nope.scala

// uncomment variance annotations -> compiler error
trait IdentityCompose[:=>[/* - */_, /* + */_]] {
  def identity[A]: A :=> A

  def compose[A, B, C](bc: B :=> C, ab: A :=> B): A :=> C
}

case class ToFrom[A, B](to: A => B, fr: B => A)

object ToFrom {
  def ToFromIdentityCompose: IdentityCompose[ToFrom] =
    new IdentityCompose[ToFrom] {
      def identity[A]: A ToFrom A =
        ToFrom(a => a, a => a)

      def compose[A, B, C](bc: B ToFrom C, ab: A ToFrom B): A ToFrom C =
        ToFrom(bc.to compose ab.to, bc.fr andThen ab.fr)
    }
}