Index: docs/UsersManual.rst
===================================================================
--- docs/UsersManual.rst
+++ docs/UsersManual.rst
@@ -2080,7 +2080,7 @@
       /W1                    Enable -Wall
       /W2                    Enable -Wall
       /W3                    Enable -Wall
-      /W4                    Enable -Wall
+      /W4                    Enable -Wall and -Wextra
       /Wall                  Enable -Wall
       /WX-                   Do not treat warnings as errors
       /WX                    Treat warnings as errors
Index: include/clang/Basic/DiagnosticGroups.td
===================================================================
--- include/clang/Basic/DiagnosticGroups.td
+++ include/clang/Basic/DiagnosticGroups.td
@@ -663,8 +663,11 @@
 // Note that putting warnings in -Wall will not disable them by default. If a
 // warning should be active _only_ when -Wall is passed in, mark it as
 // DefaultIgnore in addition to putting it here.
-def : DiagGroup<"all", [Most, Parentheses, Switch, SwitchBool]>;
+def All : DiagGroup<"all", [Most, Parentheses, Switch, SwitchBool]>;
 
+// Warnings that should be in clang-cl /w4.
+def : DiagGroup<"CL4", [All, Extra]>;
+
 // Warnings enabled by -pedantic.  This is magically filled in by TableGen.
 def Pedantic : DiagGroup<"pedantic">;
 
Index: include/clang/Driver/CLCompatOptions.td
===================================================================
--- include/clang/Driver/CLCompatOptions.td
+++ include/clang/Driver/CLCompatOptions.td
@@ -119,7 +119,7 @@
 def _SLASH_W1 : CLFlag<"W1">, HelpText<"Enable -Wall">, Alias<Wall>;
 def _SLASH_W2 : CLFlag<"W2">, HelpText<"Enable -Wall">, Alias<Wall>;
 def _SLASH_W3 : CLFlag<"W3">, HelpText<"Enable -Wall">, Alias<Wall>;
-def _SLASH_W4 : CLFlag<"W4">, HelpText<"Enable -Wall">, Alias<Wall>;
+def _SLASH_W4 : CLFlag<"W4">, HelpText<"Enable -Wall">, Alias<WCL4>;
 def _SLASH_Wall : CLFlag<"Wall">, HelpText<"Enable -Wall">, Alias<Wall>;
 def _SLASH_WX : CLFlag<"WX">, HelpText<"Treat warnings as errors">,
   Alias<W_Joined>, AliasArgs<["error"]>;
Index: include/clang/Driver/Options.td
===================================================================
--- include/clang/Driver/Options.td
+++ include/clang/Driver/Options.td
@@ -295,6 +295,7 @@
   HelpText<"Pass the comma separated arguments in <arg> to the assembler">,
   MetaVarName<"<arg>">;
 def Wall : Flag<["-"], "Wall">, Group<W_Group>, Flags<[CC1Option]>;
+def WCL4 : Flag<["-"], "WCL4">, Group<W_Group>, Flags<[CC1Option]>;
 def Wdeprecated : Flag<["-"], "Wdeprecated">, Group<W_Group>, Flags<[CC1Option]>;
 def Wno_deprecated : Flag<["-"], "Wno-deprecated">, Group<W_Group>, Flags<[CC1Option]>;
 def Wextra : Flag<["-"], "Wextra">, Group<W_Group>, Flags<[CC1Option]>;
Index: test/Driver/cl-options.c
===================================================================
--- test/Driver/cl-options.c
+++ test/Driver/cl-options.c
@@ -173,9 +173,10 @@
 // RUN: %clang_cl /W1 -### -- %s 2>&1 | FileCheck -check-prefix=W1 %s
 // RUN: %clang_cl /W2 -### -- %s 2>&1 | FileCheck -check-prefix=W1 %s
 // RUN: %clang_cl /W3 -### -- %s 2>&1 | FileCheck -check-prefix=W1 %s
-// RUN: %clang_cl /W4 -### -- %s 2>&1 | FileCheck -check-prefix=W1 %s
+// RUN: %clang_cl /W4 -### -- %s 2>&1 | FileCheck -check-prefix=W4 %s
 // RUN: %clang_cl /Wall -### -- %s 2>&1 | FileCheck -check-prefix=W1 %s
 // W1: -Wall
+// W4: -WCL4
 
 // RUN: %clang_cl /WX -### -- %s 2>&1 | FileCheck -check-prefix=WX %s
 // WX: -Werror