Rationale (February, 2019):

This issue is covered in more detail in issue 2395.

The Standard is not clear on the treatment of an example like the following, and there is implementation variance:

  template<class ...Types> struct Tuple_ { // _VARIADIC_TEMPLATE
    template<Types ...T, int> int f() {
      return sizeof...(Types);
    }
  };

  int main() {
    Tuple_<char,int> a;
    int b = a.f<1, 2, 3>();
  }