Disambiguate __gnu_cxx::append_ partial specialization

Disambiguate __gnu_cxx::append_ partial specialization

This patch corrects ambiguous partial specializations of
typelist::detail::append_.  Previously, neither append_<chain<Hd, Tl>,
Typelist_Chain> nor append_<Typelist_Chain, null_type> was a better
match for append_<chain<Hd, Tl>, null_type>.

2019-02-22  Tom Honermann  <tom@honermann.net>

	* include/ext/typelist.h: Constrain a partial specialization of
	typelist::detail::append_ to only match chain<T1,T2>.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@269088 138bc75d-0d04-0410-961f-82ee72b054a4