Index: include/llvm/Support/YAMLTraits.h
===================================================================
--- include/llvm/Support/YAMLTraits.h
+++ include/llvm/Support/YAMLTraits.h
@@ -863,8 +863,13 @@
     mapOptionalWithContext(Key, Val, Ctx);
   }
 
+  // NB: The std::enable_if is used on the Default argument to prevent this
+  // argument from being used in template parameter deduction. This allows the
+  // default value to be specified without a cast even if it's type does not
+  // exactly match the type of Val.
   template <typename T>
-  void mapOptional(const char *Key, T &Val, const T &Default) {
+  void mapOptional(const char *Key, T &Val,
+                   const typename std::enable_if<true, T>::type &Default) {
     EmptyContext Ctx;
     mapOptionalWithContext(Key, Val, Default, Ctx);
   }
Index: unittests/Support/YAMLIOTest.cpp
===================================================================
--- unittests/Support/YAMLIOTest.cpp
+++ unittests/Support/YAMLIOTest.cpp
@@ -823,7 +823,7 @@
       io.mapRequired("f1", c.f1);
       io.mapRequired("f2", c.f2);
       io.mapRequired("f3", c.f3);
-      io.mapOptional("f4", c.f4, MyFlags(flagRound));
+      io.mapOptional("f4", c.f4, flagRound);
      }
   };
 }
@@ -1327,8 +1327,8 @@
     static void mapping(IO &io, TotalSeconds &secs) {
       MappingNormalization<NormalizedSeconds, TotalSeconds> keys(io, secs);
 
-      io.mapOptional("hours",    keys->hours,    (uint32_t)0);
-      io.mapOptional("minutes",  keys->minutes,  (uint8_t)0);
+      io.mapOptional("hours", keys->hours, 0);
+      io.mapOptional("minutes", keys->minutes, 0);
       io.mapRequired("seconds",  keys->seconds);
     }
   };