This repo illustrates the issue reported here: gogo/protobuf#411

Specifically, message Span contains two fields TraceID and SpanID defined as bytes. They are both backed by custom types TraceID and SpanID, where TraceID is a two-field struct and SpanID is an alias to uint64.

The test ids_test.go does round-trip serialization using proto buffers and JSON. If the method func (s *SpanID) UnmarshalJSONPB at the end of ids.go is commented out or removed, the test fails, even though SpanID implements func (t *T) UnmarshalJSON(data []byte) error {} required for custom types per https://github.com/gogo/protobuf/blob/master/custom_types.md.