Zero-cost, subtyping-free conversion from product type to sum type

Option.scala

sealed trait Option[A]

sealed abstract case class Some[A](value: A) extends Option[A]
sealed abstract case class None[A]()         extends Option[A]

sealed trait Id[A] { self: Option[A] =>
  final def toOption: Option[A] = this
  def value: A
}

object Id {
  def apply[A](value: A) = new Some[A](value) with Id[A] {}

  def unapply[A](id: Id[A]) = new Unapply(id.value)

  final class Unapply[A](val value: A) extends AnyVal {
    def isEmpty = false
    def get     = value
  }
}

object Some {
  def apply[A](value: A): Option[A] = Id(value).toOption
}

object None {
  private[this] val None = new None[Nothing]() {}
  def apply[A]: Option[A] = None.asInstanceOf[Option[A]]
}