[clang] NFC: add test for cwg2398 ambiguity issue

Author: mizvekov

clang/test/SemaTemplate/cwg2398.cpp

@@ -59,6 +59,21 @@ namespace templ {
   template struct C<B<int>>;
 } // namespace templ
 
+namespace class_template {
+  template <class T1, class T2 = float> struct A;
+
+  template <class T3> struct B;
+
+  template <template <class T4> class TT1, class T5> struct B<TT1<T5>>;
+  // new-note@-1 {{partial specialization matches}}
+
+  template <class T6, class T7> struct B<A<T6, T7>> {};
+  // new-note@-1 {{partial specialization matches}}
+
+  template struct B<A<int>>;
+  // new-error@-1 {{ambiguous partial specialization}}
+} // namespace class_template
+
 namespace type_pack1 {
   template<class T2> struct A;
   template<template<class ...T3s> class TT1, class T4> struct A<TT1<T4>>   ;