code: func (g *oneofGroup) onWritingAlternative(alt *oneofAlternative, msg reflect.Value) reflect.Value {
  	wrap := reflect.New(alt.outerTyp.Elem()) // ~ wrap = &testprotos.Msg_FieldA{}
  	msg.Field(g.fieldIdx).Set(wrap)          // ~ msg.Wrapper = wrap
  	return wrap.Elem().Field(0)              // ~ ref to wrap.A, to be set to
  }
docs: // onWritingAlternative is called when writing a value to the proto message.
  //
  // It instantiates the oneof wrapping struct of a correct type (based on the
  // picked alternative) and returns a reference to the value to be set there.