[Clang][Sema] Avoid duplicate diagnostics for incomplete types in nested name specifier (C++20+)

clang/include/clang/Sema/Sema.h

@@ -1555,6 +1555,12 @@ class Sema final : public SemaBase {
   Sema(const Sema &) = delete;
   void operator=(const Sema &) = delete;
 
+  /// Tracks (TagDecl, SourceLocation) pairs that have already triggered
+  /// an "incomplete type in nested name specifier" diagnostic,
+  /// to prevent emitting duplicate errors in C++20 and later,
+  /// where the same scope may be processed multiple times.
+  llvm::DenseSet<std::pair<const clang::TagDecl *, clang::SourceLocation>> DiagnosedIncompleteTypeSet;
+
   /// The handler for the FileChanged preprocessor events.
   ///
   /// Used for diagnostics that implement custom semantic analysis for #include

clang/lib/Sema/SemaCXXScopeSpec.cpp

@@ -209,10 +209,22 @@ bool Sema::RequireCompleteDeclContext(CXXScopeSpec &SS,
   SourceLocation loc = SS.getLastQualifierNameLoc();
   if (loc.isInvalid()) loc = SS.getRange().getBegin();
 
+  // If the DeclContext is a tag declaration, form a unique key
+  // from the TagDecl and the source location where the scope starts.
+  // If this key has already been diagnosed, skip emitting the error again.
+  const TagDecl *Tag = dyn_cast_or_null<TagDecl>(DC);
+  if (Tag) {
+    auto Key = std::make_pair(Tag, SS.getBeginLoc());
+    if (!DiagnosedIncompleteTypeSet.insert(Key).second)
+      return true; // Already diagnosed
+  }
+
+
   // The type must be complete.
   if (RequireCompleteType(loc, type, diag::err_incomplete_nested_name_spec,
                           SS.getRange())) {
     SS.SetInvalid(SS.getRange());
+
     return true;
   }
 

clang/test/SemaCXX/nested-name-spec.cpp

@@ -1,3 +1,10 @@
+// RUN: %clang_cc1 -std=c++20 -fsyntax-only -verify %s
+
+struct incomplete;
+incomplete::type var; // expected-error{{incomplete type 'incomplete' named in nested name specifier}}
+// expected-note@-2{{forward declaration of 'incomplete'}}
+
+
 // RUN: %clang_cc1 -fsyntax-only -std=c++98 -verify -fblocks %s
 namespace A {
   struct C {