A type inferencer bug?

Hi,

I've run into a strange typing issue with an HList implementation. It's
not the first issue I found and had to work around but this one looks
like a bug rather than a limitation of the type system or the type
inferencer. I have a Church encoding for natural numbers:

sealed trait Nat {
type Fold[U, F[_ <: U] <: U, Z <: U] <: U
}

final object Zero extends Nat {
type Fold[U, F[_ <: U] <: U, Z <: U] = Z
}

final class Succ[N <: Nat] extends Nat {
type Fold[U, F[_ <: U] <: U, Z <: U] = F[N#Fold[U, F, Z]]
}

And an HList which defines Drop and Apply operations using those numbers:

trait HList {
type Head
type Tail <: HList
type Drop[N <: Nat] = N#Fold[HList, ({ type L[X <: HList] =
X#Tail })#L, this.type]
type Apply[N <: Nat] = Drop[N]#Head
}

class #: [H, T <: HList] extends HList { type Head = H; type Tail = T }

object HNil extends HList { type Head = Nothing; type Tail = Nothing }

Testing the implementation of Drop:

val l1 = null: Int #: String #: Boolean #: String #: HNil.type

type _2 = Succ[Succ[Zero.type]]

val t1: Boolean = null.asInstanceOf[ l1.type#Drop[_2]#Head ]

Until this point everything works fine. Now I try to use Apply[N]
instead of Drop[N]#Head (which should be the exact same thing) and it fails:

HLTest.scala:31: error: type mismatch;
found : HLTest.l1.Apply[HLTest._2]
required: Boolean
val t2: Boolean = null.asInstanceOf[ l1.type#Apply[_2] ]
^

It seems that a type xs#Apply[...] is always resolved to xs#Head,
completely ignoring the Drop[N] part. Am I doing something wrong or is
this a bug? I get the same results on 2.9.1 and on master.

Cheers,
Stefan