[clang] [Clang][Sema] Diagnose declarative nested-name-specifiers naming alias templates (PR #80842)

[Vlad Serebrennikov via cfe-commits]

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@@ -6295,22 +6297,36 @@ bool Sema::diagnoseQualifiedDeclaration(CXXScopeSpec &SS, DeclContext *DC,
         << FixItHint::CreateRemoval(TemplateId->TemplateKWLoc);
 
   NestedNameSpecifierLoc SpecLoc(SS.getScopeRep(), SS.location_data());
-  while (SpecLoc.getPrefix()) {
+  do {
     if (SpecLoc.getNestedNameSpecifier()->getKind() ==
         NestedNameSpecifier::TypeSpecWithTemplate)
       Diag(Loc, diag::ext_template_after_declarative_nns)
           << FixItHint::CreateRemoval(
                  SpecLoc.getTypeLoc().getTemplateKeywordLoc());
 
-    SpecLoc = SpecLoc.getPrefix();
-  }
-  // C++11 [dcl.meaning]p1:
-  //   [...] "The nested-name-specifier of the qualified declarator-id shall
-  //   not begin with a decltype-specifer"
-  if (isa_and_nonnull<DecltypeType>(
-          SpecLoc.getNestedNameSpecifier()->getAsType()))
-    Diag(Loc, diag::err_decltype_in_declarator)
-      << SpecLoc.getTypeLoc().getSourceRange();
+    if (const Type *T = SpecLoc.getNestedNameSpecifier()->getAsType()) {
+      if (const auto *TST = T->getAsAdjusted<TemplateSpecializationType>()) {
+        // C++23 [expr.prim.id.qual]p3:
+        //   [...] If a nested-name-specifier N is declarative and has a
+        //   simple-template-id with a template argument list A that involves a
+        //   template parameter, let T be the template nominated by N without A.
+        //   T shall be a class template.
+        if (TST->isDependentType() && TST->isTypeAlias())
+          Diag(Loc, diag::ext_alias_template_in_declarative_nns)
+              << SpecLoc.getLocalSourceRange();
+      } else if (T->isDecltypeType()) {
+        // C++23 [expr.prim.id.qual]p2:
+        //   [...] A declarative nested-name-specifier shall not have a
+        //   decltype-specifier.
+        //
+        // FIXME: This wording appears to be defective, as it does not
----------------
Endilll wrote:

It would make sense to me to implement the resolution you proposed right away, because it really feels a wording defect rather than an intentional omission.
CC @cor3ntin 

https://github.com/llvm/llvm-project/pull/80842