diff --git a/clang/include/clang/Driver/Options.td b/clang/include/clang/Driver/Options.td
--- a/clang/include/clang/Driver/Options.td
+++ b/clang/include/clang/Driver/Options.td
@@ -775,6 +775,7 @@
   MetaVarName<"<arg>">;
 def Wall : Flag<["-"], "Wall">, Group<W_Group>, Flags<[CC1Option, HelpHidden]>;
 def WCL4 : Flag<["-"], "WCL4">, Group<W_Group>, Flags<[CC1Option, HelpHidden]>;
+def Wsystem_headers : Flag<["-"], "Wsystem-headers">, Group<W_Group>, Flags<[CC1Option, HelpHidden]>;
 def Wdeprecated : Flag<["-"], "Wdeprecated">, Group<W_Group>, Flags<[CC1Option]>,
   HelpText<"Enable warnings for deprecated constructs and define __DEPRECATED">;
 def Wno_deprecated : Flag<["-"], "Wno-deprecated">, Group<W_Group>, Flags<[CC1Option]>;
@@ -6458,6 +6459,11 @@
   Alias<W_Joined>, AliasArgs<["invalid-source-encoding"]>;
 def _SLASH_validate_charset_ : CLFlag<"validate-charset-">,
   Alias<W_Joined>, AliasArgs<["no-invalid-source-encoding"]>;
+def _SLASH_external_W0 : CLIgnoredFlag<"external:W0">;
+def _SLASH_external_W1 : CLFlag<"external:W1">, HelpText<"Enable -Wsystem-headers">, Alias<Wsystem_headers>;
+def _SLASH_external_W2 : CLFlag<"external:W2">, HelpText<"Enable -Wsystem-headers">, Alias<Wsystem_headers>;
+def _SLASH_external_W3 : CLFlag<"external:W3">, HelpText<"Enable -Wsystem-headers">, Alias<Wsystem_headers>;
+def _SLASH_external_W4 : CLFlag<"external:W4">, HelpText<"Enable -Wsystem-headers">, Alias<Wsystem_headers>;
 def _SLASH_W0 : CLFlag<"W0">, HelpText<"Disable all warnings">, Alias<w>;
 def _SLASH_W1 : CLFlag<"W1">, HelpText<"Enable -Wall">, Alias<Wall>;
 def _SLASH_W2 : CLFlag<"W2">, HelpText<"Enable -Wall">, Alias<Wall>;
diff --git a/clang/test/Driver/cl-zc.cpp b/clang/test/Driver/cl-zc.cpp
--- a/clang/test/Driver/cl-zc.cpp
+++ b/clang/test/Driver/cl-zc.cpp
@@ -104,6 +104,12 @@
 // RUN: %clang_cl /Zc:dllexportInlines /c -### -- %s 2>&1 | FileCheck -check-prefix=DllExportInlines %s
 // DllExportInlines-NOT: "-fno-dllexport-inlines"
 
+// RUN: %clang_cl /external:W1 /c -### -- %s 2>&1 | FileCheck -check-prefix=EXTERNAL_Wn %s
+// RUN: %clang_cl /external:W2 /c -### -- %s 2>&1 | FileCheck -check-prefix=EXTERNAL_Wn %s
+// RUN: %clang_cl /external:W3 /c -### -- %s 2>&1 | FileCheck -check-prefix=EXTERNAL_Wn %s
+// RUN: %clang_cl /external:W4 /c -### -- %s 2>&1 | FileCheck -check-prefix=EXTERNAL_Wn %s
+// EXTERNAL_Wn: "-Wsystem-headers"
+
 // We recognize -f[no-]delayed-template-parsing.
 // /Zc:twoPhase[-] has the opposite meaning.
 // RUN: %clang_cl -c -### -- %s 2>&1 | FileCheck -check-prefix=DELAYEDDEFAULT %s