Allowed to use undefined in comparison

[ratfactor]

Normally using undefined in a comparison produces a compiler error:

./foo.zig:9:9: error: use of undefined value here causes undefined behavior
    if(1==undefined) return;

But when using "this one weird trick" (comparing an optional pointer struct field), there are no errors and it compiles and runs:

const std = @import("std");

const Foo = struct{ a: ?*u8 = undefined };

pub fn main() void {
    var foo = Foo{};
    if(foo.a == undefined) std.debug.print("foo.a is undef {}\n",.{foo.a});
}

[LakeByTheWoods]

It also works if a is a non-optional pointer: https://godbolt.org/z/vExKb7

[RetroDev256]

Confirmed this bug lack of error message is in self-hosted, 0.13+.

[tau-dev]

This has been fixed in master:

const std = @import("std");

const Foo = struct{ a: ?*u8 = undefined };

pub fn main() void {
    var foo = Foo{};
    const c: u8 = 1;
    if(foo.a == undefined) std.debug.print("foo.a is undef {}\n",.{foo.a orelse &c});
    _ = &foo;
}

/tmp/lol.zig:8:14: error: use of undefined value here causes undefined behavior
    if(foo.a == undefined) std.debug.print("foo.a is undef {}\n",.{foo.a orelse &c});
       ~~~~~~^~~~~~~~~~~~
referenced by:
    posixCallMainAndExit: zig-out/lib/zig/std/start.zig:647:22
    _start: zig-out/lib/zig/std/start.zig:464:40