[Bug c++/86288] Recognize __gnu and/or __gnu__ as attribute-namespace

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86288

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
I think Jakub's patch does only allow __gnu__, using the same canonicalization
for attribute-namespaces as for attributes.