sealed trait MyType
object Type1 extends MyType
object Type2 extends MyType
case class Type3(i:Int) extends MyType

def show[A :< MyType](a: A)(implicit sh: Show[A]) = sh.show(a) // Will not compile

implicit val type3CanShow: Show[Type3] =
new Show[Type3] {
  def show(t: Type3): String = s"show ${t.i}"
}

def show[A :< MyType](a: A)(implicit sh: Show[A]) =
  a match {
     case t: Type1 =>  sh.show(t)
     case t: Type2 =>  sh.show(t)
     case t: Type3 =>  sh.show(t)
  }