https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67093
--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> --- And right before the quote you gave: "An application that relies on such a feature or behavior cannot be assured to be portable across conforming implementations." So this might not be a full requirement of POSIX, it is not a portable to do it. So GCC warns about this case.
