    const error_union_info = switch (ip.indexToKey(ty)) {
        .error_union_type => |x| x,
        else => return .none,
    };
    if (error_union_info.error_set_type != .adhoc_inferred_error_set_type)
        return .none;

    try sema.resolveInferredErrorSetPtr(block, src, ies);
    const new_ty = try pt.intern(.{ .error_union_type = .{
        .error_set_type = ies.resolved,
        .payload_type = error_union_info.payload_type,
    } });
    return new_ty;
}

fn resolveInferredErrorSetTy(
    sema: *Sema,
    block: *Block,
    src: LazySrcLoc,
    ty: InternPool.Index,
) CompileError!InternPool.Index {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const ip = &zcu.intern_pool;
    if (ty == .anyerror_type) return ty;
    switch (ip.indexToKey(ty)) {
        .error_set_type => return ty,
        .inferred_error_set_type => return sema.resolveInferredErrorSet(block, src, ty),
        else => unreachable,
    }
}

fn structZirInfo(zir: Zir, zir_index: Zir.Inst.Index) struct {
    /// fields_len
    usize,
    Zir.Inst.StructDecl.Small,
    /// extra_index
    usize,
} {
    const extended = zir.instructions.items(.data)[@intFromEnum(zir_index)].extended;
    assert(extended.opcode == .struct_decl);
    const small: Zir.Inst.StructDecl.Small = @bitCast(extended.small);
    var extra_index: usize = extended.operand + @typeInfo(Zir.Inst.StructDecl).@"struct".fields.len;

    const captures_len = if (small.has_captures_len) blk: {
        const captures_len = zir.extra[extra_index];
        extra_index += 1;
        break :blk captures_len;
    } else 0;

    const fields_len = if (small.has_fields_len) blk: {
        const fields_len = zir.extra[extra_index];
        extra_index += 1;
        break :blk fields_len;
    } else 0;

    const decls_len = if (small.has_decls_len) decls_len: {
        const decls_len = zir.extra[extra_index];
        extra_index += 1;
        break :decls_len decls_len;
    } else 0;

    extra_index += captures_len * 2;

    // The backing integer cannot be handled until `resolveStructLayout()`.
    if (small.has_backing_int) {
        const backing_int_body_len = zir.extra[extra_index];
        extra_index += 1; // backing_int_body_len
        if (backing_int_body_len == 0) {
            extra_index += 1; // backing_int_ref
        } else {
            extra_index += backing_int_body_len; // backing_int_body_inst
        }
    }

    // Skip over decls.
    extra_index += decls_len;

    return .{ fields_len, small, extra_index };
}

fn structFields(
    sema: *Sema,
    struct_type: InternPool.LoadedStructType,
) CompileError!void {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const gpa = zcu.gpa;
    const ip = &zcu.intern_pool;
    const namespace_index = struct_type.namespace;
    const zir = zcu.namespacePtr(namespace_index).fileScope(zcu).zir.?;
    const zir_index = struct_type.zir_index.resolve(ip) orelse return error.AnalysisFail;

    const fields_len, _, var extra_index = structZirInfo(zir, zir_index);

    if (fields_len == 0) switch (struct_type.layout) {
        .@"packed" => {
            try sema.backingIntType(struct_type);
            return;
        },
        .auto, .@"extern" => {
            struct_type.setLayoutResolved(ip, 0, .none);
            return;
        },
    };

    var block_scope: Block = .{
        .parent = null,
        .sema = sema,
        .namespace = namespace_index,
        .instructions = .{},
        .inlining = null,
        .comptime_reason = .{ .reason = .{
            .src = .{
                .base_node_inst = struct_type.zir_index,
                .offset = .nodeOffset(.zero),
            },
            .r = .{ .simple = .struct_fields },
        } },
        .src_base_inst = struct_type.zir_index,
        .type_name_ctx = struct_type.name,
    };
    defer assert(block_scope.instructions.items.len == 0);

    const Field = struct {
        type_body_len: u32 = 0,
        align_body_len: u32 = 0,
        init_body_len: u32 = 0,
        type_ref: Zir.Inst.Ref = .none,
    };
    const fields = try sema.arena.alloc(Field, fields_len);

    var any_inits = false;
    var any_aligned = false;

    {
        const bits_per_field = 4;
        const fields_per_u32 = 32 / bits_per_field;
        const bit_bags_count = std.math.divCeil(usize, fields_len, fields_per_u32) catch unreachable;
        const flags_index = extra_index;
        var bit_bag_index: usize = flags_index;
        extra_index += bit_bags_count;
        var cur_bit_bag: u32 = undefined;
        var field_i: u32 = 0;
        while (field_i < fields_len) : (field_i += 1) {
            if (field_i % fields_per_u32 == 0) {
                cur_bit_bag = zir.extra[bit_bag_index];
                bit_bag_index += 1;
            }
            const has_align = @as(u1, @truncate(cur_bit_bag)) != 0;
            cur_bit_bag >>= 1;
            const has_init = @as(u1, @truncate(cur_bit_bag)) != 0;
            cur_bit_bag >>= 1;
            const is_comptime = @as(u1, @truncate(cur_bit_bag)) != 0;
            cur_bit_bag >>= 1;
            const has_type_body = @as(u1, @truncate(cur_bit_bag)) != 0;
            cur_bit_bag >>= 1;

            if (is_comptime) struct_type.setFieldComptime(ip, field_i);

            const field_name_zir: [:0]const u8 = zir.nullTerminatedString(@enumFromInt(zir.extra[extra_index]));
            extra_index += 1; // field_name

            fields[field_i] = .{};

            if (has_type_body) {
                fields[field_i].type_body_len = zir.extra[extra_index];
            } else {
                fields[field_i].type_ref = @enumFromInt(zir.extra[extra_index]);
            }
            extra_index += 1;

            // This string needs to outlive the ZIR code.
            const field_name = try ip.getOrPutString(gpa, pt.tid, field_name_zir, .no_embedded_nulls);
            assert(struct_type.addFieldName(ip, field_name) == null);

            if (has_align) {
                fields[field_i].align_body_len = zir.extra[extra_index];
                extra_index += 1;
                any_aligned = true;
            }
            if (has_init) {
                fields[field_i].init_body_len = zir.extra[extra_index];
                extra_index += 1;
                any_inits = true;
            }
        }
    }

    // Next we do only types and alignments, saving the inits for a second pass,
    // so that init values may depend on type layout.

    for (fields, 0..) |zir_field, field_i| {
        const ty_src: LazySrcLoc = .{
            .base_node_inst = struct_type.zir_index,
            .offset = .{ .container_field_type = @intCast(field_i) },
        };
        const field_ty: Type = ty: {
            if (zir_field.type_ref != .none) {
                break :ty try sema.resolveType(&block_scope, ty_src, zir_field.type_ref);
            }
            assert(zir_field.type_body_len != 0);
            const body = zir.bodySlice(extra_index, zir_field.type_body_len);
            extra_index += body.len;
            const ty_ref = try sema.resolveInlineBody(&block_scope, body, zir_index);
            break :ty try sema.analyzeAsType(&block_scope, ty_src, ty_ref);
        };

        struct_type.field_types.get(ip)[field_i] = field_ty.toIntern();

        if (field_ty.zigTypeTag(zcu) == .@"opaque") {
            const msg = msg: {
                const msg = try sema.errMsg(ty_src, "opaque types have unknown size and therefore cannot be directly embedded in structs", .{});
                errdefer msg.destroy(sema.gpa);

                try sema.addDeclaredHereNote(msg, field_ty);
                break :msg msg;
            };
            return sema.failWithOwnedErrorMsg(&block_scope, msg);
        }
        if (field_ty.zigTypeTag(zcu) == .noreturn) {
            const msg = msg: {
                const msg = try sema.errMsg(ty_src, "struct fields cannot be 'noreturn'", .{});
                errdefer msg.destroy(sema.gpa);

                try sema.addDeclaredHereNote(msg, field_ty);
                break :msg msg;
            };
            return sema.failWithOwnedErrorMsg(&block_scope, msg);
        }
        switch (struct_type.layout) {
            .@"extern" => if (!try sema.validateExternType(field_ty, .struct_field)) {
                const msg = msg: {
                    const msg = try sema.errMsg(ty_src, "extern structs cannot contain fields of type '{}'", .{field_ty.fmt(pt)});
                    errdefer msg.destroy(sema.gpa);

                    try sema.explainWhyTypeIsNotExtern(msg, ty_src, field_ty, .struct_field);

                    try sema.addDeclaredHereNote(msg, field_ty);
                    break :msg msg;
                };
                return sema.failWithOwnedErrorMsg(&block_scope, msg);
            },
            .@"packed" => if (!try sema.validatePackedType(field_ty)) {
                const msg = msg: {
                    const msg = try sema.errMsg(ty_src, "packed structs cannot contain fields of type '{}'", .{field_ty.fmt(pt)});
                    errdefer msg.destroy(sema.gpa);

                    try sema.explainWhyTypeIsNotPacked(msg, ty_src, field_ty);

                    try sema.addDeclaredHereNote(msg, field_ty);
                    break :msg msg;
                };
                return sema.failWithOwnedErrorMsg(&block_scope, msg);
            },
            else => {},
        }

        if (zir_field.align_body_len > 0) {
            const body = zir.bodySlice(extra_index, zir_field.align_body_len);
            extra_index += body.len;
            const align_ref = try sema.resolveInlineBody(&block_scope, body, zir_index);
            const align_src: LazySrcLoc = .{
                .base_node_inst = struct_type.zir_index,
                .offset = .{ .container_field_align = @intCast(field_i) },
            };
            const field_align = try sema.analyzeAsAlign(&block_scope, align_src, align_ref);
            struct_type.field_aligns.get(ip)[field_i] = field_align;
        }

        extra_index += zir_field.init_body_len;
    }

    struct_type.clearFieldTypesWip(ip);
    if (!any_inits) struct_type.setHaveFieldInits(ip);

    try sema.flushExports();
}

// This logic must be kept in sync with `structFields`
fn structFieldInits(
    sema: *Sema,
    struct_type: InternPool.LoadedStructType,
) CompileError!void {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const ip = &zcu.intern_pool;

    assert(!struct_type.haveFieldInits(ip));

    const namespace_index = struct_type.namespace;
    const zir = zcu.namespacePtr(namespace_index).fileScope(zcu).zir.?;
    const zir_index = struct_type.zir_index.resolve(ip) orelse return error.AnalysisFail;
    const fields_len, _, var extra_index = structZirInfo(zir, zir_index);

    var block_scope: Block = .{
        .parent = null,
        .sema = sema,
        .namespace = namespace_index,
        .instructions = .{},
        .inlining = null,
        .comptime_reason = undefined, // set when `block_scope` is used
        .src_base_inst = struct_type.zir_index,
        .type_name_ctx = struct_type.name,
    };
    defer assert(block_scope.instructions.items.len == 0);

    const Field = struct {
        type_body_len: u32 = 0,
        align_body_len: u32 = 0,
        init_body_len: u32 = 0,
    };
    const fields = try sema.arena.alloc(Field, fields_len);

    var any_inits = false;

    {
        const bits_per_field = 4;
        const fields_per_u32 = 32 / bits_per_field;
        const bit_bags_count = std.math.divCeil(usize, fields_len, fields_per_u32) catch unreachable;
        const flags_index = extra_index;
        var bit_bag_index: usize = flags_index;
        extra_index += bit_bags_count;
        var cur_bit_bag: u32 = undefined;
        var field_i: u32 = 0;
        while (field_i < fields_len) : (field_i += 1) {
            if (field_i % fields_per_u32 == 0) {
                cur_bit_bag = zir.extra[bit_bag_index];
                bit_bag_index += 1;
            }
            const has_align = @as(u1, @truncate(cur_bit_bag)) != 0;
            cur_bit_bag >>= 1;
            const has_init = @as(u1, @truncate(cur_bit_bag)) != 0;
            cur_bit_bag >>= 2;
            const has_type_body = @as(u1, @truncate(cur_bit_bag)) != 0;
            cur_bit_bag >>= 1;

            extra_index += 1; // field_name

            fields[field_i] = .{};

            if (has_type_body) fields[field_i].type_body_len = zir.extra[extra_index];
            extra_index += 1;

            if (has_align) {
                fields[field_i].align_body_len = zir.extra[extra_index];
                extra_index += 1;
            }
            if (has_init) {
                fields[field_i].init_body_len = zir.extra[extra_index];
                extra_index += 1;
                any_inits = true;
            }
        }
    }

    if (any_inits) {
        for (fields, 0..) |zir_field, field_i| {
            extra_index += zir_field.type_body_len;
            extra_index += zir_field.align_body_len;
            const body = zir.bodySlice(extra_index, zir_field.init_body_len);
            extra_index += zir_field.init_body_len;

            if (body.len == 0) continue;

            // Pre-populate the type mapping the body expects to be there.
            // In init bodies, the zir index of the struct itself is used
            // to refer to the current field type.

            const field_ty: Type = .fromInterned(struct_type.field_types.get(ip)[field_i]);
            const type_ref = Air.internedToRef(field_ty.toIntern());
            try sema.inst_map.ensureSpaceForInstructions(sema.gpa, &.{zir_index});
            sema.inst_map.putAssumeCapacity(zir_index, type_ref);

            const init_src: LazySrcLoc = .{
                .base_node_inst = struct_type.zir_index,
                .offset = .{ .container_field_value = @intCast(field_i) },
            };

            block_scope.comptime_reason = .{ .reason = .{
                .src = init_src,
                .r = .{ .simple = .struct_field_default_value },
            } };
            const init = try sema.resolveInlineBody(&block_scope, body, zir_index);
            const coerced = try sema.coerce(&block_scope, field_ty, init, init_src);
            const default_val = try sema.resolveConstValue(&block_scope, init_src, coerced, null);

            if (default_val.canMutateComptimeVarState(zcu)) {
                const field_name = struct_type.fieldName(ip, field_i).unwrap().?;
                return sema.failWithContainsReferenceToComptimeVar(&block_scope, init_src, field_name, "field default value", default_val);
            }
            struct_type.field_inits.get(ip)[field_i] = default_val.toIntern();
        }
    }

    try sema.flushExports();
}

fn unionFields(
    sema: *Sema,
    union_ty: InternPool.Index,
    union_type: InternPool.LoadedUnionType,
) CompileError!void {
    const tracy = trace(@src());
    defer tracy.end();

    const pt = sema.pt;
    const zcu = pt.zcu;
    const gpa = zcu.gpa;
    const ip = &zcu.intern_pool;
    const zir = zcu.namespacePtr(union_type.namespace).fileScope(zcu).zir.?;
    const zir_index = union_type.zir_index.resolve(ip) orelse return error.AnalysisFail;
    const extended = zir.instructions.items(.data)[@intFromEnum(zir_index)].extended;
    assert(extended.opcode == .union_decl);
    const small: Zir.Inst.UnionDecl.Small = @bitCast(extended.small);
    const extra = zir.extraData(Zir.Inst.UnionDecl, extended.operand);
    var extra_index: usize = extra.end;

    const tag_type_ref: Zir.Inst.Ref = if (small.has_tag_type) blk: {
        const ty_ref: Zir.Inst.Ref = @enumFromInt(zir.extra[extra_index]);
        extra_index += 1;
        break :blk ty_ref;
    } else .none;

    const captures_len = if (small.has_captures_len) blk: {
        const captures_len = zir.extra[extra_index];
        extra_index += 1;
        break :blk captures_len;
    } else 0;

    const body_len = if (small.has_body_len) blk: {
        const body_len = zir.extra[extra_index];
        extra_index += 1;
        break :blk body_len;
    } else 0;

    const fields_len = if (small.has_fields_len) blk: {
        const fields_len = zir.extra[extra_index];
        extra_index += 1;
        break :blk fields_len;
    } else 0;

    const decls_len = if (small.has_decls_len) decls_len: {
        const decls_len = zir.extra[extra_index];
        extra_index += 1;
        break :decls_len decls_len;
    } else 0;

    // Skip over captures and decls.
    extra_index += captures_len * 2 + decls_len;

    const body = zir.bodySlice(extra_index, body_len);
    extra_index += body.len;

    const src: LazySrcLoc = .{
        .base_node_inst = union_type.zir_index,
        .offset = .nodeOffset(.zero),
    };

    var block_scope: Block = .{
        .parent = null,
        .sema = sema,
        .namespace = union_type.namespace,
        .instructions = .{},
        .inlining = null,
        .comptime_reason = .{ .reason = .{
            .src = src,
            .r = .{ .simple = .union_fields },
        } },
        .src_base_inst = union_type.zir_index,
        .type_name_ctx = union_type.name,
    };
    defer assert(block_scope.instructions.items.len == 0);

    if (body.len != 0) {
        _ = try sema.analyzeInlineBody(&block_scope, body, zir_index);
    }

    var int_tag_ty: Type = undefined;
    var enum_field_names: []InternPool.NullTerminatedString = &.{};
    var enum_field_vals: std.AutoArrayHashMapUnmanaged(InternPool.Index, void) = .empty;
    var explicit_tags_seen: []bool = &.{};
    if (tag_type_ref != .none) {
        const tag_ty_src: LazySrcLoc = .{
            .base_node_inst = union_type.zir_index,
            .offset = .{ .node_offset_container_tag = .zero },
        };
        const provided_ty = try sema.resolveType(&block_scope, tag_ty_src, tag_type_ref);
        if (small.auto_enum_tag) {
            // The provided type is an integer type and we must construct the enum tag type here.
            int_tag_ty = provided_ty;
            if (int_tag_ty.zigTypeTag(zcu) != .int and int_tag_ty.zigTypeTag(zcu) != .comptime_int) {
                return sema.fail(&block_scope, tag_ty_src, "expected integer tag type, found '{}'", .{int_tag_ty.fmt(pt)});
            }

            if (fields_len > 0) {
                const field_count_val = try pt.intValue(.comptime_int, fields_len - 1);
                if (!(try sema.intFitsInType(field_count_val, int_tag_ty, null))) {
                    const msg = msg: {
                        const msg = try sema.errMsg(tag_ty_src, "specified integer tag type cannot represent every field", .{});
                        errdefer msg.destroy(sema.gpa);
                        try sema.errNote(tag_ty_src, msg, "type '{}' cannot fit values in range 0...{d}", .{
                            int_tag_ty.fmt(pt),
                            fields_len - 1,
                        });
                        break :msg msg;
                    };
                    return sema.failWithOwnedErrorMsg(&block_scope, msg);
                }
                enum_field_names = try sema.arena.alloc(InternPool.NullTerminatedString, fields_len);
                try enum_field_vals.ensureTotalCapacity(sema.arena, fields_len);
            }
        } else {
            // The provided type is the enum tag type.
            const enum_type = switch (ip.indexToKey(provided_ty.toIntern())) {
                .enum_type => ip.loadEnumType(provided_ty.toIntern()),
                else => return sema.fail(&block_scope, tag_ty_src, "expected enum tag type, found '{}'", .{provided_ty.fmt(pt)}),
            };
            union_type.setTagType(ip, provided_ty.toIntern());
            // The fields of the union must match the enum exactly.
            // A flag per field is used to check for missing and extraneous fields.
            explicit_tags_seen = try sema.arena.alloc(bool, enum_type.names.len);
            @memset(explicit_tags_seen, false);
        }
    } else {
        // If auto_enum_tag is false, this is an untagged union. However, for semantic analysis
        // purposes, we still auto-generate an enum tag type the same way. That the union is
        // untagged is represented by the Type tag (union vs union_tagged).
        enum_field_names = try sema.arena.alloc(InternPool.NullTerminatedString, fields_len);
    }

    var field_types: std.ArrayListUnmanaged(InternPool.Index) = .empty;
    var field_aligns: std.ArrayListUnmanaged(InternPool.Alignment) = .empty;

    try field_types.ensureTotalCapacityPrecise(sema.arena, fields_len);
    if (small.any_aligned_fields)
        try field_aligns.ensureTotalCapacityPrecise(sema.arena, fields_len);

    const bits_per_field = 4;
    const fields_per_u32 = 32 / bits_per_field;
    const bit_bags_count = std.math.divCeil(usize, fields_len, fields_per_u32) catch unreachable;
    var bit_bag_index: usize = extra_index;
    extra_index += bit_bags_count;
    var cur_bit_bag: u32 = undefined;
    var field_i: u32 = 0;
    var last_tag_val: ?Value = null;
    while (field_i < fields_len) : (field_i += 1) {
        if (field_i % fields_per_u32 == 0) {
            cur_bit_bag = zir.extra[bit_bag_index];
            bit_bag_index += 1;
        }
        const has_type = @as(u1, @truncate(cur_bit_bag)) != 0;
        cur_bit_bag >>= 1;
        const has_align = @as(u1, @truncate(cur_bit_bag)) != 0;
        cur_bit_bag >>= 1;
        const has_tag = @as(u1, @truncate(cur_bit_bag)) != 0;
        cur_bit_bag >>= 1;
        const unused = @as(u1, @truncate(cur_bit_bag)) != 0;
        cur_bit_bag >>= 1;
        _ = unused;

        const field_name_index: Zir.NullTerminatedString = @enumFromInt(zir.extra[extra_index]);
        const field_name_zir = zir.nullTerminatedString(field_name_index);
        extra_index += 1;

        const field_type_ref: Zir.Inst.Ref = if (has_type) blk: {
            const field_type_ref: Zir.Inst.Ref = @enumFromInt(zir.extra[extra_index]);
            extra_index += 1;
            break :blk field_type_ref;
        } else .none;

        const align_ref: Zir.Inst.Ref = if (has_align) blk: {
            const align_ref: Zir.Inst.Ref = @enumFromInt(zir.extra[extra_index]);
            extra_index += 1;
            break :blk align_ref;
        } else .none;

        const tag_ref: Air.Inst.Ref = if (has_tag) blk: {
            const tag_ref: Zir.Inst.Ref = @enumFromInt(zir.extra[extra_index]);
            extra_index += 1;
            break :blk try sema.resolveInst(tag_ref);
        } else .none;

        const name_src: LazySrcLoc = .{
            .base_node_inst = union_type.zir_index,
            .offset = .{ .container_field_name = field_i },
        };
        const value_src: LazySrcLoc = .{
            .base_node_inst = union_type.zir_index,
            .offset = .{ .container_field_value = field_i },
        };
        const align_src: LazySrcLoc = .{
            .base_node_inst = union_type.zir_index,
            .offset = .{ .container_field_align = field_i },
        };
        const type_src: LazySrcLoc = .{
            .base_node_inst = union_type.zir_index,
            .offset = .{ .container_field_type = field_i },
        };

        if (enum_field_vals.capacity() > 0) {
            const enum_tag_val = if (tag_ref != .none) blk: {
                const coerced = try sema.coerce(&block_scope, int_tag_ty, tag_ref, value_src);
                const val = try sema.resolveConstDefinedValue(&block_scope, value_src, coerced, .{ .simple = .enum_field_tag_value });
                last_tag_val = val;

                break :blk val;
            } else blk: {
                if (last_tag_val) |last_tag| {
                    const result = try arith.incrementDefinedInt(sema, int_tag_ty, last_tag);
                    if (result.overflow) return sema.fail(
                        &block_scope,
                        value_src,
                        "enumeration value '{}' too large for type '{}'",
                        .{ result.val.fmtValueSema(pt, sema), int_tag_ty.fmt(pt) },
                    );
                    last_tag_val = result.val;
                } else {
                    last_tag_val = try pt.intValue(int_tag_ty, 0);
                }
                break :blk last_tag_val.?;
            };
            const gop = enum_field_vals.getOrPutAssumeCapacity(enum_tag_val.toIntern());
            if (gop.found_existing) {
                const other_value_src: LazySrcLoc = .{
                    .base_node_inst = union_type.zir_index,
                    .offset = .{ .container_field_value = @intCast(gop.index) },
                };
                const msg = msg: {
                    const msg = try sema.errMsg(
                        value_src,
                        "enum tag value {} already taken",
                        .{enum_tag_val.fmtValueSema(pt, sema)},
                    );
                    errdefer msg.destroy(gpa);
                    try sema.errNote(other_value_src, msg, "other occurrence here", .{});
                    break :msg msg;
                };
                return sema.failWithOwnedErrorMsg(&block_scope, msg);
            }
        }

        // This string needs to outlive the ZIR code.
        const field_name = try ip.getOrPutString(gpa, pt.tid, field_name_zir, .no_embedded_nulls);
        if (enum_field_names.len != 0) {
            enum_field_names[field_i] = field_name;
        }

        const field_ty: Type = if (!has_type)
            .void
        else if (field_type_ref == .none)
            .noreturn
        else
            try sema.resolveType(&block_scope, type_src, field_type_ref);

        if (explicit_tags_seen.len > 0) {
            const tag_ty = union_type.tagTypeUnordered(ip);
            const tag_info = ip.loadEnumType(tag_ty);
            const enum_index = tag_info.nameIndex(ip, field_name) orelse {
                return sema.fail(&block_scope, name_src, "no field named '{}' in enum '{}'", .{
                    field_name.fmt(ip), Type.fromInterned(tag_ty).fmt(pt),
                });
            };

            // No check for duplicate because the check already happened in order
            // to create the enum type in the first place.
            assert(!explicit_tags_seen[enum_index]);
            explicit_tags_seen[enum_index] = true;

            // Enforce the enum fields and the union fields being in the same order.
            if (enum_index != field_i) {
                const msg = msg: {
                    const enum_field_src: LazySrcLoc = .{
                        .base_node_inst = Type.fromInterned(tag_ty).typeDeclInstAllowGeneratedTag(zcu).?,
                        .offset = .{ .container_field_name = enum_index },
                    };
                    const msg = try sema.errMsg(name_src, "union field '{}' ordered differently than corresponding enum field", .{
                        field_name.fmt(ip),
                    });
                    errdefer msg.destroy(sema.gpa);
                    try sema.errNote(enum_field_src, msg, "enum field here", .{});
                    break :msg msg;
                };
                return sema.failWithOwnedErrorMsg(&block_scope, msg);
            }
        }

        if (field_ty.zigTypeTag(zcu) == .@"opaque") {
            const msg = msg: {
                const msg = try sema.errMsg(type_src, "opaque types have unknown size and therefore cannot be directly embedded in unions", .{});
                errdefer msg.destroy(sema.gpa);

                try sema.addDeclaredHereNote(msg, field_ty);
                break :msg msg;
            };
            return sema.failWithOwnedErrorMsg(&block_scope, msg);
        }
        const layout = union_type.flagsUnordered(ip).layout;
        if (layout == .@"extern" and
            !try sema.validateExternType(field_ty, .union_field))
        {
            const msg = msg: {
                const msg = try sema.errMsg(type_src, "extern unions cannot contain fields of type '{}'", .{field_ty.fmt(pt)});
                errdefer msg.destroy(sema.gpa);

                try sema.explainWhyTypeIsNotExtern(msg, type_src, field_ty, .union_field);

                try sema.addDeclaredHereNote(msg, field_ty);
                break :msg msg;
            };
            return sema.failWithOwnedErrorMsg(&block_scope, msg);
        } else if (layout == .@"packed" and !try sema.validatePackedType(field_ty)) {
            const msg = msg: {
                const msg = try sema.errMsg(type_src, "packed unions cannot contain fields of type '{}'", .{field_ty.fmt(pt)});
                errdefer msg.destroy(sema.gpa);

                try sema.explainWhyTypeIsNotPacked(msg, type_src, field_ty);

                try sema.addDeclaredHereNote(msg, field_ty);
                break :msg msg;
            };
            return sema.failWithOwnedErrorMsg(&block_scope, msg);
        }

        field_types.appendAssumeCapacity(field_ty.toIntern());

        if (small.any_aligned_fields) {
            field_aligns.appendAssumeCapacity(if (align_ref != .none)
                try sema.resolveAlign(&block_scope, align_src, align_ref)
            else
                .none);
        } else {
            assert(align_ref == .none);
        }
    }

    union_type.setFieldTypes(ip, field_types.items);
    union_type.setFieldAligns(ip, field_aligns.items);

    if (explicit_tags_seen.len > 0) {
        const tag_ty = union_type.tagTypeUnordered(ip);
        const tag_info = ip.loadEnumType(tag_ty);
        if (tag_info.names.len > fields_len) {
            const msg = msg: {
                const msg = try sema.errMsg(src, "enum field(s) missing in union", .{});
                errdefer msg.destroy(sema.gpa);

                for (tag_info.names.get(ip), 0..) |field_name, field_index| {
                    if (explicit_tags_seen[field_index]) continue;
                    try sema.addFieldErrNote(.fromInterned(tag_ty), field_index, msg, "field '{}' missing, declared here", .{
                        field_name.fmt(ip),
                    });
                }
                try sema.addDeclaredHereNote(msg, .fromInterned(tag_ty));
                break :msg msg;
            };
            return sema.failWithOwnedErrorMsg(&block_scope, msg);
        }
    } else if (enum_field_vals.count() > 0) {
        const enum_ty = try sema.generateUnionTagTypeNumbered(&block_scope, enum_field_names, enum_field_vals.keys(), union_ty, union_type.name);
        union_type.setTagType(ip, enum_ty);
    } else {
        const enum_ty = try sema.generateUnionTagTypeSimple(&block_scope, enum_field_names, union_ty, union_type.name);
        union_type.setTagType(ip, enum_ty);
    }

    try sema.flushExports();
}

fn generateUnionTagTypeNumbered(
    sema: *Sema,
    block: *Block,
    enum_field_names: []const InternPool.NullTerminatedString,
    enum_field_vals: []const InternPool.Index,
    union_type: InternPool.Index,
    union_name: InternPool.NullTerminatedString,
) !InternPool.Index {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const gpa = sema.gpa;
    const ip = &zcu.intern_pool;

    const name = try ip.getOrPutStringFmt(
        gpa,
        pt.tid,
        "@typeInfo({}).@\"union\".tag_type.?",
        .{union_name.fmt(ip)},
        .no_embedded_nulls,
    );

    const enum_ty = try ip.getGeneratedTagEnumType(gpa, pt.tid, .{
        .name = name,
        .owner_union_ty = union_type,
        .tag_ty = if (enum_field_vals.len == 0)
            (try pt.intType(.unsigned, 0)).toIntern()
        else
            ip.typeOf(enum_field_vals[0]),
        .names = enum_field_names,
        .values = enum_field_vals,
        .tag_mode = .explicit,
        .parent_namespace = block.namespace,
    });

    return enum_ty;
}

fn generateUnionTagTypeSimple(
    sema: *Sema,
    block: *Block,
    enum_field_names: []const InternPool.NullTerminatedString,
    union_type: InternPool.Index,
    union_name: InternPool.NullTerminatedString,
) !InternPool.Index {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const ip = &zcu.intern_pool;
    const gpa = sema.gpa;

    const name = try ip.getOrPutStringFmt(
        gpa,
        pt.tid,
        "@typeInfo({}).@\"union\".tag_type.?",
        .{union_name.fmt(ip)},
        .no_embedded_nulls,
    );

    const enum_ty = try ip.getGeneratedTagEnumType(gpa, pt.tid, .{
        .name = name,
        .owner_union_ty = union_type,
        .tag_ty = (try pt.smallestUnsignedInt(enum_field_names.len -| 1)).toIntern(),
        .names = enum_field_names,
        .values = &.{},
        .tag_mode = .auto,
        .parent_namespace = block.namespace,
    });

    return enum_ty;
}

/// There is another implementation of this in `Type.onePossibleValue`. This one
/// in `Sema` is for calling during semantic analysis, and performs field resolution
/// to get the answer. The one in `Type` is for calling during codegen and asserts
/// that the types are already resolved.
/// TODO assert the return value matches `ty.onePossibleValue`
pub fn typeHasOnePossibleValue(sema: *Sema, ty: Type) CompileError!?Value {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const ip = &zcu.intern_pool;
    return switch (ty.toIntern()) {
        .u0_type,
        .i0_type,
        => try pt.intValue(ty, 0),
        .u1_type,
        .u8_type,
        .i8_type,
        .u16_type,
        .i16_type,
        .u29_type,
        .u32_type,
        .i32_type,
        .u64_type,
        .i64_type,
        .u80_type,
        .u128_type,
        .i128_type,
        .u256_type,
        .usize_type,
        .isize_type,
        .c_char_type,
        .c_short_type,
        .c_ushort_type,
        .c_int_type,
        .c_uint_type,
        .c_long_type,
        .c_ulong_type,
        .c_longlong_type,
        .c_ulonglong_type,
        .c_longdouble_type,
        .f16_type,
        .f32_type,
        .f64_type,
        .f80_type,
        .f128_type,
        .anyopaque_type,
        .bool_type,
        .type_type,
        .anyerror_type,
        .adhoc_inferred_error_set_type,
        .comptime_int_type,
        .comptime_float_type,
        .enum_literal_type,
        .ptr_usize_type,
        .ptr_const_comptime_int_type,
        .manyptr_u8_type,
        .manyptr_const_u8_type,
        .manyptr_const_u8_sentinel_0_type,
        .slice_const_u8_type,
        .slice_const_u8_sentinel_0_type,
        .vector_8_i8_type,
        .vector_16_i8_type,
        .vector_32_i8_type,
        .vector_64_i8_type,
        .vector_1_u8_type,
        .vector_2_u8_type,
        .vector_4_u8_type,
        .vector_8_u8_type,
        .vector_16_u8_type,
        .vector_32_u8_type,
        .vector_64_u8_type,
        .vector_2_i16_type,
        .vector_4_i16_type,
        .vector_8_i16_type,
        .vector_16_i16_type,
        .vector_32_i16_type,
        .vector_4_u16_type,
        .vector_8_u16_type,
        .vector_16_u16_type,
        .vector_32_u16_type,
        .vector_2_i32_type,
        .vector_4_i32_type,
        .vector_8_i32_type,
        .vector_16_i32_type,
        .vector_4_u32_type,
        .vector_8_u32_type,
        .vector_16_u32_type,
        .vector_2_i64_type,
        .vector_4_i64_type,
        .vector_8_i64_type,
        .vector_2_u64_type,
        .vector_4_u64_type,
        .vector_8_u64_type,
        .vector_1_u128_type,
        .vector_2_u128_type,
        .vector_1_u256_type,
        .vector_4_f16_type,
        .vector_8_f16_type,
        .vector_16_f16_type,
        .vector_32_f16_type,
        .vector_2_f32_type,
        .vector_4_f32_type,
        .vector_8_f32_type,
        .vector_16_f32_type,
        .vector_2_f64_type,
        .vector_4_f64_type,
        .vector_8_f64_type,
        .anyerror_void_error_union_type,
        => null,
        .void_type => Value.void,
        .noreturn_type => Value.@"unreachable",
        .anyframe_type => unreachable,
        .null_type => Value.null,
        .undefined_type => Value.undef,
        .optional_noreturn_type => try pt.nullValue(ty),
        .generic_poison_type => unreachable,
        .empty_tuple_type => Value.empty_tuple,
        // values, not types
        .undef,
        .undef_bool,
        .undef_usize,
        .undef_u1,
        .zero,
        .zero_usize,
        .zero_u1,
        .zero_u8,
        .one,
        .one_usize,
        .one_u1,
        .one_u8,
        .four_u8,
        .negative_one,
        .void_value,
        .unreachable_value,
        .null_value,
        .bool_true,
        .bool_false,
        .empty_tuple,
        // invalid
        .none,
        => unreachable,

        _ => switch (ty.toIntern().unwrap(ip).getTag(ip)) {
            .removed => unreachable,

            .type_int_signed, // i0 handled above
            .type_int_unsigned, // u0 handled above
            .type_pointer,
            .type_slice,
            .type_optional, // ?noreturn handled above
            .type_anyframe,
            .type_error_union,
            .type_anyerror_union,
            .type_error_set,
            .type_inferred_error_set,
            .type_opaque,
            .type_function,
            => null,

            .simple_type, // handled above
            // values, not types
            .undef,
            .simple_value,
            .ptr_nav,
            .ptr_uav,
            .ptr_uav_aligned,
            .ptr_comptime_alloc,
            .ptr_comptime_field,
            .ptr_int,
            .ptr_eu_payload,
            .ptr_opt_payload,
            .ptr_elem,
            .ptr_field,
            .ptr_slice,
            .opt_payload,
            .opt_null,
            .int_u8,
            .int_u16,
            .int_u32,
            .int_i32,
            .int_usize,
            .int_comptime_int_u32,
            .int_comptime_int_i32,
            .int_small,
            .int_positive,
            .int_negative,
            .int_lazy_align,
            .int_lazy_size,
            .error_set_error,
            .error_union_error,
            .error_union_payload,
            .enum_literal,
            .enum_tag,
            .float_f16,
            .float_f32,
            .float_f64,
            .float_f80,
            .float_f128,
            .float_c_longdouble_f80,
            .float_c_longdouble_f128,
            .float_comptime_float,
            .variable,
            .@"extern",
            .func_decl,
            .func_instance,
            .func_coerced,
            .only_possible_value,
            .union_value,
            .bytes,
            .aggregate,
            .repeated,
            // memoized value, not types
            .memoized_call,
            => unreachable,

            .type_array_big,
            .type_array_small,
            .type_vector,
            .type_enum_auto,
            .type_enum_explicit,
            .type_enum_nonexhaustive,
            .type_struct,
            .type_struct_packed,
            .type_struct_packed_inits,
            .type_tuple,
            .type_union,
            => switch (ip.indexToKey(ty.toIntern())) {
                inline .array_type, .vector_type => |seq_type, seq_tag| {
                    const has_sentinel = seq_tag == .array_type and seq_type.sentinel != .none;
                    if (seq_type.len + @intFromBool(has_sentinel) == 0) return Value.fromInterned(try pt.intern(.{ .aggregate = .{
                        .ty = ty.toIntern(),
                        .storage = .{ .elems = &.{} },
                    } }));

                    if (try sema.typeHasOnePossibleValue(.fromInterned(seq_type.child))) |opv| {
                        return Value.fromInterned(try pt.intern(.{ .aggregate = .{
                            .ty = ty.toIntern(),
                            .storage = .{ .repeated_elem = opv.toIntern() },
                        } }));
                    }
                    return null;
                },

                .struct_type => {
                    // Resolving the layout first helps to avoid loops.
                    // If the type has a coherent layout, we can recurse through fields safely.
                    try ty.resolveLayout(pt);

                    const struct_type = ip.loadStructType(ty.toIntern());

                    if (struct_type.field_types.len == 0) {
                        // In this case the struct has no fields at all and
                        // therefore has one possible value.
                        return Value.fromInterned(try pt.intern(.{ .aggregate = .{
                            .ty = ty.toIntern(),
                            .storage = .{ .elems = &.{} },
                        } }));
                    }

                    const field_vals = try sema.arena.alloc(
                        InternPool.Index,
                        struct_type.field_types.len,
                    );
                    for (field_vals, 0..) |*field_val, i| {
                        if (struct_type.fieldIsComptime(ip, i)) {
                            try ty.resolveStructFieldInits(pt);
                            field_val.* = struct_type.field_inits.get(ip)[i];
                            continue;
                        }
                        const field_ty: Type = .fromInterned(struct_type.field_types.get(ip)[i]);
                        if (try sema.typeHasOnePossibleValue(field_ty)) |field_opv| {
                            field_val.* = field_opv.toIntern();
                        } else return null;
                    }

                    // In this case the struct has no runtime-known fields and
                    // therefore has one possible value.
                    return Value.fromInterned(try pt.intern(.{ .aggregate = .{
                        .ty = ty.toIntern(),
                        .storage = .{ .elems = field_vals },
                    } }));
                },

                .tuple_type => |tuple| {
                    for (tuple.values.get(ip)) |val| {
                        if (val == .none) return null;
                    }
                    // In this case the struct has all comptime-known fields and
                    // therefore has one possible value.
                    // TODO: write something like getCoercedInts to avoid needing to dupe
                    return Value.fromInterned(try pt.intern(.{ .aggregate = .{
                        .ty = ty.toIntern(),
                        .storage = .{ .elems = try sema.arena.dupe(InternPool.Index, tuple.values.get(ip)) },
                    } }));
                },

                .union_type => {
                    // Resolving the layout first helps to avoid loops.
                    // If the type has a coherent layout, we can recurse through fields safely.
                    try ty.resolveLayout(pt);

                    const union_obj = ip.loadUnionType(ty.toIntern());
                    const tag_val = (try sema.typeHasOnePossibleValue(.fromInterned(union_obj.tagTypeUnordered(ip)))) orelse
                        return null;
                    if (union_obj.field_types.len == 0) {
                        const only = try pt.intern(.{ .empty_enum_value = ty.toIntern() });
                        return Value.fromInterned(only);
                    }
                    const only_field_ty: Type = .fromInterned(union_obj.field_types.get(ip)[0]);
                    const val_val = (try sema.typeHasOnePossibleValue(only_field_ty)) orelse
                        return null;
                    const only = try pt.internUnion(.{
                        .ty = ty.toIntern(),
                        .tag = tag_val.toIntern(),
                        .val = val_val.toIntern(),
                    });
                    return Value.fromInterned(only);
                },

                .enum_type => {
                    const enum_type = ip.loadEnumType(ty.toIntern());
                    switch (enum_type.tag_mode) {
                        .nonexhaustive => {
                            if (enum_type.tag_ty == .comptime_int_type) return null;

                            if (try sema.typeHasOnePossibleValue(.fromInterned(enum_type.tag_ty))) |int_opv| {
                                const only = try pt.intern(.{ .enum_tag = .{
                                    .ty = ty.toIntern(),
                                    .int = int_opv.toIntern(),
                                } });
                                return Value.fromInterned(only);
                            }

                            return null;
                        },
                        .auto, .explicit => {
                            if (Type.fromInterned(enum_type.tag_ty).hasRuntimeBits(zcu)) return null;

                            return Value.fromInterned(switch (enum_type.names.len) {
                                0 => try pt.intern(.{ .empty_enum_value = ty.toIntern() }),
                                1 => try pt.intern(.{ .enum_tag = .{
                                    .ty = ty.toIntern(),
                                    .int = if (enum_type.values.len == 0)
                                        (try pt.intValue(.fromInterned(enum_type.tag_ty), 0)).toIntern()
                                    else
                                        try ip.getCoercedInts(
                                            zcu.gpa,
                                            pt.tid,
                                            ip.indexToKey(enum_type.values.get(ip)[0]).int,
                                            enum_type.tag_ty,
                                        ),
                                } }),
                                else => return null,
                            });
                        },
                    }
                },

                else => unreachable,
            },
        },
    };
}

/// Returns the type of the AIR instruction.
fn typeOf(sema: *Sema, inst: Air.Inst.Ref) Type {
    return sema.getTmpAir().typeOf(inst, &sema.pt.zcu.intern_pool);
}

pub fn getTmpAir(sema: Sema) Air {
    return .{
        .instructions = sema.air_instructions.slice(),
        .extra = sema.air_extra,
    };
}

pub fn addExtra(sema: *Sema, extra: anytype) Allocator.Error!u32 {
    const fields = std.meta.fields(@TypeOf(extra));
    try sema.air_extra.ensureUnusedCapacity(sema.gpa, fields.len);
    return sema.addExtraAssumeCapacity(extra);
}

pub fn addExtraAssumeCapacity(sema: *Sema, extra: anytype) u32 {
    const result: u32 = @intCast(sema.air_extra.items.len);
    sema.air_extra.appendSliceAssumeCapacity(&payloadToExtraItems(extra));
    return result;
}

fn payloadToExtraItems(data: anytype) [@typeInfo(@TypeOf(data)).@"struct".fields.len]u32 {
    const fields = @typeInfo(@TypeOf(data)).@"struct".fields;
    var result: [fields.len]u32 = undefined;
    inline for (&result, fields) |*val, field| {
        val.* = switch (field.type) {
            u32 => @field(data, field.name),
            i32, Air.CondBr.BranchHints => @bitCast(@field(data, field.name)),
            Air.Inst.Ref, InternPool.Index => @intFromEnum(@field(data, field.name)),
            else => @compileError("bad field type: " ++ @typeName(field.type)),
        };
    }
    return result;
}

fn appendRefsAssumeCapacity(sema: *Sema, refs: []const Air.Inst.Ref) void {
    sema.air_extra.appendSliceAssumeCapacity(@ptrCast(refs));
}

fn getBreakBlock(sema: *Sema, inst_index: Air.Inst.Index) ?Air.Inst.Index {
    const air_datas = sema.air_instructions.items(.data);
    const air_tags = sema.air_instructions.items(.tag);
    switch (air_tags[@intFromEnum(inst_index)]) {
        .br => return air_datas[@intFromEnum(inst_index)].br.block_inst,
        else => return null,
    }
}

fn isComptimeKnown(
    sema: *Sema,
    inst: Air.Inst.Ref,
) !bool {
    return (try sema.resolveValue(inst)) != null;
}

fn analyzeComptimeAlloc(
    sema: *Sema,
    block: *Block,
    src: LazySrcLoc,
    var_type: Type,
    alignment: Alignment,
) CompileError!Air.Inst.Ref {
    const pt = sema.pt;
    const zcu = pt.zcu;

    // Needed to make an anon decl with type `var_type` (the `finish()` call below).
    _ = try sema.typeHasOnePossibleValue(var_type);

    const ptr_type = try pt.ptrTypeSema(.{
        .child = var_type.toIntern(),
        .flags = .{
            .alignment = alignment,
            .address_space = target_util.defaultAddressSpace(zcu.getTarget(), .global_constant),
        },
    });

    const alloc = try sema.newComptimeAlloc(block, src, var_type, alignment);

    return Air.internedToRef((try pt.intern(.{ .ptr = .{
        .ty = ptr_type.toIntern(),
        .base_addr = .{ .comptime_alloc = alloc },
        .byte_offset = 0,
    } })));
}

fn resolveAddressSpace(
    sema: *Sema,
    block: *Block,
    src: LazySrcLoc,
    zir_ref: Zir.Inst.Ref,
    ctx: std.builtin.AddressSpace.Context,
) !std.builtin.AddressSpace {
    const air_ref = try sema.resolveInst(zir_ref);
    return sema.analyzeAsAddressSpace(block, src, air_ref, ctx);
}

pub fn analyzeAsAddressSpace(
    sema: *Sema,
    block: *Block,
    src: LazySrcLoc,
    air_ref: Air.Inst.Ref,
    ctx: std.builtin.AddressSpace.Context,
) !std.builtin.AddressSpace {
    const pt = sema.pt;
    const addrspace_ty = try sema.getBuiltinType(src, .AddressSpace);
    const coerced = try sema.coerce(block, addrspace_ty, air_ref, src);
    const addrspace_val = try sema.resolveConstDefinedValue(block, src, coerced, .{ .simple = .@"addrspace" });
    const address_space = try sema.interpretBuiltinType(block, src, addrspace_val, std.builtin.AddressSpace);
    const target = pt.zcu.getTarget();

    if (!target.cpu.supportsAddressSpace(address_space, ctx)) {
        // TODO error messages could be made more elaborate here
        const entity = switch (ctx) {
            .function => "functions",
            .variable => "mutable values",
            .constant => "constant values",
            .pointer => "pointers",
        };
        return sema.fail(
            block,
            src,
            "{s} with address space '{s}' are not supported on {s}",
            .{ entity, @tagName(address_space), @tagName(target.cpu.arch.family()) },
        );
    }

    return address_space;
}

/// Asserts the value is a pointer and dereferences it.
/// Returns `null` if the pointer contents cannot be loaded at comptime.
fn pointerDeref(sema: *Sema, block: *Block, src: LazySrcLoc, ptr_val: Value, ptr_ty: Type) CompileError!?Value {
    // TODO: audit use sites to eliminate this coercion
    const pt = sema.pt;
    const coerced_ptr_val = try pt.getCoerced(ptr_val, ptr_ty);
    switch (try sema.pointerDerefExtra(block, src, coerced_ptr_val)) {
        .runtime_load => return null,
        .val => |v| return v,
        .needed_well_defined => |ty| return sema.fail(
            block,
            src,
            "comptime dereference requires '{}' to have a well-defined layout",
            .{ty.fmt(pt)},
        ),
        .out_of_bounds => |ty| return sema.fail(
            block,
            src,
            "dereference of '{}' exceeds bounds of containing decl of type '{}'",
            .{ ptr_ty.fmt(pt), ty.fmt(pt) },
        ),
    }
}

const DerefResult = union(enum) {
    runtime_load,
    val: Value,
    needed_well_defined: Type,
    out_of_bounds: Type,
};

fn pointerDerefExtra(sema: *Sema, block: *Block, src: LazySrcLoc, ptr_val: Value) CompileError!DerefResult {
    const pt = sema.pt;
    const ip = &pt.zcu.intern_pool;
    switch (try sema.loadComptimePtr(block, src, ptr_val)) {
        .success => |mv| return .{ .val = try mv.intern(pt, sema.arena) },
        .runtime_load => return .runtime_load,
        .undef => return sema.failWithUseOfUndef(block, src),
        .err_payload => |err_name| return sema.fail(block, src, "attempt to unwrap error: {}", .{err_name.fmt(ip)}),
        .null_payload => return sema.fail(block, src, "attempt to use null value", .{}),
        .inactive_union_field => return sema.fail(block, src, "access of inactive union field", .{}),
        .needed_well_defined => |ty| return .{ .needed_well_defined = ty },
        .out_of_bounds => |ty| return .{ .out_of_bounds = ty },
        .exceeds_host_size => return sema.fail(block, src, "bit-pointer target exceeds host size", .{}),
    }
}

/// Used to convert a u64 value to a usize value, emitting a compile error if the number
/// is too big to fit.
fn usizeCast(sema: *Sema, block: *Block, src: LazySrcLoc, int: u64) CompileError!usize {
    if (@bitSizeOf(u64) <= @bitSizeOf(usize)) return int;
    return std.math.cast(usize, int) orelse return sema.fail(block, src, "expression produces integer value '{d}' which is too big for this compiler implementation to handle", .{int});
}

/// For pointer-like optionals, it returns the pointer type. For pointers,
/// the type is returned unmodified.
/// This can return `error.AnalysisFail` because it sometimes requires resolving whether
/// a type has zero bits, which can cause a "foo depends on itself" compile error.
/// This logic must be kept in sync with `Type.isPtrLikeOptional`.
fn typePtrOrOptionalPtrTy(sema: *Sema, ty: Type) !?Type {
    const pt = sema.pt;
    const zcu = pt.zcu;
    return switch (zcu.intern_pool.indexToKey(ty.toIntern())) {
        .ptr_type => |ptr_type| switch (ptr_type.flags.size) {
            .one, .many, .c => ty,
            .slice => null,
        },
        .opt_type => |opt_child| switch (zcu.intern_pool.indexToKey(opt_child)) {
            .ptr_type => |ptr_type| switch (ptr_type.flags.size) {
                .slice, .c => null,
                .many, .one => {
                    if (ptr_type.flags.is_allowzero) return null;

                    // optionals of zero sized types behave like bools, not pointers
                    const payload_ty: Type = .fromInterned(opt_child);
                    if ((try sema.typeHasOnePossibleValue(payload_ty)) != null) {
                        return null;
                    }

                    return payload_ty;
                },
            },
            else => null,
        },
        else => null,
    };
}

fn unionFieldIndex(
    sema: *Sema,
    block: *Block,
    union_ty: Type,
    field_name: InternPool.NullTerminatedString,
    field_src: LazySrcLoc,
) !u32 {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const ip = &zcu.intern_pool;
    try union_ty.resolveFields(pt);
    const union_obj = zcu.typeToUnion(union_ty).?;
    const field_index = union_obj.loadTagType(ip).nameIndex(ip, field_name) orelse
        return sema.failWithBadUnionFieldAccess(block, union_ty, union_obj, field_src, field_name);
    return @intCast(field_index);
}

fn structFieldIndex(
    sema: *Sema,
    block: *Block,
    struct_ty: Type,
    field_name: InternPool.NullTerminatedString,
    field_src: LazySrcLoc,
) !u32 {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const ip = &zcu.intern_pool;
    try struct_ty.resolveFields(pt);
    const struct_type = zcu.typeToStruct(struct_ty).?;
    return struct_type.nameIndex(ip, field_name) orelse
        return sema.failWithBadStructFieldAccess(block, struct_ty, struct_type, field_src, field_name);
}

const IntFromFloatMode = enum { exact, truncate };

fn intFromFloat(
    sema: *Sema,
    block: *Block,
    src: LazySrcLoc,
    val: Value,
    float_ty: Type,
    int_ty: Type,
    mode: IntFromFloatMode,
) CompileError!Value {
    const pt = sema.pt;
    const zcu = pt.zcu;
    if (float_ty.zigTypeTag(zcu) == .vector) {
        const result_data = try sema.arena.alloc(InternPool.Index, float_ty.vectorLen(zcu));
        for (result_data, 0..) |*scalar, i| {
            const elem_val = try val.elemValue(pt, i);
            scalar.* = (try sema.intFromFloatScalar(block, src, elem_val, int_ty.scalarType(zcu), mode)).toIntern();
        }
        return Value.fromInterned(try pt.intern(.{ .aggregate = .{
            .ty = int_ty.toIntern(),
            .storage = .{ .elems = result_data },
        } }));
    }
    return sema.intFromFloatScalar(block, src, val, int_ty, mode);
}

// float is expected to be finite and non-NaN
fn float128IntPartToBigInt(
    arena: Allocator,
    float: f128,
) !std.math.big.int.Managed {
    const is_negative = std.math.signbit(float);
    const floored = @floor(@abs(float));

    var rational = try std.math.big.Rational.init(arena);
    defer rational.q.deinit();
    rational.setFloat(f128, floored) catch |err| switch (err) {
        error.NonFiniteFloat => unreachable,
        error.OutOfMemory => return error.OutOfMemory,
    };

    // The float is reduced in rational.setFloat, so we assert that denominator is equal to one
    const big_one = std.math.big.int.Const{ .limbs = &.{1}, .positive = true };
    assert(rational.q.toConst().eqlAbs(big_one));

    if (is_negative) {
        rational.negate();
    }
    return rational.p;
}

fn intFromFloatScalar(
    sema: *Sema,
    block: *Block,
    src: LazySrcLoc,
    val: Value,
    int_ty: Type,
    mode: IntFromFloatMode,
) CompileError!Value {
    const pt = sema.pt;
    const zcu = pt.zcu;

    if (val.isUndef(zcu)) return sema.failWithUseOfUndef(block, src);

    if (mode == .exact and val.floatHasFraction(zcu)) return sema.fail(
        block,
        src,
        "fractional component prevents float value '{}' from coercion to type '{}'",
        .{ val.fmtValueSema(pt, sema), int_ty.fmt(pt) },
    );

    const float = val.toFloat(f128, zcu);
    if (std.math.isNan(float)) {
        return sema.fail(block, src, "float value NaN cannot be stored in integer type '{}'", .{
            int_ty.fmt(pt),
        });
    }
    if (std.math.isInf(float)) {
        return sema.fail(block, src, "float value Inf cannot be stored in integer type '{}'", .{
            int_ty.fmt(pt),
        });
    }

    var big_int = try float128IntPartToBigInt(sema.arena, float);
    defer big_int.deinit();

    const cti_result = try pt.intValue_big(.comptime_int, big_int.toConst());

    if (!(try sema.intFitsInType(cti_result, int_ty, null))) {
        return sema.fail(block, src, "float value '{}' cannot be stored in integer type '{}'", .{
            val.fmtValueSema(pt, sema), int_ty.fmt(pt),
        });
    }
    return pt.getCoerced(cti_result, int_ty);
}

/// Asserts the value is an integer, and the destination type is ComptimeInt or Int.
/// Vectors are also accepted. Vector results are reduced with AND.
///
/// If provided, `vector_index` reports the first element that failed the range check.
fn intFitsInType(
    sema: *Sema,
    val: Value,
    ty: Type,
    vector_index: ?*usize,
) CompileError!bool {
    const pt = sema.pt;
    const zcu = pt.zcu;
    if (ty.toIntern() == .comptime_int_type) return true;
    const info = ty.intInfo(zcu);
    switch (val.toIntern()) {
        .zero_usize, .zero_u8 => return true,
        else => switch (zcu.intern_pool.indexToKey(val.toIntern())) {
            .undef => return true,
            .variable, .@"extern", .func, .ptr => {
                const target = zcu.getTarget();
                const ptr_bits = target.ptrBitWidth();
                return switch (info.signedness) {
                    .signed => info.bits > ptr_bits,
                    .unsigned => info.bits >= ptr_bits,
                };
            },
            .int => |int| switch (int.storage) {
                .u64, .i64, .big_int => {
                    var buffer: InternPool.Key.Int.Storage.BigIntSpace = undefined;
                    const big_int = int.storage.toBigInt(&buffer);
                    return big_int.fitsInTwosComp(info.signedness, info.bits);
                },
                .lazy_align => |lazy_ty| {
                    const max_needed_bits = @as(u16, 16) + @intFromBool(info.signedness == .signed);
                    // If it is u16 or bigger we know the alignment fits without resolving it.
                    if (info.bits >= max_needed_bits) return true;
                    const x = try Type.fromInterned(lazy_ty).abiAlignmentSema(pt);
                    if (x == .none) return true;
                    const actual_needed_bits = @as(usize, x.toLog2Units()) + 1 + @intFromBool(info.signedness == .signed);
                    return info.bits >= actual_needed_bits;
                },
                .lazy_size => |lazy_ty| {
                    const max_needed_bits = @as(u16, 64) + @intFromBool(info.signedness == .signed);
                    // If it is u64 or bigger we know the size fits without resolving it.
                    if (info.bits >= max_needed_bits) return true;
                    const x = try Type.fromInterned(lazy_ty).abiSizeSema(pt);
                    if (x == 0) return true;
                    const actual_needed_bits = std.math.log2(x) + 1 + @intFromBool(info.signedness == .signed);
                    return info.bits >= actual_needed_bits;
                },
            },
            .aggregate => |aggregate| {
                assert(ty.zigTypeTag(zcu) == .vector);
                return switch (aggregate.storage) {
                    .bytes => |bytes| for (bytes.toSlice(ty.vectorLen(zcu), &zcu.intern_pool), 0..) |byte, i| {
                        if (byte == 0) continue;
                        const actual_needed_bits = std.math.log2(byte) + 1 + @intFromBool(info.signedness == .signed);
                        if (info.bits >= actual_needed_bits) continue;
                        if (vector_index) |vi| vi.* = i;
                        break false;
                    } else true,
                    .elems, .repeated_elem => for (switch (aggregate.storage) {
                        .bytes => unreachable,
                        .elems => |elems| elems,
                        .repeated_elem => |elem| @as(*const [1]InternPool.Index, &elem),
                    }, 0..) |elem, i| {
                        if (try sema.intFitsInType(Value.fromInterned(elem), ty.scalarType(zcu), null)) continue;
                        if (vector_index) |vi| vi.* = i;
                        break false;
                    } else true,
                };
            },
            else => unreachable,
        },
    }
}

fn intInRange(sema: *Sema, tag_ty: Type, int_val: Value, end: usize) !bool {
    const pt = sema.pt;
    if (!(try int_val.compareAllWithZeroSema(.gte, pt))) return false;
    const end_val = try pt.intValue(tag_ty, end);
    if (!(try sema.compareAll(int_val, .lt, end_val, tag_ty))) return false;
    return true;
}

/// Asserts the type is an enum.
fn enumHasInt(sema: *Sema, ty: Type, int: Value) CompileError!bool {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const enum_type = zcu.intern_pool.loadEnumType(ty.toIntern());
    assert(enum_type.tag_mode != .nonexhaustive);
    // The `tagValueIndex` function call below relies on the type being the integer tag type.
    // `getCoerced` assumes the value will fit the new type.
    if (!(try sema.intFitsInType(int, .fromInterned(enum_type.tag_ty), null))) return false;
    const int_coerced = try pt.getCoerced(int, .fromInterned(enum_type.tag_ty));

    return enum_type.tagValueIndex(&zcu.intern_pool, int_coerced.toIntern()) != null;
}

/// Asserts the values are comparable. Both operands have type `ty`.
/// For vectors, returns true if the comparison is true for ALL elements.
///
/// Note that `!compareAll(.eq, ...) != compareAll(.neq, ...)`
fn compareAll(
    sema: *Sema,
    lhs: Value,
    op: std.math.CompareOperator,
    rhs: Value,
    ty: Type,
) CompileError!bool {
    const pt = sema.pt;
    const zcu = pt.zcu;
    if (ty.zigTypeTag(zcu) == .vector) {
        var i: usize = 0;
        while (i < ty.vectorLen(zcu)) : (i += 1) {
            const lhs_elem = try lhs.elemValue(pt, i);
            const rhs_elem = try rhs.elemValue(pt, i);
            if (!(try sema.compareScalar(lhs_elem, op, rhs_elem, ty.scalarType(zcu)))) {
                return false;
            }
        }
        return true;
    }
    return sema.compareScalar(lhs, op, rhs, ty);
}

/// Asserts the values are comparable. Both operands have type `ty`.
fn compareScalar(
    sema: *Sema,
    lhs: Value,
    op: std.math.CompareOperator,
    rhs: Value,
    ty: Type,
) CompileError!bool {
    const pt = sema.pt;
    const coerced_lhs = try pt.getCoerced(lhs, ty);
    const coerced_rhs = try pt.getCoerced(rhs, ty);

    // Equality comparisons of signed zero and NaN need to use floating point semantics
    if (coerced_lhs.isFloat(pt.zcu) or coerced_rhs.isFloat(pt.zcu))
        return Value.compareHeteroSema(coerced_lhs, op, coerced_rhs, pt);

    switch (op) {
        .eq => return sema.valuesEqual(coerced_lhs, coerced_rhs, ty),
        .neq => return !(try sema.valuesEqual(coerced_lhs, coerced_rhs, ty)),
        else => return Value.compareHeteroSema(coerced_lhs, op, coerced_rhs, pt),
    }
}

fn valuesEqual(
    sema: *Sema,
    lhs: Value,
    rhs: Value,
    ty: Type,
) CompileError!bool {
    return lhs.eql(rhs, ty, sema.pt.zcu);
}

/// Asserts the values are comparable vectors of type `ty`.
fn compareVector(
    sema: *Sema,
    lhs: Value,
    op: std.math.CompareOperator,
    rhs: Value,
    ty: Type,
) !Value {
    const pt = sema.pt;
    const zcu = pt.zcu;
    assert(ty.zigTypeTag(zcu) == .vector);
    const result_data = try sema.arena.alloc(InternPool.Index, ty.vectorLen(zcu));
    for (result_data, 0..) |*scalar, i| {
        const lhs_elem = try lhs.elemValue(pt, i);
        const rhs_elem = try rhs.elemValue(pt, i);
        if (lhs_elem.isUndef(zcu) or rhs_elem.isUndef(zcu)) {
            scalar.* = .undef_bool;
        } else {
            const res_bool = try sema.compareScalar(lhs_elem, op, rhs_elem, ty.scalarType(zcu));
            scalar.* = Value.makeBool(res_bool).toIntern();
        }
    }
    return Value.fromInterned(try pt.intern(.{ .aggregate = .{
        .ty = (try pt.vectorType(.{ .len = ty.vectorLen(zcu), .child = .bool_type })).toIntern(),
        .storage = .{ .elems = result_data },
    } }));
}

/// Merge lhs with rhs.
/// Asserts that lhs and rhs are both error sets and are resolved.
fn errorSetMerge(sema: *Sema, lhs: Type, rhs: Type) !Type {
    const pt = sema.pt;
    const ip = &pt.zcu.intern_pool;
    const arena = sema.arena;
    const lhs_names = lhs.errorSetNames(pt.zcu);
    const rhs_names = rhs.errorSetNames(pt.zcu);
    var names: InferredErrorSet.NameMap = .{};
    try names.ensureUnusedCapacity(arena, lhs_names.len);

    for (0..lhs_names.len) |lhs_index| {
        names.putAssumeCapacityNoClobber(lhs_names.get(ip)[lhs_index], {});
    }
    for (0..rhs_names.len) |rhs_index| {
        try names.put(arena, rhs_names.get(ip)[rhs_index], {});
    }

    return pt.errorSetFromUnsortedNames(names.keys());
}

/// Avoids crashing the compiler when asking if inferred allocations are noreturn.
fn isNoReturn(sema: *Sema, ref: Air.Inst.Ref) bool {
    if (ref == .unreachable_value) return true;
    if (ref.toIndex()) |inst| switch (sema.air_instructions.items(.tag)[@intFromEnum(inst)]) {
        .inferred_alloc, .inferred_alloc_comptime => return false,
        else => {},
    };
    return sema.typeOf(ref).isNoReturn(sema.pt.zcu);
}

/// Avoids crashing the compiler when asking if inferred allocations are known to be a certain zig type.
fn isKnownZigType(sema: *Sema, ref: Air.Inst.Ref, tag: std.builtin.TypeId) bool {
    if (ref.toIndex()) |inst| switch (sema.air_instructions.items(.tag)[@intFromEnum(inst)]) {
        .inferred_alloc, .inferred_alloc_comptime => return false,
        else => {},
    };
    return sema.typeOf(ref).zigTypeTag(sema.pt.zcu) == tag;
}

pub fn declareDependency(sema: *Sema, dependee: InternPool.Dependee) !void {
    const pt = sema.pt;
    if (!pt.zcu.comp.incremental) return;

    const gop = try sema.dependencies.getOrPut(sema.gpa, dependee);
    if (gop.found_existing) return;

    // Avoid creating dependencies on ourselves. This situation can arise when we analyze the fields
    // of a type and they use `@This()`. This dependency would be unnecessary, and in fact would
    // just result in over-analysis since `Zcu.findOutdatedToAnalyze` would never be able to resolve
    // the loop.
    // Note that this also disallows a `nav_val`
    switch (sema.owner.unwrap()) {
        .nav_val => |this_nav| switch (dependee) {
            .nav_val => |other_nav| if (this_nav == other_nav) return,
            else => {},
        },
        .nav_ty => |this_nav| switch (dependee) {
            .nav_ty => |other_nav| if (this_nav == other_nav) return,
            else => {},
        },
        else => {},
    }

    try pt.addDependency(sema.owner, dependee);
}

fn isComptimeMutablePtr(sema: *Sema, val: Value) bool {
    return switch (sema.pt.zcu.intern_pool.indexToKey(val.toIntern())) {
        .slice => |slice| sema.isComptimeMutablePtr(Value.fromInterned(slice.ptr)),
        .ptr => |ptr| switch (ptr.base_addr) {
            .uav, .nav, .int => false,
            .comptime_field => true,
            .comptime_alloc => |alloc_index| !sema.getComptimeAlloc(alloc_index).is_const,
            .eu_payload, .opt_payload => |base| sema.isComptimeMutablePtr(Value.fromInterned(base)),
            .arr_elem, .field => |bi| sema.isComptimeMutablePtr(Value.fromInterned(bi.base)),
        },
        else => false,
    };
}

fn checkRuntimeValue(sema: *Sema, ptr: Air.Inst.Ref) bool {
    const val = ptr.toInterned() orelse return true;
    return !Value.fromInterned(val).canMutateComptimeVarState(sema.pt.zcu);
}

fn validateRuntimeValue(sema: *Sema, block: *Block, val_src: LazySrcLoc, val: Air.Inst.Ref) CompileError!void {
    if (sema.checkRuntimeValue(val)) return;
    return sema.failWithOwnedErrorMsg(block, msg: {
        const msg = try sema.errMsg(val_src, "runtime value contains reference to comptime var", .{});
        errdefer msg.destroy(sema.gpa);
        try sema.errNote(val_src, msg, "comptime var pointers are not available at runtime", .{});
        const pt = sema.pt;
        const zcu = pt.zcu;
        const val_str = try zcu.intern_pool.getOrPutString(zcu.gpa, pt.tid, "runtime_value", .no_embedded_nulls);
        try sema.explainWhyValueContainsReferenceToComptimeVar(msg, val_src, val_str, .fromInterned(val.toInterned().?));
        break :msg msg;
    });
}

fn failWithContainsReferenceToComptimeVar(sema: *Sema, block: *Block, src: LazySrcLoc, value_name: InternPool.NullTerminatedString, kind_of_value: []const u8, val: ?Value) CompileError {
    return sema.failWithOwnedErrorMsg(block, msg: {
        const msg = try sema.errMsg(src, "{s} contains reference to comptime var", .{kind_of_value});
        errdefer msg.destroy(sema.gpa);
        if (val) |v| try sema.explainWhyValueContainsReferenceToComptimeVar(msg, src, value_name, v);
        break :msg msg;
    });
}

fn explainWhyValueContainsReferenceToComptimeVar(sema: *Sema, msg: *Zcu.ErrorMsg, src: LazySrcLoc, value_name: InternPool.NullTerminatedString, val: Value) Allocator.Error!void {
    // Our goal is something like this:
    //   note: '(value.? catch unreachable)[0]' points to 'v0.?.foo'
    //   note: '(v0.?.bar catch unreachable)' points to 'v1'
    //   note: 'v1.?' points to a comptime var

    var intermediate_value_count: u32 = 0;
    var cur_val: Value = val;
    while (true) {
        switch (try sema.notePathToComptimeAllocPtr(msg, src, cur_val, intermediate_value_count, value_name)) {
            .done => return,
            .new_val => |new_val| {
                intermediate_value_count += 1;
                cur_val = new_val;
            },
        }
    }
}

fn notePathToComptimeAllocPtr(sema: *Sema, msg: *Zcu.ErrorMsg, src: LazySrcLoc, val: Value, intermediate_value_count: u32, start_value_name: InternPool.NullTerminatedString) Allocator.Error!union(enum) {
    done,
    new_val: Value,
} {
    const arena = sema.arena;
    const pt = sema.pt;
    const zcu = pt.zcu;
    const ip = &zcu.intern_pool;

    var first_path: std.ArrayListUnmanaged(u8) = .empty;
    if (intermediate_value_count == 0) {
        try first_path.writer(arena).print("{i}", .{start_value_name.fmt(ip)});
    } else {
        try first_path.writer(arena).print("v{}", .{intermediate_value_count - 1});
    }

    const comptime_ptr = try sema.notePathToComptimeAllocPtrInner(val, &first_path);

    switch (ip.indexToKey(comptime_ptr.toIntern()).ptr.base_addr) {
        .comptime_field => {
            try sema.errNote(src, msg, "'{s}' points to comptime field", .{first_path.items});
            return .done;
        },
        .comptime_alloc => |idx| {
            const cta = sema.getComptimeAlloc(idx);
            if (!cta.is_const) {
                try sema.errNote(cta.src, msg, "'{s}' points to comptime var declared here", .{first_path.items});
                return .done;
            }
        },
        else => {}, // there will be another stage
    }

    const derivation = comptime_ptr.pointerDerivationAdvanced(arena, pt, false, sema) catch |err| switch (err) {
        error.OutOfMemory => |e| return e,
        error.AnalysisFail => unreachable,
    };

    var second_path: std.ArrayListUnmanaged(u8) = .empty;
    const inter_name = try std.fmt.allocPrint(arena, "v{d}", .{intermediate_value_count});
    const deriv_start = @import("print_value.zig").printPtrDerivation(
        derivation,
        second_path.writer(arena),
        pt,
        .lvalue,
        .{ .str = inter_name },
        20,
    ) catch |err| switch (err) {
        error.OutOfMemory => |e| return e,
        error.AnalysisFail => unreachable,
        error.ComptimeReturn => unreachable,
        error.ComptimeBreak => unreachable,
    };

    switch (deriv_start) {
        .int, .nav_ptr => unreachable,
        .uav_ptr => |uav| {
            try sema.errNote(src, msg, "'{s}' points to '{s}', where", .{ first_path.items, second_path.items });
            return .{ .new_val = .fromInterned(uav.val) };
        },
        .comptime_alloc_ptr => |cta_info| {
            try sema.errNote(src, msg, "'{s}' points to '{s}', where", .{ first_path.items, second_path.items });
            const cta = sema.getComptimeAlloc(cta_info.idx);
            if (cta.is_const) {
                return .{ .new_val = cta_info.val };
            } else {
                try sema.errNote(cta.src, msg, "'{s}' is a comptime var declared here", .{inter_name});
                return .done;
            }
        },
        .comptime_field_ptr => {
            try sema.errNote(src, msg, "'{s}' points to '{s}', where", .{ first_path.items, second_path.items });
            try sema.errNote(src, msg, "'{s}' is a comptime field", .{inter_name});
            return .done;
        },
        .eu_payload_ptr,
        .opt_payload_ptr,
        .field_ptr,
        .elem_ptr,
        .offset_and_cast,
        => unreachable,
    }
}

fn notePathToComptimeAllocPtrInner(sema: *Sema, val: Value, path: *std.ArrayListUnmanaged(u8)) Allocator.Error!Value {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const ip = &zcu.intern_pool;
    const arena = sema.arena;
    assert(val.canMutateComptimeVarState(zcu));
    switch (ip.indexToKey(val.toIntern())) {
        .ptr => return val,
        .error_union => |eu| {
            try path.insert(arena, 0, '(');
            try path.appendSlice(arena, " catch unreachable)");
            return sema.notePathToComptimeAllocPtrInner(.fromInterned(eu.val.payload), path);
        },
        .slice => |slice| {
            try path.appendSlice(arena, ".ptr");
            return sema.notePathToComptimeAllocPtrInner(.fromInterned(slice.ptr), path);
        },
        .opt => |opt| {
            try path.appendSlice(arena, ".?");
            return sema.notePathToComptimeAllocPtrInner(.fromInterned(opt.val), path);
        },
        .un => |un| {
            assert(un.tag != .none);
            const union_ty: Type = .fromInterned(un.ty);
            const backing_enum = union_ty.unionTagTypeHypothetical(zcu);
            const field_idx = backing_enum.enumTagFieldIndex(.fromInterned(un.tag), zcu).?;
            const field_name = backing_enum.enumFieldName(field_idx, zcu);
            try path.writer(arena).print(".{i}", .{field_name.fmt(ip)});
            return sema.notePathToComptimeAllocPtrInner(.fromInterned(un.val), path);
        },
        .aggregate => |agg| {
            const elem: InternPool.Index, const elem_idx: usize = switch (agg.storage) {
                .bytes => unreachable,
                .repeated_elem => |elem| .{ elem, 0 },
                .elems => |elems| for (elems, 0..) |elem, elem_idx| {
                    if (Value.fromInterned(elem).canMutateComptimeVarState(zcu)) {
                        break .{ elem, elem_idx };
                    }
                } else unreachable,
            };
            const agg_ty: Type = .fromInterned(agg.ty);
            switch (agg_ty.zigTypeTag(zcu)) {
                .array, .vector => try path.writer(arena).print("[{d}]", .{elem_idx}),
                .pointer => switch (elem_idx) {
                    Value.slice_ptr_index => try path.appendSlice(arena, ".ptr"),
                    Value.slice_len_index => try path.appendSlice(arena, ".len"),
                    else => unreachable,
                },
                .@"struct" => if (agg_ty.isTuple(zcu)) {
                    try path.writer(arena).print("[{d}]", .{elem_idx});
                } else {
                    const name = agg_ty.structFieldName(elem_idx, zcu).unwrap().?;
                    try path.writer(arena).print(".{i}", .{name.fmt(ip)});
                },
                else => unreachable,
            }
            return sema.notePathToComptimeAllocPtrInner(.fromInterned(elem), path);
        },
        else => unreachable,
    }
}

/// Returns true if any value contained in `val` is undefined.
fn anyUndef(sema: *Sema, block: *Block, src: LazySrcLoc, val: Value) !bool {
    const pt = sema.pt;
    const zcu = pt.zcu;
    return switch (zcu.intern_pool.indexToKey(val.toIntern())) {
        .undef => true,
        .simple_value => |v| v == .undefined,
        .slice => {
            // If the slice contents are runtime-known, reification will fail later on with a
            // specific error message.
            const arr = try sema.maybeDerefSliceAsArray(block, src, val) orelse return false;
            return sema.anyUndef(block, src, arr);
        },
        .aggregate => |aggregate| for (0..aggregate.storage.values().len) |i| {
            const elem = zcu.intern_pool.indexToKey(val.toIntern()).aggregate.storage.values()[i];
            if (try sema.anyUndef(block, src, Value.fromInterned(elem))) break true;
        } else false,
        else => false,
    };
}

/// Asserts that `slice_val` is a slice of `u8`.
fn sliceToIpString(
    sema: *Sema,
    block: *Block,
    src: LazySrcLoc,
    slice_val: Value,
    reason: ComptimeReason,
) CompileError!InternPool.NullTerminatedString {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const slice_ty = slice_val.typeOf(zcu);
    assert(slice_ty.isSlice(zcu));
    assert(slice_ty.childType(zcu).toIntern() == .u8_type);
    const array_val = try sema.derefSliceAsArray(block, src, slice_val, reason);
    const array_ty = array_val.typeOf(zcu);
    return array_val.toIpString(array_ty, pt);
}

/// Given a slice value, attempts to dereference it into a comptime-known array.
/// Emits a compile error if the contents of the slice are not comptime-known.
/// Asserts that `slice_val` is a slice.
fn derefSliceAsArray(
    sema: *Sema,
    block: *Block,
    src: LazySrcLoc,
    slice_val: Value,
    reason: ComptimeReason,
) CompileError!Value {
    return try sema.maybeDerefSliceAsArray(block, src, slice_val) orelse {
        return sema.failWithNeededComptime(block, src, reason);
    };
}

/// Given a slice value, attempts to dereference it into a comptime-known array.
/// Returns `null` if the contents of the slice are not comptime-known.
/// Asserts that `slice_val` is a slice.
fn maybeDerefSliceAsArray(
    sema: *Sema,
    block: *Block,
    src: LazySrcLoc,
    slice_val: Value,
) CompileError!?Value {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const ip = &zcu.intern_pool;
    assert(slice_val.typeOf(zcu).isSlice(zcu));
    const slice = switch (ip.indexToKey(slice_val.toIntern())) {
        .undef => return sema.failWithUseOfUndef(block, src),
        .slice => |slice| slice,
        else => unreachable,
    };
    const elem_ty = Type.fromInterned(slice.ty).childType(zcu);
    const len = try Value.fromInterned(slice.len).toUnsignedIntSema(pt);
    const array_ty = try pt.arrayType(.{
        .child = elem_ty.toIntern(),
        .len = len,
    });
    const ptr_ty = try pt.ptrTypeSema(p: {
        var p = Type.fromInterned(slice.ty).ptrInfo(zcu);
        p.flags.size = .one;
        p.child = array_ty.toIntern();
        p.sentinel = .none;
        break :p p;
    });
    const casted_ptr = try pt.getCoerced(Value.fromInterned(slice.ptr), ptr_ty);
    return sema.pointerDeref(block, src, casted_ptr, ptr_ty);
}

fn analyzeUnreachable(sema: *Sema, block: *Block, src: LazySrcLoc, safety_check: bool) !void {
    if (safety_check and block.wantSafety()) {
        // We only apply the first hint in a branch.
        // This allows user-provided hints to override implicit cold hints.
        if (sema.branch_hint == null) {
            sema.branch_hint = .cold;
        }

        try sema.safetyPanic(block, src, .reached_unreachable);
    } else {
        _ = try block.addNoOp(.unreach);
    }
}

/// This should be called exactly once, at the end of a `Sema`'s lifetime.
/// It takes the exports stored in `sema.export` and flushes them to the `Zcu`
/// to be processed by the linker after the update.
pub fn flushExports(sema: *Sema) !void {
    if (sema.exports.items.len == 0) return;

    const zcu = sema.pt.zcu;
    const gpa = zcu.gpa;

    // There may be existing exports. For instance, a struct may export
    // things during both field type resolution and field default resolution.
    //
    // So, pick up and delete any existing exports. This strategy performs
    // redundant work, but that's okay, because this case is exceedingly rare.
    if (zcu.single_exports.get(sema.owner)) |export_idx| {
        try sema.exports.append(gpa, export_idx.ptr(zcu).*);
    } else if (zcu.multi_exports.get(sema.owner)) |info| {
        try sema.exports.appendSlice(gpa, zcu.all_exports.items[info.index..][0..info.len]);
    }
    zcu.deleteUnitExports(sema.owner);

    // `sema.exports` is completed; store the data into the `Zcu`.
    if (sema.exports.items.len == 1) {
        try zcu.single_exports.ensureUnusedCapacity(gpa, 1);
        const export_idx: Zcu.Export.Index = zcu.free_exports.pop() orelse idx: {
            _ = try zcu.all_exports.addOne(gpa);
            break :idx @enumFromInt(zcu.all_exports.items.len - 1);
        };
        export_idx.ptr(zcu).* = sema.exports.items[0];
        zcu.single_exports.putAssumeCapacityNoClobber(sema.owner, export_idx);
    } else {
        try zcu.multi_exports.ensureUnusedCapacity(gpa, 1);
        const exports_base = zcu.all_exports.items.len;
        try zcu.all_exports.appendSlice(gpa, sema.exports.items);
        zcu.multi_exports.putAssumeCapacityNoClobber(sema.owner, .{
            .index = @intCast(exports_base),
            .len = @intCast(sema.exports.items.len),
        });
    }
}

/// Called as soon as a `declared` enum type is created.
/// Resolves the tag type and field inits.
/// Marks the `src_inst` dependency on the enum's declaration, so call sites need not do this.
pub fn resolveDeclaredEnum(
    pt: Zcu.PerThread,
    wip_ty: InternPool.WipEnumType,
    inst: Zir.Inst.Index,
    tracked_inst: InternPool.TrackedInst.Index,
    namespace: InternPool.NamespaceIndex,
    type_name: InternPool.NullTerminatedString,
    small: Zir.Inst.EnumDecl.Small,
    body: []const Zir.Inst.Index,
    tag_type_ref: Zir.Inst.Ref,
    any_values: bool,
    fields_len: u32,
    zir: Zir,
    body_end: usize,
) Zcu.SemaError!void {
    const zcu = pt.zcu;
    const gpa = zcu.gpa;

    const src: LazySrcLoc = .{ .base_node_inst = tracked_inst, .offset = LazySrcLoc.Offset.nodeOffset(.zero) };

    var arena: std.heap.ArenaAllocator = .init(gpa);
    defer arena.deinit();

    var comptime_err_ret_trace: std.ArrayList(Zcu.LazySrcLoc) = .init(gpa);
    defer comptime_err_ret_trace.deinit();

    var sema: Sema = .{
        .pt = pt,
        .gpa = gpa,
        .arena = arena.allocator(),
        .code = zir,
        .owner = .wrap(.{ .type = wip_ty.index }),
        .func_index = .none,
        .func_is_naked = false,
        .fn_ret_ty = .void,
        .fn_ret_ty_ies = null,
        .comptime_err_ret_trace = &comptime_err_ret_trace,
    };
    defer sema.deinit();

    if (zcu.comp.debugIncremental()) {
        const info = try zcu.incremental_debug_state.getUnitInfo(gpa, sema.owner);
        info.last_update_gen = zcu.generation;
    }

    try sema.declareDependency(.{ .src_hash = tracked_inst });

    var block: Block = .{
        .parent = null,
        .sema = &sema,
        .namespace = namespace,
        .instructions = .{},
        .inlining = null,
        .comptime_reason = .{ .reason = .{
            .src = src,
            .r = .{ .simple = .enum_fields },
        } },
        .src_base_inst = tracked_inst,
        .type_name_ctx = type_name,
    };
    defer block.instructions.deinit(gpa);

    sema.resolveDeclaredEnumInner(
        &block,
        wip_ty,
        inst,
        tracked_inst,
        src,
        small,
        body,
        tag_type_ref,
        any_values,
        fields_len,
        zir,
        body_end,
    ) catch |err| switch (err) {
        error.ComptimeBreak => unreachable,
        error.ComptimeReturn => unreachable,
        error.OutOfMemory => |e| return e,
        error.AnalysisFail => {
            if (!zcu.failed_analysis.contains(sema.owner)) {
                try zcu.transitive_failed_analysis.put(gpa, sema.owner, {});
            }
            return error.AnalysisFail;
        },
    };
}

fn resolveDeclaredEnumInner(
    sema: *Sema,
    block: *Block,
    wip_ty: InternPool.WipEnumType,
    inst: Zir.Inst.Index,
    tracked_inst: InternPool.TrackedInst.Index,
    src: LazySrcLoc,
    small: Zir.Inst.EnumDecl.Small,
    body: []const Zir.Inst.Index,
    tag_type_ref: Zir.Inst.Ref,
    any_values: bool,
    fields_len: u32,
    zir: Zir,
    body_end: usize,
) Zcu.CompileError!void {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const gpa = zcu.gpa;
    const ip = &zcu.intern_pool;

    const bit_bags_count = std.math.divCeil(usize, fields_len, 32) catch unreachable;

    const tag_ty_src: LazySrcLoc = .{ .base_node_inst = tracked_inst, .offset = .{ .node_offset_container_tag = .zero } };

    const int_tag_ty = ty: {
        if (body.len != 0) {
            _ = try sema.analyzeInlineBody(block, body, inst);
        }

        if (tag_type_ref != .none) {
            const ty = try sema.resolveType(block, tag_ty_src, tag_type_ref);
            if (ty.zigTypeTag(zcu) != .int and ty.zigTypeTag(zcu) != .comptime_int) {
                return sema.fail(block, tag_ty_src, "expected integer tag type, found '{}'", .{ty.fmt(pt)});
            }
            break :ty ty;
        } else if (fields_len == 0) {
            break :ty try pt.intType(.unsigned, 0);
        } else {
            const bits = std.math.log2_int_ceil(usize, fields_len);
            break :ty try pt.intType(.unsigned, bits);
        }
    };

    wip_ty.setTagTy(ip, int_tag_ty.toIntern());

    var extra_index = body_end + bit_bags_count;
    var bit_bag_index: usize = body_end;
    var cur_bit_bag: u32 = undefined;
    var last_tag_val: ?Value = null;
    for (0..fields_len) |field_i_usize| {
        const field_i: u32 = @intCast(field_i_usize);
        if (field_i % 32 == 0) {
            cur_bit_bag = zir.extra[bit_bag_index];
            bit_bag_index += 1;
        }
        const has_tag_value = @as(u1, @truncate(cur_bit_bag)) != 0;
        cur_bit_bag >>= 1;

        const field_name_index: Zir.NullTerminatedString = @enumFromInt(zir.extra[extra_index]);
        const field_name_zir = zir.nullTerminatedString(field_name_index);
        extra_index += 1; // field name

        const field_name = try ip.getOrPutString(gpa, pt.tid, field_name_zir, .no_embedded_nulls);

        const value_src: LazySrcLoc = .{
            .base_node_inst = tracked_inst,
            .offset = .{ .container_field_value = field_i },
        };

        const tag_overflow = if (has_tag_value) overflow: {
            const tag_val_ref: Zir.Inst.Ref = @enumFromInt(zir.extra[extra_index]);
            extra_index += 1;
            const tag_inst = try sema.resolveInst(tag_val_ref);
            last_tag_val = try sema.resolveConstDefinedValue(block, .{
                .base_node_inst = tracked_inst,
                .offset = .{ .container_field_name = field_i },
            }, tag_inst, .{ .simple = .enum_field_tag_value });
            if (!(try sema.intFitsInType(last_tag_val.?, int_tag_ty, null))) break :overflow true;
            last_tag_val = try pt.getCoerced(last_tag_val.?, int_tag_ty);
            if (wip_ty.nextField(ip, field_name, last_tag_val.?.toIntern())) |conflict| {
                assert(conflict.kind == .value); // AstGen validated names are unique
                const other_field_src: LazySrcLoc = .{
                    .base_node_inst = tracked_inst,
                    .offset = .{ .container_field_value = conflict.prev_field_idx },
                };
                const msg = msg: {
                    const msg = try sema.errMsg(value_src, "enum tag value {} already taken", .{last_tag_val.?.fmtValueSema(pt, sema)});
                    errdefer msg.destroy(gpa);
                    try sema.errNote(other_field_src, msg, "other occurrence here", .{});
                    break :msg msg;
                };
                return sema.failWithOwnedErrorMsg(block, msg);
            }
            break :overflow false;
        } else if (any_values) overflow: {
            if (last_tag_val) |last_tag| {
                const result = try arith.incrementDefinedInt(sema, int_tag_ty, last_tag);
                last_tag_val = result.val;
                if (result.overflow) break :overflow true;
            } else {
                last_tag_val = try pt.intValue(int_tag_ty, 0);
            }
            if (wip_ty.nextField(ip, field_name, last_tag_val.?.toIntern())) |conflict| {
                assert(conflict.kind == .value); // AstGen validated names are unique
                const other_field_src: LazySrcLoc = .{
                    .base_node_inst = tracked_inst,
                    .offset = .{ .container_field_value = conflict.prev_field_idx },
                };
                const msg = msg: {
                    const msg = try sema.errMsg(value_src, "enum tag value {} already taken", .{last_tag_val.?.fmtValueSema(pt, sema)});
                    errdefer msg.destroy(gpa);
                    try sema.errNote(other_field_src, msg, "other occurrence here", .{});
                    break :msg msg;
                };
                return sema.failWithOwnedErrorMsg(block, msg);
            }
            break :overflow false;
        } else overflow: {
            assert(wip_ty.nextField(ip, field_name, .none) == null);
            last_tag_val = try pt.intValue(.comptime_int, field_i);
            if (!try sema.intFitsInType(last_tag_val.?, int_tag_ty, null)) break :overflow true;
            last_tag_val = try pt.getCoerced(last_tag_val.?, int_tag_ty);
            break :overflow false;
        };

        if (tag_overflow) {
            const msg = try sema.errMsg(value_src, "enumeration value '{}' too large for type '{}'", .{
                last_tag_val.?.fmtValueSema(pt, sema), int_tag_ty.fmt(pt),
            });
            return sema.failWithOwnedErrorMsg(block, msg);
        }
    }
    if (small.nonexhaustive and int_tag_ty.toIntern() != .comptime_int_type) {
        if (fields_len >= 1 and std.math.log2_int(u64, fields_len) == int_tag_ty.bitSize(zcu)) {
            return sema.fail(block, src, "non-exhaustive enum specifies every value", .{});
        }
    }
}

pub const bitCastVal = @import("Sema/bitcast.zig").bitCast;
pub const bitCastSpliceVal = @import("Sema/bitcast.zig").bitCastSplice;

const loadComptimePtr = @import("Sema/comptime_ptr_access.zig").loadComptimePtr;
const ComptimeLoadResult = @import("Sema/comptime_ptr_access.zig").ComptimeLoadResult;
const storeComptimePtr = @import("Sema/comptime_ptr_access.zig").storeComptimePtr;
const ComptimeStoreResult = @import("Sema/comptime_ptr_access.zig").ComptimeStoreResult;

pub fn getBuiltinType(sema: *Sema, src: LazySrcLoc, decl: Zcu.BuiltinDecl) SemaError!Type {
    assert(decl.kind() == .type);
    try sema.ensureMemoizedStateResolved(src, decl.stage());
    return .fromInterned(sema.pt.zcu.builtin_decl_values.get(decl));
}
pub fn getBuiltin(sema: *Sema, src: LazySrcLoc, decl: Zcu.BuiltinDecl) SemaError!InternPool.Index {
    assert(decl.kind() != .type);
    try sema.ensureMemoizedStateResolved(src, decl.stage());
    return sema.pt.zcu.builtin_decl_values.get(decl);
}

pub const NavPtrModifiers = struct {
    alignment: Alignment,
    @"linksection": InternPool.OptionalNullTerminatedString,
    @"addrspace": std.builtin.AddressSpace,
};

pub fn resolveNavPtrModifiers(
    sema: *Sema,
    block: *Block,
    zir_decl: Zir.Inst.Declaration.Unwrapped,
    decl_inst: Zir.Inst.Index,
    nav_ty: Type,
) CompileError!NavPtrModifiers {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const gpa = zcu.gpa;
    const ip = &zcu.intern_pool;

    const align_src = block.src(.{ .node_offset_var_decl_align = .zero });
    const section_src = block.src(.{ .node_offset_var_decl_section = .zero });
    const addrspace_src = block.src(.{ .node_offset_var_decl_addrspace = .zero });

    const alignment: InternPool.Alignment = a: {
        const align_body = zir_decl.align_body orelse break :a .none;
        const align_ref = try sema.resolveInlineBody(block, align_body, decl_inst);
        break :a try sema.analyzeAsAlign(block, align_src, align_ref);
    };

    const @"linksection": InternPool.OptionalNullTerminatedString = ls: {
        const linksection_body = zir_decl.linksection_body orelse break :ls .none;
        const linksection_ref = try sema.resolveInlineBody(block, linksection_body, decl_inst);
        const bytes = try sema.toConstString(block, section_src, linksection_ref, .{ .simple = .@"linksection" });
        if (std.mem.indexOfScalar(u8, bytes, 0) != null) {
            return sema.fail(block, section_src, "linksection cannot contain null bytes", .{});
        } else if (bytes.len == 0) {
            return sema.fail(block, section_src, "linksection cannot be empty", .{});
        }
        break :ls try ip.getOrPutStringOpt(gpa, pt.tid, bytes, .no_embedded_nulls);
    };

    const @"addrspace": std.builtin.AddressSpace = as: {
        const addrspace_ctx: std.builtin.AddressSpace.Context = switch (zir_decl.kind) {
            .@"var" => .variable,
            else => switch (nav_ty.zigTypeTag(zcu)) {
                .@"fn" => .function,
                else => .constant,
            },
        };
        const target = zcu.getTarget();
        const addrspace_body = zir_decl.addrspace_body orelse break :as switch (addrspace_ctx) {
            .function => target_util.defaultAddressSpace(target, .function),
            .variable => target_util.defaultAddressSpace(target, .global_mutable),
            .constant => target_util.defaultAddressSpace(target, .global_constant),
            else => unreachable,
        };
        const addrspace_ref = try sema.resolveInlineBody(block, addrspace_body, decl_inst);
        break :as try sema.analyzeAsAddressSpace(block, addrspace_src, addrspace_ref, addrspace_ctx);
    };

    return .{
        .alignment = alignment,
        .@"linksection" = @"linksection",
        .@"addrspace" = @"addrspace",
    };
}

pub fn analyzeMemoizedState(sema: *Sema, block: *Block, simple_src: LazySrcLoc, builtin_namespace: InternPool.NamespaceIndex, stage: InternPool.MemoizedStateStage) CompileError!bool {
    const pt = sema.pt;
    const zcu = pt.zcu;
    const ip = &zcu.intern_pool;
    const gpa = zcu.gpa;

    var any_changed = false;

    inline for (comptime std.enums.values(Zcu.BuiltinDecl)) |builtin_decl| {
        if (stage == comptime builtin_decl.stage()) {
            const parent_ns: Zcu.Namespace.Index, const parent_name: []const u8, const name: []const u8 = switch (comptime builtin_decl.access()) {
                .direct => |name| .{ builtin_namespace, "std.builtin", name },
                .nested => |nested| access: {
                    const parent_ty: Type = .fromInterned(zcu.builtin_decl_values.get(nested[0]));
                    const parent_ns = parent_ty.getNamespace(zcu).unwrap() orelse {
                        return sema.fail(block, simple_src, "std.builtin.{s} is not a container type", .{@tagName(nested[0])});
                    };
                    break :access .{ parent_ns, "std.builtin." ++ @tagName(nested[0]), nested[1] };
                },
            };

            const name_nts = try ip.getOrPutString(gpa, pt.tid, name, .no_embedded_nulls);
            const nav = try sema.namespaceLookup(block, simple_src, parent_ns, name_nts) orelse
                return sema.fail(block, simple_src, "{s} missing {s}", .{ parent_name, name });

            const src: LazySrcLoc = .{
                .base_node_inst = ip.getNav(nav).srcInst(ip),
                .offset = .nodeOffset(.zero),
            };

            const result = try sema.analyzeNavVal(block, src, nav);

            const uncoerced_val = try sema.resolveConstDefinedValue(block, src, result, null);
            const maybe_lazy_val: Value = switch (builtin_decl.kind()) {
                .type => if (uncoerced_val.typeOf(zcu).zigTypeTag(zcu) != .type) {
                    return sema.fail(block, src, "{s}.{s} is not a type", .{ parent_name, name });
                } else val: {
                    try uncoerced_val.toType().resolveFully(pt);
                    break :val uncoerced_val;
                },
                .func => val: {
                    const func_ty = try sema.getExpectedBuiltinFnType(builtin_decl);
                    const coerced = try sema.coerce(block, func_ty, Air.internedToRef(uncoerced_val.toIntern()), src);
                    break :val .fromInterned(coerced.toInterned().?);
                },
                .string => val: {
                    const coerced = try sema.coerce(block, .slice_const_u8, Air.internedToRef(uncoerced_val.toIntern()), src);
                    break :val .fromInterned(coerced.toInterned().?);
                },
            };
            const val = try sema.resolveLazyValue(maybe_lazy_val);

            const prev = zcu.builtin_decl_values.get(builtin_decl);
            if (val.toIntern() != prev) {
                zcu.builtin_decl_values.set(builtin_decl, val.toIntern());
                any_changed = true;
            }
        }
    }

    return any_changed;
}

/// Given that `decl.kind() == .func`, get the type expected of the function.
fn getExpectedBuiltinFnType(sema: *Sema, decl: Zcu.BuiltinDecl) CompileError!Type {
    const pt = sema.pt;
    return switch (decl) {
        // `noinline fn () void`
        .returnError => try pt.funcType(.{
            .param_types = &.{},
            .return_type = .void_type,
            .is_noinline = true,
        }),

        // `fn ([]const u8, ?usize) noreturn`
        .@"panic.call" => try pt.funcType(.{
            .param_types = &.{
                .slice_const_u8_type,
                (try pt.optionalType(.usize_type)).toIntern(),
            },
            .return_type = .noreturn_type,
        }),

        // `fn (anytype, anytype) noreturn`
        .@"panic.sentinelMismatch",
        .@"panic.inactiveUnionField",
        => try pt.funcType(.{
            .param_types = &.{ .generic_poison_type, .generic_poison_type },
            .return_type = .noreturn_type,
            .is_generic = true,
        }),

        // `fn (anyerror) noreturn`
        .@"panic.unwrapError" => try pt.funcType(.{
            .param_types = &.{.anyerror_type},
            .return_type = .noreturn_type,
        }),

        // `fn (usize) noreturn`
        .@"panic.sliceCastLenRemainder" => try pt.funcType(.{
            .param_types = &.{.usize_type},
            .return_type = .noreturn_type,
        }),

        // `fn (usize, usize) noreturn`
        .@"panic.outOfBounds",
        .@"panic.startGreaterThanEnd",
        => try pt.funcType(.{
            .param_types = &.{ .usize_type, .usize_type },
            .return_type = .noreturn_type,
        }),

        // `fn () noreturn`
        .@"panic.reachedUnreachable",
        .@"panic.unwrapNull",
        .@"panic.castToNull",
        .@"panic.incorrectAlignment",
        .@"panic.invalidErrorCode",
        .@"panic.integerOutOfBounds",
        .@"panic.integerOverflow",
        .@"panic.shlOverflow",
        .@"panic.shrOverflow",
        .@"panic.divideByZero",
        .@"panic.exactDivisionRemainder",
        .@"panic.integerPartOutOfBounds",
        .@"panic.corruptSwitch",
        .@"panic.shiftRhsTooBig",
        .@"panic.invalidEnumValue",
        .@"panic.forLenMismatch",
        .@"panic.copyLenMismatch",
        .@"panic.memcpyAlias",
        .@"panic.noreturnReturned",
        => try pt.funcType(.{
            .param_types = &.{},
            .return_type = .noreturn_type,
        }),

        else => unreachable,
    };
}