Add a failing testcase where the template arguments of the return type f fail to be deduced

from the rhs of the type alias.

This is a FIXME for implementing "If deduction fails for another reason, proceed with an empty set of deduced template arguments."

Author: hokein

clang/test/SemaCXX/cxx20-ctad-type-alias.cpp

@@ -40,4 +40,26 @@ struct container {
 
 template<class T> using vector = container<T, int>;
 vector v(0, 0);
-} // namespace test3
+} // namespace test3
+
+
+namespace test4 {
+template<class T>
+struct X
+{
+  T t;
+  X(T) {}
+};
+
+template <class T>
+X(T) -> X<double>;
+
+template <class T>
+using AX = X<T>;
+
+void test1() {
+  AX s = {1};
+  // FIXME: should select X<double> deduction guide
+  // static_assert(__is_same(decltype(s.t), double));
+}
+}