1 files changed, 26 insertions, 10 deletions

diff --git a/clang/lib/Sema/SemaDecl.cpp b/clang/lib/Sema/SemaDecl.cpp
index 558109d..10b5c27 100644
--- a/clang/lib/Sema/SemaDecl.cpp
+++ b/clang/lib/Sema/SemaDecl.cpp
@@ -6210,6 +6210,8 @@ bool Sema::diagnoseQualifiedDeclaration(CXXScopeSpec &SS, DeclContext *DC,
                                         SourceLocation Loc,
                                         TemplateIdAnnotation *TemplateId,
                                         bool IsMemberSpecialization) {
+  assert(SS.isValid() && "diagnoseQualifiedDeclaration called for declaration "
+                         "without nested-name-specifier");
   DeclContext *Cur = CurContext;
   while (isa<LinkageSpecDecl>(Cur) || isa<CapturedDecl>(Cur))
     Cur = Cur->getParent();
@@ -6298,22 +6300,36 @@ 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 forbid
+        // declarative nested-name-specifiers with pack-index-specifiers.
+        // See https://github.com/cplusplus/CWG/issues/499.
+        Diag(Loc, diag::err_decltype_in_declarator)
+            << SpecLoc.getTypeLoc().getSourceRange();
+      }
+    }
+  } while ((SpecLoc = SpecLoc.getPrefix()));
 
   return false;
 }