Index: include/clang/Basic/DiagnosticSemaKinds.td
===================================================================
--- include/clang/Basic/DiagnosticSemaKinds.td
+++ include/clang/Basic/DiagnosticSemaKinds.td
@@ -2956,6 +2956,10 @@
   "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">,
+  InGroup<DiagGroup<"signed-enum-bitfield">>;
 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: lib/Sema/SemaChecking.cpp
===================================================================
--- lib/Sema/SemaChecking.cpp
+++ lib/Sema/SemaChecking.cpp
@@ -7828,6 +7828,24 @@
     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 integer type was not explicitly stated in the source
+    // code, 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;
 
@@ -7858,7 +7876,7 @@
 
   // 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: test/CXX/drs/dr0xx.cpp
===================================================================
--- test/CXX/drs/dr0xx.cpp
+++ test/CXX/drs/dr0xx.cpp
@@ -1,7 +1,7 @@
-// RUN: %clang_cc1 -std=c++98 %s -verify -fexceptions -fcxx-exceptions -pedantic-errors -Wno-bind-to-temporary-copy
-// RUN: %clang_cc1 -std=c++11 %s -verify -fexceptions -fcxx-exceptions -pedantic-errors -triple %itanium_abi_triple
-// RUN: %clang_cc1 -std=c++14 %s -verify -fexceptions -fcxx-exceptions -pedantic-errors -triple %itanium_abi_triple
-// RUN: %clang_cc1 -std=c++1z %s -verify -fexceptions -fcxx-exceptions -pedantic-errors -triple %itanium_abi_triple
+// RUN: %clang_cc1 -std=c++98 %s -verify -fexceptions -fcxx-exceptions -pedantic-errors -Wno-signed-enum-bitfield -Wno-bind-to-temporary-copy
+// RUN: %clang_cc1 -std=c++11 %s -verify -fexceptions -fcxx-exceptions -pedantic-errors -Wno-signed-enum-bitfield -triple %itanium_abi_triple
+// RUN: %clang_cc1 -std=c++14 %s -verify -fexceptions -fcxx-exceptions -pedantic-errors -Wno-signed-enum-bitfield -triple %itanium_abi_triple
+// RUN: %clang_cc1 -std=c++1z %s -verify -fexceptions -fcxx-exceptions -pedantic-errors -Wno-signed-enum-bitfield -triple %itanium_abi_triple
 
 namespace dr1 { // dr1: no
   namespace X { extern "C" void dr1_f(int a = 1); }
Index: test/SemaCXX/warn-msvc-enum-bitfield.cpp
===================================================================
--- test/SemaCXX/warn-msvc-enum-bitfield.cpp
+++ test/SemaCXX/warn-msvc-enum-bitfield.cpp
@@ -0,0 +1,40 @@
+// RUN: %clang_cc1 -fsyntax-only -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}}
+  s.f1 = F1; // expected-warning {{enums in the Microsoft ABI are signed integers by default; consider giving the enum F an unsigned underlying type}}
+
+  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;
+}