Index: lib/Sema/SemaDecl.cpp
===================================================================
--- lib/Sema/SemaDecl.cpp
+++ lib/Sema/SemaDecl.cpp
@@ -16372,6 +16372,17 @@
         << FixItHint::CreateInsertion(FD->getLocation(), "*");
       QualType T = Context.getObjCObjectPointerType(FD->getType());
       FD->setType(T);
+    } else if (Record && Record->isUnion() &&
+               FD->getType().hasNonTrivialObjCLifetime() &&
+               getSourceManager().isInSystemHeader(FD->getLocation()) &&
+               !getLangOpts().CPlusPlus && !FD->hasAttr<UnavailableAttr>() &&
+               !FD->getType()->getAs<AttributedType>()) {
+      // For backward compatibility, fields of C unions declared in system
+      // headers that have non-trivial ObjC ownership qualifications are marked
+      // as unavailable unless the qualification is explicit.
+      FD->addAttr(UnavailableAttr::CreateImplicit(
+          Context, "", UnavailableAttr::IR_ARCFieldWithOwnership,
+          FD->getLocation()));
     } else if (getLangOpts().ObjC &&
                getLangOpts().getGC() != LangOptions::NonGC &&
                Record && !Record->hasObjectMember()) {
Index: test/SemaObjC/Inputs/non-trivial-c-union.h
===================================================================
--- /dev/null
+++ test/SemaObjC/Inputs/non-trivial-c-union.h
@@ -0,0 +1,13 @@
+// For backward compatibility, fields of C unions declared in system headers
+// that have non-trivial ObjC ownership qualifications are marked as unavailable
+// unless the qualification is explicit.
+
+#pragma clang system_header
+
+typedef union {
+  id f0;
+} U0_SystemHeader;
+
+typedef union { // expected-note {{'U1_SystemHeader' has subobjects that are non-trivial to destruct}} expected-note {{'U1_SystemHeader' has subobjects that are non-trivial to copy}}
+  __strong id f0; // expected-note {{f0 has type '__strong id' that is non-trivial to destruct}} expected-note {{f0 has type '__strong id' that is non-trivial to copy}}
+} U1_SystemHeader;
Index: test/SemaObjC/non-trivial-c-union.m
===================================================================
--- test/SemaObjC/non-trivial-c-union.m
+++ test/SemaObjC/non-trivial-c-union.m
@@ -1,4 +1,6 @@
-// RUN: %clang_cc1 -fsyntax-only -fblocks -fobjc-arc -fobjc-runtime-has-weak -verify %s
+// RUN: %clang_cc1 -fsyntax-only -fblocks -fobjc-arc -fobjc-runtime-has-weak -I %S/Inputs -verify %s
+
+#include "non-trivial-c-union.h"
 
 typedef union { // expected-note 12 {{'U0' has subobjects that are non-trivial to default-initialize}} expected-note 36 {{'U0' has subobjects that are non-trivial to destruct}} expected-note 28 {{'U0' has subobjects that are non-trivial to copy}}
   id f0; // expected-note 12 {{f0 has type '__strong id' that is non-trivial to default-initialize}} expected-note 36 {{f0 has type '__strong id' that is non-trivial to destruct}} expected-note 28 {{f0 has type '__strong id' that is non-trivial to copy}}
@@ -80,3 +82,7 @@
 void testVolatileLValueToRValue(volatile U0 *a) {
   (void)*a; // expected-error {{cannot use volatile type 'volatile U0' where it causes an lvalue-to-rvalue conversion since it is a union that is non-trivial to destruct}} // expected-error {{cannot use volatile type 'volatile U0' where it causes an lvalue-to-rvalue conversion since it is a union that is non-trivial to copy}}
 }
+
+void unionInSystemHeader0(U0_SystemHeader);
+
+void unionInSystemHeader1(U1_SystemHeader); // expected-error {{cannot use type 'U1_SystemHeader' for a function/method parameter since it is a union that is non-trivial to destruct}} expected-error {{cannot use type 'U1_SystemHeader' for a function/method parameter since it is a union that is non-trivial to copy}}