Import objc forward declarations

Author: Nuri Amari

include/swift/Basic/LangOptions.h

@@ -786,7 +786,7 @@ namespace swift {
 
     /// If true, forward declarations will be imported using unavailable types
     /// instead of dropped altogether when possible.
-    bool ImportForwardDeclarations = false;
+    bool ImportForwardDeclarations = true;
 
     /// If true ignore the swift bridged attribute.
     bool DisableSwiftBridgeAttr = false;

lib/ClangImporter/ImportDecl.cpp

@@ -4372,12 +4372,33 @@ namespace {
                                                            clangModule))
             return native;
 
-        Impl.addImportDiagnostic(
-            decl, Diagnostic(diag::forward_declared_protocol_label, decl),
-            decl->getSourceRange().getBegin());
+        if (Impl.ImportForwardDeclarations) {
+          auto result = Impl.createDeclWithClangNode<ProtocolDecl>(
+              decl, AccessLevel::Public,
+              Impl.getClangModuleForDecl(decl->getCanonicalDecl(),
+                                         /*allowForwardDeclaration=*/true),
+              SourceLoc(), SourceLoc(), name,
+              ArrayRef<PrimaryAssociatedTypeName>(), None,
+              /*TrailingWhere=*/nullptr);
+
+          Impl.ImportedDecls[{decl->getCanonicalDecl(), getVersion()}] = result;
+          result->setSuperclass(Impl.getNSObjectProtocolType());
+          result->setAddedImplicitInitializers(); // suppress all initializers
+          addObjCAttribute(result,
+                           Impl.importIdentifier(decl->getIdentifier()));
+          SmallVector<InheritedEntry, 4> inheritedTypes = {
+              TypeLoc::withoutLoc(Impl.getNSObjectProtocolType())};
+          result->setInherited(Impl.SwiftContext.AllocateCopy(inheritedTypes));
+          result->setImplicit();
+          return result;
+        } else {
+          Impl.addImportDiagnostic(
+              decl, Diagnostic(diag::forward_declared_protocol_label, decl),
+              decl->getSourceRange().getBegin());
 
-        forwardDeclaration = true;
-        return nullptr;
+          forwardDeclaration = true;
+          return nullptr;
+        }
       }
 
       decl = decl->getDefinition();
@@ -4443,7 +4464,7 @@ namespace {
                                                         nullptr, dc,
                                                         /*isActor*/false);
         Impl.ImportedDecls[{decl->getCanonicalDecl(), getVersion()}] = result;
-        result->setSuperclass(Type());
+        result->setSuperclass(Impl.getNSObjectType());
         result->setAddedImplicitInitializers(); // suppress all initializers
         result->setHasMissingVTableEntries(false);
         addObjCAttribute(result, Impl.importIdentifier(decl->getIdentifier()));
@@ -4497,15 +4518,23 @@ namespace {
         }
 
         if (Impl.ImportForwardDeclarations) {
-          // Fake it by making an unavailable opaque @objc root class.
-          auto result = createFakeRootClass(name);
+          auto result = Impl.createDeclWithClangNode<ClassDecl>(
+              decl, AccessLevel::Public, SourceLoc(), name, SourceLoc(), None,
+              nullptr,
+              Impl.getClangModuleForDecl(decl->getCanonicalDecl(),
+                                         /*allowForwardDeclaration=*/true),
+              /*isActor*/ false);
+          Type superclassType = Impl.getNSObjectType();
+          Impl.ImportedDecls[{decl->getCanonicalDecl(), getVersion()}] = result;
+          SmallVector<InheritedEntry, 4> inheritedTypes{
+              TypeLoc::withoutLoc(superclassType)};
+          result->setInherited(Impl.SwiftContext.AllocateCopy(inheritedTypes));
+          result->setSuperclass(superclassType);
+          result->setAddedImplicitInitializers(); // suppress all initializers
+          result->setHasMissingVTableEntries(false);
+          addObjCAttribute(result,
+                           Impl.importIdentifier(decl->getIdentifier()));
           result->setImplicit();
-          auto attr = AvailableAttr::createPlatformAgnostic(Impl.SwiftContext,
-              "This Objective-C class has only been forward-declared; "
-              "import its owning module to use it");
-          result->getAttrs().add(attr);
-          result->getAttrs().add(
-              new (Impl.SwiftContext) ForbidSerializingReferenceAttr(true));
           return result;
         } else {
           Impl.addImportDiagnostic(