diff --git a/lib/Basic/Diagnostic.cpp b/lib/Basic/Diagnostic.cpp
index 8d0d813..4351f66 100644
--- a/lib/Basic/Diagnostic.cpp
+++ b/lib/Basic/Diagnostic.cpp
@@ -203,6 +203,7 @@
   AllExtensionsSilenced = 0;
   IgnoreAllWarnings = false;
   WarningsAsErrors = false;
+  ErrorsAsFatal = false;
   SuppressSystemWarnings = false;
   SuppressAllDiagnostics = false;
   ExtBehavior = Ext_Ignore;
@@ -326,9 +327,13 @@
       return Diagnostic::Ignored;
     Result = Diagnostic::Warning;
     if (ExtBehavior == Ext_Error) Result = Diagnostic::Error;
+    if (Result == Diagnostic::Error && ErrorsAsFatal)
+      Result = Diagnostic::Fatal;
     break;
   case diag::MAP_ERROR:
     Result = Diagnostic::Error;
+    if (ErrorsAsFatal)
+      Result = Diagnostic::Fatal;
     break;
   case diag::MAP_FATAL:
     Result = Diagnostic::Fatal;
@@ -349,6 +354,8 @@
 
     if (WarningsAsErrors)
       Result = Diagnostic::Error;
+    if (Result == Diagnostic::Error && ErrorsAsFatal)
+      Result = Diagnostic::Fatal;
     break;
 
   case diag::MAP_WARNING_NO_WERROR:
@@ -361,6 +368,12 @@
       return Diagnostic::Ignored;
 
     break;
+
+  case diag::MAP_ERROR_NO_WFATAL:
+    // Diagnostics specified as -Wno-fatal-error=foo should be errors, but
+    // unaffected by -Wfatal-errors.
+    Result = Diagnostic::Error;
+    break;
   }
 
   // Okay, we're about to return this as a "diagnostic to emit" one last check:
diff --git a/lib/Frontend/Warnings.cpp b/lib/Frontend/Warnings.cpp
index ff44c90..4d12bcf 100644
--- a/lib/Frontend/Warnings.cpp
+++ b/lib/Frontend/Warnings.cpp
@@ -47,8 +47,6 @@
   else
     Diags.setExtensionHandlingBehavior(Diagnostic::Ext_Ignore);
 
-  // FIXME: -Wfatal-errors / -Wfatal-errors=foo
-
   for (unsigned i = 0, e = Opts.Warnings.size(); i != e; ++i) {
     const std::string &Opt = Opts.Warnings[i];
     const char *OptStart = &Opt[0];
@@ -98,6 +96,31 @@
       OptStart = Specifier;
     }
 
+    // -Wfatal-errors is yet another special case.
+    if (OptEnd-OptStart >= 12 && memcmp(OptStart, "fatal-errors", 12) == 0) {
+      const char* Specifier = 0;
+      if (OptEnd-OptStart != 12) {
+        if ((OptStart[12] != '=' && OptStart[12] != '-') ||
+            OptEnd-OptStart == 13) {
+          fprintf(stderr,
+                  "warning: unknown -Wfatal-errors warning specifier: -W%s\n",
+                  Opt.c_str());
+          continue;
+        }
+        Specifier = OptStart + 13;
+      }
+
+      if (Specifier == 0) {
+        Diags.setErrorsAsFatal(isPositive);
+        continue;
+      }
+
+      // -Wfatal-errors=foo maps foo to Fatal, -Wno-fatal-errors=foo
+      // maps it to Error.
+      Mapping = isPositive ? diag::MAP_FATAL : diag::MAP_ERROR_NO_WFATAL;
+      OptStart = Specifier;
+    }
+
     if (Diags.setDiagnosticGroupMapping(OptStart, Mapping))
       Diags.Report(diag::warn_unknown_warning_option) << ("-W" + Opt);
   }