Combine two partial functions narrowing domain

combined.sc

import scala.util.{Success, Try}

val pf1: PartialFunction[Try[String], String] = {
  case Success(s) => s
}

val pf2: PartialFunction[String, String] = {
  case s if s.contains("pass") => s
}

class Combined[-A, B, +C] (pf: PartialFunction[A, B], k: PartialFunction[B, C]) extends PartialFunction[A, C] with Serializable {
  import Combined._

  def isDefinedAt(x: A) = {
    val b: B = pf.applyOrElse(x, checkFallback[B])
    if (!fallbackOccurred(b)) k.isDefinedAt(b) else false
  }

  def apply(x: A): C = k(pf(x))

  override def applyOrElse[A1 <: A, B1 >: C](x: A1, default: A1 => B1) = {
    val pfv = pf.applyOrElse(x, checkFallback[B])
    if (!fallbackOccurred(pfv)) k.applyOrElse(pfv, (_: B) => default(x)) else default(x)
  }
}

object Combined {
  private[this] val fallback_pf: PartialFunction[Any, Any] = { case _ => fallback_pf }
  private def checkFallback[B] = fallback_pf.asInstanceOf[PartialFunction[Any, B]]
  private def fallbackOccurred[B](x: B) = fallback_pf eq x.asInstanceOf[AnyRef]
}

implicit class PartialFunctionCombine[A, B](pf: PartialFunction[A, B]) {
  def combine[C](pf2: PartialFunction[B, C]): Combined[A, B, C] = new Combined(pf, pf2)
}

val pf = pf1 combine pf2

println(pf.applyOrElse(Try("reject"), (_: Try[String]) => "not defined"))
println(pf.applyOrElse(Try("pass"), (_: Try[String]) => "not defined"))