type parameters and type fields

proxy.scala

trait Service {

  type A
  type B

  def getA:Option[A]
  def getB:Option[B]

}

trait SService[AA, BB] extends Service {
  type A = AA
  type B = BB
}

abstract class AbstractService[AA, BB <: String](x:String) extends SService[AA, BB] {
  override def getA: Option[A] = None

  override def getB: Option[B] = None
}

trait ServiceProxy[M <: Service {
  type A
  type B <: String
}] {
  type proxy = AbstractService[M#A, M#B]
}

trait ServiceWrapper[M[_, _ <: String]] {
  type wrapped[A, B <: String] = M[A, B]
}

object test {
  
  type StringService = SService[String, String]
  
}


object Main extends App {

  type ToS = ServiceProxy[StringService]#proxy
  type ToW = ServiceWrapper[AbstractService]#wrapped[String, String]

  class S extends ToS("test") {

  }

  class T[A, B <: String] extends AbstractService[A, B]("test")

  val l = List[StringService](new S)
  println(l)
  println(l.head.isInstanceOf[AbstractService[String, String]])

}