[Bug c++/59480] Missing error diagnostic: friend declaration specifying a default argument must be a definition

[paolo.carlini at oracle dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=59480

--- Comment #17 from Paolo Carlini <paolo.carlini at oracle dot com> ---
If we can exploit DECL_HIDDEN_FRIEND_P of the olddecl everything is much
easier: see attached draft which I'm probably going to resubmit as-is of with
only minor modifications.