broken LLVM module on use of optional slice

[daurnimator]

const std = @import("std");
const testing = std.testing;
test "crash" {
    const foo = struct {
        pub fn bar(self: *@This()) ?[]u8 {
            return null;
        }
    }{};
    const expected = @as(?[]const u8, null);
    const actual = foo.bar();
    testing.expectEqual(expected, actual);
}

$ zig test bad-codegen.zig 
Code Generation [494/713] std.mem.Allocator.alignedAlloc...broken LLVM module found: Call parameter type does not match function signature!
  %actual = alloca %"?[]u8", align 8
 %"?[]const u8"*  call fastcc void @std.testing.expectEqual(%"?[]const u8"* @65, %"?[]u8"* %actual), !dbg !2101

This is a bug in the Zig compiler.
/home/daurnimator/src/zig/src-self-hosted/stage2.zig:37:5: 0x5586e997fe3c in stage2_panic (zigstage2)
    @panic(ptr[0..len]);
    ^
/home/daurnimator/src/zig/src/util.cpp:20:17: 0x5586e993b489 in zig_panic (/home/daurnimator/src/zig/src/util.cpp)
    stage2_panic("", 0);
                ^
/home/daurnimator/src/zig/src/codegen.cpp:7997:18: 0x5586e987581c in do_code_gen (/home/daurnimator/src/zig/src/codegen.cpp)
    if (LLVMVerifyModule(g->module, LLVMReturnStatusAction, &error)) {
                 ^
/home/daurnimator/src/zig/src/codegen.cpp:10774:24: 0x5586e9880afb in codegen_build_and_link (/home/daurnimator/src/zig/src/codegen.cpp)

                       ^
/home/daurnimator/src/zig/src/main.cpp:1669:39: 0x5586e98514ea in main0 (/home/daurnimator/src/zig/src/main.cpp)
                codegen_translate_c(g, in_file_buf);
                                      ^
/home/daurnimator/src/zig/src/main.cpp:1747:24: 0x5586e985197d in main (/home/daurnimator/src/zig/src/main.cpp)
        return stage2_cmd_targets(target_string, mcpu, dynamic_linker);
                       ^
???:?:?: 0x7f346be58022 in ??? (???)
Aborted (core dumped)

Possible duplicate of #4904

[LemonBoy]

The test case can be further simplified:

fn fu(x: var, y: @TypeOf(x)) void {}
test "crash" {
    var x0: ?[]const u8 = null;
    var x1: ?[]u8 = null;
    fu(x0, x1);
}

[shakesoda]

certainly not simpler, but looks like I ran into this when updating zig-vulkan-triangle, too: https://gist.github.com/shakesoda/28eb50428eb683ce677b8b1903a57473

[xackus]

Even simpler case:

fn fu(x: ?[]const u8) void {}

export fn bar() void {
    var x1: ?[]u8 = null;
    fu(x1);
}

The problem is caused by zig generating two different LLVM types for const and non-const optional slices.

[Vexu]

$ git grep "cast slice to const slice" test
test/behavior/optional.zig:test "cast slice to const slice nested in error union and optional" {