https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82272
--- Comment #2 from H. Peter Anvin <hpa at zytor dot com> --- Since it doesn't seem to be clear from the text, perhaps an interpretation request to the committee is in order. If this indeed is the requirement, I would suggest implementing it as a gnu99/gnu11 extension and push for a relaxation of the requirement in future versions of the C standard. Either that or push for actual _True and _False in a future version of the C standard, equivalent to ((_Bool)1) and ((_Bool)0) except usable in the C preprocessor. That would also let people rely on, for example, sizeof(true) == sizeof(bool).
