Compiler crash on comptime var assginment with tagged union

[vancluever]

Zig Version

0.12.0-dev.3518+d2be725e4

Steps to Reproduce and Observed Behavior

The following code will cause SIGTRAP on zig build. The repro was set up off of the zig init scaffolding:

const std = @import("std");

const A = struct {
    a: i32,
};

const B = struct {
    a: i32,
};

const AB_type = enum {
    a,
    b,
};

const AB = union(AB_type) {
    a: A,
    b: B,
};

pub fn main() !void {
    comptime var foo: AB = .{ .a = .{ .a = 32 } };
    std.debug.print("{d}\n", .{foo.a.a});
    foo = .{ .a = .{ .a = 64 } };
    std.debug.print("{d}\n", .{foo.a.a});
}

Removing the comptime from the comptime var assignment fixes things, allowing the build/run to succeed.

Expected Behavior

Build should succeed, or tell me why what I am doing is a bad idea and/or not allowed. In the real world situation where this came up, I think for some reason the compiler used to be telling me that the union needed to be comptime, even though I probably just needed to add a type to it or something else. 😉

[mlugg]

Likely related to #19452 - this example should be tested when that branch is complete.

[vancluever]

@mlugg if it helps, I did roll back to 0.12.0-dev.3405+31791ae15 during troubleshooting, which helped surface some of the errors (I think that re-assignment here actually caused a full-on segfault). 0.12.0-dev.2811+3cafb9655 was working completely without errors.

[Vexu]

Related #19511