4 files changed, 102 insertions, 20 deletions
diff --git a/clang/lib/Serialization/ASTReader.cpp b/clang/lib/Serialization/ASTReader.cpp
index 3cb96df12e4d..29aec144aec1 100644
--- a/clang/lib/Serialization/ASTReader.cpp
+++ b/clang/lib/Serialization/ASTReader.cpp
@@ -3921,6 +3921,13 @@ llvm::Error ASTReader::ReadASTBlock(ModuleFile &F,
       }
       break;
 
+    case VTABLES_TO_EMIT:
+      if (F.Kind == MK_MainFile ||
+          getContext().getLangOpts().BuildingPCHWithObjectFile)
+        for (unsigned I = 0, N = Record.size(); I != N;)
+          VTablesToEmit.push_back(ReadDeclID(F, Record, I));
+      break;
+
     case IMPORTED_MODULES:
       if (!F.isModule()) {
         // If we aren't loading a module (which has its own exports), make
@@ -8110,6 +8117,10 @@ void ASTReader::PassInterestingDeclToConsumer(Decl *D) {
     Consumer->HandleInterestingDecl(DeclGroupRef(D));
 }
 
+void ASTReader::PassVTableToConsumer(CXXRecordDecl *RD) {
+  Consumer->HandleVTable(RD);
+}
+
 void ASTReader::StartTranslationUnit(ASTConsumer *Consumer) {
   this->Consumer = Consumer;
 
diff --git a/clang/lib/Serialization/ASTReaderDecl.cpp b/clang/lib/Serialization/ASTReaderDecl.cpp
index 31ab6c651d59..c118f3818467 100644
--- a/clang/lib/Serialization/ASTReaderDecl.cpp
+++ b/clang/lib/Serialization/ASTReaderDecl.cpp
@@ -3684,6 +3684,54 @@ static void inheritDefaultTemplateArguments(ASTContext &Context,
   }
 }
 
+// [basic.link]/p10:
+//    If two declarations of an entity are attached to different modules,
+//    the program is ill-formed;
+static void checkMultipleDefinitionInNamedModules(ASTReader &Reader, Decl *D,
+                                                  Decl *Previous) {
+  Module *M = Previous->getOwningModule();
+
+  // We only care about the case in named modules.
+  if (!M || !M->isNamedModule())
+    return;
+
+  // If it is previous implcitly introduced, it is not meaningful to
+  // diagnose it.
+  if (Previous->isImplicit())
+    return;
+
+  // FIXME: Get rid of the enumeration of decl types once we have an appropriate
+  // abstract for decls of an entity. e.g., the namespace decl and using decl
+  // doesn't introduce an entity.
+  if (!isa<VarDecl, FunctionDecl, TagDecl, RedeclarableTemplateDecl>(Previous))
+    return;
+
+  // Skip implicit instantiations since it may give false positive diagnostic
+  // messages.
+  // FIXME: Maybe this shows the implicit instantiations may have incorrect
+  // module owner ships. But given we've finished the compilation of a module,
+  // how can we add new entities to that module?
+  if (auto *VTSD = dyn_cast<VarTemplateSpecializationDecl>(Previous);
+      VTSD && !VTSD->isExplicitSpecialization())
+    return;
+  if (auto *CTSD = dyn_cast<ClassTemplateSpecializationDecl>(Previous);
+      CTSD && !CTSD->isExplicitSpecialization())
+    return;
+  if (auto *Func = dyn_cast<FunctionDecl>(Previous))
+    if (auto *FTSI = Func->getTemplateSpecializationInfo();
+        FTSI && !FTSI->isExplicitSpecialization())
+      return;
+
+  // It is fine if they are in the same module.
+  if (Reader.getContext().isInSameModule(M, D->getOwningModule()))
+    return;
+
+  Reader.Diag(Previous->getLocation(),
+              diag::err_multiple_decl_in_different_modules)
+      << cast<NamedDecl>(Previous) << M->Name;
+  Reader.Diag(D->getLocation(), diag::note_also_found);
+}
+
 void ASTDeclReader::attachPreviousDecl(ASTReader &Reader, Decl *D,
                                        Decl *Previous, Decl *Canon) {
   assert(D && Previous);
@@ -3697,22 +3745,7 @@ void ASTDeclReader::attachPreviousDecl(ASTReader &Reader, Decl *D,
 #include "clang/AST/DeclNodes.inc"
   }
 
-  // [basic.link]/p10:
-  //    If two declarations of an entity are attached to different modules,
-  //    the program is ill-formed;
-  //
-  // FIXME: Get rid of the enumeration of decl types once we have an appropriate
-  // abstract for decls of an entity. e.g., the namespace decl and using decl
-  // doesn't introduce an entity.
-  if (Module *M = Previous->getOwningModule();
-      M && M->isNamedModule() &&
-      isa<VarDecl, FunctionDecl, TagDecl, RedeclarableTemplateDecl>(Previous) &&
-      !Reader.getContext().isInSameModule(M, D->getOwningModule())) {
-    Reader.Diag(Previous->getLocation(),
-                diag::err_multiple_decl_in_different_modules)
-        << cast<NamedDecl>(Previous) << M->Name;
-    Reader.Diag(D->getLocation(), diag::note_also_found);
-  }
+  checkMultipleDefinitionInNamedModules(Reader, D, Previous);
 
   // If the declaration was visible in one module, a redeclaration of it in
   // another module remains visible even if it wouldn't be visible by itself.
@@ -4209,6 +4242,13 @@ void ASTReader::PassInterestingDeclsToConsumer() {
 
   // If we add any new potential interesting decl in the last call, consume it.
   ConsumingPotentialInterestingDecls();
+
+  for (GlobalDeclID ID : VTablesToEmit) {
+    auto *RD = cast<CXXRecordDecl>(GetDecl(ID));
+    assert(!RD->shouldEmitInExternalSource());
+    PassVTableToConsumer(RD);
+  }
+  VTablesToEmit.clear();
 }
 
 void ASTReader::loadDeclUpdateRecords(PendingUpdateRecord &Record) {
diff --git a/clang/lib/Serialization/ASTWriter.cpp b/clang/lib/Serialization/ASTWriter.cpp
index c78d8943d6d9..7c0636962459 100644
--- a/clang/lib/Serialization/ASTWriter.cpp
+++ b/clang/lib/Serialization/ASTWriter.cpp
@@ -927,6 +927,7 @@ void ASTWriter::WriteBlockInfoBlock() {
   RECORD(DECLS_TO_CHECK_FOR_DEFERRED_DIAGS);
   RECORD(PP_ASSUME_NONNULL_LOC);
   RECORD(PP_UNSAFE_BUFFER_USAGE);
+  RECORD(VTABLES_TO_EMIT);
 
   // SourceManager Block.
   BLOCK(SOURCE_MANAGER_BLOCK);
@@ -3961,6 +3962,10 @@ void ASTWriter::WriteIdentifierTable(Preprocessor &PP,
     Stream.EmitRecord(INTERESTING_IDENTIFIERS, InterestingIdents);
 }
 
+void ASTWriter::handleVTable(CXXRecordDecl *RD) {
+  PendingEmittingVTables.push_back(RD);
+}
+
 //===----------------------------------------------------------------------===//
 // DeclContext's Name Lookup Table Serialization
 //===----------------------------------------------------------------------===//
@@ -5163,6 +5168,13 @@ void ASTWriter::PrepareWritingSpecialDecls(Sema &SemaRef) {
   // Write all of the DeclsToCheckForDeferredDiags.
   for (auto *D : SemaRef.DeclsToCheckForDeferredDiags)
     GetDeclRef(D);
+
+  // Write all classes that need to emit the vtable definitions if required.
+  if (isWritingStdCXXNamedModules())
+    for (CXXRecordDecl *RD : PendingEmittingVTables)
+      GetDeclRef(RD);
+  else
+    PendingEmittingVTables.clear();
 }
 
 void ASTWriter::WriteSpecialDeclRecords(Sema &SemaRef) {
@@ -5317,6 +5329,17 @@ void ASTWriter::WriteSpecialDeclRecords(Sema &SemaRef) {
   }
   if (!DeleteExprsToAnalyze.empty())
     Stream.EmitRecord(DELETE_EXPRS_TO_ANALYZE, DeleteExprsToAnalyze);
+
+  RecordData VTablesToEmit;
+  for (CXXRecordDecl *RD : PendingEmittingVTables) {
+    if (!wasDeclEmitted(RD))
+      continue;
+
+    AddDeclRef(RD, VTablesToEmit);
+  }
+
+  if (!VTablesToEmit.empty())
+    Stream.EmitRecord(VTABLES_TO_EMIT, VTablesToEmit);
 }
 
 ASTFileSignature ASTWriter::WriteASTCore(Sema &SemaRef, StringRef isysroot,
@@ -6559,10 +6582,12 @@ void ASTRecordWriter::AddCXXDefinitionData(const CXXRecordDecl *D) {
   // computed.
   Record->push_back(D->getODRHash());
 
-  bool ModulesDebugInfo =
-      Writer->Context->getLangOpts().ModulesDebugInfo && !D->isDependentType();
-  Record->push_back(ModulesDebugInfo);
-  if (ModulesDebugInfo)
+  bool ModulesCodegen =
+      !D->isDependentType() &&
+     (Writer->Context->getLangOpts().ModulesDebugInfo ||
+      D->isInNamedModule());
+  Record->push_back(ModulesCodegen);
+  if (ModulesCodegen)
     Writer->AddDeclRef(D, Writer->ModularCodegenDecls);
 
   // IsLambda bit is already saved.
diff --git a/clang/lib/Serialization/ASTWriterDecl.cpp b/clang/lib/Serialization/ASTWriterDecl.cpp
index 17c774038571..8a4ca54349e3 100644
--- a/clang/lib/Serialization/ASTWriterDecl.cpp
+++ b/clang/lib/Serialization/ASTWriterDecl.cpp
@@ -1529,8 +1529,14 @@ void ASTDeclWriter::VisitCXXRecordDecl(CXXRecordDecl *D) {
   if (D->isThisDeclarationADefinition())
     Record.AddCXXDefinitionData(D);
 
+  if (D->isCompleteDefinition() && D->isInNamedModule())
+    Writer.AddDeclRef(D, Writer.ModularCodegenDecls);
+
   // Store (what we currently believe to be) the key function to avoid
   // deserializing every method so we can compute it.
+  //
+  // FIXME: Avoid adding the key function if the class is defined in
+  // module purview since in that case the key function is meaningless.
   if (D->isCompleteDefinition())
     Record.AddDeclRef(Context.getCurrentKeyFunction(D));