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>.

From-SVN: r269088

libstdc++-v3/ChangeLog

+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>.
+
 2019-02-21  Jonathan Wakely  <jwakely@redhat.com>
 
 	PR libstdc++/89416

libstdc++-v3/include/ext/typelist.h

@@ -215,10 +215,10 @@ namespace detail
       typedef Typelist_Chain 			      		type;
     };
 
-  template<typename Typelist_Chain>
-    struct append_<Typelist_Chain, null_type>
+  template<typename Hd, typename Tl>
+    struct append_<chain<Hd, Tl>, null_type>
     {
-      typedef Typelist_Chain 					type;
+      typedef chain<Hd, Tl>  					type;
     };
 
   template<>