// IsAProtocolMessage<T>::value is a compile-time bool constant that's
// true iff T is type ProtocolMessage, proto2::Message, or a subclass
// of those.
template <typename T>
struct IsAProtocolMessage
    : public bool_constant<
  ImplicitlyConvertible<const T*, const ::ProtocolMessage*>::value ||
  ImplicitlyConvertible<const T*, const ::proto2::Message*>::value> {
};

// When the compiler sees expression IsContainerTest<C>(0), if C is an