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From: Nikolay Ivchenkov on 27 Mar 2010 06:18
Is it allowed to use default template-arguments and template parameter
pack in templated alias declaration?

template <class T, class... Types>
struct X
{ typedef typename T<Types...>::type type; };

// is it legal?
template <class T = void, class... Types>
using A = typename X<T, Types...>::type;

I don't see any special rules about this. For example, N3035 - 14.2/11
contains the following statements:

"If a template-parameter of a class template has a default template-
argument, each subsequent template-parameter shall either have a
default template-argument supplied or be a template parameter pack. If
a template-parameter of a class template is a template parameter pack,
it shall be the last template-parameter."

I think, the same restrictions shall be applied to template aliases
(which are not class templates themselves).

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