function-like.scala

trait Fn[A,B] {
  // AA is contravariant; BB is covariant
  // Similar to Function1[-AA, +BB]
  def apply[AA >: A, BB <: B](t: AA): BB
}

class Sup
class Sub extends Sup

def test[S >: Sub](a: Sub): Fn[S,S] =
  new Fn[S,S] {
    override def apply[AA >: S, BB <: S](b: AA): BB = if (true) a else b
  }