presence.scala

scala> trait Presence {
     |   type Apply[_]
     | }
defined trait Presence

scala>

scala> trait Yes extends Presence {
     |   type Apply[A] = A
     | }
defined trait Yes

scala>

scala> trait No extends Presence {
     |   type Apply[A] = Unit
     | }
defined trait No

scala>

scala> trait Optional extends Presence {
     |   type Apply[A] = Option[A]
     | }
defined trait Optional

scala>

scala> case class Blurb[AIsPresent <: Presence, BIsPresent <: Presence](a: AIsPresent#Apply[Int], b: BIsPresent#Apply[String])
defined class Blurb

scala> Blurb[Yes, Yes](2, "ola")
res1: Blurb[Yes,Yes] = Blurb(2,ola)

scala> Blurb[Yes, No](2, ())
res2: Blurb[Yes,No] = Blurb(2,())

scala> Blurb[Optional, No](Some(3), ())
res3: Blurb[Optional,No] = Blurb(Some(3),())