diff --git a/lib/AST/DeclBase.cpp b/lib/AST/DeclBase.cpp
index b794150..024f370 100644
--- a/lib/AST/DeclBase.cpp
+++ b/lib/AST/DeclBase.cpp
@@ -779,9 +779,16 @@
     return this;
 
   case Decl::ObjCInterface:
+    if (ObjCInterfaceDecl *Def = cast<ObjCInterfaceDecl>(this)->getDefinition())
+      return Def;
+      
+    return this;
+      
   case Decl::ObjCProtocol:
+    // FIXME: Update when protocols properly model forward declarations.
+    // For now, it's fine to fall through
+      
   case Decl::ObjCCategory:
-    // FIXME: Can Objective-C interfaces be forward-declared?
     return this;
 
   case Decl::ObjCImplementation:
diff --git a/lib/AST/DeclObjC.cpp b/lib/AST/DeclObjC.cpp
index decbd54..1da0fab 100644
--- a/lib/AST/DeclObjC.cpp
+++ b/lib/AST/DeclObjC.cpp
@@ -224,7 +224,15 @@
 
 void ObjCInterfaceDecl::allocateDefinitionData() {
   assert(!hasDefinition() && "ObjC class already has a definition");
-  Definition = new (getASTContext()) DefinitionData();  
+  Definition.setPointer(new (getASTContext()) DefinitionData());
+  Definition.setInt(true);
+  
+  // Update all of the declarations with a pointer to the definition.
+  for (redecl_iterator RD = redecls_begin(), RDEnd = redecls_end();
+       RD != RDEnd; ++RD) {
+    if (*RD != this)
+      RD->Definition.setPointer(Definition.getPointer());
+  }
 }
 
 void ObjCInterfaceDecl::startDefinition() {
diff --git a/lib/Serialization/ASTCommon.h b/lib/Serialization/ASTCommon.h
index 367f57f..2e3d70b 100644
--- a/lib/Serialization/ASTCommon.h
+++ b/lib/Serialization/ASTCommon.h
@@ -26,7 +26,8 @@
   UPD_CXX_ADDED_IMPLICIT_MEMBER,
   UPD_CXX_ADDED_TEMPLATE_SPECIALIZATION,
   UPD_CXX_ADDED_ANONYMOUS_NAMESPACE,
-  UPD_CXX_INSTANTIATED_STATIC_DATA_MEMBER
+  UPD_CXX_INSTANTIATED_STATIC_DATA_MEMBER,
+  UPD_OBJC_SET_CLASS_DEFINITIONDATA
 };
 
 TypeIdx TypeIdxFromBuiltin(const BuiltinType *BT);
diff --git a/lib/Serialization/ASTReaderDecl.cpp b/lib/Serialization/ASTReaderDecl.cpp
index f7f93af..6fc5764 100644
--- a/lib/Serialization/ASTReaderDecl.cpp
+++ b/lib/Serialization/ASTReaderDecl.cpp
@@ -554,6 +554,7 @@
 }
 
 void ASTDeclReader::VisitObjCInterfaceDecl(ObjCInterfaceDecl *ID) {
+  VisitRedeclarable(ID);
   VisitObjCContainerDecl(ID);
   ID->setTypeForDecl(Reader.readType(F, Record, Idx).getTypePtrOrNull());
   
@@ -618,10 +619,9 @@
       // pending references were linked.
       Reader.PendingForwardRefs.erase(ID);
 #endif
-    
     } else if (Def) {
-      if (Def->Definition) {
-        ID->Definition = Def->Definition;
+      if (Def->Definition.getPointer()) {
+        ID->Definition.setPointer(Def->Definition.getPointer());
       } else {
         // The definition is still initializing.
         Reader.PendingForwardRefs[Def].push_back(ID);
@@ -2067,6 +2067,19 @@
       cast<VarDecl>(D)->getMemberSpecializationInfo()->setPointOfInstantiation(
           Reader.ReadSourceLocation(ModuleFile, Record, Idx));
       break;
+    
+    case UPD_OBJC_SET_CLASS_DEFINITIONDATA: {
+      ObjCInterfaceDecl *ID = cast<ObjCInterfaceDecl>(D);
+      ObjCInterfaceDecl *Def
+        = Reader.ReadDeclAs<ObjCInterfaceDecl>(ModuleFile, Record, Idx);
+      if (Def->Definition.getPointer()) {
+        ID->Definition.setPointer(Def->Definition.getPointer());
+      } else {
+        // The definition is still initializing.
+        Reader.PendingForwardRefs[Def].push_back(ID);
+      }
+      break;
+    }
     }
   }
 }
diff --git a/lib/Serialization/ASTWriter.cpp b/lib/Serialization/ASTWriter.cpp
index 6cce0af..b3f76f7 100644
--- a/lib/Serialization/ASTWriter.cpp
+++ b/lib/Serialization/ASTWriter.cpp
@@ -3439,6 +3439,7 @@
       case UPD_CXX_ADDED_IMPLICIT_MEMBER:
       case UPD_CXX_ADDED_TEMPLATE_SPECIALIZATION:
       case UPD_CXX_ADDED_ANONYMOUS_NAMESPACE:
+      case UPD_OBJC_SET_CLASS_DEFINITIONDATA:
         URec[Idx] = GetDeclRef(reinterpret_cast<Decl *>(URec[Idx]));
         ++Idx;
         break;
@@ -4379,10 +4380,26 @@
 
 void ASTWriter::CompletedObjCForwardRef(const ObjCContainerDecl *D) {
   assert(!WritingAST && "Already writing the AST!");
-  if (!D->isFromASTFile())
-    return; // Declaration not imported from PCH.
+  if (D->isFromASTFile())
+    RewriteDecl(D);
 
-  RewriteDecl(D);
+  if (const ObjCInterfaceDecl *ID = dyn_cast<ObjCInterfaceDecl>(D)) {
+    for (ObjCInterfaceDecl::redecl_iterator I = ID->redecls_begin(), 
+                                            E = ID->redecls_end(); 
+         I != E; ++I) {
+      if (*I == ID)
+        continue;
+      
+      // We are interested when a PCH decl is modified.
+      if (I->isFromASTFile()) {
+        UpdateRecord &Record = DeclUpdates[*I];
+        Record.push_back(UPD_OBJC_SET_CLASS_DEFINITIONDATA);
+        assert((*I)->hasDefinition());
+        assert((*I)->getDefinition() == D);
+        Record.push_back(reinterpret_cast<uint64_t>(D)); // the DefinitionDecl
+      }
+    }
+  }
 }
 
 void ASTWriter::AddedObjCPropertyInClassExtension(const ObjCPropertyDecl *Prop,
diff --git a/lib/Serialization/ASTWriterDecl.cpp b/lib/Serialization/ASTWriterDecl.cpp
index 9396958..e641c10 100644
--- a/lib/Serialization/ASTWriterDecl.cpp
+++ b/lib/Serialization/ASTWriterDecl.cpp
@@ -447,6 +447,7 @@
 }
 
 void ASTDeclWriter::VisitObjCInterfaceDecl(ObjCInterfaceDecl *D) {
+  VisitRedeclarable(D);
   VisitObjCContainerDecl(D);
   Writer.AddTypeRef(QualType(D->getTypeForDecl(), 0), Record);