[Bug c++/102660] New: Valid template default parameter of a friend function is rejected

[fchelnokov at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102660

            Bug ID: 102660
           Summary: Valid template default parameter of a friend function
                    is rejected
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: fchelnokov at gmail dot com
  Target Milestone: ---

This is a presumably valid program:
```
struct A{
    template<int R>
    friend void show();

private:
    int c;
};

template<int R = decltype(A{}.c){0} >
void show(){
}

int main(){
    show<>();
}
```
It is accepted by Clang, MSVC, but rejected in GCC, which complains about
default parameter of R (asking for forward declaration of the function). Demo:
https://gcc.godbolt.org/z/q1r1a9Ycn

Related discussion: https://stackoverflow.com/q/69341936/7325599