Index: cfe/trunk/include/clang/AST/Type.h
===================================================================
--- cfe/trunk/include/clang/AST/Type.h
+++ cfe/trunk/include/clang/AST/Type.h
@@ -770,6 +770,10 @@
   /// Return true if this is a trivially copyable type (C++0x [basic.types]p9)
   bool isTriviallyCopyableType(const ASTContext &Context) const;
 
+  /// Return true if this has unique object representations according to (C++17
+  /// [meta.unary.prop]p9)
+  bool hasUniqueObjectRepresentations(const ASTContext &Context) const;
+
   // Don't promise in the API that anything besides 'const' can be
   // easily added.
 
@@ -1114,6 +1118,8 @@
   QualType getAtomicUnqualifiedType() const;
 
 private:
+  bool unionHasUniqueObjectRepresentations(const ASTContext& Context) const;
+  bool structHasUniqueObjectRepresentations(const ASTContext& Context) const;
   // These methods are implemented in a separate translation unit;
   // "static"-ize them to avoid creating temporary QualTypes in the
   // caller.
Index: cfe/trunk/include/clang/Basic/TokenKinds.def
===================================================================
--- cfe/trunk/include/clang/Basic/TokenKinds.def
+++ cfe/trunk/include/clang/Basic/TokenKinds.def
@@ -455,6 +455,8 @@
 TYPE_TRAIT_1(__is_polymorphic, IsPolymorphic, KEYCXX)
 TYPE_TRAIT_1(__is_trivial, IsTrivial, KEYCXX)
 TYPE_TRAIT_1(__is_union, IsUnion, KEYCXX)
+TYPE_TRAIT_1(__has_unique_object_representations,
+             HasUniqueObjectRepresentations, KEYCXX)
 
 // Clang-only C++ Type Traits
 TYPE_TRAIT_N(__is_trivially_constructible, IsTriviallyConstructible, KEYCXX)
Index: cfe/trunk/include/clang/Basic/TypeTraits.h
===================================================================
--- cfe/trunk/include/clang/Basic/TypeTraits.h
+++ cfe/trunk/include/clang/Basic/TypeTraits.h
@@ -70,7 +70,8 @@
     UTT_IsUnsigned,
     UTT_IsVoid,
     UTT_IsVolatile,
-    UTT_Last = UTT_IsVolatile,
+    UTT_HasUniqueObjectRepresentations,
+    UTT_Last = UTT_HasUniqueObjectRepresentations,
     BTT_IsBaseOf,
     BTT_IsConvertible,
     BTT_IsConvertibleTo,
Index: cfe/trunk/lib/AST/Type.cpp
===================================================================
--- cfe/trunk/lib/AST/Type.cpp
+++ cfe/trunk/lib/AST/Type.cpp
@@ -2166,6 +2166,152 @@
   return false;
 }
 
+bool QualType::unionHasUniqueObjectRepresentations(
+    const ASTContext &Context) const {
+  assert((*this)->isUnionType() && "must be union type");
+  CharUnits UnionSize = Context.getTypeSizeInChars(*this);
+  const RecordDecl *Union = getTypePtr()->getAs<RecordType>()->getDecl();
+
+  for (const auto *Field : Union->fields()) {
+    if (!Field->getType().hasUniqueObjectRepresentations(Context))
+      return false;
+    CharUnits FieldSize = Context.getTypeSizeInChars(Field->getType());
+    if (FieldSize != UnionSize)
+      return false;
+  }
+  return true;
+}
+
+bool isStructEmpty(QualType Ty) {
+  assert(Ty.getTypePtr()->isStructureOrClassType() &&
+         "Must be struct or class");
+  const RecordDecl *RD = Ty.getTypePtr()->getAs<RecordType>()->getDecl();
+
+  if (!RD->field_empty())
+    return false;
+
+  if (const CXXRecordDecl *ClassDecl = dyn_cast<CXXRecordDecl>(RD)) {
+    return ClassDecl->isEmpty();
+  }
+
+  return true;
+}
+
+bool QualType::structHasUniqueObjectRepresentations(
+    const ASTContext &Context) const {
+  assert((*this)->isStructureOrClassType() && "Must be struct or class");
+  const RecordDecl *RD = getTypePtr()->getAs<RecordType>()->getDecl();
+
+  if (isStructEmpty(*this))
+    return false;
+
+  // Check base types.
+  CharUnits BaseSize{};
+  if (const CXXRecordDecl *ClassDecl = dyn_cast<CXXRecordDecl>(RD)) {
+    for (const auto Base : ClassDecl->bases()) {
+      if (Base.isVirtual())
+        return false;
+
+      // Empty bases are permitted, otherwise ensure base has unique
+      // representation. Also, Empty Base Optimization means that an
+      // Empty base takes up 0 size.
+      if (!isStructEmpty(Base.getType())) {
+        if (!Base.getType().structHasUniqueObjectRepresentations(Context))
+          return false;
+        BaseSize += Context.getTypeSizeInChars(Base.getType());
+      }
+    }
+  }
+
+  CharUnits StructSize = Context.getTypeSizeInChars(*this);
+
+  // This struct obviously has bases that keep it from being 'empty', so
+  // checking fields is no longer required.  Ensure that the struct size
+  // is the sum of the bases.
+  if (RD->field_empty())
+    return StructSize == BaseSize;
+  ;
+
+  CharUnits CurOffset =
+      Context.toCharUnitsFromBits(Context.getFieldOffset(*RD->field_begin()));
+
+  // If the first field isn't at the sum of the size of the bases, there
+  // is padding somewhere.
+  if (BaseSize != CurOffset)
+    return false;
+
+  for (const auto *Field : RD->fields()) {
+    if (!Field->getType().hasUniqueObjectRepresentations(Context))
+      return false;
+    CharUnits FieldSize = Context.getTypeSizeInChars(Field->getType());
+    CharUnits FieldOffset =
+        Context.toCharUnitsFromBits(Context.getFieldOffset(Field));
+    // Has padding between fields.
+    if (FieldOffset != CurOffset)
+      return false;
+    CurOffset += FieldSize;
+  }
+  // Check for tail padding.
+  return CurOffset == StructSize;
+}
+
+bool QualType::hasUniqueObjectRepresentations(const ASTContext &Context) const {
+  // C++17 [meta.unary.prop]:
+  //   The predicate condition for a template specialization
+  //   has_unique_object_representations<T> shall be
+  //   satisfied if and only if:
+  //     (9.1) — T is trivially copyable, and
+  //     (9.2) — any two objects of type T with the same value have the same
+  //     object representation, where two objects
+  //   of array or non-union class type are considered to have the same value
+  //   if their respective sequences of
+  //   direct subobjects have the same values, and two objects of union type
+  //   are considered to have the same
+  //   value if they have the same active member and the corresponding members
+  //   have the same value.
+  //   The set of scalar types for which this condition holds is
+  //   implementation-defined. [ Note: If a type has padding
+  //   bits, the condition does not hold; otherwise, the condition holds true
+  //   for unsigned integral types. — end
+  //   note ]
+  if (isNull())
+    return false;
+
+  // Arrays are unique only if their element type is unique.
+  if ((*this)->isArrayType())
+    return Context.getBaseElementType(*this).hasUniqueObjectRepresentations(
+        Context);
+
+  // (9.1) — T is trivially copyable, and
+  if (!isTriviallyCopyableType(Context))
+    return false;
+
+  // Functions are not unique.
+  if ((*this)->isFunctionType())
+    return false;
+
+  // All integrals and enums are unique!
+  if ((*this)->isIntegralOrEnumerationType())
+    return true;
+
+  // All pointers are unique, since they're just integrals.
+  if ((*this)->isPointerType() || (*this)->isMemberPointerType())
+    return true;
+
+  if ((*this)->isRecordType()) {
+    const RecordDecl *Record = getTypePtr()->getAs<RecordType>()->getDecl();
+
+    // Lambda types are not unique, so exclude them immediately.
+    if (Record->isLambda())
+      return false;
+
+    if (Record->isUnion())
+      return unionHasUniqueObjectRepresentations(Context);
+    return structHasUniqueObjectRepresentations(Context);
+  }
+  return false;
+}
+
 bool QualType::isNonWeakInMRRWithObjCWeak(const ASTContext &Context) const {
   return !Context.getLangOpts().ObjCAutoRefCount &&
          Context.getLangOpts().ObjCWeak &&
Index: cfe/trunk/lib/Parse/ParseExpr.cpp
===================================================================
--- cfe/trunk/lib/Parse/ParseExpr.cpp
+++ cfe/trunk/lib/Parse/ParseExpr.cpp
@@ -716,6 +716,7 @@
 ///                   '__is_sealed'                           [MS]
 ///                   '__is_trivial'
 ///                   '__is_union'
+///                   '__has_unique_object_representations'
 ///
 /// [Clang] unary-type-trait:
 ///                   '__is_aggregate'
Index: cfe/trunk/lib/Sema/SemaExprCXX.cpp
===================================================================
--- cfe/trunk/lib/Sema/SemaExprCXX.cpp
+++ cfe/trunk/lib/Sema/SemaExprCXX.cpp
@@ -4175,6 +4175,7 @@
   case UTT_IsDestructible:
   case UTT_IsNothrowDestructible:
   case UTT_IsTriviallyDestructible:
+  case UTT_HasUniqueObjectRepresentations:
     if (ArgTy->isIncompleteArrayType() || ArgTy->isVoidType())
       return true;
 
@@ -4614,6 +4615,8 @@
     //   Returns True if and only if T is a complete type at the point of the
     //   function call.
     return !T->isIncompleteType();
+  case UTT_HasUniqueObjectRepresentations:
+    return T.hasUniqueObjectRepresentations(C);
   }
 }
 
Index: cfe/trunk/test/SemaCXX/type-traits.cpp
===================================================================
--- cfe/trunk/test/SemaCXX/type-traits.cpp
+++ cfe/trunk/test/SemaCXX/type-traits.cpp
@@ -2352,3 +2352,236 @@
   { int arr[F(__is_trivially_destructible(void))]; }
   { int arr[F(__is_trivially_destructible(const volatile void))]; }
 }
+
+// Instantiation of __has_unique_object_representations
+template <typename T>
+struct has_unique_object_representations {
+  static const bool value = __has_unique_object_representations(T);
+};
+
+static_assert(!has_unique_object_representations<void>::value, "void is never unique");
+static_assert(!has_unique_object_representations<const void>::value, "void is never unique");
+static_assert(!has_unique_object_representations<volatile void>::value, "void is never unique");
+static_assert(!has_unique_object_representations<const volatile void>::value, "void is never unique");
+
+static_assert(has_unique_object_representations<int>::value, "integrals are");
+static_assert(has_unique_object_representations<const int>::value, "integrals are");
+static_assert(has_unique_object_representations<volatile int>::value, "integrals are");
+static_assert(has_unique_object_representations<const volatile int>::value, "integrals are");
+
+static_assert(has_unique_object_representations<void *>::value, "as are pointers");
+static_assert(has_unique_object_representations<const void *>::value, "as are pointers");
+static_assert(has_unique_object_representations<volatile void *>::value, "are pointers");
+static_assert(has_unique_object_representations<const volatile void *>::value, "as are pointers");
+
+static_assert(has_unique_object_representations<int *>::value, "as are pointers");
+static_assert(has_unique_object_representations<const int *>::value, "as are pointers");
+static_assert(has_unique_object_representations<volatile int *>::value, "as are pointers");
+static_assert(has_unique_object_representations<const volatile int *>::value, "as are pointers");
+
+class C {};
+using FP = int (*)(int);
+using PMF = int (C::*)(int);
+using PMD = int C::*;
+
+static_assert(has_unique_object_representations<FP>::value, "even function pointers");
+static_assert(has_unique_object_representations<const FP>::value, "even function pointers");
+static_assert(has_unique_object_representations<volatile FP>::value, "even function pointers");
+static_assert(has_unique_object_representations<const volatile FP>::value, "even function pointers");
+
+static_assert(has_unique_object_representations<PMF>::value, "and pointer to members");
+static_assert(has_unique_object_representations<const PMF>::value, "and pointer to members");
+static_assert(has_unique_object_representations<volatile PMF>::value, "and pointer to members");
+static_assert(has_unique_object_representations<const volatile PMF>::value, "and pointer to members");
+
+static_assert(has_unique_object_representations<PMD>::value, "and pointer to members");
+static_assert(has_unique_object_representations<const PMD>::value, "and pointer to members");
+static_assert(has_unique_object_representations<volatile PMD>::value, "and pointer to members");
+static_assert(has_unique_object_representations<const volatile PMD>::value, "and pointer to members");
+
+static_assert(has_unique_object_representations<bool>::value, "yes, all integral types");
+static_assert(has_unique_object_representations<char>::value, "yes, all integral types");
+static_assert(has_unique_object_representations<signed char>::value, "yes, all integral types");
+static_assert(has_unique_object_representations<unsigned char>::value, "yes, all integral types");
+static_assert(has_unique_object_representations<short>::value, "yes, all integral types");
+static_assert(has_unique_object_representations<unsigned short>::value, "yes, all integral types");
+static_assert(has_unique_object_representations<int>::value, "yes, all integral types");
+static_assert(has_unique_object_representations<unsigned int>::value, "yes, all integral types");
+static_assert(has_unique_object_representations<long>::value, "yes, all integral types");
+static_assert(has_unique_object_representations<unsigned long>::value, "yes, all integral types");
+static_assert(has_unique_object_representations<long long>::value, "yes, all integral types");
+static_assert(has_unique_object_representations<unsigned long long>::value, "yes, all integral types");
+static_assert(has_unique_object_representations<wchar_t>::value, "yes, all integral types");
+static_assert(has_unique_object_representations<char16_t>::value, "yes, all integral types");
+static_assert(has_unique_object_representations<char32_t>::value, "yes, all integral types");
+
+static_assert(!has_unique_object_representations<void>::value, "but not void!");
+static_assert(!has_unique_object_representations<decltype(nullptr)>::value, "or nullptr_t");
+static_assert(!has_unique_object_representations<float>::value, "definitely not Floating Point");
+static_assert(!has_unique_object_representations<double>::value, "definitely not Floating Point");
+static_assert(!has_unique_object_representations<long double>::value, "definitely not Floating Point");
+
+struct NoPadding {
+  int a;
+  int b;
+};
+
+static_assert(has_unique_object_representations<NoPadding>::value, "types without padding are");
+
+struct InheritsFromNoPadding : NoPadding {
+  int c;
+  int d;
+};
+
+static_assert(has_unique_object_representations<InheritsFromNoPadding>::value, "types without padding are");
+
+struct VirtuallyInheritsFromNoPadding : virtual NoPadding {
+  int c;
+  int d;
+};
+
+static_assert(!has_unique_object_representations<VirtuallyInheritsFromNoPadding>::value, "No virtual inheritence");
+
+struct Padding {
+  char a;
+  int b;
+};
+
+static_assert(!has_unique_object_representations<Padding>::value, "but not with padding");
+
+struct InheritsFromPadding : Padding {
+  int c;
+  int d;
+};
+
+static_assert(!has_unique_object_representations<InheritsFromPadding>::value, "or its subclasses");
+
+struct TailPadding {
+  int a;
+  char b;
+};
+
+static_assert(!has_unique_object_representations<TailPadding>::value, "even at the end");
+
+struct TinyStruct {
+  char a;
+};
+
+static_assert(has_unique_object_representations<TinyStruct>::value, "Should be no padding");
+
+struct InheritsFromTinyStruct : TinyStruct {
+  int b;
+};
+
+static_assert(!has_unique_object_representations<InheritsFromTinyStruct>::value, "Inherit causes padding");
+
+union NoPaddingUnion {
+  int a;
+  unsigned int b;
+};
+
+static_assert(has_unique_object_representations<NoPaddingUnion>::value, "unions follow the same rules as structs");
+
+union PaddingUnion {
+  int a;
+  long long b;
+};
+
+static_assert(!has_unique_object_representations<PaddingUnion>::value, "unions follow the same rules as structs");
+
+struct NotTriviallyCopyable {
+  int x;
+  NotTriviallyCopyable(const NotTriviallyCopyable &) {}
+};
+
+static_assert(!has_unique_object_representations<NotTriviallyCopyable>::value, "must be trivially copyable");
+
+struct HasNonUniqueMember {
+  float x;
+};
+
+static_assert(!has_unique_object_representations<HasNonUniqueMember>::value, "all members must be unique");
+
+enum ExampleEnum { xExample,
+                   yExample };
+enum LLEnum : long long { xLongExample,
+                          yLongExample };
+
+static_assert(has_unique_object_representations<ExampleEnum>::value, "Enums are integrals, so unique!");
+static_assert(has_unique_object_representations<LLEnum>::value, "Enums are integrals, so unique!");
+
+enum class ExampleEnumClass { xExample,
+                              yExample };
+enum class LLEnumClass : long long { xLongExample,
+                                     yLongExample };
+
+static_assert(has_unique_object_representations<ExampleEnumClass>::value, "Enums are integrals, so unique!");
+static_assert(has_unique_object_representations<LLEnumClass>::value, "Enums are integrals, so unique!");
+
+// because reference types aren't object types
+static_assert(!has_unique_object_representations<int &>::value, "No references!");
+static_assert(!has_unique_object_representations<const int &>::value, "No references!");
+static_assert(!has_unique_object_representations<volatile int &>::value, "No references!");
+static_assert(!has_unique_object_representations<const volatile int &>::value, "No references!");
+
+static_assert(!has_unique_object_representations<Empty>::value, "No empty types!");
+
+class Compressed : Empty {
+  int x;
+};
+
+static_assert(has_unique_object_representations<Compressed>::value, "But inheriting from one is ok");
+
+class EmptyInheritor : Compressed {};
+
+static_assert(has_unique_object_representations<EmptyInheritor>::value, "As long as the base has items, empty is ok");
+
+class Dynamic {
+  virtual void A();
+  int i;
+};
+
+static_assert(!has_unique_object_representations<Dynamic>::value, "Dynamic types are not valid");
+
+class InheritsDynamic : Dynamic {
+  int j;
+};
+
+static_assert(!has_unique_object_representations<InheritsDynamic>::value, "Dynamic types are not valid");
+
+static_assert(has_unique_object_representations<int[42]>::value, "Arrays are fine, as long as their value type is");
+static_assert(has_unique_object_representations<int[]>::value, "Arrays are fine, as long as their value type is");
+static_assert(has_unique_object_representations<int[][42]>::value, "Arrays are fine, as long as their value type is");
+static_assert(!has_unique_object_representations<double[42]>::value, "So no array of doubles!");
+static_assert(!has_unique_object_representations<double[]>::value, "So no array of doubles!");
+static_assert(!has_unique_object_representations<double[][42]>::value, "So no array of doubles!");
+
+static_assert(!has_unique_object_representations<int(int)>::value, "Functions are not unique");
+static_assert(!has_unique_object_representations<int(int) const>::value, "Functions are not unique");
+static_assert(!has_unique_object_representations<int(int) volatile>::value, "Functions are not unique");
+static_assert(!has_unique_object_representations<int(int) const volatile>::value, "Functions are not unique");
+static_assert(!has_unique_object_representations<int(int) &>::value, "Functions are not unique");
+static_assert(!has_unique_object_representations<int(int) const &>::value, "Functions are not unique");
+static_assert(!has_unique_object_representations<int(int) volatile &>::value, "Functions are not unique");
+static_assert(!has_unique_object_representations<int(int) const volatile &>::value, "Functions are not unique");
+static_assert(!has_unique_object_representations<int(int) &&>::value, "Functions are not unique");
+static_assert(!has_unique_object_representations<int(int) const &&>::value, "Functions are not unique");
+static_assert(!has_unique_object_representations<int(int) volatile &&>::value, "Functions are not unique");
+static_assert(!has_unique_object_representations<int(int) const volatile &&>::value, "Functions are not unique");
+
+static_assert(!has_unique_object_representations<int(int, ...)>::value, "Functions are not unique");
+static_assert(!has_unique_object_representations<int(int, ...) const>::value, "Functions are not unique");
+static_assert(!has_unique_object_representations<int(int, ...) volatile>::value, "Functions are not unique");
+static_assert(!has_unique_object_representations<int(int, ...) const volatile>::value, "Functions are not unique");
+static_assert(!has_unique_object_representations<int(int, ...) &>::value, "Functions are not unique");
+static_assert(!has_unique_object_representations<int(int, ...) const &>::value, "Functions are not unique");
+static_assert(!has_unique_object_representations<int(int, ...) volatile &>::value, "Functions are not unique");
+static_assert(!has_unique_object_representations<int(int, ...) const volatile &>::value, "Functions are not unique");
+static_assert(!has_unique_object_representations<int(int, ...) &&>::value, "Functions are not unique");
+static_assert(!has_unique_object_representations<int(int, ...) const &&>::value, "Functions are not unique");
+static_assert(!has_unique_object_representations<int(int, ...) volatile &&>::value, "Functions are not unique");
+static_assert(!has_unique_object_representations<int(int, ...) const volatile &&>::value, "Functions are not unique");
+
+static auto lambda = []() {};
+static_assert(!has_unique_object_representations<decltype(lambda)>::value, "Lambdas are not unique");
+