Rationale (October, 2012):

The latter interpretation (a list of specializations) is the correct interpretation; a parameter pack can't be substituted into anything, including an alias template specialization. CWG felt that this is clear enough in the current wording.

The relative order of template parameter pack expansion and alias template substitution is not clear in the current wording. For example, in

  template<typename T> using Int = int;

  template<typename ...Ts> struct S {
    typedef S<Int<Ts>...> other;
  };

it is not clear whether int is substituted for Int<Ts> first, leaving the ellipsis with no parameter pack to expand, or whether the pack expansion is to be applied first, producing a list of specializations of Int<T>.

(See also issue 1558.)