Open source protobuf messages derive from google::protobuf::MessageLite.

GTest enables ProtbufPrinter based on:

template <typename T>
struct IsAProtocolMessage
   : public std::is_convertible<const T*, const ::proto2::MessageLite*> {};

So open-source protobufs are printed as bytes.

This was previously closed as "inactive": #1102

The issue still occurs at current master.