[Sema] Diagnose default-initialization, destruction, and copying of

non-trivial C union types

This recommits r365985, which was reverted because it broke a few
projects using unions containing non-trivial ObjC pointer fields in
system headers. We now have a patch to fix the problem (see
https://reviews.llvm.org/D65256).

Original commit message:

This patch diagnoses uses of non-trivial C unions and structs/unions
containing non-trivial C unions in the following contexts, which require
default-initialization, destruction, or copying of the union objects,
instead of disallowing fields of non-trivial types in C unions, which is
what we currently do:

- function parameters.
- function returns.
- assignments.
- compound literals.
- block captures except capturing of `__block` variables by non-escaping blocks.
- local and global variable definitions.
- lvalue-to-rvalue conversions of volatile types.

See the discussion in https://reviews.llvm.org/D62988 for more background.

rdar://problem/50679094

Differential Revision: https://reviews.llvm.org/D63753

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@371275 91177308-0d34-0410-b5e6-96231b3b80d8

18 files changed, 615 insertions, 122 deletions

diff --git a/include/clang/AST/Decl.h b/include/clang/AST/Decl.h
index 0d9da7d0dc..096df484cb 100644
--- a/include/clang/AST/Decl.h
+++ b/include/clang/AST/Decl.h
@@ -3754,6 +3754,30 @@ public:
     RecordDeclBits.NonTrivialToPrimitiveDestroy = V;
   }
 
+  bool hasNonTrivialToPrimitiveDefaultInitializeCUnion() const {
+    return RecordDeclBits.HasNonTrivialToPrimitiveDefaultInitializeCUnion;
+  }
+
+  void setHasNonTrivialToPrimitiveDefaultInitializeCUnion(bool V) {
+    RecordDeclBits.HasNonTrivialToPrimitiveDefaultInitializeCUnion = V;
+  }
+
+  bool hasNonTrivialToPrimitiveDestructCUnion() const {
+    return RecordDeclBits.HasNonTrivialToPrimitiveDestructCUnion;
+  }
+
+  void setHasNonTrivialToPrimitiveDestructCUnion(bool V) {
+    RecordDeclBits.HasNonTrivialToPrimitiveDestructCUnion = V;
+  }
+
+  bool hasNonTrivialToPrimitiveCopyCUnion() const {
+    return RecordDeclBits.HasNonTrivialToPrimitiveCopyCUnion;
+  }
+
+  void setHasNonTrivialToPrimitiveCopyCUnion(bool V) {
+    RecordDeclBits.HasNonTrivialToPrimitiveCopyCUnion = V;
+  }
+
   /// Determine whether this class can be passed in registers. In C++ mode,
   /// it must have at least one trivial, non-deleted copy or move constructor.
   /// FIXME: This should be set as part of completeDefinition.
diff --git a/include/clang/AST/DeclBase.h b/include/clang/AST/DeclBase.h
index 26edb7790c..d64d0cb425 100644
--- a/include/clang/AST/DeclBase.h
+++ b/include/clang/AST/DeclBase.h
@@ -1440,6 +1440,13 @@ class DeclContext {
     uint64_t NonTrivialToPrimitiveCopy : 1;
     uint64_t NonTrivialToPrimitiveDestroy : 1;
 
+    /// The following bits indicate whether this is or contains a C union that
+    /// is non-trivial to default-initialize, destruct, or copy. These bits
+    /// imply the associated basic non-triviality predicates declared above.
+    uint64_t HasNonTrivialToPrimitiveDefaultInitializeCUnion : 1;
+    uint64_t HasNonTrivialToPrimitiveDestructCUnion : 1;
+    uint64_t HasNonTrivialToPrimitiveCopyCUnion : 1;
+
     /// Indicates whether this struct is destroyed in the callee.
     uint64_t ParamDestroyedInCallee : 1;
 
@@ -1448,7 +1455,7 @@ class DeclContext {
   };
 
   /// Number of non-inherited bits in RecordDeclBitfields.
-  enum { NumRecordDeclBits = 11 };
+  enum { NumRecordDeclBits = 14 };
 
   /// Stores the bits used by OMPDeclareReductionDecl.
   /// If modified NumOMPDeclareReductionDeclBits and the accessor
diff --git a/include/clang/AST/Type.h b/include/clang/AST/Type.h
index dc1db0500f..88f022b083 100644
--- a/include/clang/AST/Type.h
+++ b/include/clang/AST/Type.h
@@ -1133,12 +1133,6 @@ public:
   };
 
   /// Check if this is a non-trivial type that would cause a C struct
-  /// transitively containing this type to be non-trivial. This function can be
-  /// used to determine whether a field of this type can be declared inside a C
-  /// union.
-  bool isNonTrivialPrimitiveCType(const ASTContext &Ctx) const;
-
-  /// Check if this is a non-trivial type that would cause a C struct
   /// transitively containing this type to be non-trivial to copy and return the
   /// kind.
   PrimitiveCopyKind isNonTrivialToPrimitiveCopy() const;
@@ -1167,6 +1161,22 @@ public:
     return isDestructedTypeImpl(*this);
   }
 
+  /// Check if this is or contains a C union that is non-trivial to
+  /// default-initialize, which is a union that has a member that is non-trivial
+  /// to default-initialize. If this returns true,
+  /// isNonTrivialToPrimitiveDefaultInitialize returns PDIK_Struct.
+  bool hasNonTrivialToPrimitiveDefaultInitializeCUnion() const;
+
+  /// Check if this is or contains a C union that is non-trivial to destruct,
+  /// which is a union that has a member that is non-trivial to destruct. If
+  /// this returns true, isDestructedType returns DK_nontrivial_c_struct.
+  bool hasNonTrivialToPrimitiveDestructCUnion() const;
+
+  /// Check if this is or contains a C union that is non-trivial to copy, which
+  /// is a union that has a member that is non-trivial to copy. If this returns
+  /// true, isNonTrivialToPrimitiveCopy returns PCK_Struct.
+  bool hasNonTrivialToPrimitiveCopyCUnion() const;
+
   /// Determine whether expressions of the given type are forbidden
   /// from being lvalues in C.
   ///
@@ -1239,6 +1249,11 @@ private:
                                                  const ASTContext &C);
   static QualType IgnoreParens(QualType T);
   static DestructionKind isDestructedTypeImpl(QualType type);
+
+  /// Check if \param RD is or contains a non-trivial C union.
+  static bool hasNonTrivialToPrimitiveDefaultInitializeCUnion(const RecordDecl *RD);
+  static bool hasNonTrivialToPrimitiveDestructCUnion(const RecordDecl *RD);
+  static bool hasNonTrivialToPrimitiveCopyCUnion(const RecordDecl *RD);
 };
 
 } // namespace clang
@@ -6255,6 +6270,24 @@ inline Qualifiers::GC QualType::getObjCGCAttr() const {
   return getQualifiers().getObjCGCAttr();
 }
 
+inline bool QualType::hasNonTrivialToPrimitiveDefaultInitializeCUnion() const {
+  if (auto *RD = getTypePtr()->getBaseElementTypeUnsafe()->getAsRecordDecl())
+    return hasNonTrivialToPrimitiveDefaultInitializeCUnion(RD);
+  return false;
+}
+
+inline bool QualType::hasNonTrivialToPrimitiveDestructCUnion() const {
+  if (auto *RD = getTypePtr()->getBaseElementTypeUnsafe()->getAsRecordDecl())
+    return hasNonTrivialToPrimitiveDestructCUnion(RD);
+  return false;
+}
+
+inline bool QualType::hasNonTrivialToPrimitiveCopyCUnion() const {
+  if (auto *RD = getTypePtr()->getBaseElementTypeUnsafe()->getAsRecordDecl())
+    return hasNonTrivialToPrimitiveCopyCUnion(RD);
+  return false;
+}
+
 inline FunctionType::ExtInfo getFunctionExtInfo(const Type &t) {
   if (const auto *PT = t.getAs<PointerType>()) {
     if (const auto *FT = PT->getPointeeType()->getAs<FunctionType>())
diff --git a/include/clang/Basic/DiagnosticSemaKinds.td b/include/clang/Basic/DiagnosticSemaKinds.td
index d04a96f527..9ccf897f99 100644
--- a/include/clang/Basic/DiagnosticSemaKinds.td
+++ b/include/clang/Basic/DiagnosticSemaKinds.td
@@ -647,8 +647,23 @@ def warn_cstruct_memaccess : Warning<
   InGroup<NonTrivialMemaccess>;
 def note_nontrivial_field : Note<
   "field is non-trivial to %select{copy|default-initialize}0">;
-def err_nontrivial_primitive_type_in_union : Error<
-  "non-trivial C types are disallowed in union">;
+def err_non_trivial_c_union_in_invalid_context : Error<
+  "cannot %select{"
+  "use type %1 for a function/method parameter|"
+  "use type %1 for function/method return|"
+  "default-initialize an object of type %1|"
+  "declare an automatic variable of type %1|"
+  "copy-initialize an object of type %1|"
+  "assign to a variable of type %1|"
+  "construct an automatic compound literal of type %1|"
+  "capture a variable of type %1|"
+  "cannot use volatile type %1 where it causes an lvalue-to-rvalue conversion"
+  "}3 "
+  "since it %select{contains|is}2 a union that is non-trivial to "
+  "%select{default-initialize|destruct|copy}0">;
+def note_non_trivial_c_union : Note<
+  "%select{%2 has subobjects that are|%3 has type %2 that is}0 "
+  "non-trivial to %select{default-initialize|destruct|copy}1">;
 def warn_dyn_class_memaccess : Warning<
   "%select{destination for|source of|first operand of|second operand of}0 this "
   "%1 call is a pointer to %select{|class containing a }2dynamic class %3; "
diff --git a/include/clang/Sema/Sema.h b/include/clang/Sema/Sema.h
index d7581a1673..59f16da8f3 100644
--- a/include/clang/Sema/Sema.h
+++ b/include/clang/Sema/Sema.h
@@ -2145,6 +2145,48 @@ public:
   bool SetParamDefaultArgument(ParmVarDecl *Param, Expr *DefaultArg,
                                SourceLocation EqualLoc);
 
+  // Contexts where using non-trivial C union types can be disallowed. This is
+  // passed to err_non_trivial_c_union_in_invalid_context.
+  enum NonTrivialCUnionContext {
+    // Function parameter.
+    NTCUC_FunctionParam,
+    // Function return.
+    NTCUC_FunctionReturn,
+    // Default-initialized object.
+    NTCUC_DefaultInitializedObject,
+    // Variable with automatic storage duration.
+    NTCUC_AutoVar,
+    // Initializer expression that might copy from another object.
+    NTCUC_CopyInit,
+    // Assignment.
+    NTCUC_Assignment,
+    // Compound literal.
+    NTCUC_CompoundLiteral,
+    // Block capture.
+    NTCUC_BlockCapture,
+    // lvalue-to-rvalue conversion of volatile type.
+    NTCUC_LValueToRValueVolatile,
+  };
+
+  /// Emit diagnostics if the initializer or any of its explicit or
+  /// implicitly-generated subexpressions require copying or
+  /// default-initializing a type that is or contains a C union type that is
+  /// non-trivial to copy or default-initialize.
+  void checkNonTrivialCUnionInInitializer(const Expr *Init, SourceLocation Loc);
+
+  // These flags are passed to checkNonTrivialCUnion.
+  enum NonTrivialCUnionKind {
+    NTCUK_Init = 0x1,
+    NTCUK_Destruct = 0x2,
+    NTCUK_Copy = 0x4,
+  };
+
+  /// Emit diagnostics if a non-trivial C union type or a struct that contains
+  /// a non-trivial C union is used in an invalid context.
+  void checkNonTrivialCUnion(QualType QT, SourceLocation Loc,
+                             NonTrivialCUnionContext UseContext,
+                             unsigned NonTrivialKind);
+
   void AddInitializerToDecl(Decl *dcl, Expr *init, bool DirectInit);
   void ActOnUninitializedDecl(Decl *dcl);
   void ActOnInitializerError(Decl *Dcl);
diff --git a/lib/AST/Decl.cpp b/lib/AST/Decl.cpp
index 6c0c828146..1d28f03b06 100644
--- a/lib/AST/Decl.cpp
+++ b/lib/AST/Decl.cpp
@@ -4268,6 +4268,9 @@ RecordDecl::RecordDecl(Kind DK, TagKind TK, const ASTContext &C,
   setNonTrivialToPrimitiveDefaultInitialize(false);
   setNonTrivialToPrimitiveCopy(false);
   setNonTrivialToPrimitiveDestroy(false);
+  setHasNonTrivialToPrimitiveDefaultInitializeCUnion(false);
+  setHasNonTrivialToPrimitiveDestructCUnion(false);
+  setHasNonTrivialToPrimitiveCopyCUnion(false);
   setParamDestroyedInCallee(false);
   setArgPassingRestrictions(APK_CanPassInRegs);
 }
diff --git a/lib/AST/Type.cpp b/lib/AST/Type.cpp
index 608626820a..411cb749a7 100644
--- a/lib/AST/Type.cpp
+++ b/lib/AST/Type.cpp
@@ -2289,60 +2289,16 @@ bool QualType::isNonWeakInMRRWithObjCWeak(const ASTContext &Context) const {
          getObjCLifetime() != Qualifiers::OCL_Weak;
 }
 
-namespace {
-// Helper class that determines whether this is a type that is non-trivial to
-// primitive copy or move, or is a struct type that has a field of such type.
-template <bool IsMove>
-struct IsNonTrivialCopyMoveVisitor
-    : CopiedTypeVisitor<IsNonTrivialCopyMoveVisitor<IsMove>, IsMove, bool> {
-  using Super =
-      CopiedTypeVisitor<IsNonTrivialCopyMoveVisitor<IsMove>, IsMove, bool>;
-  IsNonTrivialCopyMoveVisitor(const ASTContext &C) : Ctx(C) {}
-  void preVisit(QualType::PrimitiveCopyKind PCK, QualType QT) {}
-
-  bool visitWithKind(QualType::PrimitiveCopyKind PCK, QualType QT) {
-    if (const auto *AT = this->Ctx.getAsArrayType(QT))
-      return this->asDerived().visit(Ctx.getBaseElementType(AT));
-    return Super::visitWithKind(PCK, QT);
-  }
-
-  bool visitARCStrong(QualType QT) { return true; }
-  bool visitARCWeak(QualType QT) { return true; }
-  bool visitTrivial(QualType QT) { return false; }
-  // Volatile fields are considered trivial.
-  bool visitVolatileTrivial(QualType QT) { return false; }
-
-  bool visitStruct(QualType QT) {
-    const RecordDecl *RD = QT->castAs<RecordType>()->getDecl();
-    // We don't want to apply the C restriction in C++ because C++
-    // (1) can apply the restriction at a finer grain by banning copying or
-    //     destroying the union, and
-    // (2) allows users to override these restrictions by declaring explicit
-    //     constructors/etc, which we're not proposing to add to C.
-    if (isa<CXXRecordDecl>(RD))
-      return false;
-    for (const FieldDecl *FD : RD->fields())
-      if (this->asDerived().visit(FD->getType()))
-        return true;
-    return false;
-  }
-
-  const ASTContext &Ctx;
-};
+bool QualType::hasNonTrivialToPrimitiveDefaultInitializeCUnion(const RecordDecl *RD) {
+  return RD->hasNonTrivialToPrimitiveDefaultInitializeCUnion();
+}
 
-} // namespace
+bool QualType::hasNonTrivialToPrimitiveDestructCUnion(const RecordDecl *RD) {
+  return RD->hasNonTrivialToPrimitiveDestructCUnion();
+}
 
-bool QualType::isNonTrivialPrimitiveCType(const ASTContext &Ctx) const {
-  if (isNonTrivialToPrimitiveDefaultInitialize())
-    return true;
-  DestructionKind DK = isDestructedType();
-  if (DK != DK_none && DK != DK_cxx_destructor)
-    return true;
-  if (IsNonTrivialCopyMoveVisitor<false>(Ctx).visit(*this))
-    return true;
-  if (IsNonTrivialCopyMoveVisitor<true>(Ctx).visit(*this))
-    return true;
-  return false;
+bool QualType::hasNonTrivialToPrimitiveCopyCUnion(const RecordDecl *RD) {
+  return RD->hasNonTrivialToPrimitiveCopyCUnion();
 }
 
 QualType::PrimitiveDefaultInitializeKind
diff --git a/lib/Sema/Sema.cpp b/lib/Sema/Sema.cpp
index 152cc7d9a2..3f4dacbfc1 100644
--- a/lib/Sema/Sema.cpp
+++ b/lib/Sema/Sema.cpp
@@ -1684,12 +1684,24 @@ static void markEscapingByrefs(const FunctionScopeInfo &FSI, Sema &S) {
   // Set the EscapingByref flag of __block variables captured by
   // escaping blocks.
   for (const BlockDecl *BD : FSI.Blocks) {
-    if (BD->doesNotEscape())
-      continue;
     for (const BlockDecl::Capture &BC : BD->captures()) {
       VarDecl *VD = BC.getVariable();
-      if (VD->hasAttr<BlocksAttr>())
+      if (VD->hasAttr<BlocksAttr>()) {
+        // Nothing to do if this is a __block variable captured by a
+        // non-escaping block.
+        if (BD->doesNotEscape())
+          continue;
         VD->setEscapingByref();
+      }
+      // Check whether the captured variable is or contains an object of
+      // non-trivial C union type.
+      QualType CapType = BC.getVariable()->getType();
+      if (CapType.hasNonTrivialToPrimitiveDestructCUnion() ||
+          CapType.hasNonTrivialToPrimitiveCopyCUnion())
+        S.checkNonTrivialCUnion(BC.getVariable()->getType(),
+                                BD->getCaretLocation(),
+                                Sema::NTCUC_BlockCapture,
+                                Sema::NTCUK_Destruct|Sema::NTCUK_Copy);
     }
   }
 
diff --git a/lib/Sema/SemaDecl.cpp b/lib/Sema/SemaDecl.cpp
index a2708223c1..279332ab93 100644
--- a/lib/Sema/SemaDecl.cpp
+++ b/lib/Sema/SemaDecl.cpp
@@ -22,6 +22,7 @@
 #include "clang/AST/DeclTemplate.h"
 #include "clang/AST/EvaluatedExprVisitor.h"
 #include "clang/AST/ExprCXX.h"
+#include "clang/AST/NonTrivialTypeVisitor.h"
 #include "clang/AST/StmtCXX.h"
 #include "clang/Basic/Builtins.h"
 #include "clang/Basic/PartialDiagnostic.h"
@@ -6612,6 +6613,11 @@ NamedDecl *Sema::ActOnVariableDeclarator(
 
     if (D.isInvalidType())
       NewVD->setInvalidDecl();
+
+    if (NewVD->getType().hasNonTrivialToPrimitiveDestructCUnion() &&
+        NewVD->hasLocalStorage())
+      checkNonTrivialCUnion(NewVD->getType(), NewVD->getLocation(),
+                            NTCUC_AutoVar, NTCUK_Destruct);
   } else {
     bool Invalid = false;
 
@@ -9061,6 +9067,12 @@ Sema::ActOnFunctionDeclarator(Scope *S, Declarator &D, DeclContext *DC,
             << FunctionType::getNameForCallConv(CC);
       }
     }
+
+   if (NewFD->getReturnType().hasNonTrivialToPrimitiveDestructCUnion() ||
+       NewFD->getReturnType().hasNonTrivialToPrimitiveCopyCUnion())
+     checkNonTrivialCUnion(NewFD->getReturnType(),
+                           NewFD->getReturnTypeSourceRange().getBegin(),
+                           NTCUC_FunctionReturn, NTCUK_Destruct|NTCUK_Copy);
   } else {
     // C++11 [replacement.functions]p3:
     //  The program's definitions shall not be specified as inline.
@@ -11207,6 +11219,264 @@ bool Sema::DeduceVariableDeclarationType(VarDecl *VDecl, bool DirectInit,
   return VDecl->isInvalidDecl();
 }
 
+void Sema::checkNonTrivialCUnionInInitializer(const Expr *Init,
+                                              SourceLocation Loc) {
+  if (auto *CE = dyn_cast<ConstantExpr>(Init))
+    Init = CE->getSubExpr();
+
+  QualType InitType = Init->getType();
+  assert((InitType.hasNonTrivialToPrimitiveDefaultInitializeCUnion() ||
+          InitType.hasNonTrivialToPrimitiveCopyCUnion()) &&
+         "shouldn't be called if type doesn't have a non-trivial C struct");
+  if (auto *ILE = dyn_cast<InitListExpr>(Init)) {
+    for (auto I : ILE->inits()) {
+      if (!I->getType().hasNonTrivialToPrimitiveDefaultInitializeCUnion() &&
+          !I->getType().hasNonTrivialToPrimitiveCopyCUnion())
+        continue;
+      SourceLocation SL = I->getExprLoc();
+      checkNonTrivialCUnionInInitializer(I, SL.isValid() ? SL : Loc);
+    }
+    return;
+  }
+
+  if (isa<ImplicitValueInitExpr>(Init)) {
+    if (InitType.hasNonTrivialToPrimitiveDefaultInitializeCUnion())
+      checkNonTrivialCUnion(InitType, Loc, NTCUC_DefaultInitializedObject,
+                            NTCUK_Init);
+  } else {
+    // Assume all other explicit initializers involving copying some existing
+    // object.
+    // TODO: ignore any explicit initializers where we can guarantee
+    // copy-elision.
+    if (InitType.hasNonTrivialToPrimitiveCopyCUnion())
+      checkNonTrivialCUnion(InitType, Loc, NTCUC_CopyInit, NTCUK_Copy);
+  }
+}
+
+namespace {
+
+struct DiagNonTrivalCUnionDefaultInitializeVisitor
+    : DefaultInitializedTypeVisitor<DiagNonTrivalCUnionDefaultInitializeVisitor,
+                                    void> {
+  using Super =
+      DefaultInitializedTypeVisitor<DiagNonTrivalCUnionDefaultInitializeVisitor,
+                                    void>;
+
+  DiagNonTrivalCUnionDefaultInitializeVisitor(
+      QualType OrigTy, SourceLocation OrigLoc,
+      Sema::NonTrivialCUnionContext UseContext, Sema &S)
+      : OrigTy(OrigTy), OrigLoc(OrigLoc), UseContext(UseContext), S(S) {}
+
+  void visitWithKind(QualType::PrimitiveDefaultInitializeKind PDIK, QualType QT,
+                     const FieldDecl *FD, bool InNonTrivialUnion) {
+    if (const auto *AT = S.Context.getAsArrayType(QT))
+      return this->asDerived().visit(S.Context.getBaseElementType(AT), FD,
+                                     InNonTrivialUnion);
+    return Super::visitWithKind(PDIK, QT, FD, InNonTrivialUnion);
+  }
+
+  void visitARCStrong(QualType QT, const FieldDecl *FD,
+                      bool InNonTrivialUnion) {
+    if (InNonTrivialUnion)
+      S.Diag(FD->getLocation(), diag::note_non_trivial_c_union)
+          << 1 << 0 << QT << FD->getName();
+  }
+
+  void visitARCWeak(QualType QT, const FieldDecl *FD, bool InNonTrivialUnion) {
+    if (InNonTrivialUnion)
+      S.Diag(FD->getLocation(), diag::note_non_trivial_c_union)
+          << 1 << 0 << QT << FD->getName();
+  }
+
+  void visitStruct(QualType QT, const FieldDecl *FD, bool InNonTrivialUnion) {
+    const RecordDecl *RD = QT->castAs<RecordType>()->getDecl();
+    if (RD->isUnion()) {
+      if (OrigLoc.isValid()) {
+        bool IsUnion = false;
+        if (auto *OrigRD = OrigTy->getAsRecordDecl())
+          IsUnion = OrigRD->isUnion();
+        S.Diag(OrigLoc, diag::err_non_trivial_c_union_in_invalid_context)
+            << 0 << OrigTy << IsUnion << UseContext;
+        // Reset OrigLoc so that this diagnostic is emitted only once.
+        OrigLoc = SourceLocation();
+      }
+      InNonTrivialUnion = true;
+    }
+
+    if (InNonTrivialUnion)
+      S.Diag(RD->getLocation(), diag::note_non_trivial_c_union)
+          << 0 << 0 << QT.getUnqualifiedType() << "";
+
+    for (const FieldDecl *FD : RD->fields())
+      asDerived().visit(FD->getType(), FD, InNonTrivialUnion);
+  }
+
+  void visitTrivial(QualType QT, const FieldDecl *FD, bool InNonTrivialUnion) {}
+
+  // The non-trivial C union type or the struct/union type that contains a
+  // non-trivial C union.
+  QualType OrigTy;
+  SourceLocation OrigLoc;
+  Sema::NonTrivialCUnionContext UseContext;
+  Sema &S;
+};
+
+struct DiagNonTrivalCUnionDestructedTypeVisitor
+    : DestructedTypeVisitor<DiagNonTrivalCUnionDestructedTypeVisitor, void> {
+  using Super =
+      DestructedTypeVisitor<DiagNonTrivalCUnionDestructedTypeVisitor, void>;
+
+  DiagNonTrivalCUnionDestructedTypeVisitor(
+      QualType OrigTy, SourceLocation OrigLoc,
+      Sema::NonTrivialCUnionContext UseContext, Sema &S)
+      : OrigTy(OrigTy), OrigLoc(OrigLoc), UseContext(UseContext), S(S) {}
+
+  void visitWithKind(QualType::DestructionKind DK, QualType QT,
+                     const FieldDecl *FD, bool InNonTrivialUnion) {
+    if (const auto *AT = S.Context.getAsArrayType(QT))
+      return this->asDerived().visit(S.Context.getBaseElementType(AT), FD,
+                                     InNonTrivialUnion);
+    return Super::visitWithKind(DK, QT, FD, InNonTrivialUnion);
+  }
+
+  void visitARCStrong(QualType QT, const FieldDecl *FD,
+                      bool InNonTrivialUnion) {
+    if (InNonTrivialUnion)
+      S.Diag(FD->getLocation(), diag::note_non_trivial_c_union)
+          << 1 << 1 << QT << FD->getName();
+  }
+
+  void visitARCWeak(QualType QT, const FieldDecl *FD, bool InNonTrivialUnion) {
+    if (InNonTrivialUnion)
+      S.Diag(FD->getLocation(), diag::note_non_trivial_c_union)
+          << 1 << 1 << QT << FD->getName();
+  }
+
+  void visitStruct(QualType QT, const FieldDecl *FD, bool InNonTrivialUnion) {
+    const RecordDecl *RD = QT->castAs<RecordType>()->getDecl();
+    if (RD->isUnion()) {
+      if (OrigLoc.isValid()) {
+        bool IsUnion = false;
+        if (auto *OrigRD = OrigTy->getAsRecordDecl())
+          IsUnion = OrigRD->isUnion();
+        S.Diag(OrigLoc, diag::err_non_trivial_c_union_in_invalid_context)
+            << 1 << OrigTy << IsUnion << UseContext;
+        // Reset OrigLoc so that this diagnostic is emitted only once.
+        OrigLoc = SourceLocation();
+      }
+      InNonTrivialUnion = true;
+    }
+
+    if (InNonTrivialUnion)
+      S.Diag(RD->getLocation(), diag::note_non_trivial_c_union)
+          << 0 << 1 << QT.getUnqualifiedType() << "";
+
+    for (const FieldDecl *FD : RD->fields())
+      asDerived().visit(FD->getType(), FD, InNonTrivialUnion);
+  }
+
+  void visitTrivial(QualType QT, const FieldDecl *FD, bool InNonTrivialUnion) {}
+  void visitCXXDestructor(QualType QT, const FieldDecl *FD,
+                          bool InNonTrivialUnion) {}
+
+  // The non-trivial C union type or the struct/union type that contains a
+  // non-trivial C union.
+  QualType OrigTy;
+  SourceLocation OrigLoc;
+  Sema::NonTrivialCUnionContext UseContext;
+  Sema &S;
+};
+
+struct DiagNonTrivalCUnionCopyVisitor
+    : CopiedTypeVisitor<DiagNonTrivalCUnionCopyVisitor, false, void> {
+  using Super = CopiedTypeVisitor<DiagNonTrivalCUnionCopyVisitor, false, void>;
+
+  DiagNonTrivalCUnionCopyVisitor(QualType OrigTy, SourceLocation OrigLoc,
+                                 Sema::NonTrivialCUnionContext UseContext,
+                                 Sema &S)
+      : OrigTy(OrigTy), OrigLoc(OrigLoc), UseContext(UseContext), S(S) {}
+
+  void visitWithKind(QualType::PrimitiveCopyKind PCK, QualType QT,
+                     const FieldDecl *FD, bool InNonTrivialUnion) {
+    if (const auto *AT = S.Context.getAsArrayType(QT))
+      return this->asDerived().visit(S.Context.getBaseElementType(AT), FD,
+                                     InNonTrivialUnion);
+    return Super::visitWithKind(PCK, QT, FD, InNonTrivialUnion);
+  }
+
+  void visitARCStrong(QualType QT, const FieldDecl *FD,
+                      bool InNonTrivialUnion) {
+    if (InNonTrivialUnion)
+      S.Diag(FD->getLocation(), diag::note_non_trivial_c_union)
+          << 1 << 2 << QT << FD->getName();
+  }
+
+  void visitARCWeak(QualType QT, const FieldDecl *FD, bool InNonTrivialUnion) {
+    if (InNonTrivialUnion)
+      S.Diag(FD->getLocation(), diag::note_non_trivial_c_union)
+          << 1 << 2 << QT << FD->getName();
+  }
+
+  void visitStruct(QualType QT, const FieldDecl *FD, bool InNonTrivialUnion) {
+    const RecordDecl *RD = QT->castAs<RecordType>()->getDecl();
+    if (RD->isUnion()) {
+      if (OrigLoc.isValid()) {
+        bool IsUnion = false;
+        if (auto *OrigRD = OrigTy->getAsRecordDecl())
+          IsUnion = OrigRD->isUnion();
+        S.Diag(OrigLoc, diag::err_non_trivial_c_union_in_invalid_context)
+            << 2 << OrigTy << IsUnion << UseContext;
+        // Reset OrigLoc so that this diagnostic is emitted only once.
+        OrigLoc = SourceLocation();
+      }
+      InNonTrivialUnion = true;
+    }
+
+    if (InNonTrivialUnion)
+      S.Diag(RD->getLocation(), diag::note_non_trivial_c_union)
+          << 0 << 2 << QT.getUnqualifiedType() << "";
+
+    for (const FieldDecl *FD : RD->fields())
+      asDerived().visit(FD->getType(), FD, InNonTrivialUnion);
+  }
+
+  void preVisit(QualType::PrimitiveCopyKind PCK, QualType QT,
+                const FieldDecl *FD, bool InNonTrivialUnion) {}
+  void visitTrivial(QualType QT, const FieldDecl *FD, bool InNonTrivialUnion) {}
+  void visitVolatileTrivial(QualType QT, const FieldDecl *FD,
+                            bool InNonTrivialUnion) {}
+
+  // The non-trivial C union type or the struct/union type that contains a
+  // non-trivial C union.
+  QualType OrigTy;
+  SourceLocation OrigLoc;
+  Sema::NonTrivialCUnionContext UseContext;
+  Sema &S;
+};
+
+} // namespace
+
+void Sema::checkNonTrivialCUnion(QualType QT, SourceLocation Loc,
+                                 NonTrivialCUnionContext UseContext,
+                                 unsigned NonTrivialKind) {
+  assert((QT.hasNonTrivialToPrimitiveDefaultInitializeCUnion() ||
+          QT.hasNonTrivialToPrimitiveDestructCUnion() ||
+          QT.hasNonTrivialToPrimitiveCopyCUnion()) &&
+         "shouldn't be called if type doesn't have a non-trivial C union");
+
+  if ((NonTrivialKind & NTCUK_Init) &&
+      QT.hasNonTrivialToPrimitiveDefaultInitializeCUnion())
+    DiagNonTrivalCUnionDefaultInitializeVisitor(QT, Loc, UseContext, *this)
+        .visit(QT, nullptr, false);
+  if ((NonTrivialKind & NTCUK_Destruct) &&
+      QT.hasNonTrivialToPrimitiveDestructCUnion())
+    DiagNonTrivalCUnionDestructedTypeVisitor(QT, Loc, UseContext, *this)
+        .visit(QT, nullptr, false);
+  if ((NonTrivialKind & NTCUK_Copy) && QT.hasNonTrivialToPrimitiveCopyCUnion())
+    DiagNonTrivalCUnionCopyVisitor(QT, Loc, UseContext, *this)
+        .visit(QT, nullptr, false);
+}
+
 /// AddInitializerToDecl - Adds the initializer Init to the
 /// declaration dcl. If DirectInit is true, this is C++ direct
 /// initialization rather than copy initialization.
@@ -11615,6 +11885,12 @@ void Sema::AddInitializerToDecl(Decl *RealDecl, Expr *Init, bool DirectInit) {
       CheckForConstantInitializer(Init, DclT);
   }
 
+  QualType InitType = Init->getType();
+  if (!InitType.isNull() &&
+      (InitType.hasNonTrivialToPrimitiveDefaultInitializeCUnion() ||
+       InitType.hasNonTrivialToPrimitiveCopyCUnion()))
+    checkNonTrivialCUnionInInitializer(Init, Init->getExprLoc());
+
   // We will represent direct-initialization similarly to copy-initialization:
   //    int x(1);  -as-> int x = 1;
   //    ClassType x(a,b,c); -as-> ClassType x = ClassType(a,b,c);
@@ -11739,7 +12015,14 @@ void Sema::ActOnUninitializedDecl(Decl *RealDecl) {
       return;
     }
 
-    switch (Var->isThisDeclarationADefinition()) {
+    VarDecl::DefinitionKind DefKind = Var->isThisDeclarationADefinition();
+    if (!Var->isInvalidDecl() && DefKind != VarDecl::DeclarationOnly &&
+        Var->getType().hasNonTrivialToPrimitiveDefaultInitializeCUnion())
+      checkNonTrivialCUnion(Var->getType(), Var->getLocation(),
+                            NTCUC_DefaultInitializedObject, NTCUK_Init);
+
+
+    switch (DefKind) {
     case VarDecl::Definition:
       if (!Var->isStaticDataMember() || !Var->getAnyInitializer())
         break;
@@ -12829,6 +13112,11 @@ ParmVarDecl *Sema::CheckParameter(DeclContext *DC, SourceLocation StartLoc,
     if (auto *LSI = getEnclosingLambda())
       LSI->LocalPacks.push_back(New);
 
+  if (New->getType().hasNonTrivialToPrimitiveDestructCUnion() ||
+      New->getType().hasNonTrivialToPrimitiveCopyCUnion())
+    checkNonTrivialCUnion(New->getType(), New->getLocation(),
+                          NTCUC_FunctionParam, NTCUK_Destruct|NTCUK_Copy);
+
   // Parameters can not be abstract class types.
   // For record types, this is done by the AbstractClassUsageDiagnoser once
   // the class has been completely parsed.
@@ -16077,7 +16365,6 @@ void Sema::ActOnFields(Scope *S, SourceLocation RecLoc, Decl *EnclosingDecl,
   // Verify that all the fields are okay.
   SmallVector<FieldDecl*, 32> RecFields;
 
-  bool ObjCFieldLifetimeErrReported = false;
   for (ArrayRef<Decl *>::iterator i = Fields.begin(), end = Fields.end();
        i != end; ++i) {
     FieldDecl *FD = cast<FieldDecl>(*i);
@@ -16216,38 +16503,12 @@ void Sema::ActOnFields(Scope *S, SourceLocation RecLoc, Decl *EnclosingDecl,
         Record->setHasObjectMember(true);
       if (Record && FDTTy->getDecl()->hasVolatileMember())
         Record->setHasVolatileMember(true);
-      if (Record && Record->isUnion() &&
-          FD->getType().isNonTrivialPrimitiveCType(Context))
-        Diag(FD->getLocation(),
-             diag::err_nontrivial_primitive_type_in_union);
     } else if (FDTy->isObjCObjectType()) {
       /// A field cannot be an Objective-c object
       Diag(FD->getLocation(), diag::err_statically_allocated_object)
         << FixItHint::CreateInsertion(FD->getLocation(), "*");
       QualType T = Context.getObjCObjectPointerType(FD->getType());
       FD->setType(T);
-    } else if (getLangOpts().allowsNonTrivialObjCLifetimeQualifiers() &&
-               Record && !ObjCFieldLifetimeErrReported && Record->isUnion() &&
-               !getLangOpts().CPlusPlus) {
-      // 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();
-      if (T.hasNonTrivialObjCLifetime()) {
-        SourceLocation loc = FD->getLocation();
-        if (getSourceManager().isInSystemHeader(loc)) {
-          if (!FD->hasAttr<UnavailableAttr>()) {
-            FD->addAttr(UnavailableAttr::CreateImplicit(Context, "",
-                          UnavailableAttr::IR_ARCFieldWithOwnership, loc));
-          }
-        } else {
-          Diag(FD->getLocation(), diag::err_arc_objc_object_in_tag)
-            << T->isBlockPointerType() << Record->getTagKind();
-        }
-        ObjCFieldLifetimeErrReported = true;
-      }
     } else if (getLangOpts().ObjC &&
                getLangOpts().getGC() != LangOptions::NonGC &&
                Record && !Record->hasObjectMember()) {
@@ -16267,14 +16528,23 @@ void Sema::ActOnFields(Scope *S, SourceLocation RecLoc, Decl *EnclosingDecl,
 
     if (Record && !getLangOpts().CPlusPlus && !FD->hasAttr<UnavailableAttr>()) {
       QualType FT = FD->getType();
-      if (FT.isNonTrivialToPrimitiveDefaultInitialize())
+      if (FT.isNonTrivialToPrimitiveDefaultInitialize()) {
         Record->setNonTrivialToPrimitiveDefaultInitialize(true);
+        if (FT.hasNonTrivialToPrimitiveDefaultInitializeCUnion() ||
+            Record->isUnion())
+          Record->setHasNonTrivialToPrimitiveDefaultInitializeCUnion(true);
+      }
       QualType::PrimitiveCopyKind PCK = FT.isNonTrivialToPrimitiveCopy();
-      if (PCK != QualType::PCK_Trivial && PCK != QualType::PCK_VolatileTrivial)
+      if (PCK != QualType::PCK_Trivial && PCK != QualType::PCK_VolatileTrivial) {
         Record->setNonTrivialToPrimitiveCopy(true);
+        if (FT.hasNonTrivialToPrimitiveCopyCUnion() || Record->isUnion())
+          Record->setHasNonTrivialToPrimitiveCopyCUnion(true);
+      }
       if (FT.isDestructedType()) {
         Record->setNonTrivialToPrimitiveDestroy(true);
         Record->setParamDestroyedInCallee(true);
+        if (FT.hasNonTrivialToPrimitiveDestructCUnion() || Record->isUnion())
+          Record->setHasNonTrivialToPrimitiveDestructCUnion(true);
       }
 
       if (const auto *RT = FT->getAs<RecordType>()) {
diff --git a/lib/Sema/SemaExpr.cpp b/lib/Sema/SemaExpr.cpp
index 3001be23cf..7f00981d34 100644
--- a/lib/Sema/SemaExpr.cpp
+++ b/lib/Sema/SemaExpr.cpp
@@ -6083,7 +6083,7 @@ Sema::BuildCompoundLiteralExpr(SourceLocation LParenLoc, TypeSourceInfo *TInfo,
         ILE->setInit(i, ConstantExpr::Create(Context, Init));
       }
 
-  Expr *E = new (Context) CompoundLiteralExpr(LParenLoc, TInfo, literalType,
+  auto *E = new (Context) CompoundLiteralExpr(LParenLoc, TInfo, literalType,
                                               VK, LiteralExpr, isFileScope);
   if (isFileScope) {
     if (!LiteralExpr->isTypeDependent() &&
@@ -6101,6 +6101,19 @@ Sema::BuildCompoundLiteralExpr(SourceLocation LParenLoc, TypeSourceInfo *TInfo,
     return ExprError();
   }
 
+  // Compound literals that have automatic storage duration are destroyed at
+  // the end of the scope. Emit diagnostics if it is or contains a C union type
+  // that is non-trivial to destruct.
+  if (!isFileScope)
+    if (E->getType().hasNonTrivialToPrimitiveDestructCUnion())
+      checkNonTrivialCUnion(E->getType(), E->getExprLoc(),
+                            NTCUC_CompoundLiteral, NTCUK_Destruct);
+
+  if (E->getType().hasNonTrivialToPrimitiveDefaultInitializeCUnion() ||
+      E->getType().hasNonTrivialToPrimitiveCopyCUnion())
+    checkNonTrivialCUnionInInitializer(E->getInitializer(),
+                                       E->getInitializer()->getExprLoc());
+
   return MaybeBindToTemporary(E);
 }
 
@@ -12782,6 +12795,10 @@ ExprResult Sema::CreateBuiltinBinOp(SourceLocation OpLoc,
           if (auto *VD = dyn_cast<VarDecl>(DRE->getDecl()))
             if (VD->hasLocalStorage() && getCurScope()->isDeclScope(VD))
               BE->getBlockDecl()->setCanAvoidCopyToHeap();
+
+      if (LHS.get()->getType().hasNonTrivialToPrimitiveCopyCUnion())
+        checkNonTrivialCUnion(LHS.get()->getType(), LHS.get()->getExprLoc(),
+                              NTCUC_Assignment, NTCUK_Copy);
     }
     RecordModifiableNonNullParam(*this, LHS.get());
     break;
@@ -14194,6 +14211,11 @@ ExprResult Sema::ActOnBlockStmtExpr(SourceLocation CaretLoc,
       !BD->isDependentContext())
     computeNRVO(Body, BSI);
 
+  if (RetTy.hasNonTrivialToPrimitiveDestructCUnion() ||
+      RetTy.hasNonTrivialToPrimitiveCopyCUnion())
+    checkNonTrivialCUnion(RetTy, BD->getCaretLocation(), NTCUC_FunctionReturn,
+                          NTCUK_Destruct|NTCUK_Copy);
+
   PopDeclContext();
 
   // Pop the block scope now but keep it alive to the end of this function.
@@ -16464,6 +16486,15 @@ static ExprResult rebuildPotentialResultsAsNonOdrUsed(Sema &S, Expr *E,
 }
 
 ExprResult Sema::CheckLValueToRValueConversionOperand(Expr *E) {
+  // Check whether the operand is or contains an object of non-trivial C union
+  // type.
+  if (E->getType().isVolatileQualified() &&
+      (E->getType().hasNonTrivialToPrimitiveDestructCUnion() ||
+       E->getType().hasNonTrivialToPrimitiveCopyCUnion()))
+    checkNonTrivialCUnion(E->getType(), E->getExprLoc(),
+                          Sema::NTCUC_LValueToRValueVolatile,
+                          NTCUK_Destruct|NTCUK_Copy);
+
   // C++2a [basic.def.odr]p4:
   //   [...] an expression of non-volatile-qualified non-class type to which
   //   the lvalue-to-rvalue conversion is applied [...]
diff --git a/lib/Sema/SemaType.cpp b/lib/Sema/SemaType.cpp
index f9c36e0183..4281a6a0b3 100644
--- a/lib/Sema/SemaType.cpp
+++ b/lib/Sema/SemaType.cpp
@@ -2456,6 +2456,11 @@ bool Sema::CheckFunctionReturnType(QualType T, SourceLocation Loc) {
     return true;
   }
 
+  if (T.hasNonTrivialToPrimitiveDestructCUnion() ||
+      T.hasNonTrivialToPrimitiveCopyCUnion())
+    checkNonTrivialCUnion(T, Loc, NTCUC_FunctionReturn,
+                          NTCUK_Destruct|NTCUK_Copy);
+
   return false;
 }
 
diff --git a/lib/Serialization/ASTReaderDecl.cpp b/lib/Serialization/ASTReaderDecl.cpp
index 15797ba348..ddfb6899ac 100644
--- a/lib/Serialization/ASTReaderDecl.cpp
+++ b/lib/Serialization/ASTReaderDecl.cpp
@@ -794,6 +794,9 @@ ASTDeclReader::VisitRecordDeclImpl(RecordDecl *RD) {
   RD->setNonTrivialToPrimitiveDefaultInitialize(Record.readInt());
   RD->setNonTrivialToPrimitiveCopy(Record.readInt());
   RD->setNonTrivialToPrimitiveDestroy(Record.readInt());
+  RD->setHasNonTrivialToPrimitiveDefaultInitializeCUnion(Record.readInt());
+  RD->setHasNonTrivialToPrimitiveDestructCUnion(Record.readInt());
+  RD->setHasNonTrivialToPrimitiveCopyCUnion(Record.readInt());
   RD->setParamDestroyedInCallee(Record.readInt());
   RD->setArgPassingRestrictions((RecordDecl::ArgPassingKind)Record.readInt());
   return Redecl;
diff --git a/lib/Serialization/ASTWriterDecl.cpp b/lib/Serialization/ASTWriterDecl.cpp
index 3d9dd7131b..b71315505d 100644
--- a/lib/Serialization/ASTWriterDecl.cpp
+++ b/lib/Serialization/ASTWriterDecl.cpp
@@ -476,6 +476,9 @@ void ASTDeclWriter::VisitRecordDecl(RecordDecl *D) {
   Record.push_back(D->isNonTrivialToPrimitiveDefaultInitialize());
   Record.push_back(D->isNonTrivialToPrimitiveCopy());
   Record.push_back(D->isNonTrivialToPrimitiveDestroy());
+  Record.push_back(D->hasNonTrivialToPrimitiveDefaultInitializeCUnion());
+  Record.push_back(D->hasNonTrivialToPrimitiveDestructCUnion());
+  Record.push_back(D->hasNonTrivialToPrimitiveCopyCUnion());
   Record.push_back(D->isParamDestroyedInCallee());
   Record.push_back(D->getArgPassingRestrictions());
 
@@ -1999,6 +2002,12 @@ void ASTWriter::WriteDeclAbbrevs() {
   Abv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::Fixed, 1));
   // isNonTrivialToPrimitiveDestroy
   Abv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::Fixed, 1));
+  // hasNonTrivialToPrimitiveDefaultInitializeCUnion
+  Abv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::Fixed, 1));
+  // hasNonTrivialToPrimitiveDestructCUnion
+  Abv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::Fixed, 1));
+  // hasNonTrivialToPrimitiveCopyCUnion
+  Abv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::Fixed, 1));
   // isParamDestroyedInCallee
   Abv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::Fixed, 1));
   // getArgPassingRestrictions
diff --git a/test/CodeGenObjC/Inputs/strong_in_union.h b/test/CodeGenObjC/Inputs/strong_in_union.h
deleted file mode 100644
index abe4549055..0000000000
--- a/test/CodeGenObjC/Inputs/strong_in_union.h
+++ /dev/null
@@ -1,10 +0,0 @@
-#ifndef STRONG_IN_UNION_H
-#define STRONG_IN_UNION_H
-#pragma clang system_header
-
-typedef union {
-  id f0;
-  int *f1;
-} U;
-
-#endif // STRONG_IN_UNION_H
diff --git a/test/CodeGenObjC/strong-in-c-struct.m b/test/CodeGenObjC/strong-in-c-struct.m
index b385177235..eae5013dd3 100644
--- a/test/CodeGenObjC/strong-in-c-struct.m
+++ b/test/CodeGenObjC/strong-in-c-struct.m
@@ -1,11 +1,10 @@
-// RUN: %clang_cc1 -triple arm64-apple-ios11 -fobjc-arc -fblocks  -fobjc-runtime=ios-11.0 -emit-llvm -o - -DUSESTRUCT -I %S/Inputs %s | FileCheck %s
+// RUN: %clang_cc1 -triple arm64-apple-ios11 -fobjc-arc -fblocks  -fobjc-runtime=ios-11.0 -emit-llvm -o - -DUSESTRUCT %s | FileCheck %s
 
-// RUN: %clang_cc1 -triple arm64-apple-ios11 -fobjc-arc -fblocks  -fobjc-runtime=ios-11.0 -emit-pch -I %S/Inputs -o %t %s
-// RUN: %clang_cc1 -triple arm64-apple-ios11 -fobjc-arc -fblocks  -fobjc-runtime=ios-11.0 -include-pch %t -emit-llvm -o - -DUSESTRUCT -I %S/Inputs %s | FileCheck %s
+// RUN: %clang_cc1 -triple arm64-apple-ios11 -fobjc-arc -fblocks  -fobjc-runtime=ios-11.0 -emit-pch -o %t %s
+// RUN: %clang_cc1 -triple arm64-apple-ios11 -fobjc-arc -fblocks  -fobjc-runtime=ios-11.0 -include-pch %t -emit-llvm -o - -DUSESTRUCT %s | FileCheck %s
 
 #ifndef HEADER
 #define HEADER
-#include "strong_in_union.h"
 
 typedef void (^BlockTy)(void);
 
@@ -695,14 +694,6 @@ void test_copy_constructor_Bitfield1(Bitfield1 *a) {
   Bitfield1 t = *a;
 }
 
-// CHECK: define void @test_strong_in_union()
-// CHECK: alloca %{{.*}}
-// CHECK-NEXT: ret void
-
-void test_strong_in_union() {
-  U t;
-}
-
 // CHECK: define void @test_copy_constructor_VolatileArray(
 // CHECK: call void @__copy_constructor_8_8_s0_AB8s4n16_tv64w32_AE(
 
diff --git a/test/PCH/non-trivial-c-union.m b/test/PCH/non-trivial-c-union.m
new file mode 100644
index 0000000000..abd335497d
--- /dev/null
+++ b/test/PCH/non-trivial-c-union.m
@@ -0,0 +1,24 @@
+// RUN: %clang_cc1 -fblocks -fobjc-arc -fobjc-runtime-has-weak -emit-pch -o %t.pch %s
+// RUN: %clang_cc1 -fblocks -fobjc-arc -fobjc-runtime-has-weak -include-pch %t.pch -verify %s
+
+#ifndef HEADER
+#define HEADER
+
+typedef union {
+  id f0;
+} U0;
+
+#else
+
+// expected-note@-6 {{'U0' has subobjects that are non-trivial to destruct}}
+// expected-note@-7 {{'U0' has subobjects that are non-trivial to copy}}
+// expected-note@-8 {{'U0' has subobjects that are non-trivial to default-initialize}}
+// expected-note@-8 {{f0 has type '__strong id' that is non-trivial to destruct}}
+// expected-note@-9 {{f0 has type '__strong id' that is non-trivial to copy}}
+// expected-note@-10 {{f0 has type '__strong id' that is non-trivial to default-initialize}}
+
+U0 foo0(void); // expected-error {{cannot use type 'U0' for function/method return since it is a union that is non-trivial to destruct}} expected-error {{cannot use type 'U0' for function/method return since it is a union that is non-trivial to copy}}
+
+U0 g0; // expected-error {{cannot default-initialize an object of type 'U0' since it is a union that is non-trivial to default-initialize}}
+
+#endif
diff --git a/test/SemaObjC/arc-decls.m b/test/SemaObjC/arc-decls.m
index 0abd45dac3..28c3de9969 100644
--- a/test/SemaObjC/arc-decls.m
+++ b/test/SemaObjC/arc-decls.m
@@ -8,11 +8,7 @@ struct A {
 };
 
 union u {
-    id u; // expected-error {{ARC forbids Objective-C objects in union}}
-};
-
-union u_nontrivial_c {
-  struct A a; // expected-error {{non-trivial C types are disallowed in union}}
+  id u;
 };
 
 // Volatile fields are fine.
diff --git a/test/SemaObjC/non-trivial-c-union.m b/test/SemaObjC/non-trivial-c-union.m
new file mode 100644
index 0000000000..7bd8277545
--- /dev/null
+++ b/test/SemaObjC/non-trivial-c-union.m
@@ -0,0 +1,82 @@
+// RUN: %clang_cc1 -fsyntax-only -fblocks -fobjc-arc -fobjc-runtime-has-weak -verify %s
+
+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}}
+  __weak id f1; // expected-note 12 {{f1 has type '__weak id' that is non-trivial to default-initialize}} expected-note 36 {{f1 has type '__weak id' that is non-trivial to destruct}} expected-note 28 {{f1 has type '__weak id' that is non-trivial to copy}}
+} U0;
+
+typedef struct {
+  U0 f0;
+  id f1;
+} S0;
+
+id g0;
+U0 ug0; // expected-error {{cannot default-initialize an object of type 'U0' since it is a union that is non-trivial to default-initialize}}
+U0 ug1 = { .f0 = 0 };
+S0 sg0; // expected-error {{cannot default-initialize an object of type 'S0' since it contains a union that is non-trivial to default-initialize}}
+S0 sg1 = { .f0 = {0}, .f1 = 0 };
+S0 sg2 = { .f1 = 0 }; // expected-error {{cannot default-initialize an object of type 'U0' since it is a union that is non-trivial to default-initialize}}
+
+U0 foo0(U0); // expected-error {{cannot use type 'U0' for a function/method parameter since it is a union that is non-trivial to destruct}} expected-error {{cannot use type 'U0' for a function/method parameter since it is a union that is non-trivial to copy}} expected-error {{cannot use type 'U0' for function/method return since it is a union that is non-trivial to destruct}} expected-error {{cannot use type 'U0' for function/method return since it is a union that is non-trivial to copy}}
+S0 foo1(S0); // expected-error {{cannot use type 'S0' for a function/method parameter since it contains a union that is non-trivial to destruct}} expected-error {{cannot use type 'S0' for a function/method parameter since it contains a union that is non-trivial to copy}} expected-error {{cannot use type 'S0' for function/method return since it contains a union that is non-trivial to destruct}} expected-error {{cannot use type 'S0' for function/method return since it contains a union that is non-trivial to copy}}
+
+@interface C
+-(U0)m0:(U0)arg; // expected-error {{cannot use type 'U0' for a function/method parameter since it is a union that is non-trivial to destruct}} expected-error {{cannot use type 'U0' for a function/method parameter since it is a union that is non-trivial to copy}} expected-error {{cannot use type 'U0' for function/method return since it is a union that is non-trivial to destruct}} expected-error {{cannot use type 'U0' for function/method return since it is a union that is non-trivial to copy}}
+-(S0)m1:(S0)arg; // expected-error {{cannot use type 'S0' for a function/method parameter since it contains a union that is non-trivial to destruct}} expected-error {{cannot use type 'S0' for a function/method parameter since it contains a union that is non-trivial to copy}} expected-error {{cannot use type 'S0' for function/method return since it contains a union that is non-trivial to destruct}} expected-error {{cannot use type 'S0' for function/method return since it contains a union that is non-trivial to copy}}
+@end
+
+void testBlockFunction(void) {
+  (void)^(U0 a){ return ug0; }; // expected-error {{cannot use type 'U0' for a function/method parameter since it is a union that is non-trivial to destruct}} expected-error {{cannot use type 'U0' for a function/method parameter since it is a union that is non-trivial to copy}} expected-error {{cannot use type 'U0' for function/method return since it is a union that is non-trivial to destruct}} expected-error {{cannot use type 'U0' for function/method return since it is a union that is non-trivial to copy}}
+  (void)^(S0 a){ return sg0; }; // expected-error {{cannot use type 'S0' for a function/method parameter since it contains a union that is non-trivial to destruct}} expected-error {{cannot use type 'S0' for a function/method parameter since it contains a union that is non-trivial to copy}} expected-error {{cannot use type 'S0' for function/method return since it contains a union that is non-trivial to destruct}} expected-error {{cannot use type 'S0' for function/method return since it contains a union that is non-trivial to copy}}
+}
+void testAutoVar(void) {
+  U0 u0; // expected-error {{cannot declare an automatic variable of type 'U0' since it is a union that is non-trivial to destruct}} expected-error {{cannot default-initialize an object of type 'U0' since it is a union that is non-trivial to default-initialize}}
+  U0 u1 = ug0; // expected-error {{cannot declare an automatic variable of type 'U0' since it is a union that is non-trivial to destruct}} expected-error {{cannot copy-initialize an object of type 'U0' since it is a union that is non-trivial to copy}}
+  U0 u2 = { g0 }; // expected-error {{cannot declare an automatic variable of type 'U0' since it is a union that is non-trivial to destruct}}
+  U0 u3 = { .f1 = g0 }; // expected-error {{cannot declare an automatic variable of type 'U0' since it is a union that is non-trivial to destruct}}
+  S0 s0; // expected-error {{cannot declare an automatic variable of type 'S0' since it contains a union that is non-trivial to destruct}} expected-error {{cannot default-initialize an object of type 'S0' since it contains a union that is non-trivial to default-initialize}}
+  S0 s1 = sg0; // expected-error {{declare an automatic variable of type 'S0' since it contains a union that is non-trivial to destruct}} expected-error {{cannot copy-initialize an object of type 'S0' since it contains a union that is non-trivial to copy}}
+  S0 s2 = { ug0 }; // expected-error {{cannot declare an automatic variable of type 'S0' since it contains a union that is non-trivial to destruct}} expected-error {{cannot copy-initialize an object of type 'U0' since it is a union that is non-trivial to copy}}
+  S0 s3 = { .f0 = ug0 }; // expected-error {{cannot declare an automatic variable of type 'S0' since it contains a union that is non-trivial to destruct}} expected-error {{cannot copy-initialize an object of type 'U0' since it is a union that is non-trivial to copy}}
+  S0 s4 = { .f1 = g0 }; // expected-error {{cannot declare an automatic variable of type 'S0' since it contains a union that is non-trivial to destruct}} expected-error {{cannot default-initialize an object of type 'U0' since it is a union that is non-trivial to default-initialize}}
+}
+
+void testAssignment(void) {
+  ug0 = ug1; // expected-error {{cannot assign to a variable of type 'U0' since it is a union that is non-trivial to copy}}
+  sg0 = sg1; // expected-error {{cannot assign to a variable of type 'S0' since it contains a union that is non-trivial to copy}}
+}
+
+U0 ug2 = (U0){ .f1 = 0 }; // expected-error {{cannot copy-initialize an object of type 'U0' since it is a union that is non-trivial to copy}}
+S0 sg3 = (S0){ .f0 = {0}, .f1 = 0 }; // expected-error {{cannot copy-initialize an object of type 'S0' since it contains a union that is non-trivial to copy}}
+S0 *sg4 = &(S0){ .f1 = 0 }; // expected-error {{cannot default-initialize an object of type 'U0' since it is a union that is non-trivial to default-initialize}}
+
+void testCompoundLiteral(void) {
+  const U0 *t0 = &(U0){ .f0 = g0 }; // expected-error {{cannot construct an automatic compound literal of type 'U0' since it is a union that is non-trivial to destruct}}
+  const U0 *t1 = &(U0){ .f1 = g0 }; // expected-error {{cannot construct an automatic compound literal of type 'U0' since it is a union that is non-trivial to destruct}}
+  const S0 *t2 = &(S0){ .f0 = ug0 }; // expected-error {{cannot construct an automatic compound literal of type 'S0' since it contains a union that is non-trivial to destruct}} expected-error {{cannot copy-initialize an object of type 'U0' since it is a union that is non-trivial to copy}}
+  const S0 *t3 = &(S0){ .f1 = g0 }; // expected-error {{cannot construct an automatic compound literal of type 'S0' since it contains a union that is non-trivial to destruct}} expected-error {{cannot default-initialize an object of type 'U0' since it is a union that is non-trivial to default-initialize}}
+}
+
+typedef void (^BlockTy)(void);
+void escapingFunc(BlockTy);
+void noescapingFunc(__attribute__((noescape)) BlockTy);
+
+void testBlockCapture(void) {
+  U0 t0; // expected-error {{cannot declare an automatic variable of type 'U0' since it is a union that is non-trivial to destruct}} expected-error {{cannot default-initialize an object of type 'U0' since it is a union that is non-trivial to default-initialize}}
+  S0 t1; // expected-error {{cannot declare an automatic variable of type 'S0' since it contains a union that is non-trivial to destruct}} expected-error {{cannot default-initialize an object of type 'S0' since it contains a union that is non-trivial to default-initialize}}
+  __block U0 t2; // expected-error {{cannot declare an automatic variable of type 'U0' since it is a union that is non-trivial to destruct}} expected-error {{cannot default-initialize an object of type 'U0' since it is a union that is non-trivial to default-initialize}}
+  __block S0 t3; // expected-error {{cannot declare an automatic variable of type 'S0' since it contains a union that is non-trivial to destruct}} expected-error {{cannot default-initialize an object of type 'S0' since it contains a union that is non-trivial to default-initialize}}
+
+  escapingFunc(^{ g0 = t0.f0; }); // expected-error {{cannot capture a variable of type 'U0' since it is a union that is non-trivial to destruct}} expected-error {{cannot capture a variable of type 'U0' since it is a union that is non-trivial to copy}}
+  escapingFunc(^{ g0 = t1.f0.f0; }); // expected-error {{cannot capture a variable of type 'S0' since it contains a union that is non-trivial to destruct}} expected-error {{cannot capture a variable of type 'S0' since it contains a union that is non-trivial to copy}}
+  escapingFunc(^{ g0 = t2.f0; }); // expected-error {{cannot capture a variable of type 'U0' since it is a union that is non-trivial to destruct}} expected-error {{cannot capture a variable of type 'U0' since it is a union that is non-trivial to copy}}
+  escapingFunc(^{ g0 = t3.f0.f0; }); // expected-error {{cannot capture a variable of type 'S0' since it contains a union that is non-trivial to destruct}} expected-error {{cannot capture a variable of type 'S0' since it contains a union that is non-trivial to copy}}
+  noescapingFunc(^{ g0 = t0.f0; }); // expected-error {{cannot capture a variable of type 'U0' since it is a union that is non-trivial to destruct}} expected-error {{cannot capture a variable of type 'U0' since it is a union that is non-trivial to copy}}
+  noescapingFunc(^{ g0 = t1.f0.f0; }); // expected-error {{cannot capture a variable of type 'S0' since it contains a union that is non-trivial to destruct}} expected-error {{cannot capture a variable of type 'S0' since it contains a union that is non-trivial to copy}}
+  noescapingFunc(^{ g0 = t2.f0; });
+  noescapingFunc(^{ g0 = t3.f0.f0; });
+}
+
+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}}
+}