[Objective-C] Add objc_subclassing_restricted attribute

This patch adds an objc_subclassing_restricted attribute into clang. This
attribute acts similarly to 'final' - Objective-C classes with this attribute
can't be subclassed. However, @interface declarations that have
objc_subclassing_restricted but don't have @implementation are allowed to
inherit other @interface declarations with objc_subclassing_restricted. This is
needed to describe the Swift class hierarchy in clang while making sure that
the Objective-C classes cannot subclass the Swift classes.

This attribute is already implemented in a fork of clang that's used for Swift
(https://github.com/apple/swift-clang) and this patch moves that code to the
upstream clang repository.

rdar://28937548

Differential Revision: https://reviews.llvm.org/D25993

llvm-svn: 285391

2 files changed, 23 insertions, 0 deletions

diff --git a/clang/lib/Sema/SemaDeclAttr.cpp b/clang/lib/Sema/SemaDeclAttr.cpp
index 9413b6e51aa..936c29767d7 100644
--- a/clang/lib/Sema/SemaDeclAttr.cpp
+++ b/clang/lib/Sema/SemaDeclAttr.cpp
@@ -5776,6 +5776,9 @@ static void ProcessDeclAttribute(Sema &S, Scope *scope, Decl *D,
   case AttributeList::AT_ObjCRootClass:
     handleSimpleAttribute<ObjCRootClassAttr>(S, D, Attr);
     break;
+  case AttributeList::AT_ObjCSubclassingRestricted:
+    handleSimpleAttribute<ObjCSubclassingRestrictedAttr>(S, D, Attr);
+    break;
   case AttributeList::AT_ObjCExplicitProtocolImpl:
     handleObjCSuppresProtocolAttr(S, D, Attr);
     break;
diff --git a/clang/lib/Sema/SemaDeclObjC.cpp b/clang/lib/Sema/SemaDeclObjC.cpp
index 79cdd6ead92..1b030801f23 100644
--- a/clang/lib/Sema/SemaDeclObjC.cpp
+++ b/clang/lib/Sema/SemaDeclObjC.cpp
@@ -3853,6 +3853,18 @@ Decl *Sema::ActOnAtEnd(Scope *S, SourceRange AtEnd, ArrayRef<Decl *> allMethods,
         Diag(IDecl->getLocation(), diag::err_objc_root_class_subclass);
       }
 
+      if (const ObjCInterfaceDecl *Super = IDecl->getSuperClass()) {
+        // An interface can subclass another interface with a
+        // objc_subclassing_restricted attribute when it has that attribute as
+        // well (because of interfaces imported from Swift). Therefore we have
+        // to check if we can subclass in the implementation as well.
+        if (IDecl->hasAttr<ObjCSubclassingRestrictedAttr>() &&
+            Super->hasAttr<ObjCSubclassingRestrictedAttr>()) {
+          Diag(IC->getLocation(), diag::err_restricted_superclass_mismatch);
+          Diag(Super->getLocation(), diag::note_class_declared);
+        }
+      }
+
       if (LangOpts.ObjCRuntime.isNonFragile()) {
         while (IDecl->getSuperClass()) {
           DiagnoseDuplicateIvars(IDecl, IDecl->getSuperClass());
@@ -3873,6 +3885,14 @@ Decl *Sema::ActOnAtEnd(Scope *S, SourceRange AtEnd, ArrayRef<Decl *> allMethods,
         ImplMethodsVsClassMethods(S, CatImplClass, Cat);
       }
     }
+  } else if (const auto *IntfDecl = dyn_cast<ObjCInterfaceDecl>(ClassDecl)) {
+    if (const ObjCInterfaceDecl *Super = IntfDecl->getSuperClass()) {
+      if (!IntfDecl->hasAttr<ObjCSubclassingRestrictedAttr>() &&
+          Super->hasAttr<ObjCSubclassingRestrictedAttr>()) {
+        Diag(IntfDecl->getLocation(), diag::err_restricted_superclass_mismatch);
+        Diag(Super->getLocation(), diag::note_class_declared);
+      }
+    }
   }
   if (isInterfaceDeclKind) {
     // Reject invalid vardecls.