Rationale (November, 2014):

CWG was not convinced that such cases are sufficiently useful to warrant the additional complexity in the rules required to support them.

According to 13.10.2 [temp.arg.explicit] paragraph 9,

Template argument deduction can extend the sequence of template arguments corresponding to a template parameter pack, even when the sequence contains explicitly specified template arguments.

However, it is not clear how to handle an example like:

  template<class...> struct Z {
    Z (int);
  };

  template<class... Ts> void f (Z<Ts...>);

  int main () {
    f<void, void> (0);
  }