Since the revision I see:

$ cat layout.ii
template <class> class Stringify;
template <long N> class Stringify<const char[N]>;

$ g++ layout.ii -c
layout.ii:2:25: error: partial specialization ‘class Stringify<const char [N]>’ is not more specialized than [-fpermissive]
    2 | template <long N> class Stringify<const char[N]>;
      |                         ^~~~~~~~~~~~~~~~~~~~~~~~
layout.ii:1:24: note: primary template ‘template<class> class Stringify’
    1 | template <class> class Stringify;
      |                        ^~~~~~~~~