Notes from the March, 2018 meeting:

CWG felt that ill-formed, no diagnostic required was the correct approach.

It is not clear how to handle parameter packs that are expanded during instantiation in parallel with those that are not yet concrete. In particular, does the following example require a diagnostic?

  template<typename ...T> struct Tuple;
  template<class T, class U> struct Outer;
  template<class ...T, class ...U>
  struct Outer<Tuple<T ...>, Tuple<U ...> > {
    template<class X, class Y> struct Inner;
    template<class ...Y> struct Inner<Tuple<T, Y> ...> { };
    template<class ...Y> struct Inner<Tuple<U, Y> ...> { };
  };
  Outer<Tuple<int, void>, Tuple<int, void> > outer;