[Objective-C] Introduce objc_runtime_visible attribute.

The objc_runtime_visible attribute deals with an odd corner case where
a particular Objective-C class is known to the Objective-C runtime
(and, therefore, accessible by name) but its symbol has been hidden
for some reason. For such classes, teach CodeGen to use
objc_lookUpClass to retrieve the Class object, rather than referencing
the class symbol directly.

Classes annotated with objc_runtime_visible have two major limitations
that fall out from places where Objective-C metadata needs to refer to
the class (or metaclass) symbol directly:

* One cannot implement a subclass of an objc_runtime_visible class.
* One cannot implement a category on an objc_runtime_visible class.

Implements rdar://problem/25494092.

llvm-svn: 265201

1 files changed, 17 insertions, 0 deletions

diff --git a/clang/lib/Sema/SemaDeclObjC.cpp b/clang/lib/Sema/SemaDeclObjC.cpp
index 6121973eb18..b62f9130bdb 100644
--- a/clang/lib/Sema/SemaDeclObjC.cpp
+++ b/clang/lib/Sema/SemaDeclObjC.cpp
@@ -1831,6 +1831,13 @@ Decl *Sema::ActOnStartCategoryImplementation(
   if (IDecl)
     DiagnoseUseOfDecl(IDecl, ClassLoc);
 
+  // If the interface has the objc_runtime_visible attribute, we
+  // cannot implement a category for it.
+  if (IDecl && IDecl->hasAttr<ObjCRuntimeVisibleAttr>()) {
+    Diag(ClassLoc, diag::err_objc_runtime_visible_category)
+      << IDecl->getDeclName();
+  }
+
   /// Check that CatName, category name, is not used in another implementation.
   if (CatIDecl) {
     if (CatIDecl->getImplementation()) {
@@ -1968,6 +1975,16 @@ Decl *Sema::ActOnStartClassImplementation(
                                         dyn_cast<NamedDecl>(IDecl), 
                                         IMPDecl->getLocation(), 1);
   }
+
+  // If the superclass has the objc_runtime_visible attribute, we
+  // cannot implement a subclass of it.
+  if (IDecl->getSuperClass() &&
+      IDecl->getSuperClass()->hasAttr<ObjCRuntimeVisibleAttr>()) {
+    Diag(ClassLoc, diag::err_objc_runtime_visible_subclass)
+      << IDecl->getDeclName()
+      << IDecl->getSuperClass()->getDeclName();
+  }
+
   return ActOnObjCContainerStartDefinition(IMPDecl);
 }