Index: clang/lib/Driver/ToolChains/Clang.cpp
===================================================================
--- clang/lib/Driver/ToolChains/Clang.cpp
+++ clang/lib/Driver/ToolChains/Clang.cpp
@@ -4646,7 +4646,7 @@
     CmdArgs.push_back("-fforbid-guard-variables");
 
   if (Args.hasFlag(options::OPT_mms_bitfields, options::OPT_mno_ms_bitfields,
-                   false)) {
+                   Triple.isWindowsGNUEnvironment())) {
     CmdArgs.push_back("-mms-bitfields");
   }
 
Index: clang/test/Driver/ms-bitfields.c
===================================================================
--- clang/test/Driver/ms-bitfields.c
+++ clang/test/Driver/ms-bitfields.c
@@ -1,4 +1,5 @@
-// RUN: %clang -### %s 2>&1 | FileCheck %s -check-prefix=NO-MSBITFIELDS
+// RUN: %clang -### -target x86_64-linux-gnu %s 2>&1 | FileCheck %s -check-prefix=NO-MSBITFIELDS
+// RUN: %clang -### -target x86_64-windows-gnu %s 2>&1 | FileCheck %s -check-prefix=MSBITFIELDS
 // RUN: %clang -### -mno-ms-bitfields -mms-bitfields %s 2>&1 | FileCheck %s -check-prefix=MSBITFIELDS
 // RUN: %clang -### -mms-bitfields -mno-ms-bitfields %s 2>&1 | FileCheck %s -check-prefix=NO-MSBITFIELDS