ConstraintsOnAssociativeType.scala

case class Hoge()
case class Foo()
case class Dummy()

trait C[A, Conv] {
  def method(a: A): Conv
}

trait Show[A] { def format(a: A): String }

object InstanceDeclarations {
  implicit object HogeInstance extends C[Hoge, String] {
    def method(a: Hoge): String = "Hoge"
  }
  implicit object FooInstance extends C[Foo, Dummy] {
    def method(a: Foo): Dummy = Dummy()
  }
  implicit object ShowString extends Show[String] {
    def format(a: String): String  = a
  }
}

import InstanceDeclarations._

object ConstraintsOnAssociativeType {
  def func[S, Conv](s: S)(implicit cs: C[S, Conv], show: Show[Conv]): String = show.format(cs.method(s))
  println(func(Hoge()))
  //println(func(Foo()))
}