A limitation of Scala 2.8's type constructor inference that could be solved by Higher Order Unification

higher-order-unification.scala

object test {
  trait Bar[M[_]]
  def baz[M[_] : Bar, A](m: M[A]) = 0
  implicit object ListBar extends Bar[List]
  baz(List(1))
  type EitherIntX[X] = Either[Int, X]
  implicit object EitherIntXBar extends Bar[EitherIntX]

  val e: Either[Int, String] = Left(1)
  //baz(e)
  //<console>:19: error: type mismatch;
  // found   : Either[Int,String]
  // required: ?M[ ?A ]
  //Note that implicit conversions are not applicable because they are ambiguous:
  // both method any2ArrowAssoc in object Predef of type [A](x: A)ArrowAssoc[A]
  // and method any2Ensuring in object Predef of type [A](x: A)Ensuring[A]
  // are possible conversion functions from Either[Int,String] to ?M[ ?A ]
  //       baz(e)
  //          ^

  baz[EitherIntX, String](e)
}