Index: include/clang/Basic/DiagnosticIDs.h
===================================================================
--- include/clang/Basic/DiagnosticIDs.h
+++ include/clang/Basic/DiagnosticIDs.h
@@ -124,8 +124,8 @@
   DiagnosticIDs();
   ~DiagnosticIDs();
 
-  /// \brief Return an ID for a diagnostic with the specified format string and
-  /// level.
+  /// \brief Return an ID for a diagnostic with the specified format string,
+  /// level and warning option.
   ///
   /// If this is the first request for this diagnostic, it is registered and
   /// created, otherwise the existing ID is returned.
@@ -133,7 +133,8 @@
   // FIXME: Replace this function with a create-only facilty like
   // createCustomDiagIDFromFormatString() to enforce safe usage. At the time of
   // writing, nearly all callers of this function were invalid.
-  unsigned getCustomDiagID(Level L, StringRef FormatString);
+  unsigned getCustomDiagID(Level L, StringRef FormatString,
+                           StringRef WarningOption = "");
 
   //===--------------------------------------------------------------------===//
   // Diagnostic classification and reporting interfaces.
@@ -142,6 +143,16 @@
   /// \brief Given a diagnostic ID, return a description of the issue.
   StringRef getDescription(unsigned DiagID) const;
 
+  /// \brief Return the lowest-level warning option that enables the specified
+  /// diagnostic.
+  ///
+  /// This method is similar to getWarningOptionForDiag, but it also handles
+  /// custom diagnostic DiagIDs.
+  ///
+  /// If there is no -Wfoo flag that controls the diagnostic, this returns an
+  /// empty string.
+  StringRef getWarningOption(unsigned DiagID) const;
+
   /// \brief Return true if the unmapped diagnostic levelof the specified
   /// diagnostic ID is a Warning or Extension.
   ///
Index: lib/Basic/DiagnosticIDs.cpp
===================================================================
--- lib/Basic/DiagnosticIDs.cpp
+++ lib/Basic/DiagnosticIDs.cpp
@@ -256,7 +256,25 @@
 namespace clang {
   namespace diag {
     class CustomDiagInfo {
-      typedef std::pair<DiagnosticIDs::Level, std::string> DiagDesc;
+      struct DiagDesc {
+        DiagnosticIDs::Level Level;
+        std::string Description;
+        std::string WarningOption;
+        DiagDesc(DiagnosticIDs::Level Level, std::string Description,
+                 std::string WarningOption)
+            : Level(Level), Description(Description),
+              WarningOption(WarningOption) {}
+        bool operator==(const DiagDesc &RHS) const {
+          return Level == RHS.Level && Description == RHS.Description &&
+                 WarningOption == RHS.WarningOption;
+        }
+        bool operator<(const DiagDesc &RHS) const {
+          return Level < RHS.Level ||
+                 (Level == RHS.Level && Description < RHS.Description) ||
+                 (Level == RHS.Level && Description == RHS.Description &&
+                  WarningOption < RHS.WarningOption);
+        }
+      };
       std::vector<DiagDesc> DiagInfo;
       std::map<DiagDesc, unsigned> DiagIDs;
     public:
@@ -266,19 +284,27 @@
       StringRef getDescription(unsigned DiagID) const {
         assert(this && DiagID-DIAG_UPPER_LIMIT < DiagInfo.size() &&
                "Invalid diagnostic ID");
-        return DiagInfo[DiagID-DIAG_UPPER_LIMIT].second;
+        return DiagInfo[DiagID-DIAG_UPPER_LIMIT].Description;
       }
 
       /// getLevel - Return the level of the specified custom diagnostic.
       DiagnosticIDs::Level getLevel(unsigned DiagID) const {
         assert(this && DiagID-DIAG_UPPER_LIMIT < DiagInfo.size() &&
                "Invalid diagnostic ID");
-        return DiagInfo[DiagID-DIAG_UPPER_LIMIT].first;
+        return DiagInfo[DiagID-DIAG_UPPER_LIMIT].Level;
       }
 
+      /// getLevel - Return the warning option of the specified custom
+      /// diagnostic.
+      StringRef getWarningOption(unsigned DiagID) const {
+        assert(this);
+        assert(DiagID - DIAG_UPPER_LIMIT < DiagInfo.size() &&
+               "Invalid diagnostic ID");
+        return DiagInfo[DiagID-DIAG_UPPER_LIMIT].WarningOption;
+      }
       unsigned getOrCreateDiagID(DiagnosticIDs::Level L, StringRef Message,
-                                 DiagnosticIDs &Diags) {
-        DiagDesc D(L, Message);
+                                 StringRef WarningOption) {
+        DiagDesc D(L, Message, WarningOption);
         // Check to see if it already exists.
         std::map<DiagDesc, unsigned>::iterator I = DiagIDs.lower_bound(D);
         if (I != DiagIDs.end() && I->first == D)
@@ -314,12 +340,19 @@
 ///
 /// \param FormatString A fixed diagnostic format string that will be hashed and
 /// mapped to a unique DiagID.
-unsigned DiagnosticIDs::getCustomDiagID(Level L, StringRef FormatString) {
+unsigned DiagnosticIDs::getCustomDiagID(Level L, StringRef FormatString,
+                                        StringRef WarningOption /*= ""*/) {
   if (CustomDiagInfo == 0)
     CustomDiagInfo = new diag::CustomDiagInfo();
-  return CustomDiagInfo->getOrCreateDiagID(L, FormatString, *this);
+  return CustomDiagInfo->getOrCreateDiagID(L, FormatString, WarningOption);
 }
 
+StringRef DiagnosticIDs::getWarningOption(unsigned DiagID) const {
+  if (DiagID >= diag::DIAG_UPPER_LIMIT)
+    return CustomDiagInfo->getWarningOption(DiagID);
+
+  return getWarningOptionForDiag(DiagID);
+}
 
 /// isBuiltinWarningOrExtension - Return true if the unmapped diagnostic
 /// level of the specified diagnostic ID is a Warning or Extension.