@fieldParentPtr always fails for zero sized type field pointers.

[alexnask]

const std = @import("std");

const Parent = struct {
    child: Child,
};

const Child = struct {};

test "" {
    var p = Parent{ .child = .{} };
    var c = &p.child;
    // Fails with "error: pointer value not based on parent struct", 'c' is resolved to be comptime and not set to point to a parent struct correctly.
    std.debug.assert(@fieldParentPtr(Parent, "child", c) == &p);
}

Workaround: add any non-zero sized state to Child

[SpexGuy]

With the decision in #6706, pointers to zero-sized types are now real pointers, so this should be fixed once the compiler is updated to honor this.