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);
   }