[Clang] Duplicate diagnostics in C++20+ mode

Author: zhy-tju

clang/include/clang/Sema/DeclSpec.h

@@ -76,13 +76,23 @@ class CXXScopeSpec {
   NestedNameSpecifierLocBuilder Builder;
   ArrayRef<TemplateParameterList *> TemplateParamLists;
 
+  /// Flag indicating whether an incomplete-type diagnostic
+  /// has already been emitted for this scope specifier.
+  bool HadIncompleteTypeError = false;
+
 public:
   SourceRange getRange() const { return Range; }
   void setRange(SourceRange R) { Range = R; }
   void setBeginLoc(SourceLocation Loc) { Range.setBegin(Loc); }
   void setEndLoc(SourceLocation Loc) { Range.setEnd(Loc); }
   SourceLocation getBeginLoc() const { return Range.getBegin(); }
   SourceLocation getEndLoc() const { return Range.getEnd(); }
+  
+  /// Return true if an incomplete-type diagnostic has already been emitted.
+  bool hasIncompleteTypeError() const { return HadIncompleteTypeError; }
+
+  /// Mark that an incomplete-type error was emitted for this scope.
+  void setIncompleteTypeError(bool v = true) { HadIncompleteTypeError = v; }
 
   void setTemplateParamLists(ArrayRef<TemplateParameterList *> L) {
     TemplateParamLists = L;

clang/lib/Sema/SemaCXXScopeSpec.cpp

@@ -209,10 +209,20 @@ bool Sema::RequireCompleteDeclContext(CXXScopeSpec &SS,
   SourceLocation loc = SS.getLastQualifierNameLoc();
   if (loc.isInvalid()) loc = SS.getRange().getBegin();
 
+  // If an incomplete-type error has already been emitted for this scope,
+  // suppress duplicate diagnostics to avoid noisy repeated messages.
+  if (SS.hasIncompleteTypeError())
+    return true;
+
   // The type must be complete.
   if (RequireCompleteType(loc, type, diag::err_incomplete_nested_name_spec,
                           SS.getRange())) {
     SS.SetInvalid(SS.getRange());
+
+    // Remember that we've already diagnosed this incomplete type,
+    // so later checks won't emit redundant diagnostics.
+    SS.setIncompleteTypeError();
+
     return true;
   }