gcc rejects the following, clang accepts it:

template<size_t,class...> struct S; template<class...A> struct S<sizeof...(A),A...>{};

both gcc and clang reject this:

template<int,int> struct S; template<int i> struct S<i,i+1>{};

gcc accepts the following, clang rejects it:

template<class,int> struct S; template<class T> struct S<T,T{}>{};

Clang now accepts the second example.

Fixed by 9b94785dedb08b006419bec1a402614d9241317a.