https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61447
--- Comment #2 from Rich Felker <bugdal at aerifal dot cx> --- Indeed, you seem to be correct in that claim. However it seems desirable that if this usage is not supported, it should be treated as a constraint violation rather than silently producing a wrong/meaningless result that appears "right" in most cases. In any case, I still think it's a bug that the wrong value is being produced for _Alignof(long long) except with -std=c11. Even if earlier versions of the standard have nothing to say about what it should produce, the principle of least surprise says that it should produce the same result as it does under C11 since only C11 defines _Alignof.
