diff --git a/lib/ARCMigrate/ARCMT.cpp b/lib/ARCMigrate/ARCMT.cpp
index 13bb33d..7f34796 100644
--- a/lib/ARCMigrate/ARCMT.cpp
+++ b/lib/ARCMigrate/ARCMT.cpp
@@ -187,6 +187,8 @@
   CInvok->getPreprocessorOpts().addMacroDef(define);
   CInvok->getLangOpts().ObjCAutoRefCount = true;
   CInvok->getDiagnosticOpts().ErrorLimit = 0;
+  CInvok->getDiagnosticOpts().Warnings.push_back(
+                                            "error=arc-unsafe-retained-assign");
   CInvok->getLangOpts().ObjCNoAutoRefCountRuntime = !HasARCRuntime(origCI);
 
   return CInvok.take();
diff --git a/test/ARCMT/checking.m b/test/ARCMT/checking.m
index 62387fa..8ee7e80 100644
--- a/test/ARCMT/checking.m
+++ b/test/ARCMT/checking.m
@@ -253,3 +253,15 @@
 void rdar9504750(id p) {
   RELEASE_MACRO(p); // expected-error {{ARC forbids explicit message send of 'release'}}
 }
+
+// rdar://8939557
+@interface TestReadonlyProperty : NSObject
+@property(assign,readonly) NSObject *value;
+@end
+
+@implementation TestReadonlyProperty
+@synthesize value;
+- (void)viewDidLoad {
+  value = [NSObject new]; // expected-error {{assigning retained object}}
+}
+@end