[Clang] Add warning message for C++17 alias template CTAD

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -8444,9 +8444,16 @@ let CategoryName = "Lambda Issue" in {
     "C++ standards before C++20">, InGroup<CXXPre20Compat>, DefaultIgnore;
 
   // C++20 class template argument deduction for alias templates.
-  def warn_cxx17_compat_ctad_for_alias_templates : Warning<
-  "class template argument deduction for alias templates is incompatible with "
-  "C++ standards before C++20">, InGroup<CXXPre20Compat>, DefaultIgnore;
+  def warn_cxx17_compat_ctad_for_alias_templates
+      : Warning<"class template argument deduction for alias templates is "
+                "incompatible with "
+                "C++ standards before C++20">,
+        InGroup<CXXPre20Compat>,
+        DefaultIgnore;
+  def ext_ctad_for_alias_templates_cxx20
+      : ExtWarn<"class template argument deduction for alias templates is a "
+                "C++20 extension">,
+        InGroup<CXX20>;
 }
 
 def err_return_in_captured_stmt : Error<

clang/lib/Sema/SemaInit.cpp

@@ -9896,7 +9896,9 @@ QualType Sema::DeduceTemplateSpecializationFromInitializer(
     if (auto *AliasTemplate = dyn_cast_or_null<TypeAliasTemplateDecl>(
             TemplateName.getAsTemplateDecl())) {
       Diag(Kind.getLocation(),
-           diag::warn_cxx17_compat_ctad_for_alias_templates);
+           getLangOpts().CPlusPlus20
+               ? diag::warn_cxx17_compat_ctad_for_alias_templates
+               : diag::ext_ctad_for_alias_templates_cxx20);
       LookupTemplateDecl = AliasTemplate;
       auto UnderlyingType = AliasTemplate->getTemplatedDecl()
                                 ->getUnderlyingType()

clang/test/SemaCXX/cxx17-compat.cpp

@@ -137,7 +137,7 @@ template<typename T> struct A { A(T); };
 template<typename T> using B = A<T>;
 B b = {1};
 #if __cplusplus <= 201703L
-  // FIXME: diagnose as well
+  // expected-warning@-2 {{class template argument deduction for alias templates is a C++20 extension}}
 #else
   // expected-warning@-4 {{class template argument deduction for alias templates is incompatible with C++ standards before C++20}}
 #endif

clang/test/SemaCXX/cxx1z-class-template-argument-deduction.cpp

@@ -113,9 +113,9 @@ namespace dependent {
   };
   template<typename T> void f() {
     typename T::X tx = 0;
-    typename T::Y ty = 0;
+    typename T::Y ty = 0; // expected-warning {{class template argument deduction for alias templates is a C++20 extension}}
   }
-  template void f<B>();
+  template void f<B>(); // expected-note {{in instantiation of function template specialization 'dependent::f<dependent::B>' requested here}}
 
   template<typename T> struct C { C(T); };
   template<typename T> C(T) -> C<T>;