Notes from the November, 2014 meeting:

CWG felt that these two declarations should not be equivalent.

In an example like

  template<typename T, typename U> using X = T;
  template<typename T> X<void, typename T::type> f();
  template<typename T> X<void, typename T::other> f();

it appears that the second declaration of f is a redeclaration of the first but distinguishable by SFINAE, i.e., equivalent but not functionally equivalent.