Index: include/clang/AST/ASTContext.h
===================================================================
--- include/clang/AST/ASTContext.h
+++ include/clang/AST/ASTContext.h
@@ -2054,6 +2054,12 @@
   /// types.
   bool areCompatibleVectorTypes(QualType FirstVec, QualType SecondVec);
 
+  /// This function finds out whether there is an AttributedType of kind
+  /// attr::ObjCOwnership in Ty. The existence of AttributedType of kind
+  /// attr::ObjCOwnership implies an ownership qualifier was explicitly
+  /// specified rather than being added implicitly by the compiler.
+  bool isObjCOwnershipAttributedType(QualType Ty) const;
+
   /// Return true if this is an \c NSObject object with its \c NSObject
   /// attribute set.
   static bool isObjCNSObjectType(QualType Ty) {
Index: lib/AST/ASTContext.cpp
===================================================================
--- lib/AST/ASTContext.cpp
+++ lib/AST/ASTContext.cpp
@@ -7866,6 +7866,18 @@
   return false;
 }
 
+bool ASTContext::isObjCOwnershipAttributedType(QualType Ty) const {
+  while (const auto *AttrTy = Ty->getAs<AttributedType>()) {
+    if (AttrTy->getAttrKind() == attr::ObjCOwnership)
+      return true;
+
+    // Peel off AttributedTypes that are not of kind ObjCOwnership.
+    Ty = AttrTy->getModifiedType();
+  }
+
+  return false;
+}
+
 //===----------------------------------------------------------------------===//
 // ObjCQualifiedIdTypesAreCompatible - Compatibility testing for qualified id's.
 //===----------------------------------------------------------------------===//
Index: lib/Sema/SemaDecl.cpp
===================================================================
--- lib/Sema/SemaDecl.cpp
+++ lib/Sema/SemaDecl.cpp
@@ -11173,7 +11173,8 @@
           << 0 << 0 << QT.getUnqualifiedType() << "";
 
     for (const FieldDecl *FD : RD->fields())
-      asDerived().visit(FD->getType(), FD, InNonTrivialUnion);
+      if (!FD->hasAttr<UnavailableAttr>())
+        asDerived().visit(FD->getType(), FD, InNonTrivialUnion);
   }
 
   void visitTrivial(QualType QT, const FieldDecl *FD, bool InNonTrivialUnion) {}
@@ -11237,7 +11238,8 @@
           << 0 << 1 << QT.getUnqualifiedType() << "";
 
     for (const FieldDecl *FD : RD->fields())
-      asDerived().visit(FD->getType(), FD, InNonTrivialUnion);
+      if (!FD->hasAttr<UnavailableAttr>())
+        asDerived().visit(FD->getType(), FD, InNonTrivialUnion);
   }
 
   void visitTrivial(QualType QT, const FieldDecl *FD, bool InNonTrivialUnion) {}
@@ -11302,7 +11304,8 @@
           << 0 << 2 << QT.getUnqualifiedType() << "";
 
     for (const FieldDecl *FD : RD->fields())
-      asDerived().visit(FD->getType(), FD, InNonTrivialUnion);
+      if (!FD->hasAttr<UnavailableAttr>())
+        asDerived().visit(FD->getType(), FD, InNonTrivialUnion);
   }
 
   void preVisit(QualType::PrimitiveCopyKind PCK, QualType QT,
@@ -16372,6 +16375,18 @@
         << 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().getObjCLifetime() != Qualifiers::OCL_Strong ||
+                !Context.isObjCOwnershipAttributedType(FD->getType()))) {
+      // For backward compatibility, fields of C unions declared in system
+      // headers that have non-trivial ObjC ownership qualifications are marked
+      // as unavailable unless the qualifier is explicit and __strong.
+      FD->addAttr(UnavailableAttr::CreateImplicit(
+          Context, "", UnavailableAttr::IR_ARCFieldWithOwnership,
+          FD->getLocation()));
     } else if (getLangOpts().ObjC &&
                getLangOpts().getGC() != LangOptions::NonGC &&
                Record && !Record->hasObjectMember()) {
Index: lib/Sema/SemaExpr.cpp
===================================================================
--- lib/Sema/SemaExpr.cpp
+++ lib/Sema/SemaExpr.cpp
@@ -15456,27 +15456,11 @@
 
   // Warn about implicitly autoreleasing indirect parameters captured by blocks.
   if (const auto *PT = CaptureType->getAs<PointerType>()) {
-    // This function finds out whether there is an AttributedType of kind
-    // attr::ObjCOwnership in Ty. The existence of AttributedType of kind
-    // attr::ObjCOwnership implies __autoreleasing was explicitly specified
-    // rather than being added implicitly by the compiler.
-    auto IsObjCOwnershipAttributedType = [](QualType Ty) {
-      while (const auto *AttrTy = Ty->getAs<AttributedType>()) {
-        if (AttrTy->getAttrKind() == attr::ObjCOwnership)
-          return true;
-
-        // Peel off AttributedTypes that are not of kind ObjCOwnership.
-        Ty = AttrTy->getModifiedType();
-      }
-
-      return false;
-    };
-
     QualType PointeeTy = PT->getPointeeType();
 
     if (!Invalid && PointeeTy->getAs<ObjCObjectPointerType>() &&
         PointeeTy.getObjCLifetime() == Qualifiers::OCL_Autoreleasing &&
-        !IsObjCOwnershipAttributedType(PointeeTy)) {
+        !S.Context.isObjCOwnershipAttributedType(PointeeTy)) {
       if (BuildAndDiagnose) {
         SourceLocation VarLoc = Var->getLocation();
         S.Diag(Loc, diag::warn_block_capture_autoreleasing);
Index: test/SemaObjC/Inputs/non-trivial-c-union.h
===================================================================
--- /dev/null
+++ test/SemaObjC/Inputs/non-trivial-c-union.h
@@ -0,0 +1,16 @@
+// For backward compatibility, fields of C unions declared in system headers
+// that have non-trivial ObjC ownership qualifications are marked as unavailable
+// unless the qualifier is explicit and __strong.
+
+#pragma clang system_header
+
+typedef union {
+  id f0;
+  _Nonnull id f1;
+  __weak id f2;
+} 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}}
+  _Nonnull id f1;
+} 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}}