Friend of function in dependent scope

Question

Is this code invalid:

template <class T> struct A;

class C {
    template <class T> friend void A<T>::foo();
};

In GCC 6.1.0 it says:

error: member 'void A<T>::foo()' declared as friend before type 'A<T>' defined

     template <class T> friend void A<T>::foo();

Clang 3.8.0:

warning: dependent nested name specifier 'A<T>::' for friend class declaration 
is not supported; turning off access control for 'C' [-Wunsupported-friend]

And Visual Studio 2015 crashes:

fatal error C1001: An internal error has occurred in the compiler.
(compiler file 'f:\dd\vctools\compiler\cxxfe\sl\p1\c\template.cpp', line 8952)
        template <class T> friend void A<T>::foo();

More specifically, is A required to be defined before the friend declaration?

  template <class T> struct A;

  class C {
    static void foo();
    template <class T> friend void A<T>::f();
  };

  template <class T> struct A {
    void f() { }
  };

If so, why?

Answer 1

You refer to A's member function foo. This function is not known to exist yet, because you only forward declare A.

In other words, you'll have to declare A<T>::foo before C, as the GCC message tries to tell you (the other two are rather cryptic). This means you have to declare the complete interface of A before C, instead of only forward declaring it.