Inject + PDTs

Inject.scala

import scalaz.{Coproduct, Inject}

trait X { self ⇒
  type O[Z]
  def :+:(that: X): X { type O[Z] = Coproduct[that.O, self.O, Z] } =
    new X { type O[Z] = Coproduct[that.O, self.O, Z] }
}
object X {
  def apply[F[_]]: X { type O[Z] = F[Z] } =
    new X { type O[Z] = F[Z] }
}

trait A[Z]
trait B[Z]
trait C[Z]

object App extends App {
  type O1[Z] = Coproduct[B, C, Z]
  type O[Z] = Coproduct[A, O1, Z]

  val x = X[A] :+: X[B] :+: X[C]
  implicitly[O[Int] =:= x.O[Int]] // Fine
  Inject[A, O] // Fine
  Inject[A, x.O] // Fails; could not find implicit value for parameter I: Inject[A, x.0]
}