[Bug c++/110037] New: GCC accepts private member access of enclosing through friend function of inner class

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110037

            Bug ID: 110037
           Summary: GCC accepts private member access of enclosing through
                    friend function of inner class
           Product: gcc
           Version: 13.2.1
            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 program is accepted by gcc and clang:
https://godbolt.org/z/Gs479M4Tf

```
template <typename T>
class Outer
{
public:
  struct Inner {
    friend bool operator==(const Inner& a, const Inner& b) {
      return a.outer_.private_member_ == b.outer_.private_member_;
    }

  Inner(Outer<T> & o):outer_{o} 
  {

  }
   private:
    Outer<T>& outer_;

  };
  friend struct Iterator;
 //friend bool operator==(const Inner& a, const Inner& b);
  private:
  int private_member_;
};

bool test(Outer<int>::Inner A, Outer<int>::Inner B)
{
    return A == B;
}

int main()
{
    Outer<int> o; 
    Outer<int>::Inner i(o); 
    Outer<int>::Inner j(o);
    if(i==j){}  
}
```

[class.nest#4] makes this ill-formed.