diff --git a/include/clang/Basic/Diagnostic.h b/include/clang/Basic/Diagnostic.h
index 1d07111..9925170 100644
--- a/include/clang/Basic/Diagnostic.h
+++ b/include/clang/Basic/Diagnostic.h
@@ -65,7 +65,16 @@
       MAP_IGNORE  = 1,     //< Map this diagnostic to nothing, ignore it.
       MAP_WARNING = 2,     //< Map this diagnostic to a warning.
       MAP_ERROR   = 3,     //< Map this diagnostic to an error.
-      MAP_FATAL   = 4      //< Map this diagnostic to a fatal error.
+      MAP_FATAL   = 4,     //< Map this diagnostic to a fatal error.
+      
+      /// Map this diagnostic to "warning", but make it immune to
+      /// -pedantic-errors.  This happens when you specify -Wfoo for an
+      /// extension warning.
+      MAP_WARNING_NO_PEDANTIC_ERROR = 5,
+      
+      /// Map this diagnostic to "warning", but make it immune to -Werror and
+      /// -pedantic-errors.  This happens when you specify -Wno-error=foo.
+      MAP_WARNING_NO_WERROR = 6
     };
   }
   
diff --git a/lib/Basic/Diagnostic.cpp b/lib/Basic/Diagnostic.cpp
index 19bfeee..6b7f732 100644
--- a/lib/Basic/Diagnostic.cpp
+++ b/lib/Basic/Diagnostic.cpp
@@ -382,7 +382,20 @@
     // If warnings are globally mapped to ignore or error, do it.
     if (IgnoreAllWarnings)
       return Diagnostic::Ignored;
-    Result = WarningsAsErrors ? Diagnostic::Error : Diagnostic::Warning;
+      
+    Result = Diagnostic::Warning;
+    if (WarningsAsErrors)
+      Result = Diagnostic::Error;
+    break;
+  case diag::MAP_WARNING_NO_WERROR:
+    // Diagnostics specified with -Wno-error=foo should be set to warnings, but
+    // not be adjusted by -Werror or -pedantic-errors.
+    Result = Diagnostic::Warning;
+      
+    // If warnings are globally mapped to ignore or error, do it.
+    if (IgnoreAllWarnings)
+      return Diagnostic::Ignored;
+      
     break;
   }
 
diff --git a/test/Misc/diag-mapping2.c b/test/Misc/diag-mapping2.c
new file mode 100644
index 0000000..7e0d774
--- /dev/null
+++ b/test/Misc/diag-mapping2.c
@@ -0,0 +1,19 @@
+// This should warn by default.
+// RUN: clang-cc %s 2>&1 | grep "warning:" &&
+
+// This should not emit anything.
+// RUN: clang-cc %s -w 2>&1 | not grep diagnostic &&
+// RUN: clang-cc %s -Wno-#warnings 2>&1 | not grep diagnostic &&
+
+// -Werror can map all warnings to error.
+// RUN: clang-cc %s -Werror 2>&1 | grep "error:" &&
+
+// -Werror can map this one warning to error.
+// RUN: clang-cc %s -Werror=#warnings 2>&1 | grep "error:" &&
+
+// -Wno-error= overrides -Werror.  rdar://3158301
+// RUN: clang-cc %s -Werror -Wno-error=#warnings 2>&1 | grep "warning:"
+
+#warning foo
+
+
diff --git a/tools/clang-cc/Warnings.cpp b/tools/clang-cc/Warnings.cpp
index ccaa653..818e774 100644
--- a/tools/clang-cc/Warnings.cpp
+++ b/tools/clang-cc/Warnings.cpp
@@ -138,7 +138,7 @@
       }
       
       // -Werror=foo maps foo to Error, -Wno-error=foo maps it to Warning.
-      Mapping = isPositive ? diag::MAP_ERROR : diag::MAP_WARNING;
+      Mapping = isPositive ? diag::MAP_ERROR : diag::MAP_WARNING_NO_WERROR;
       OptStart = Specifier;
     }