Index: cfe/trunk/include/clang/Basic/DiagnosticGroups.td
===================================================================
--- cfe/trunk/include/clang/Basic/DiagnosticGroups.td
+++ cfe/trunk/include/clang/Basic/DiagnosticGroups.td
@@ -543,6 +543,7 @@
 def CharSubscript : DiagGroup<"char-subscripts">;
 def LargeByValueCopy : DiagGroup<"large-by-value-copy">;
 def DuplicateArgDecl : DiagGroup<"duplicate-method-arg">;
+def SignedEnumBitfield : DiagGroup<"signed-enum-bitfield">;
 
 // Unreachable code warning groups.
 //
@@ -661,6 +662,7 @@
     ReturnType,
     SelfAssignment,
     SelfMove,
+    SignedEnumBitfield,
     SizeofArrayArgument,
     SizeofArrayDecay,
     StringPlusInt,
Index: cfe/trunk/include/clang/Basic/DiagnosticSemaKinds.td
===================================================================
--- cfe/trunk/include/clang/Basic/DiagnosticSemaKinds.td
+++ cfe/trunk/include/clang/Basic/DiagnosticSemaKinds.td
@@ -3020,6 +3020,10 @@
   "cast to %1 from smaller integer type %0">,
   InGroup<IntToVoidPointerCast>;
 
+def warn_no_underlying_type_specified_for_enum_bitfield : Warning<
+  "enums in the Microsoft ABI are signed integers by default; consider giving "
+  "the enum %0 an unsigned underlying type to make this code portable">,
+  InGroup<DiagGroup<"signed-enum-bitfield">>, DefaultIgnore;
 def warn_attribute_packed_for_bitfield : Warning<
   "'packed' attribute was ignored on bit-fields with single-byte alignment "
   "in older versions of GCC and Clang">,
Index: cfe/trunk/lib/Sema/SemaChecking.cpp
===================================================================
--- cfe/trunk/lib/Sema/SemaChecking.cpp
+++ cfe/trunk/lib/Sema/SemaChecking.cpp
@@ -8518,6 +8518,24 @@
     return false;
 
   // White-list bool bitfields.
+  QualType BitfieldType = Bitfield->getType();
+  if (BitfieldType->isBooleanType())
+     return false;
+
+  if (BitfieldType->isEnumeralType()) {
+    EnumDecl *BitfieldEnumDecl = BitfieldType->getAs<EnumType>()->getDecl();
+    // If the underlying enum type was not explicitly specified as an unsigned
+    // type and the enum contain only positive values, MSVC++ will cause an
+    // inconsistency by storing this as a signed type.
+    if (S.getLangOpts().CPlusPlus11 &&
+        !BitfieldEnumDecl->getIntegerTypeSourceInfo() &&
+        BitfieldEnumDecl->getNumPositiveBits() > 0 &&
+        BitfieldEnumDecl->getNumNegativeBits() == 0) {
+      S.Diag(InitLoc, diag::warn_no_underlying_type_specified_for_enum_bitfield)
+        << BitfieldEnumDecl->getNameAsString();
+    }
+  }
+
   if (Bitfield->getType()->isBooleanType())
     return false;
 
@@ -8547,7 +8565,7 @@
 
   // Compute the value which the bitfield will contain.
   llvm::APSInt TruncatedValue = Value.trunc(FieldWidth);
-  TruncatedValue.setIsSigned(Bitfield->getType()->isSignedIntegerType());
+  TruncatedValue.setIsSigned(BitfieldType->isSignedIntegerType());
 
   // Check whether the stored value is equal to the original value.
   TruncatedValue = TruncatedValue.extend(OriginalWidth);
Index: cfe/trunk/test/SemaCXX/warn-msvc-enum-bitfield.cpp
===================================================================
--- cfe/trunk/test/SemaCXX/warn-msvc-enum-bitfield.cpp
+++ cfe/trunk/test/SemaCXX/warn-msvc-enum-bitfield.cpp
@@ -0,0 +1,40 @@
+// RUN: %clang_cc1 -fsyntax-only -Wsigned-enum-bitfield -verify %s --std=c++11
+
+// Enums used in bitfields with no explicitly specified underlying type.
+void test0() {
+  enum E { E1, E2 };
+  enum F { F1, F2 };
+  struct { E e1 : 1; E e2; F f1 : 1; F f2; } s;
+
+  s.e1 = E1; // expected-warning {{enums in the Microsoft ABI are signed integers by default; consider giving the enum E an unsigned underlying type to make this code portable}}
+  s.f1 = F1; // expected-warning {{enums in the Microsoft ABI are signed integers by default; consider giving the enum F an unsigned underlying type to make this code portable}}
+
+  s.e2 = E2;
+  s.f2 = F2;
+}
+
+// Enums used in bitfields with an explicit signed underlying type.
+void test1() {
+  enum E : signed { E1, E2 };
+  enum F : long { F1, F2 };
+  struct { E e1 : 1; E e2; F f1 : 1; F f2; } s;
+
+  s.e1 = E1;
+  s.f1 = F1;
+
+  s.e2 = E2;
+  s.f2 = F2;
+}
+
+// Enums used in bitfields with an explicitly unsigned underlying type.
+void test3() {
+  enum E : unsigned { E1, E2 };
+  enum F : unsigned long { F1, F2 };
+  struct { E e1 : 1; E e2; F f1 : 1; F f2; } s;
+
+  s.e1 = E1;
+  s.f1 = F1;
+
+  s.e2 = E2;
+  s.f2 = F2;
+}