Index: lib/Sema/SemaDecl.cpp
===================================================================
--- lib/Sema/SemaDecl.cpp
+++ lib/Sema/SemaDecl.cpp
@@ -14351,7 +14351,13 @@
           // we perform a non-narrowing conversion as part of converted constant
           // expression checking.
           if (!isRepresentableIntegerValue(Context, EnumVal, EltTy)) {
-            if (getLangOpts().MSVCCompat) {
+            if (getLangOpts().MSVCCompat ||
+                Context.getTargetInfo().getCXXABI().isMicrosoft()) {
+              // Treat as an extension in MSVC compat mode or if using the
+              // MSVC ABI. We already treat the equivalent case in C as an
+              // extension if we're not in Microsoft mode. Several Windows
+              // headers define enums with initializers like 0x80000000 and
+              // 0xffffffff.
               Diag(IdLoc, diag::ext_enumerator_too_large) << EltTy;
               Val = ImpCastExprToType(Val, EltTy, CK_IntegralCast).get();
             } else