gistfile1.scala

  import annotation.implicitNotFound

  @implicitNotFound(msg = "This message can never appear!")
  trait ~>[A, B] { self =>
    def apply(a: A): B
    def invert(b: B): A = inverse.apply(b)
    def inverse: B ~> A = new ~>[B, A] {
      def apply(b: B) = self.invert(b)
      override def invert(a: A) = self(a)
    }
  }

  trait Init[A, B] extends (A ~> B)
  implicit def inverseOf[A, B](implicit forward: Init[A, B]): B ~> A = forward.inverse

  implicit def i2d: Init[Int, Double] = ???

  implicitly[Int ~> String] // "This message can never appear!"
  implicitly[String ~> Int] // "This message can never appear!"

  implicitly[Int ~> Double] 
  implicitly[Double ~> Int] // conjured