How to resolve implicitly the companion of a protobuf class. The issue on ScalaPB regarding this proposal is: https://github.com/scalapb/ScalaPB/issues/229

KafkaProtobufEncoder.scala

trait Event

object KafkaEncoder {

  type KafkaProtobufEvent[T <: Event] = GeneratedMessage with Message[T] with Updatable[T] with Event
  type KafkaProtobufEncoder[E <: KafkaProtobufEvent[E]] = GeneratedMessageCompanion[E]

  def apply[T <: KafkaProtobufEvent[T]](implicit enc: KafkaEncoder[T]) = enc

  def serialize[T <: KafkaProtobufEvent[T]](data: T)(implicit companion: KafkaProtobufEncoder[T]): Array[Byte] =
    companion.toByteArray(data)

  def deserealize[T <: KafkaProtobufEvent[T]](bytes: Array[Byte])(implicit companion: KafkaProtobufEncoder[T]): Try[T] =
    Try(companion.parseFrom(bytes))

}

trait KafkaEncoder[T <: KafkaEncoder.KafkaProtobufEvent[T]] {

  val companion: KafkaEncoder.KafkaProtobufEncoder[T]

  def serialize(data: T): Array[Byte] = companion.toByteArray(data)

  def deserealize(bytes: Array[Byte]): Try[T] = Try(companion.parseFrom(bytes))
}

object Test {
  import KafkaEncoder._

  //I thought I was supposed to do this but compiles without it
  //implicit val uc: GeneratedMessageCompanion[TESTCASE] = TESTCASE

  //Implicit instance that defines the companion, which is not necessary thanks to the implicit def messageCompanion
  implicit val testCaseEncoder: KafkaEncoder[TESTCASE] = new KafkaEncoder[TESTCASE] {
    override val companion = TESTCASE
  }

  val byDeclaringImplicitInstance: Array[Byte] = KafkaEncoder[TESTCASE].serialize(TESTCASE())
  //compiles
  val bytes: Array[Byte] = serialize(TESTCASE())
  val testCase: Try[TESTCASE] = deserealize[TESTCASE](bytes)

}

case class TESTCASE() extends GeneratedMessage with Message[TESTCASE] with Updatable[TESTCASE] with Event {
  override def writeTo(output: CodedOutputStream): Unit = ???

  override def getField(field: FieldDescriptor): Any = ???

  override def companion: GeneratedMessageCompanion[_] = ???

  override def serializedSize: Int = ???

  override def mergeFrom(input: CodedInputStream): TESTCASE = ???
}

object TESTCASE extends GeneratedMessageCompanion[TESTCASE] {
  implicit def messageCompanion: GeneratedMessageCompanion[TESTCASE] = this
  
  override def fromFieldsMap(fields: Map[FieldDescriptor, Any]): TESTCASE = ???

  override def descriptor: Descriptor = ???

  override def messageCompanionForField(field: FieldDescriptor): GeneratedMessageCompanion[_] = ???

  override def enumCompanionForField(field: FieldDescriptor): GeneratedEnumCompanion[_] = ???

  override def defaultInstance: TESTCASE = ???
}