Attempt to use kind-projector to support @mandubian's use case as presented here: https://gist.github.com/mandubian/55f8b1fb9f9bec2c0a99

xyz.scala

package xyz

import language.higherKinds
import scalaz._
import Scalaz._
import scala.concurrent.Future

// see https://github.com/non/kind-projector for more info about what is going on here

object Test {

  // consumes a type *
  def type0[U]: Int = 0

  // consumes a type * -> *
  def type1[U[_]]: Int = 1

  // OptionT part
  type1[({ type L[B] = OptionT[Future, B] })#L]
  type1[OptionT[Future, ?]]

  // ListT part
  type1[({ type M[A] = ListT[({ type L[+B] = OptionT[Future, B] })#L, A] })#M]
  type1[({ type M[A] = ListT[Lambda[+[B] => OptionT[Future, B]], A] })#M]
  type1[({ type M[A] = ListT[OptionT[Future, +?], A] })#M]
  type1[Lambda[A => ListT[OptionT[Future, +?], A]]]
  type1[ListT[OptionT[Future, +?], ?]]

  // MonadPlus part
  type0[MonadPlus[ListT[OptionT[Future, +?], ?]]]
}

I think this is the first "in-the-wild" use of the covariance/contravariance support I added awhile ago.