GCC

After some experiment, using typename for internal representation is not enough. It cannot distinguish between the following two cases:

template <class T> struct D { ... };
template <class T> class C {
  friend class D<T>::X;                    // use typename here?
  template <class U> friend class D<U>::Y; // also use typename?
};

My latest idea is to invent a new tree node to represent the second friend.