Implement unions as members of structs.

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 a8f95b2300bdb8143a8adecbb89afa9345f75830..c8810a6469baad436515e8016adc0b978ea937a6 100644
--- a/mojo/public/cpp/bindings/lib/bindings_internal.h
+++ b/mojo/public/cpp/bindings/lib/bindings_internal.h
@@ -76,27 +76,76 @@ struct IsHandle {
   enum { value = IsBaseOf<Handle, H>::value };
 };
 
-template <typename T, bool move_only = IsMoveOnlyType<T>::value>
+template <typename T>
+struct IsUnionDataType {
+  template <typename U>
+  static YesType Test(const typename U::MojomUnionDataType*);
+
+  template <typename U>
+  static NoType Test(...);
+
+  static const bool value =
+      sizeof(Test<T>(0)) == sizeof(YesType) && !IsConst<T>::value;
+};
+
+template <typename T>
+struct IsWrappedUnionPtr {
+  static const bool value = false;
+};
+
+template <typename S>
+struct IsWrappedUnionPtr<StructPtr<S>> {

[yzshen1, 2015/02/25 21:07:00]

I feel that you could use IsUnionDataType and WrapperTraits  to implement this
IsWrappedUnionPtr, instead of doing the overloaded Test() trick again.

[azani, 2015/03/03 00:44:15]

Done.

+  template <typename U>
+  static YesType Test(const typename U::Data_::MojomUnionDataType*);
+
+  template <typename U>
+  static NoType Test(...);
+
+  static const bool value = sizeof(Test<S>(0)) == sizeof(YesType);
+};
+
+template <typename S>
+struct IsWrappedUnionPtr<InlinedStructPtr<S>> {

[yzshen1, 2015/02/25 21:07:00]

ditto.

[azani, 2015/03/03 00:44:15]

Done.

+  template <typename U>
+  static YesType Test(const typename U::Data_::MojomUnionDataType*);
+
+  template <typename U>
+  static NoType Test(...);
+
+  static const bool value = sizeof(Test<S>(0)) == sizeof(YesType);
+};
+
+template <typename T,
+          bool move_only = IsMoveOnlyType<T>::value,
+          bool is_union_wrapper = IsWrappedUnionPtr<T>::value>
 struct WrapperTraits;
 
 template <typename T>
-struct WrapperTraits<T, false> {
+struct WrapperTraits<T, false, false> {
   typedef T DataType;
 };
 template <typename H>
-struct WrapperTraits<ScopedHandleBase<H>, true> {
+struct WrapperTraits<ScopedHandleBase<H>, true, false> {
   typedef H DataType;
 };
 template <typename S>
-struct WrapperTraits<StructPtr<S>, true> {
+struct WrapperTraits<StructPtr<S>, true, false> {
   typedef typename S::Data_* DataType;
 };
 template <typename S>
-struct WrapperTraits<InlinedStructPtr<S>, true> {
+struct WrapperTraits<InlinedStructPtr<S>, true, false> {
   typedef typename S::Data_* DataType;
 };
 template <typename S>
-struct WrapperTraits<S, true> {
+struct WrapperTraits<StructPtr<S>, true, true> {
+  typedef typename S::Data_ DataType;

[yzshen1, 2015/02/25 21:07:00]

I looked at array_serialization, and didn't see why we need it to be S::Data_
instead of S::Data_* (but I didn't look very hard). Would you please point me to
it?

[azani, 2015/03/03 00:44:15]

Done.

+};
+template <typename S>
+struct WrapperTraits<InlinedStructPtr<S>, true, true> {
+  typedef typename S::Data_ DataType;
+};
+template <typename S>
+struct WrapperTraits<S, true, false> {
   typedef typename S::Data_* DataType;
 };