Project typeclasses over collections?

gistfile1.scala

import scala.collection.immutable._

object TypeclassProjection {

  trait Show[T] {
    def show(t: T): String
  }
  object Show {
    def apply[T](showFn: T => String): Show[T] = new Show[T] { def show(t: T): String = showFn(t) }
  }

  def executeShow[T:Show](t: T): String = implicitly[Show[T]].show(t)

  implicit val showInt: Show[Int] = Show(i => i.toString)

  executeShow(42)  // works OK

  // everything below fails:

  // attempt 1
  implicit def showSeqA[T:Show](ss: Seq[T]): Show[Seq[T]] = 
    Show(_.map(implicitly[Show[T]].show).mkString(", "))

  // attempt 2
  implicit def showSeqB[A <% T, T:Show](ss: Seq[A]): Show[Seq[A]] =
    Show(_.map(_.asInstanceOf[T]).map(implicitly[Show[T]].show).mkString(", "))

  executeShow(Seq(19,42))  // can't perform typeclass lookup

}