diff --git a/docs/checkedc/Test-Baselines.md b/docs/checkedc/Test-Baselines.md
index 2816b1569823..88e3e4a4cda4 100644
--- a/docs/checkedc/Test-Baselines.md
+++ b/docs/checkedc/Test-Baselines.md
@@ -1,6 +1,6 @@
 # Test base lines
 
-## Current baseline
+## Testing baseline for the base line branch
 
 
 Here is the current baseline for testing just clang with the x86 target (using the check-clang project)
@@ -52,7 +52,14 @@ Here is the current base line for testing LLVM + clang on all targets (check-all
            Unexpected Failures: 6
 ```
 
+## Differences in test results between branches
+
+The  master branch in the checkedc-clang repo has additional tests for
+Checked C.  It is expected that the master branch will have more
+`Expected Passes` than the baseline branch.
+
 ## In-progress baseline updates
 
-This section records the test results for an in-progress update to latest sources in the baseline branch.  It is currently empty because
-no update is in-progress.
+This section records the test results for an in-progress update to latest
+sources in the baseline branch.  It is currently empty because no update
+is in-progress.
diff --git a/docs/checkedc/Testing.md b/docs/checkedc/Testing.md
index 47d110431892..afafa0b4a8f7 100644
--- a/docs/checkedc/Testing.md
+++ b/docs/checkedc/Testing.md
@@ -5,7 +5,9 @@ target to the build system for running the test suite: check-checkedc.
 
 The Checked C version of clang/LLVM should pass the same tests as the baseline version
 of clang/LLVM (in the baseline branch) and the Checked C specific tests.   The testing
-results for the baseline branch are recorded in [testing baselines](Test-Baselines.md)
+results for the baseline branch are recorded in [testing baselines](Test-Baselines.md).
+A developer should confirm that no new unexpected failures occur as a result of a change
+before committing a change.
 
 When testing a change, the testing should be sufficient for the type of change.  For changes
 to parsing and typechecking, it is usually sufficient to pass the Checked C and clang tests.
@@ -49,8 +51,9 @@ The clang-specific documentation on running tests appears to be out-of-date, so
 		llvm-lit d:\autobahn1\llvm\tools\clang\test
 
 ## Test baselines
-We have observed a few tests fail on Windows on a clean LLVM/clang enlistment that don't seem to fail on the buildbots.  
-We have not tracked down the source of the failures.  For now, we are using [testing baselines](Test-Baselines.md) to exclude these
-tests.  LLVM/clang have an optimistic check-in policy, so it is possible that a few tests may fail in the 
-main-line repos when we update to the latest sources.
-
+We have observed a few tests fail unexpectedly on Windows on a clean LLVM/clang
+enlistment.  These tests don't seem to fail on the buildbots. We have not 
+tracked down the source of the failures.  For now, we are using
+[testing baselines](Test-Baselines.md) to exclude these tests.  LLVM/clang has
+an optimistic check-in policy, so it is possible that a few tests may fail in
+the main-line repos when we update to the latest sources.
diff --git a/docs/checkedc/Update-to-latest-LLVM-sources.md b/docs/checkedc/Update-to-latest-LLVM-sources.md
index 08e1ad6c0c2c..66304852de55 100644
--- a/docs/checkedc/Update-to-latest-LLVM-sources.md
+++ b/docs/checkedc/Update-to-latest-LLVM-sources.md
@@ -44,12 +44,12 @@ Then do
 
 ## Update the baseline branch on Github
 
-You will then need to build and run tests to establish test baselines.   Assuming that the tests  results are good, you can push them to your personal 
-Github forks:
+You will then need to build and run tests to establish test baselines.   Assuming that the tests  results are good, 
+you can push them to your personal Github forks:
 
 	git push origin baseline
 
-You can then issue a pull request to pull the changes into the mainline change.
+You can then issue pull requests to pull the changes into the Microsift Github repos.
 
 ## Update the master branch.
 
@@ -58,6 +58,12 @@ After you have updated the baseline branch, you can update the master branch. Ch
 	git checkout master
 	git merge baseline
 
-Then set up the build system and compile.  Fix any issues that you encounter.  Then run tests.  Once you are passing the same
-testing as the baseline branch, push this up to personal Github fork and issue a pull request.
+Set up the build system and compile.  Fix any issues that you encounter.  
 
+Then run tests.  We have added tests for Checked C to the clang master branch, so these additional tests need to be taken
+into account during testing.  Make sure the code passes the following tests:
+
+- The same tests as the baseline branch, _plus_ the Checked C specific tests for clang in the master branch.
+- The Checked C languages tests for the Checked C project.
+
+Once the tests are passing, push the changes up to a personal Github fork and issue a pull request.
diff --git a/lib/Frontend/CompilerInvocation.cpp b/lib/Frontend/CompilerInvocation.cpp
index 779bbc34a289..a46533ea8ccc 100644
--- a/lib/Frontend/CompilerInvocation.cpp
+++ b/lib/Frontend/CompilerInvocation.cpp
@@ -1701,7 +1701,28 @@ static void ParseLangArgs(LangOptions &Opts, ArgList &Args, InputKind IK,
   Opts.PascalStrings = Args.hasArg(OPT_fpascal_strings);
   Opts.VtorDispMode = getLastArgIntValue(Args, OPT_vtordisp_mode_EQ, 1, Diags);
   Opts.Borland = Args.hasArg(OPT_fborland_extensions);
-  Opts.CheckedC = Args.hasArg(OPT_fcheckedc_extension);
+  if (Args.hasArg(OPT_fcheckedc_extension)) {
+    std::string disallowed;
+    if (Opts.CUDA)
+      disallowed = "CUDA";
+    else if (Opts.OpenCL)
+      disallowed = "OpenCL";
+    else if (Opts.ObjC1 || Opts.ObjC2) {
+      if (Opts.CPlusPlus)
+        disallowed = "Objective C/C++";
+      else
+        disallowed = "Objective C";
+    }
+    else if (Opts.CPlusPlus) {
+      disallowed = "C++";
+    }
+
+    if (disallowed.size() > 0) {
+      Diags.Report(diag::err_drv_argument_not_allowed_with) <<
+        "-fcheckedc-extension" << disallowed;
+    } else
+      Opts.CheckedC = true;
+  }
   Opts.WritableStrings = Args.hasArg(OPT_fwritable_strings);
   Opts.ConstStrings = Args.hasFlag(OPT_fconst_strings, OPT_fno_const_strings,
                                    Opts.ConstStrings);
diff --git a/test/Driver/checkedc-notsupported.cl b/test/Driver/checkedc-notsupported.cl
new file mode 100644
index 000000000000..964589285fb0
--- /dev/null
+++ b/test/Driver/checkedc-notsupported.cl
@@ -0,0 +1,14 @@
+// Checked C extension is not supported for OpenCL.   Make sure driver
+// rejects the flag.
+//
+// RUN: not %clang -fcheckedc-extension %s 2>&1 | FileCheck %s
+// CHECK: error: invalid argument '-fcheckedc-extension' not allowed with 'OpenCL'
+//
+// Have clang compile this file as a C file.
+// RUN: %clang -c -fcheckedc-extension -x c %s
+//
+// Have clang-cl compile this file as a C file.
+// RUN: %clang_cl -c -Xclang -fcheckedc-extension /TC %s
+
+extern void f() {}
+
diff --git a/test/Driver/checkedc-notsupported.cpp b/test/Driver/checkedc-notsupported.cpp
new file mode 100644
index 000000000000..16af4b5f31b2
--- /dev/null
+++ b/test/Driver/checkedc-notsupported.cpp
@@ -0,0 +1,15 @@
+// Checked C extension is not supported for C++.   Make sure driver
+// rejects the flag.
+//
+// RUN: not %clang -fcheckedc-extension %s 2>&1 | FileCheck %s
+// CHECK: error: invalid argument '-fcheckedc-extension' not allowed with 'C++'
+//
+// Have clang compile this file as a C file.
+// RUN: %clang -c -fcheckedc-extension -x c %s
+//
+// Have clang-cl compile this file as a C file.
+// RUN: %clang_cl -c -Xclang -fcheckedc-extension /TC %s
+
+extern void f() {}
+
+
diff --git a/test/Driver/checkedc-notsupported.cu b/test/Driver/checkedc-notsupported.cu
new file mode 100644
index 000000000000..7a73f0872fe9
--- /dev/null
+++ b/test/Driver/checkedc-notsupported.cu
@@ -0,0 +1,15 @@
+// Checked C extension is not supported for CUDA.   Make sure driver
+// rejects the flag.
+//
+// RUN: not %clang -fcheckedc-extension %s 2>&1 | FileCheck %s
+// CHECK: error: invalid argument '-fcheckedc-extension' not allowed with 'CUDA'
+//
+// Have clang compile this file as a C file.
+// RUN: %clang -c -fcheckedc-extension -x c %s
+//
+// Have clang-cl compile this file as a C file.
+// RUN: %clang_cl -c -Xclang -fcheckedc-extension /TC %s
+
+extern void f() {}
+
+
diff --git a/test/Driver/checkedc-notsupported.m b/test/Driver/checkedc-notsupported.m
new file mode 100644
index 000000000000..85e3ce03a4a1
--- /dev/null
+++ b/test/Driver/checkedc-notsupported.m
@@ -0,0 +1,15 @@
+// Checked C extension is not supported for Objective C.   Make sure driver
+// rejects the flag.
+//
+// RUN: not %clang -fcheckedc-extension %s 2>&1 | FileCheck %s
+// CHECK: error: invalid argument '-fcheckedc-extension' not allowed with 'Objective C'
+//
+// Have clang compile this file as a C file.
+// RUN: %clang -c -fcheckedc-extension -x c %s
+//
+// Have clang-cl compile this file as a C file.
+// RUN: %clang_cl -c -Xclang -fcheckedc-extension /TC %s
+
+extern void f() {}
+
+
diff --git a/test/Driver/checkedc-supported.c b/test/Driver/checkedc-supported.c
new file mode 100644
index 000000000000..ea7a69b1713a
--- /dev/null
+++ b/test/Driver/checkedc-supported.c
@@ -0,0 +1,35 @@
+// Checked C extension is supported for C.   Make sure driver
+// accepts the flag for C and rejects it when the file is
+// compiled as another language.
+//
+// RUN: %clang -c -fcheckedc-extension %s
+// RUN: %clang_cl -c -Xclang -fcheckedc-extension %s
+//
+// Have clang compile this file as C++ file.
+// RUN: not %clang -c -fcheckedc-extension -x c++ %s 2>&1 \
+// RUN:  | FileCheck %s -check-prefix=check-cpp
+// check-cpp: error: invalid argument '-fcheckedc-extension' not allowed with 'C++'
+//
+// Have clang-cl compile this file as a C++ file.
+// RUN: not %clang_cl -c -Xclang -fcheckedc-extension /TP %s 2>&1 \
+// RUN:  | FileCheck %s -check-prefix=clcheck-cpp
+// clcheck-cpp: error: invalid argument '-fcheckedc-extension' not allowed with 'C++'
+//
+// RUN: not %clang -c -fcheckedc-extension -x cuda %s 2>&1 \
+// RUN:  | FileCheck %s -check-prefix=check-cuda
+// check-cuda: error: invalid argument '-fcheckedc-extension' not allowed with 'CUDA'
+//
+// RUN: not %clang -c -fcheckedc-extension -x cl %s 2>&1 \
+// RUN:  | FileCheck %s -check-prefix=check-opencl
+// check-opencl: error: invalid argument '-fcheckedc-extension' not allowed with 'OpenCL'
+//
+// RUN: not %clang -c -fcheckedc-extension -x objective-c %s 2>&1 \
+// RUN:  | FileCheck %s -check-prefix=check-objc
+// check-objc: error: invalid argument '-fcheckedc-extension' not allowed with 'Objective C'
+//
+// RUN: not %clang -c -fcheckedc-extension -x objective-c++ %s 2>&1 \
+// RUN:  | FileCheck %s -check-prefix=check-objcpp
+// check-objcpp: error: invalid argument '-fcheckedc-extension' not allowed with 'Objective C/C++'
+
+
+extern void f(ptr<int> p) {}