Switched to using multiclasses for both messages

Author: GeorgeKA

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -49,6 +49,8 @@ defm constexpr_ctor_missing_init : CXX20Compat<
 defm adl_only_template_id : CXX20Compat<
   "use of function template name with no prior declaration in function call "
   "with explicit template arguments is">;
+defm ctad_for_alias_templates
+    : CXX20Compat<"class template argument deduction for alias templates is">;
 
 // C++23 compatibility with C++20 and earlier.
 defm constexpr_static_var : CXX23Compat<
@@ -8442,18 +8444,6 @@ let CategoryName = "Lambda Issue" in {
   def warn_cxx17_compat_lambda_def_ctor_assign : Warning<
     "%select{default construction|assignment}0 of lambda is incompatible with "
     "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 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

@@ -9897,8 +9897,8 @@ QualType Sema::DeduceTemplateSpecializationFromInitializer(
             TemplateName.getAsTemplateDecl())) {
       Diag(Kind.getLocation(),
            getLangOpts().CPlusPlus20
-               ? diag::warn_cxx17_compat_ctad_for_alias_templates
-               : diag::ext_ctad_for_alias_templates_cxx20);
+               ? diag::compat_cxx20_ctad_for_alias_templates
+               : diag::compat_pre_cxx20_ctad_for_alias_templates);
       LookupTemplateDecl = AliasTemplate;
       auto UnderlyingType = AliasTemplate->getTemplatedDecl()
                                 ->getUnderlyingType()