https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86288
--- Comment #5 from Jonathan Wakely <redi at gcc dot gnu.org> --- Clang has now implemented this as a special case, only recognizing "__gnu__" as a synonym for the "gnu" attribute-namespace. That's fine for libstdc++'s purposes, as I don't expect we'll start using any other vendor attributes. EDG have also been told about this, so compilers based on their front end should also be able to recognize [[__gnu__::__blah__]] at some point.
