diff --git a/include/clang/Driver/ArgList.h b/include/clang/Driver/ArgList.h
index f969804..3affb00 100644
--- a/include/clang/Driver/ArgList.h
+++ b/include/clang/Driver/ArgList.h
@@ -205,9 +205,17 @@
                                     StringRef Default = "") const;
 
     /// getLastArgValue - Return the value of the last argument as an integer,
+    /// or a default. If Diags is non-null, emits an error if the argument
+    /// is given, but non-integral.
+    int getLastArgIntValue(OptSpecifier Id, int Default,
+                           DiagnosticsEngine *Diags = 0) const;
+
+    /// getLastArgValue - Return the value of the last argument as an integer,
     /// or a default. Emits an error if the argument is given, but non-integral.
     int getLastArgIntValue(OptSpecifier Id, int Default,
-                           DiagnosticsEngine &Diags) const;
+                           DiagnosticsEngine &Diags) const {
+      return getLastArgIntValue(Id, Default, &Diags);
+    }
 
     /// getAllArgValues - Get the values of all instances of the given argument
     /// as strings.
diff --git a/include/clang/Frontend/CompilerInvocation.h b/include/clang/Frontend/CompilerInvocation.h
index 0099aae..0d2260a 100644
--- a/include/clang/Frontend/CompilerInvocation.h
+++ b/include/clang/Frontend/CompilerInvocation.h
@@ -33,6 +33,17 @@
 
 class CompilerInvocation;
 class DiagnosticsEngine;
+
+namespace driver {
+class ArgList;
+}
+
+/// CompilerInvocation - Fill out Opts based on the options given in Args.
+/// Args must have been created from the OptTable returned by
+/// createCC1OptTable(). When errors are encountered, return false and,
+/// if Diags is non-null, report the error(s).
+bool ParseDiagnosticArgs(DiagnosticOptions &Opts, driver::ArgList &Args,
+                         DiagnosticsEngine *Diags = 0);
   
 class CompilerInvocationBase : public RefCountedBase<CompilerInvocation> {
 protected:
diff --git a/lib/Driver/ArgList.cpp b/lib/Driver/ArgList.cpp
index d26da25..55a0ddf 100644
--- a/lib/Driver/ArgList.cpp
+++ b/lib/Driver/ArgList.cpp
@@ -154,13 +154,15 @@
 }
 
 int ArgList::getLastArgIntValue(OptSpecifier Id, int Default,
-                                clang::DiagnosticsEngine &Diags) const {
+                                clang::DiagnosticsEngine *Diags) const {
   int Res = Default;
 
   if (Arg *A = getLastArg(Id)) {
-    if (StringRef(A->getValue(*this)).getAsInteger(10, Res))
-      Diags.Report(diag::err_drv_invalid_int_value)
-        << A->getAsString(*this) << A->getValue(*this);
+    if (StringRef(A->getValue(*this)).getAsInteger(10, Res)) {
+      if (Diags)
+        Diags->Report(diag::err_drv_invalid_int_value)
+          << A->getAsString(*this) << A->getValue(*this);
+    }
   }
 
   return Res;
diff --git a/lib/Frontend/CompilerInvocation.cpp b/lib/Frontend/CompilerInvocation.cpp
index 7f7f35c..13a9897 100644
--- a/lib/Frontend/CompilerInvocation.cpp
+++ b/lib/Frontend/CompilerInvocation.cpp
@@ -1277,8 +1277,8 @@
   Opts.DOTOutputFile = Args.getLastArgValue(OPT_dependency_dot);
 }
 
-static bool ParseDiagnosticArgs(DiagnosticOptions &Opts, ArgList &Args,
-                                DiagnosticsEngine &Diags) {
+bool clang::ParseDiagnosticArgs(DiagnosticOptions &Opts, ArgList &Args,
+                                DiagnosticsEngine *Diags) {
   using namespace cc1options;
   bool Success = true;
 
@@ -1312,10 +1312,11 @@
   else if (ShowOverloads == "all")
     Opts.ShowOverloads = DiagnosticsEngine::Ovl_All;
   else {
-    Diags.Report(diag::err_drv_invalid_value)
+    Success = false;
+    if (Diags)
+      Diags->Report(diag::err_drv_invalid_value)
       << Args.getLastArg(OPT_fshow_overloads_EQ)->getAsString(Args)
       << ShowOverloads;
-    Success = false;
   }
 
   StringRef ShowCategory =
@@ -1327,10 +1328,11 @@
   else if (ShowCategory == "name")
     Opts.ShowCategories = 2;
   else {
-    Diags.Report(diag::err_drv_invalid_value)
+    Success = false;
+    if (Diags)
+      Diags->Report(diag::err_drv_invalid_value)
       << Args.getLastArg(OPT_fdiagnostics_show_category)->getAsString(Args)
       << ShowCategory;
-    Success = false;
   }
 
   StringRef Format =
@@ -1342,10 +1344,11 @@
   else if (Format == "vi")
     Opts.Format = DiagnosticOptions::Vi;
   else {
-    Diags.Report(diag::err_drv_invalid_value)
+    Success = false;
+    if (Diags)
+      Diags->Report(diag::err_drv_invalid_value)
       << Args.getLastArg(OPT_fdiagnostics_format)->getAsString(Args)
       << Format;
-    Success = false;
   }
   
   Opts.ShowSourceRanges = Args.hasArg(OPT_fdiagnostics_print_source_range_info);
@@ -1366,13 +1369,23 @@
   Opts.TabStop = Args.getLastArgIntValue(OPT_ftabstop,
                                     DiagnosticOptions::DefaultTabStop, Diags);
   if (Opts.TabStop == 0 || Opts.TabStop > DiagnosticOptions::MaxTabStop) {
-    Diags.Report(diag::warn_ignoring_ftabstop_value)
-      << Opts.TabStop << DiagnosticOptions::DefaultTabStop;
     Opts.TabStop = DiagnosticOptions::DefaultTabStop;
+    if (Diags)
+      Diags->Report(diag::warn_ignoring_ftabstop_value)
+      << Opts.TabStop << DiagnosticOptions::DefaultTabStop;
   }
   Opts.MessageLength = Args.getLastArgIntValue(OPT_fmessage_length, 0, Diags);
   Opts.DumpBuildInformation = Args.getLastArgValue(OPT_dump_build_information);
-  Opts.Warnings = Args.getAllArgValues(OPT_W);
+
+  for (arg_iterator it = Args.filtered_begin(OPT_W),
+         ie = Args.filtered_end(); it != ie; ++it) {
+    StringRef V = (*it)->getValue(Args);
+    // "-Wl," and such are not warnings options.
+    if (V.startswith("l,") || V.startswith("a,") || V.startswith("p,"))
+      continue;
+
+    Opts.Warnings.push_back(V);
+  }
 
   return Success;
 }
@@ -2174,7 +2187,7 @@
   Success = ParseAnalyzerArgs(Res.getAnalyzerOpts(), *Args, Diags) && Success;
   Success = ParseMigratorArgs(Res.getMigratorOpts(), *Args) && Success;
   ParseDependencyOutputArgs(Res.getDependencyOutputOpts(), *Args);
-  Success = ParseDiagnosticArgs(Res.getDiagnosticOpts(), *Args, Diags)
+  Success = ParseDiagnosticArgs(Res.getDiagnosticOpts(), *Args, &Diags)
             && Success;
   ParseFileSystemArgs(Res.getFileSystemOpts(), *Args);
   // FIXME: We shouldn't have to pass the DashX option around here
diff --git a/test/Driver/diagnostics.c b/test/Driver/diagnostics.c
new file mode 100644
index 0000000..1330041
--- /dev/null
+++ b/test/Driver/diagnostics.c
@@ -0,0 +1,9 @@
+// Parse diagnostic arguments in the driver
+// PR12181
+
+// RUN: !%clang -target x86_64-apple-darwin10 \
+// RUN:   -fsyntax-only -fzyzzybalubah \ 
+// RUN:   -Werror=unused-command-line-argument %s 
+
+// RUN: !%clang -target x86_64-apple-darwin10 \
+// RUN:   -fsyntax-only -fzyzzybalubah -Werror %s 
diff --git a/test/Driver/no-objc-arr.m b/test/Driver/no-objc-arr.m
index 15fbdc2..e449393 100644
--- a/test/Driver/no-objc-arr.m
+++ b/test/Driver/no-objc-arr.m
@@ -1,4 +1,4 @@
-// RUN: %clang  -Werror -fobjc-arc -fsyntax-only -fno-objc-arc -verify %s
+// RUN: %clang  -Werror -fobjc-arc -fsyntax-only -fno-objc-arc -Xclang -verify %s
 // rdar://8949617
 
 void * FOO() {
diff --git a/tools/driver/driver.cpp b/tools/driver/driver.cpp
index 9e666eb..8c05fff 100644
--- a/tools/driver/driver.cpp
+++ b/tools/driver/driver.cpp
@@ -12,11 +12,16 @@
 //
 //===----------------------------------------------------------------------===//
 
+#include "clang/Driver/ArgList.h"
+#include "clang/Driver/CC1Options.h"
 #include "clang/Driver/Compilation.h"
 #include "clang/Driver/Driver.h"
 #include "clang/Driver/Option.h"
+#include "clang/Driver/OptTable.h"
+#include "clang/Frontend/CompilerInvocation.h"
 #include "clang/Frontend/DiagnosticOptions.h"
 #include "clang/Frontend/TextDiagnosticPrinter.h"
+#include "clang/Frontend/Utils.h"
 
 #include "llvm/ADT/ArrayRef.h"
 #include "llvm/ADT/SmallString.h"
@@ -370,11 +375,27 @@
 
   llvm::sys::Path Path = GetExecutablePath(argv[0], CanonicalPrefixes);
 
+  DiagnosticOptions DiagOpts;
+  {
+    // Note that ParseDiagnosticArgs() uses the cc1 option table.
+    OwningPtr<OptTable> CC1Opts(createCC1OptTable());
+    unsigned MissingArgIndex, MissingArgCount;
+    OwningPtr<InputArgList> Args(CC1Opts->ParseArgs(argv.begin()+1, argv.end(),
+                                            MissingArgIndex, MissingArgCount));
+    // We ignore MissingArgCount and the return value of ParseDiagnosticArgs.
+    // Any errors that would be diagnosed here will also be diagnosed later,
+    // when the DiagnosticsEngine actually exists.
+    (void) ParseDiagnosticArgs(DiagOpts, *Args);
+  }
+  // Now we can create the DiagnosticsEngine with a properly-filled-out
+  // DiagnosticOptions instance.
   TextDiagnosticPrinter *DiagClient
-    = new TextDiagnosticPrinter(llvm::errs(), DiagnosticOptions());
+    = new TextDiagnosticPrinter(llvm::errs(), DiagOpts);
   DiagClient->setPrefix(llvm::sys::path::stem(Path.str()));
   IntrusiveRefCntPtr<DiagnosticIDs> DiagID(new DiagnosticIDs());
+
   DiagnosticsEngine Diags(DiagID, DiagClient);
+  ProcessWarningOptions(Diags, DiagOpts);
 
 #ifdef CLANG_IS_PRODUCTION
   const bool IsProduction = true;