[C++20][Modules] Refine the condition of in-place numbering of anonymous members.

Author: mpark

clang/include/clang/AST/Decl.h

@@ -4521,6 +4521,23 @@ class RecordDecl : public TagDecl {
     return field_begin() == field_end();
   }
 
+  /// noload_fields - Iterate over the fields stored in this record
+  /// that are currently loaded; don't attempt to retrieve anything
+  /// from an external source.
+  field_range noload_fields() const {
+    return field_range(noload_field_begin(), noload_field_end());
+  }
+
+  field_iterator noload_field_begin() const;
+  field_iterator noload_field_end() const {
+    return field_iterator(decl_iterator());
+  }
+
+  // Whether there are any fields (non-static data members) in this record.
+  bool noload_field_empty() const {
+    return noload_field_begin() == noload_field_end();
+  }
+
   /// Note that the definition of this type is now complete.
   virtual void completeDefinition();
 

clang/lib/AST/Decl.cpp

@@ -5161,6 +5161,10 @@ RecordDecl::field_iterator RecordDecl::field_begin() const {
   return field_iterator(decl_iterator(FirstDecl));
 }
 
+RecordDecl::field_iterator RecordDecl::noload_field_begin() const {
+  return field_iterator(decl_iterator(getDefinitionOrSelf()->FirstDecl));
+}
+
 /// completeDefinition - Notes that the definition of this type is now
 /// complete.
 void RecordDecl::completeDefinition() {

clang/lib/Serialization/ASTReaderDecl.cpp

@@ -3432,7 +3432,19 @@ NamedDecl *ASTDeclReader::getAnonymousDeclForMerging(ASTReader &Reader,
   // If this is the first time, but we have parsed a declaration of the context,
   // build the anonymous declaration list from the parsed declaration.
   auto *PrimaryDC = getPrimaryDCForAnonymousDecl(DC);
-  if (PrimaryDC && !cast<Decl>(PrimaryDC)->isFromASTFile()) {
+  auto needToNumberAnonymousDeclsWithin = [](Decl *D) {
+    if (!D->isFromASTFile())
+      return true;
+    // If this is a class template specialization from an AST file, has at least
+    // one field, but none of the fields have been loaded from external storage,
+    // this is a situation where the class template specialization decl
+    // was imported but the definition was instantiated within the source.
+    // In such a case, we still need to number the anonymous decls.
+    auto *CTSD = dyn_cast<ClassTemplateSpecializationDecl>(D);
+    return CTSD && !CTSD->noload_field_empty() &&
+           !CTSD->hasLoadedFieldsFromExternalStorage();
+  };
+  if (PrimaryDC && needToNumberAnonymousDeclsWithin(cast<Decl>(PrimaryDC))) {
     numberAnonymousDeclsWithin(PrimaryDC, [&](NamedDecl *ND, unsigned Number) {
       if (Previous.size() == Number)
         Previous.push_back(cast<NamedDecl>(ND->getCanonicalDecl()));