Index: lib/Sema/SemaCast.cpp
===================================================================
--- lib/Sema/SemaCast.cpp
+++ lib/Sema/SemaCast.cpp
@@ -872,7 +872,7 @@
     }
     SrcExpr = ExprError();
   } else if (tcr == TC_Success) {
-    if (Self.getLangOpts().ObjCAutoRefCount)
+    if (Self.getLangOpts().ObjCAutoRefCount || Self.getLangOpts().ObjCWeak)
       checkObjCConversion(Sema::CCK_OtherCast);
     DiagnoseReinterpretUpDownCast(Self, SrcExpr.get(), DestType, OpRange);
   }
@@ -936,7 +936,7 @@
   } else if (tcr == TC_Success) {
     if (Kind == CK_BitCast)
       checkCastAlign();
-    if (Self.getLangOpts().ObjCAutoRefCount)
+    if (Self.getLangOpts().ObjCAutoRefCount || Self.getLangOpts().ObjCWeak)
       checkObjCConversion(Sema::CCK_OtherCast);
   } else if (Kind == CK_BitCast) {
     checkCastAlign();
Index: lib/Sema/SemaDecl.cpp
===================================================================
--- lib/Sema/SemaDecl.cpp
+++ lib/Sema/SemaDecl.cpp
@@ -14493,7 +14493,7 @@
   // Verify that all the fields are okay.
   SmallVector<FieldDecl*, 32> RecFields;
 
-  bool ARCErrReported = false;
+  bool ObjCFieldLifetimeErrReported = false;
   for (ArrayRef<Decl *>::iterator i = Fields.begin(), end = Fields.end();
        i != end; ++i) {
     FieldDecl *FD = cast<FieldDecl>(*i);
@@ -14628,16 +14628,20 @@
         << FixItHint::CreateInsertion(FD->getLocation(), "*");
       QualType T = Context.getObjCObjectPointerType(FD->getType());
       FD->setType(T);
-    } else if (getLangOpts().ObjCAutoRefCount && Record && !ARCErrReported &&
+    } else if ((getLangOpts().ObjCAutoRefCount || getLangOpts().ObjCWeak) &&
+               Record && !ObjCFieldLifetimeErrReported &&
                (!getLangOpts().CPlusPlus || Record->isUnion())) {
-      // It's an error in ARC if a field has lifetime.
+      // It's an error in ARC or Weak if a field has lifetime.
       // We don't want to report this in a system header, though,
       // so we just make the field unavailable.
       // FIXME: that's really not sufficient; we need to make the type
       // itself invalid to, say, initialize or copy.
       QualType T = FD->getType();
       Qualifiers::ObjCLifetime lifetime = T.getObjCLifetime();
-      if (lifetime && lifetime != Qualifiers::OCL_ExplicitNone) {
+      if (lifetime &&
+          ((getLangOpts().ObjCAutoRefCount &&
+            lifetime != Qualifiers::OCL_ExplicitNone) ||
+           (getLangOpts().ObjCWeak && lifetime == Qualifiers::OCL_Weak))) {
         SourceLocation loc = FD->getLocation();
         if (getSourceManager().isInSystemHeader(loc)) {
           if (!FD->hasAttr<UnavailableAttr>()) {
@@ -14648,7 +14652,7 @@
           Diag(FD->getLocation(), diag::err_arc_objc_object_in_tag)
             << T->isBlockPointerType() << Record->getTagKind();
         }
-        ARCErrReported = true;
+        ObjCFieldLifetimeErrReported = true;
       }
     } else if (getLangOpts().ObjC1 &&
                getLangOpts().getGC() != LangOptions::NonGC &&
Index: lib/Sema/SemaDeclCXX.cpp
===================================================================
--- lib/Sema/SemaDeclCXX.cpp
+++ lib/Sema/SemaDeclCXX.cpp
@@ -7150,8 +7150,10 @@
     //   [...] nontrivally ownership-qualified types are [...] not trivially
     //   default constructible, copy constructible, move constructible, copy
     //   assignable, move assignable, or destructible [...]
-    if (S.getLangOpts().ObjCAutoRefCount &&
-        FieldType.hasNonTrivialObjCLifetime()) {
+    if ((S.getLangOpts().ObjCAutoRefCount &&
+         FieldType.hasNonTrivialObjCLifetime()) ||
+        (S.getLangOpts().ObjCWeak &&
+         FieldType.getObjCLifetime() == Qualifiers::OCL_Weak)) {
       if (Diagnose)
         S.Diag(FI->getLocation(), diag::note_nontrivial_objc_ownership)
           << RD << FieldType.getObjCLifetime();
Index: lib/Sema/SemaInit.cpp
===================================================================
--- lib/Sema/SemaInit.cpp
+++ lib/Sema/SemaInit.cpp
@@ -6680,6 +6680,8 @@
       // full-expression's cleanups.
       if ((S.getLangOpts().ObjCAutoRefCount &&
            MTE->getType()->isObjCLifetimeType()) ||
+          (S.getLangOpts().ObjCWeak &&
+           MTE->getType().getObjCLifetime() == Qualifiers::OCL_Weak) ||
           (MTE->getStorageDuration() == SD_Automatic &&
            MTE->getType().isDestructedType()))
         S.Cleanup.setExprNeedsCleanups(true);
Index: test/SemaObjCXX/objc-weak.mm
===================================================================
--- /dev/null
+++ test/SemaObjCXX/objc-weak.mm
@@ -0,0 +1,28 @@
+// RUN: %clang_cc1 -fsyntax-only -fobjc-runtime-has-weak -fobjc-weak -fblocks -Wno-objc-root-class -std=c++98 -Wno-c++0x-extensions -verify %s
+
+@interface AnObject
+@property(weak) id value;
+@end
+
+__attribute__((objc_arc_weak_reference_unavailable))
+@interface NOWEAK : AnObject // expected-note 2 {{class is declared here}}
+@end
+
+struct S {
+  __weak id a; // expected-note {{because type 'S' has a member with __weak ownership}}
+};
+
+union U {
+  __weak id a; // expected-error {{ARC forbids Objective-C objects in union}}
+  S b;         // expected-error {{union member 'b' has a non-trivial copy constructor}}
+};
+
+void testCast(AnObject *o) {
+  __weak id a = reinterpret_cast<__weak NOWEAK *>(o); // expected-error {{class is incompatible with __weak references}} \
+                                                      // expected-error {{explicit ownership qualifier on cast result has no effect}} \
+                                                      // expected-error {{assignment of a weak-unavailable object to a __weak object}}
+
+  __weak id b = static_cast<__weak NOWEAK *>(o); // expected-error {{class is incompatible with __weak references}} \
+                                                 // expected-error {{explicit ownership qualifier on cast result has no effect}} \
+                                                 // expected-error {{assignment of a weak-unavailable object to a __weak object}}
+}