[Bug c++/102995] New: Template friend class declaration of same class with different template parameters fails to allow private methods access

[jdapena at igalia dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102995

            Bug ID: 102995
           Summary: Template friend class declaration of same class with
                    different template parameters fails to allow private
                    methods access
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: jdapena at igalia dot com
  Target Milestone: ---

Created attachment 51698
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=51698&action=edit
Test case

Declaring a template friend class to the same class fails to resolve the access
to private members in its methods.

The provided example fails with this error:

  $ g++ main.cc 
  main.cc: In instantiation of 'bool operator==(const First<A, B>&, const 
  First<C, B>&) [with C = void*; A = int; B = bool]':
  main.cc:23:15:   required from here
  main.cc:13:61: error: 'int First<A, B>::GetId() const [with A = void*; B = 
  bool]' is private within this context
     13 |                              return lhs.GetId() == rhs.GetId();
        |                                                    ~~~~~~~~~^~
  main.cc:6:9: note: declared private here
      6 |     int GetId() const {
        |         ^~~~~


The same case works in Clang  (verified with Clang 9.0.1).

This issue is exposed in Chromium, compiling base/raw_ptr.h unit tests on GCC.
I created a simplified version of the problem. See:

https://chromium.googlesource.com/chromium/src/base/+/821f1e481ce7c51d31486f26a74d343ffe4cf70e/memory/raw_ptr.h#496
(declaration of the method that should have friend access to rhs private
methods).

https://chromium.googlesource.com/chromium/src/base/+/821f1e481ce7c51d31486f26a74d343ffe4cf70e/memory/raw_ptr.h#584
(the friend class declaration).

https://chromium.googlesource.com/chromium/src/base/+/821f1e481ce7c51d31486f26a74d343ffe4cf70e/memory/raw_ptr_unittest.cc#328
(failing test compilation).