memcpy crash when dest is a multi ptr

[travisstaloch]

Zig Version

0.11.0-dev.2912+d65b42e07

Steps to Reproduce and Observed Behavior

I'm not confident that the title is the correct way to frame the issue. But the following code leads to compiler crash below.

// /tmp/tmp.zig
test {
    var value: f64 = undefined;
    var word: u64 = undefined;
    @memcpy(@ptrCast([*]u8, &value), @ptrCast([*]const u8, &word)[0..@sizeOf(f64)]);
    // properly slicing dest does not cause the crash
    // @memcpy(@ptrCast([*]u8, &value)[0..@sizeOf(f64)], @ptrCast([*]const u8, &word)[0..@sizeOf(f64)]);
}

$ zig version
0.11.0-dev.2912+d65b42e07

$ zig test /tmp/tmp.zig
thread 72774 panic: reached unreachable code
~/projects/zig/zig/src/codegen/llvm.zig:5807:26: 0x1828d6f in sliceOrArrayLenInBytes (zig)
            .Many, .C => unreachable,
                         ^
~/projects/zig/zig/src/codegen/llvm.zig:8553:48: 0x18290a2 in airMemcpy (zig)
        const len = self.sliceOrArrayLenInBytes(dest_slice, dest_ptr_ty);
                                               ^
~/projects/zig/zig/src/codegen/llvm.zig:4678:54: 0x11cfcfd in genBody (zig)
                .memcpy         => try self.airMemcpy(inst),
                                                     ^
~/projects/zig/zig/src/codegen/llvm.zig:1232:35: 0x11c9e51 in updateFunc (zig)
        fg.genBody(air.getMainBody()) catch |err| switch (err) {
                                  ^
~/projects/zig/zig/src/link/Elf.zig:2533:74: 0x11d2b2e in updateFunc (zig)
        if (self.llvm_object) |llvm_object| return llvm_object.updateFunc(module, func, air, liveness);
                                                                         ^
~/projects/zig/zig/src/link.zig:578:77: 0xf0794d in updateFunc (zig)
            .elf   => return @fieldParentPtr(Elf,   "base", base).updateFunc(module, func, air, liveness),
                                                                            ^
~/projects/zig/zig/src/Module.zig:4383:37: 0xcca74c in ensureFuncBodyAnalyzed (zig)
            comp.bin_file.updateFunc(mod, func, air, liveness) catch |err| switch (err) {
                                    ^
~/projects/zig/zig/src/Compilation.zig:3127:42: 0xcc7c5d in processOneJob (zig)
            module.ensureFuncBodyAnalyzed(func) catch |err| switch (err) {
                                         ^
~/projects/zig/zig/src/Compilation.zig:3064:30: 0xb5961e in performAllTheWork (zig)
            try processOneJob(comp, work_item, main_progress_node);
                             ^
~/projects/zig/zig/src/Compilation.zig:2020:31: 0xb5596d in update (zig)
    try comp.performAllTheWork(main_progress_node);
                              ^
~/projects/zig/zig/src/main.zig:3820:24: 0xb87104 in updateModule (zig)
        try comp.update(main_progress_node);
                       ^
~/projects/zig/zig/src/main.zig:3255:17: 0x9f397e in buildOutputType (zig)
    updateModule(comp) catch |err| switch (err) {
                ^
~/projects/zig/zig/src/main.zig:273:31: 0x9c4806 in mainArgs (zig)
        return buildOutputType(gpa, arena, args, .zig_test);
                              ^
~/projects/zig/zig/src/main.zig:211:20: 0x9c3a85 in main (zig)
    return mainArgs(gpa, arena, args);
                   ^
~/projects/zig/zig/lib/std/start.zig:609:37: 0x9c63a4 in main (zig)
            const result = root.main() catch |err| {
                                    ^
Aborted

Expected Behavior

no crash. i would expect this to compile as dest is allowed to be a multiptr in the docs.