[Bug c++/95484] New: Friend declaration of member function template has no effect

[herring at lanl dot gov]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95484

            Bug ID: 95484
           Summary: Friend declaration of member function template has no
                    effect
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Keywords: rejects-valid
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: herring at lanl dot gov
  Target Milestone: ---

The current build at Compiler Explorer rejects

  struct A {
    template<class T> operator T*() const;
  };

  class B {
    constexpr static decltype(nullptr) null=nullptr;
    template<class T> friend A::operator T*() const;
  };

  template<class T> A::operator T*() const {return B::null;}

  int main() {
    int *p=A();
    return !!p;
  }

with

  <source>: In instantiation of 'A::operator T*() const [with T = int]':
  <source>:13:12:   required from here
  <source>:10:53: error: 'constexprstd::nullptr_t B::null' is private within
this context
     10 | template<class T> A::operator T*() const {return B::null;}
        |                                                  ~~~^~~~
  <source>:6:38: note: declared private here
      6 |   constexpr static decltype(nullptr) null=nullptr;
        |                                      ^~~~

(Note also the missing space in the restated declaration of B::null.)

Declaration matching has certainly succeeded: omitting the 'const' from the
friend declaration or changing "T*" to "T**", "T", or "T&" produces a (correct)
error about the member function template not being found.