[Bug c++/109649] New: GCC accepts invalid inaccessible friend declaration of member function

["jlame646 at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109649

            Bug ID: 109649
           Summary: GCC accepts invalid inaccessible friend declaration of
                    member function
           Product: gcc
           Version: 13.1.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: jlame646 at gmail dot com
  Target Milestone: ---

The following invalid programs explained here:
https://stackoverflow.com/a/76120963/12002570 can be compiled with gcc but
rejected by msvc and clang. See demo: https://godbolt.org/z/Mz8cq3G5n

```
template <typename>
class X {

    void f(){}
};
class Y
{
    friend void X<int>::f();  
};

int main()
{
    X<int> t;
    t.f();
    Y b;
}
```

As we can note the above program compiles with gcc trunk.