@fieldParentPtr gives wrong result for zero-size unions

[vadim-za]

Zig Version

zig-0.12.0-dev.3489+c808e546a

Steps to Reproduce and Observed Behavior

Compile and run the following code

const std = @import("std");

const A = struct{
    fn u(self: *@This()) *U {
        return @fieldParentPtr(U,"a",self);
    }
};
const B = struct{
    fn u(self: *@This()) *U {
        return @fieldParentPtr(U,"b",self);
    }
};
const U = union(enum) { a: A, b: B };

pub fn main() !void {
    var u: U = .{ .a = .{} };
    const pa = &u.a;
    const pu = pa.u();
    std.debug.print("{X} {X}\n",.{@intFromPtr(pu),@intFromPtr(&u)});
}

The code prints two address values, the first being 1 smaller than the second one. E.g.

3B477FFB76 3B477FFB77

Tested on x86_64 and Aarch64.

Expected Behavior

Expected both values to be equal.

[Vexu]

My guess for the cause is this optimization that only applies to structs:

[jacobly0]

Note that this already has llvm-disabled test coverage in #19470 with test "@fieldParentPtr tagged union all zero-bit fields".