A simplified code of what I have:

template 
struct M {
  struct V : public X {};  // error: invalid use of incomplete type ‘struct T::U’

  struct I {
    V* p;
  };

  // For simplicity assume that V is somehow used
  V v;
};

struct T {
  struct U {
    typename M::I i;
  };

  M m;
};