Minimised scalac crash.

Test.scala

import scalaz._
import Scalaz._
import scalaz.effect._

object Test {
  implicit def eitherTMonadIO[M[_], E](implicit M: MonadIO[M]): MonadIO[({type l[A]=EitherT[M, E, A]})#l] = ???

  def x = for {
    _ <- true whenM IO.putStrLn("World").liftIO[({type e[A]=EitherT[IO, String, A]})#e]
  } yield ()
}

/*
scala> :l Test.scala
Loading Test.scala...
import scalaz._
import Scalaz._
import scalaz.effect._
error: type mismatch;
 found   : scalaz.EitherT[scalaz.effect.IO,String,Unit]
 required: ?M
Note that implicit conversions are not applicable because they are ambiguous:
 both method ToBitraverseVFromKleisliLike in trait ToBitraverseOps of type [G[_], F[G[_], _, _], A, B](v: F[G,A,B])(implicit F0: scalaz.Bitraverse[[α, β]F[G,α,β]])scalaz.syntax.BitraverseOps[[α, β]F[G,α,β],A,B]
 and method ToArrowVFromKleisliLike in trait ToArrowOps of type [G[_], F[G[_], _, _], A, B](v: F[G,A,B])(implicit F0: scalaz.Arrow[[α, β]F[G,α,β]])scalaz.syntax.ArrowOps[[α, β]F[G,α,β],A,B]
 are possible conversion functions from scalaz.EitherT[scalaz.effect.IO,String,Unit] to ?M
*/

Here's a related followup: https://issues.scala-lang.org/browse/SI-8286?focusedCommentId=68190&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-68190