问精化型说明符中依赖类型的嵌套名称说明符

EN

template <typename T>
struct S1 { struct I { }; };

template <typename T>
struct S2 { typedef struct S1<T>::I I; }; // okay

template <typename T>
struct S3 { typedef struct S2<T>::I I; }; // okay at definition time
                                          // usually error at instantiation time
template <>
struct S2<short> : S1<short> { };

int main() {
  S1<int>::I a;
  S2<int>::I &b = a; // okay
  S3<int>::I &c = b; // error
  S1<short>::I d;
  S2<short>::I &e = d; // okay
  S3<short>::I &f = e; // okay
}