Notes from the February, 2016 meeting:

See also issue 1554; the resolution of this issue should also deal with alias templates.

The handling of an example like

  template<typename T, std::size_t S = sizeof(T)> struct X {};
  template<typename T> X<T> foo(T*);
  void foo(...);

  void test() { struct S *s; foo(s); }

varies among implementations, presumably because the meaning of “immediate context” in determining whether an error is a substitution failure or a hard error is not clearly defined.