[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, 3 insertions, 0 deletions

diff --git a/clang/lib/Sema/SemaDeclAttr.cpp b/clang/lib/Sema/SemaDeclAttr.cpp
index 89c7eafa037..9066e761ad6 100644
--- a/clang/lib/Sema/SemaDeclAttr.cpp
+++ b/clang/lib/Sema/SemaDeclAttr.cpp
@@ -5548,6 +5548,9 @@ static void ProcessDeclAttribute(Sema &S, Scope *scope, Decl *D,
   case AttributeList::AT_ObjCRuntimeName:
     handleObjCRuntimeName(S, D, Attr);
     break;
+   case AttributeList::AT_ObjCRuntimeVisible:
+    handleSimpleAttribute<ObjCRuntimeVisibleAttr>(S, D, Attr);
+    break;
   case AttributeList::AT_ObjCBoxable:
     handleObjCBoxable(S, D, Attr);
     break;