[Bug c++/72104] Inner templated class with stream operator fails to compile

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=72104

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Romain Geissler from comment #0)
> Is this a gcc bug ? Or is this invalid C++ ?

GCC 6, Clang and EDG all reject it for the same reason, so it's probably
invalid, and is now rejected due to G++ getting better at identifying invalid
friend declarations.