[Clang] Add warning message for C++17 alias template CTAD (#133806)

Alias template class template argument deduction is a documented C++20
feature. C++17 also happens to support it, but there is no message
output to indicate the officially supported version. This PR adds that.

Also updated relevant CTAD test cases.

Closes #125913

clang/docs/ReleaseNotes.rst

@@ -403,6 +403,7 @@ Bug Fixes to C++ Support
 - Clang no longer crashes when establishing subsumption between some constraint expressions. (#GH122581)
 - Clang now issues an error when placement new is used to modify a const-qualified variable
   in a ``constexpr`` function. (#GH131432)
+- Clang now emits a warning when class template argument deduction for alias templates is used in C++17. (#GH133806)
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^

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<
@@ -8448,11 +8450,6 @@ 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 err_return_in_captured_stmt : Error<

clang/lib/Sema/SemaInit.cpp

@@ -9919,8 +9919,7 @@ QualType Sema::DeduceTemplateSpecializationFromInitializer(
   if (!Template) {
     if (auto *AliasTemplate = dyn_cast_or_null<TypeAliasTemplateDecl>(
             TemplateName.getAsTemplateDecl())) {
-      Diag(Kind.getLocation(),
-           diag::warn_cxx17_compat_ctad_for_alias_templates);
+      DiagCompat(Kind.getLocation(), diag_compat::ctad_for_alias_templates);
       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>;