diff --git a/clang/lib/Basic/DiagnosticIDs.cpp b/clang/lib/Basic/DiagnosticIDs.cpp
--- a/clang/lib/Basic/DiagnosticIDs.cpp
+++ b/clang/lib/Basic/DiagnosticIDs.cpp
@@ -356,42 +356,56 @@
 
 namespace clang {
   namespace diag {
-    class CustomDiagInfo {
-      typedef std::pair<DiagnosticIDs::Level, std::string> DiagDesc;
-      std::vector<DiagDesc> DiagInfo;
-      std::map<DiagDesc, unsigned> DiagIDs;
-    public:
-
-      /// getDescription - Return the description of the specified custom
-      /// diagnostic.
-      StringRef getDescription(unsigned DiagID) const {
-        assert(DiagID - DIAG_UPPER_LIMIT < DiagInfo.size() &&
-               "Invalid diagnostic ID");
-        return DiagInfo[DiagID-DIAG_UPPER_LIMIT].second;
-      }
-
-      /// getLevel - Return the level of the specified custom diagnostic.
-      DiagnosticIDs::Level getLevel(unsigned DiagID) const {
-        assert(DiagID - DIAG_UPPER_LIMIT < DiagInfo.size() &&
-               "Invalid diagnostic ID");
-        return DiagInfo[DiagID-DIAG_UPPER_LIMIT].first;
-      }
-
-      unsigned getOrCreateDiagID(DiagnosticIDs::Level L, StringRef Message,
-                                 DiagnosticIDs &Diags) {
-        DiagDesc D(L, std::string(Message));
-        // Check to see if it already exists.
-        std::map<DiagDesc, unsigned>::iterator I = DiagIDs.lower_bound(D);
-        if (I != DiagIDs.end() && I->first == D)
-          return I->second;
-
-        // If not, assign a new ID.
-        unsigned ID = DiagInfo.size()+DIAG_UPPER_LIMIT;
-        DiagIDs.insert(std::make_pair(D, ID));
-        DiagInfo.push_back(D);
-        return ID;
-      }
-    };
+  using DiagDesc = std::pair<DiagnosticIDs::Level, std::string>;
+  struct DiagDescRef {
+    DiagDescRef(DiagnosticIDs::Level Level, StringRef FormatString)
+        : Level(Level), FormatString(FormatString) {}
+    DiagDescRef(const DiagDesc &D) : DiagDescRef(D.first, D.second) {}
+    DiagnosticIDs::Level Level;
+    StringRef FormatString;
+  };
+  static bool operator<(const DiagDesc &LHS, const DiagDescRef &RHS) {
+    if (LHS.first == RHS.Level)
+      return LHS.second < RHS.FormatString;
+    return LHS.first < RHS.Level;
+  }
+  class CustomDiagInfo {
+    std::vector<DiagDescRef> DiagInfo;
+    std::map<DiagDesc, unsigned, std::less<>> DiagIDs;
+
+  public:
+    /// getDescription - Return the description of the specified custom
+    /// diagnostic.
+    StringRef getDescription(unsigned DiagID) const {
+      assert(DiagID - DIAG_UPPER_LIMIT < DiagInfo.size() &&
+             "Invalid diagnostic ID");
+      return DiagInfo[DiagID - DIAG_UPPER_LIMIT].FormatString;
+    }
+
+    /// getLevel - Return the level of the specified custom diagnostic.
+    DiagnosticIDs::Level getLevel(unsigned DiagID) const {
+      assert(DiagID - DIAG_UPPER_LIMIT < DiagInfo.size() &&
+             "Invalid diagnostic ID");
+      return DiagInfo[DiagID - DIAG_UPPER_LIMIT].Level;
+    }
+
+    unsigned getOrCreateDiagID(DiagnosticIDs::Level L, StringRef Message,
+                               DiagnosticIDs &Diags) {
+      // Check to see if it already exists.
+
+      auto I = DiagIDs.lower_bound(DiagDescRef(L, Message));
+      if (I != DiagIDs.end() && I->first.first == L &&
+          I->first.second == Message)
+        return I->second;
+
+      // If not, assign a new ID.
+      unsigned ID = DiagInfo.size() + DIAG_UPPER_LIMIT;
+      auto InsertRet =
+          DiagIDs.insert(std::make_pair(DiagDesc{L, Message.str()}, ID));
+      DiagInfo.emplace_back(InsertRet.first->first);
+      return ID;
+    }
+  };
 
   } // end diag namespace
 } // end clang namespace