C++のテンプレートパラメータの減算

template<int I, typename Head, typename... T>
struct Tget {
  using type_t =
    typename
    Tget<I - 1, T...>
    ::type_t;
};

template<typename Head, typename... T>
struct Tget<0, Head, T...> {
  using type_t = Head;
};

int main()
{
  using t1 = Tget<2, char, int, double>::type_t;

  std::cout << "sizeof " << sizeof( t1 ) << std::endl;

  int i;
  std::cin >> i;

  return 0;
}