[Bug c++/87571] [8/9 Regression] ICE in friend_accessible_p, accessing protected member of template friend inside template class

[aoliva at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87571

Alexandre Oliva <aoliva at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2018-11-28
                 CC|                            |aoliva at gcc dot gnu.org
     Ever confirmed|0                           |1

--- Comment #1 from Alexandre Oliva <aoliva at gcc dot gnu.org> ---
The reason private works where protected doesn't is that private access
checking uses is_friend, whereas protected access checking uses
friend_accessible_p.  The latter is supposed to grant access whenever is_friend
would (that's the assert that fails), but while is_friend checks various
possibilities of template specialization with is_specialization_of_friend,
friend_accessible_p seems to only handle one layer of template friends.

I'm afraid I don't quite see how to extend e.g. protected_accessible_p, used by
friend_accessible_p, to deal with additional template layers.