implicit class X(str: String) {
  def toPositiveNumeric[T >: Int with Long with Float with Double <: AnyVal]
  (implicit ev: T => Ordered[T], ctag: ClassTag[T]): T = ???
}

Array(Array(Int.getClass, "123"), Array(Double.getClass, "123"))
  .foreach { case Array(tpe, str) =>
     assert(str.toPositiveNumeric[tpe] == 123.asInstanceOf[tpe])