variadic template parameter pack expanding for function calls

Unfortunately, as you noticed, expanding a parameter pack is only valid in certain contexts where the parser expects a comma-separated list of entries – contexts where the comma is just a syntactic separator, not the comma operator. This is arguably a deficiency in the current text.

An ugly workaround:

func((some(p), 0)...);

Do note that the evaluation order of function arguments, and thus the order of the some invocations, is unspecified, so you have to be careful with any side effects.

How about a small helper class:

template <typename Func, typename A, typename ...Args> struct Caller
{
  static void call(Func & f, A && a, Args && ...args)
  {
    f(std::forward<A>(a));
    Caller<Func, Args...>::call(f, std::forward<Args>(args)...);
  }
};

template <typename Func, typename A> struct Caller<Func, A>
{
  static void call(Func & f, A && a)
  {
    f(std::forward<A>(a));
  }
};

template <typename Func, typename ...Args>
void Call(Func & f, Args && ...args)
{
  Caller<Func, Args...>::call(f, std::forward<Args>(args)...);
}

Then you can put the following in your client code:

void foo(A);
Call(foo, a1, a2, a3);