ExistantialTest.scala

object ExistentialTest {

  case class Box[T]()
  case class Pair[T](a: Box[T], b: Box[T])

  val pair: Pair[_] = ???

  val pair2 = Pair(pair.a, pair.b)
  
}

    object ExistentialTest {

    case class Box[T]()

    case class Pair[F[x] <: Box[x], T](a: F[T], b: F[T]) {
      type Inner = T
    }


    val pair = Pair(Box[Int], Box[Int])

    val pair2 = Pair(pair.a, pair.b)

  }

object ExistentialTest {
    import scala.language.existentials


    case class Box[T](value: T)
    case class Pair(a: Box[T] forSome { type T } , b: Box[T]  forSome { type T } ) {
    }

    val pair: Pair = Pair(Box[Int](42), Box[Int](42))

    val pair2 = Pair(pair.a, pair.b)

  }