[clang] Disable alias template CTAD for C++17 (PR #133597)

[Matheus Izvekov via cfe-commits]

================
@@ -9895,26 +9895,30 @@ 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);
-      LookupTemplateDecl = AliasTemplate;
-      auto UnderlyingType = AliasTemplate->getTemplatedDecl()
-                                ->getUnderlyingType()
-                                .getCanonicalType();
-      // C++ [over.match.class.deduct#3]: ..., the defining-type-id of A must be
-      // of the form
-      //   [typename] [nested-name-specifier] [template] simple-template-id
-      if (const auto *TST =
-              UnderlyingType->getAs<TemplateSpecializationType>()) {
-        Template = dyn_cast_or_null<ClassTemplateDecl>(
-            TST->getTemplateName().getAsTemplateDecl());
-      } else if (const auto *RT = UnderlyingType->getAs<RecordType>()) {
-        // Cases where template arguments in the RHS of the alias are not
-        // dependent. e.g.
-        //   using AliasFoo = Foo<bool>;
-        if (const auto *CTSD = llvm::dyn_cast<ClassTemplateSpecializationDecl>(
-                RT->getAsCXXRecordDecl()))
-          Template = CTSD->getSpecializedTemplate();
+      if (getLangOpts().CPlusPlus20) {
+        LookupTemplateDecl = AliasTemplate;
+        auto UnderlyingType = AliasTemplate->getTemplatedDecl()
+                                  ->getUnderlyingType()
+                                  .getCanonicalType();
+        // C++ [over.match.class.deduct#3]: ..., the defining-type-id of A must
+        // be of the form
+        //   [typename] [nested-name-specifier] [template] simple-template-id
+        if (const auto *TST =
+                UnderlyingType->getAs<TemplateSpecializationType>()) {
+          Template = dyn_cast_or_null<ClassTemplateDecl>(
+              TST->getTemplateName().getAsTemplateDecl());
+        } else if (const auto *RT = UnderlyingType->getAs<RecordType>()) {
+          // Cases where template arguments in the RHS of the alias are not
+          // dependent. e.g.
+          //   using AliasFoo = Foo<bool>;
+          if (const auto *CTSD =
+                  llvm::dyn_cast<ClassTemplateSpecializationDecl>(
+                      RT->getAsCXXRecordDecl()))
+            Template = CTSD->getSpecializedTemplate();
+        }
+      } else {
+        Diag(Kind.getLocation(),
+             diag::warn_cxx17_compat_ctad_for_alias_templates);
----------------
mizvekov wrote:

This being still a warning is incompatible with the purpose of the change.

https://github.com/llvm/llvm-project/pull/133597