[ASTImporter] Fix redecl failures of Class and ClassTemplate

[martong]

Summary

Redecl chains of classes and class templates are not handled well
currently. We want to handle them similarly to functions, i.e. try to
keep the structure of the original AST as much as possible. The aim is
to not squash a prototype with a definition, rather we create both and
put them in a redecl chain.

[a_sidorin]

Hi Gabor,
I don't see any problems with the patch. Thanks! I think it will be good to get Shafik's approval as well.

[martong]

@shafik, Could you please take a look?
I have run the LLDB tests on our macOS and I could not discover any regression.

[shafik]

LGTM, thank you !

cfe/trunk/lib/AST/ASTImporter.cpp

ExpectedDecl ASTNodeImporter::VisitRecordDecl(RecordDecl *D) {
   bool IsFriendTemplate = false;
   if (auto *DCXX = dyn_cast<CXXRecordDecl>(D)) {
     IsFriendTemplate =
         DCXX->getDescribedClassTemplate() &&
         DCXX->getDescribedClassTemplate()->getFriendObjectKind() !=
             Decl::FOK_None;
   }
 
-  // If this record has a definition in the translation unit we're coming from,
-  // but this particular declaration is not that definition, import the
-  // definition and map to that.
-  TagDecl *Definition = D->getDefinition();
-  if (Definition && Definition != D &&
-      // Friend template declaration must be imported on its own.
-      !IsFriendTemplate &&
-      // In contrast to a normal CXXRecordDecl, the implicit
-      // CXXRecordDecl of ClassTemplateSpecializationDecl is its redeclaration.
-      // The definition of the implicit CXXRecordDecl in this case is the
-      // ClassTemplateSpecializationDecl itself. Thus, we start with an extra
-      // condition in order to be able to import the implict Decl.
-      !D->isImplicit()) {
-    ExpectedDecl ImportedDefOrErr = import(Definition);
-    if (!ImportedDefOrErr)
-      return ImportedDefOrErr.takeError();
-
-    return Importer.MapImported(D, *ImportedDefOrErr);
-  }
-
   // Import the major distinguishing characteristics of this record.
   DeclContext *DC, *LexicalDC;
   DeclarationName Name;
   SourceLocation Loc;
   NamedDecl *ToD;
   if (Error Err = ImportDeclParts(D, DC, LexicalDC, Name, ToD, Loc))
     return std::move(Err);
   if (ToD)
ExpectedDecl ASTNodeImporter::VisitRecordDecl(RecordDecl *D) {
 
   // We may already have a record of the same name; try to find and match it.
   RecordDecl *PrevDecl = nullptr;
   if (!DC->isFunctionOrMethod()) {
     SmallVector<NamedDecl *, 4> ConflictingDecls;
     auto FoundDecls =
         Importer.findDeclsInToCtx(DC, SearchName);
     if (!FoundDecls.empty()) {
-      // We're going to have to compare D against potentially conflicting Decls, so complete it.
+      // We're going to have to compare D against potentially conflicting Decls,
+      // so complete it.
       if (D->hasExternalLexicalStorage() && !D->isCompleteDefinition())
         D->getASTContext().getExternalSource()->CompleteType(D);
     }
 
     for (auto *FoundDecl : FoundDecls) {
       if (!FoundDecl->isInIdentifierNamespace(IDNS))
         continue;
 
static ClassTemplateDecl *getDefinition(ClassTemplateDecl *D) {
   ClassTemplateDecl *TemplateWithDef =
       ToTemplatedDef->getDescribedClassTemplate();
   return TemplateWithDef;
 }
 
 ExpectedDecl ASTNodeImporter::VisitClassTemplateDecl(ClassTemplateDecl *D) {
   bool IsFriend = D->getFriendObjectKind() != Decl::FOK_None;
 
-  // If this template has a definition in the translation unit we're coming
-  // from, but this particular declaration is not that definition, import the
-  // definition and map to that.
-  ClassTemplateDecl *Definition = getDefinition(D);
-  if (Definition && Definition != D && !IsFriend) {
-    if (ExpectedDecl ImportedDefOrErr = import(Definition))
-      return Importer.MapImported(D, *ImportedDefOrErr);
-    else
-      return ImportedDefOrErr.takeError();
-  }
-
   // Import the major distinguishing characteristics of this class template.
   DeclContext *DC, *LexicalDC;
   DeclarationName Name;
   SourceLocation Loc;
   NamedDecl *ToD;
   if (Error Err = ImportDeclParts(D, DC, LexicalDC, Name, ToD, Loc))
     return std::move(Err);
   if (ToD)

cfe/trunk/unittests/AST/ASTImporterTest.cpp

ASTIMPORTER_INSTANTIATE_TYPED_TEST_CASE(RedeclChain, FunctionTemplate,
                                         ImportPrototypeAfterImportedPrototype)
 ASTIMPORTER_INSTANTIATE_TYPED_TEST_CASE(RedeclChain, ClassTemplate, ,
                                         ImportPrototypeAfterImportedPrototype)
 ASTIMPORTER_INSTANTIATE_TYPED_TEST_CASE(RedeclChain, FunctionTemplateSpec, ,
                                         ImportPrototypeAfterImportedPrototype)
 
 ASTIMPORTER_INSTANTIATE_TYPED_TEST_CASE(RedeclChain, Function, ,
                                         ImportDefinitionAfterImportedPrototype)
-// FIXME This does not pass, possible error with Class import.
-ASTIMPORTER_INSTANTIATE_TYPED_TEST_CASE(RedeclChain, Class, DISABLED_,
+ASTIMPORTER_INSTANTIATE_TYPED_TEST_CASE(RedeclChain, Class, ,
                                         ImportDefinitionAfterImportedPrototype)
 ASTIMPORTER_INSTANTIATE_TYPED_TEST_CASE(RedeclChain, Variable, ,
                                         ImportDefinitionAfterImportedPrototype)
 // FIXME Enable this test, once we import function templates chains correctly.
 ASTIMPORTER_INSTANTIATE_TYPED_TEST_CASE(RedeclChain, FunctionTemplate,
                                         DISABLED_,
                                         ImportDefinitionAfterImportedPrototype)
-// FIXME This does not pass, possible error with ClassTemplate import.
-ASTIMPORTER_INSTANTIATE_TYPED_TEST_CASE(RedeclChain, ClassTemplate, DISABLED_,
+ASTIMPORTER_INSTANTIATE_TYPED_TEST_CASE(RedeclChain, ClassTemplate, ,
                                         ImportDefinitionAfterImportedPrototype)
 ASTIMPORTER_INSTANTIATE_TYPED_TEST_CASE(RedeclChain, FunctionTemplateSpec, ,
                                         ImportDefinitionAfterImportedPrototype)
 
 ASTIMPORTER_INSTANTIATE_TYPED_TEST_CASE(RedeclChain, Function, ,
                                         ImportPrototypeAfterImportedDefinition)
-// FIXME This does not pass, possible error with Class import.
-ASTIMPORTER_INSTANTIATE_TYPED_TEST_CASE(RedeclChain, Class, DISABLED_,
+ASTIMPORTER_INSTANTIATE_TYPED_TEST_CASE(RedeclChain, Class, ,
                                         ImportPrototypeAfterImportedDefinition)
 ASTIMPORTER_INSTANTIATE_TYPED_TEST_CASE(RedeclChain, Variable, ,
                                         ImportPrototypeAfterImportedDefinition)
 // FIXME Enable this test, once we import function templates chains correctly.
 ASTIMPORTER_INSTANTIATE_TYPED_TEST_CASE(RedeclChain, FunctionTemplate,
                                         DISABLED_,
                                         ImportPrototypeAfterImportedDefinition)
-// FIXME This does not pass, possible error with ClassTemplate import.
-ASTIMPORTER_INSTANTIATE_TYPED_TEST_CASE(RedeclChain, ClassTemplate, DISABLED_,
+ASTIMPORTER_INSTANTIATE_TYPED_TEST_CASE(RedeclChain, ClassTemplate, ,
                                         ImportPrototypeAfterImportedDefinition)
 ASTIMPORTER_INSTANTIATE_TYPED_TEST_CASE(RedeclChain, FunctionTemplateSpec, ,
                                         ImportPrototypeAfterImportedDefinition)
 
 ASTIMPORTER_INSTANTIATE_TYPED_TEST_CASE(RedeclChain, Function, ,
                                         ImportPrototypes)
 ASTIMPORTER_INSTANTIATE_TYPED_TEST_CASE(RedeclChain, Class, , ImportPrototypes)
 ASTIMPORTER_INSTANTIATE_TYPED_TEST_CASE(RedeclChain, Variable, ,