http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56335
--- Comment #2 from joseph at codesourcery dot com <joseph at codesourcery dot com> 2013-02-15 15:47:00 UTC --- See <http://gcc.gnu.org/ml/gcc-patches/2011-11/msg00841.html>, where I discuss what I think would be the appropriate checks for supported alignment (which I deferred for the initial implementation of _Alignas / _Alignof). I think the checks should be errors for both the C11 _Alignas syntax and the __attribute__ syntax.
