The instantiation of default arguments for friends defined in a templated entity is not covered by 13.7.1 [temp.decls.general] paragraph 3 or 13.9.2 [temp.inst] paragraph 2. Consider:

  template <typename T>
  struct A {
    friend void foo(A &&, int = T::happy) { }
  };

  int main(void) { foo(A<int>(), 0); }

There is implementation divergence in the treatment of this example.