Type class constraint in extension classes.

extensions.scala

trait Ops[A] {
    def typeClassInstance: Show[A]
    def self: A
    def show: String = typeClassInstance.show(self)
  }

  trait ToShowOps {
    implicit def toShow[A](target: A)(implicit tc: Show[A]): Ops[A] = new Ops[A] {
      val self = target
      val typeClassInstance = tc
    }
  }