Contravariance + Implcicits kills kittens

contravariant-implicit-priority.scala

trait FoldRight[-M[_]]

{
  implicit val StreamFoldRight: FoldRight[Stream] = new FoldRight[Stream] {}
  implicit val IterableFoldRight: FoldRight[Iterable] = new FoldRight[Iterable] {}

  assert(implicitly[FoldRight[Stream]] eq IterableFoldRight) // want StreamFoldRight
}

{
  abstract class Low {
    implicit val IterableFoldRight: FoldRight[Iterable] = new FoldRight[Iterable] {}
  }
  object FoldRight extends Low {
    implicit val StreamFoldRight: FoldRight[Stream] = new FoldRight[Stream] {}
  }
  import FoldRight._
  //assert(implicitly[FoldRight[Stream]] eq IterableFoldRight) 
  // UNCOMMENT FOR:
  //  error: ambiguous implicit values:
}