It's a known issue, but it's shown up in a few places (GPU code, cc/ipc/transferable_resource_struct_traits.h, and probably a few more I'm not thinking of atm).

While Mojo currently intentionally does not support it, we might want to consider if/how to incorporate this. It's nice to explicitly not define members if getting/setting them is actually a logical error on certain platforms (because they do nothing, etc).