Mojo C++ bindings: support enum validation.

mojo/public/cpp/bindings/lib/bindings_internal.h

Index: mojo/public/cpp/bindings/lib/bindings_internal.h
diff --git a/mojo/public/cpp/bindings/lib/bindings_internal.h b/mojo/public/cpp/bindings/lib/bindings_internal.h
index 6d2562f1b585fa710d9cf892776c721f9b29f4dc..6786d4dfc828390a0713bcec1dc192e16b552564 100644
--- a/mojo/public/cpp/bindings/lib/bindings_internal.h
+++ b/mojo/public/cpp/bindings/lib/bindings_internal.h
@@ -94,6 +94,8 @@ struct IsHandle {
 
 template <typename T>
 struct IsUnionDataType {
+  typedef char (*EnsureTypeIsComplete)[sizeof(T)];

[Ken Rockot(use gerrit already), 2016/01/26 15:39:20]

nit: any reason not to use a c++11 alias here and below? just convention?

[yzshen1, 2016/01/26 18:16:04]

Done. I also refactored a little bit. Thanks!

+
   template <typename U>
   static YesType Test(const typename U::MojomUnionDataType*);
 
@@ -104,6 +106,20 @@ struct IsUnionDataType {
       sizeof(Test<T>(0)) == sizeof(YesType) && !IsConst<T>::value;
 };
 
+template <typename T>
+struct IsEnumDataType {
+  typedef char (*EnsureTypeIsComplete)[sizeof(T)];
+
+  template <typename U>
+  static YesType Test(const typename U::MojomEnumDataType*);
+
+  template <typename U>
+  static NoType Test(...);
+
+  static const bool value =
+      sizeof(Test<T>(0)) == sizeof(YesType) && !IsConst<T>::value;
+};
+
 template <typename T, bool move_only = IsMoveOnlyType<T>::value>
 struct WrapperTraits;