This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 116a. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.

2024-12-19

2265. Delayed pack expansion and member redeclarations

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;

Notes from the March, 2018 meeting:

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