Can I expand a parameters pack and define an arguments list with it?

Question

From [temp.variadic] (working draft) it seemed to me that a parameters pack can be expanded while defining an arguments list of another template class or function.

Consi

Answer 1

The definition of the template S, and the instantiation of S<int, char>, are valid.

See [temp.param]/15: "A template parameter pack that is a parameter-declaration whose type contains one or more unexpanded parameter packs is a pack expansion."

This means that template<T ...I> can mean one of two different things: if T is a non-pack type, then it declares a normal parameter pack, accepting any number of Ts. However, if T contains an unexpanded parameter pack, then the parameter declaration is instead expanded into a sequence of parameters when the outer template is instantiated.

Your first call to m is valid, but your second and third calls to m are ill-formed

The instantiation of S<int, char> looks like this:

template<>
struct S<int, char> {
  template<int I$0, char I$1>
  void m() {}
};

(where I$0 and I$1 are the first and second slices of the pack I).

Therefore (because neither I$0 nor I$1 can be deduced from a call to m), s.m<0,'c'>() is valid but s.m<0>() and s.m<>() are ill-formed.