[Bug c++/60695] std::atomic doesn't work when X is of zero size

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60695

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
That type isn't valid in ISO C++ and I don't see how you can usefully use it as
an atomic object, so I don't think this failure matters.

My preference would be to add this to std::atomic<_Tp>:

  static_assert( sizeof(_Tp) > 0, "Don't be silly" );