Index: cfe/trunk/docs/UsersManual.rst
===================================================================
--- cfe/trunk/docs/UsersManual.rst
+++ cfe/trunk/docs/UsersManual.rst
@@ -2024,8 +2024,9 @@
 
 To suppress warnings about unused arguments, use the ``-Qunused-arguments`` option.
 
-Options that are not known to clang-cl will cause errors. If they are spelled with a
-leading ``/``, they will be mistaken for a filename:
+Options that are not known to clang-cl will be ignored by default. Use the
+``-Werror=unknown-argument`` option in order to treat them as errors. If these
+options are spelled with a leading ``/``, they will be mistaken for a filename:
 
   ::
 
Index: cfe/trunk/include/clang/Basic/DiagnosticDriverKinds.td
===================================================================
--- cfe/trunk/include/clang/Basic/DiagnosticDriverKinds.td
+++ cfe/trunk/include/clang/Basic/DiagnosticDriverKinds.td
@@ -93,6 +93,9 @@
 def err_drv_I_dash_not_supported : Error<
   "'%0' not supported, please use -iquote instead">;
 def err_drv_unknown_argument : Error<"unknown argument: '%0'">;
+def warn_drv_unknown_argument_clang_cl : Warning<
+  "unknown argument ignored in clang-cl: '%0'">,
+  InGroup<UnknownArgument>;
 def err_drv_invalid_value : Error<"invalid value '%1' in '%0'">;
 def err_drv_invalid_int_value : Error<"invalid integral value '%1' in '%0'">;
 def err_drv_invalid_remap_file : Error<
Index: cfe/trunk/include/clang/Basic/DiagnosticGroups.td
===================================================================
--- cfe/trunk/include/clang/Basic/DiagnosticGroups.td
+++ cfe/trunk/include/clang/Basic/DiagnosticGroups.td
@@ -848,3 +848,5 @@
 def InvalidOrNonExistentDirectory : DiagGroup<"invalid-or-nonexistent-directory">;
 
 def OptionIgnored : DiagGroup<"option-ignored">;
+
+def UnknownArgument : DiagGroup<"unknown-argument">;
Index: cfe/trunk/lib/Driver/Driver.cpp
===================================================================
--- cfe/trunk/lib/Driver/Driver.cpp
+++ cfe/trunk/lib/Driver/Driver.cpp
@@ -146,7 +146,9 @@
   }
 
   for (const Arg *A : Args.filtered(options::OPT_UNKNOWN))
-    Diags.Report(diag::err_drv_unknown_argument) << A->getAsString(Args);
+    Diags.Report(IsCLMode() ? diag::warn_drv_unknown_argument_clang_cl :
+                              diag::err_drv_unknown_argument)
+      << A->getAsString(Args);
 
   return Args;
 }
@@ -1710,8 +1712,11 @@
           continue;
       }
 
-      Diag(clang::diag::warn_drv_unused_argument)
-          << A->getAsString(C.getArgs());
+      // In clang-cl, don't mention unknown arguments here since they have
+      // already been warned about.
+      if (!IsCLMode() || !A->getOption().matches(options::OPT_UNKNOWN))
+        Diag(clang::diag::warn_drv_unused_argument)
+            << A->getAsString(C.getArgs());
     }
   }
 }
Index: cfe/trunk/lib/Driver/Tools.cpp
===================================================================
--- cfe/trunk/lib/Driver/Tools.cpp
+++ cfe/trunk/lib/Driver/Tools.cpp
@@ -9708,6 +9708,10 @@
                                options::OPT__SLASH_MT, options::OPT__SLASH_MTd))
     A->render(Args, CmdArgs);
 
+  // Pass through all unknown arguments so that the fallback command can see
+  // them too.
+  Args.AddAllArgs(CmdArgs, options::OPT_UNKNOWN);
+
   // Input filename.
   assert(Inputs.size() == 1);
   const InputInfo &II = Inputs[0];
Index: cfe/trunk/test/Driver/cl-fallback.c
===================================================================
--- cfe/trunk/test/Driver/cl-fallback.c
+++ cfe/trunk/test/Driver/cl-fallback.c
@@ -3,6 +3,7 @@
 
 // RUN: %clang_cl --target=i686-pc-win32 /fallback /Dfoo=bar /Ubaz /Ifoo /O0 /Ox /GR /GR- /Gy /Gy- \
 // RUN:   /Gw /Gw- /LD /LDd /EHs /EHs- /Zl /MD /MDd /MTd /MT /FImyheader.h /Zi \
+// RUN:   -garbage -moregarbage \
 // RUN:   -### -- %s 2>&1 \
 // RUN:   | FileCheck %s
 // CHECK: "-fdiagnostics-format" "msvc-fallback"
@@ -31,6 +32,8 @@
 // CHECK: "/EHs-"
 // CHECK: "/Zl"
 // CHECK: "/MT"
+// CHECK: "-garbage"
+// CHECK: "-moregarbage"
 // CHECK: "/Tc" "{{.*cl-fallback.c}}"
 // CHECK: "/Fo{{.*cl-fallback.*.obj}}"
 
Index: cfe/trunk/test/Driver/unknown-arg.c
===================================================================
--- cfe/trunk/test/Driver/unknown-arg.c
+++ cfe/trunk/test/Driver/unknown-arg.c
@@ -1,5 +1,11 @@
 // RUN: not %clang %s -cake-is-lie -%0 -%d -HHHH -munknown-to-clang-option -print-stats -funknown-to-clang-option 2>&1 | \
 // RUN: FileCheck %s
+// RUN: %clang_cl -cake-is-lie -%0 -%d -HHHH -munknown-to-clang-option -print-stats -funknown-to-clang-option -c -- %s 2>&1 | \
+// RUN: FileCheck %s --check-prefix=CL
+// RUN: not %clang_cl -cake-is-lie -%0 -%d -HHHH -munknown-to-clang-option -print-stats -funknown-to-clang-option -c -Werror=unknown-argument -- %s 2>&1 | \
+// RUN: FileCheck %s --check-prefix=CL
+// RUN: %clang_cl -cake-is-lie -%0 -%d -HHHH -munknown-to-clang-option -print-stats -funknown-to-clang-option -c -Wno-unknown-argument -- %s 2>&1 | \
+// RUN: FileCheck %s --check-prefix=SILENT --allow-empty
 
 // CHECK: unknown argument: '-cake-is-lie'
 // CHECK: unknown argument: '-%0'
@@ -8,6 +14,15 @@
 // CHECK: unknown argument: '-munknown-to-clang-option'
 // CHECK: unknown argument: '-print-stats'
 // CHECK: unknown argument: '-funknown-to-clang-option'
+// CL: unknown argument ignored in clang-cl: '-cake-is-lie'
+// CL: unknown argument ignored in clang-cl: '-%0'
+// CL: unknown argument ignored in clang-cl: '-%d'
+// CL: unknown argument ignored in clang-cl: '-HHHH'
+// CL: unknown argument ignored in clang-cl: '-munknown-to-clang-option'
+// CL: unknown argument ignored in clang-cl: '-print-stats'
+// CL: unknown argument ignored in clang-cl: '-funknown-to-clang-option'
+// SILENT-NOT: error
+// SILENT-NOT: warning
 
 
 // RUN: %clang -S %s -o %t.s  -Wunknown-to-clang-option 2>&1 | FileCheck --check-prefix=IGNORED %s
Index: cfe/trunk/test/Misc/serialized-diags-driver.c
===================================================================
--- cfe/trunk/test/Misc/serialized-diags-driver.c
+++ cfe/trunk/test/Misc/serialized-diags-driver.c
@@ -5,10 +5,10 @@
 // doesn't litter the user's system with preprocessed output.
 
 // RUN: rm -f %t
-// RUN: %clang -Wx-unknown-warning -Wall -fsyntax-only --serialize-diagnostics %t.diag %s
+// RUN: %clang -Wx-typoed-warning -Wall -fsyntax-only --serialize-diagnostics %t.diag %s
 // RUN: c-index-test -read-diagnostics %t.diag 2>&1 | FileCheck %s
 
-// CHECK: warning: unknown warning option '-Wx-unknown-warning' [-Wunknown-warning-option] []
+// CHECK: warning: unknown warning option '-Wx-typoed-warning' [-Wunknown-warning-option] []
 
 // CHECK: warning: variable 'voodoo' is uninitialized when used here [-Wuninitialized]
 // CHECK: note: initialize the variable 'voodoo' to silence this warning []