Weird recursive type parameter definitions

[s5bug]

I was thinking about how to build a Tuple/HList more friendly to various subtyping restrictions. I settled on

// An HList is defined by a function on an HList
sealed trait HList[+F[x <: HList[F]] <: Any]

// HNil can support any bound
type Maximal[x <: HList[Maximal]] = Nothing
case object HNil extends HList[Maximal]

// HCons's head element is bound by the function applied to the tail
case class HCons[+F[x <: HList[F]], +H <: F[T], +T <: HList[F]](h: H, t: T) extends HList[F]

as a first attempt. This would let one emulate the normal tuple like so:

type Minimal[x <: HList[Minimal]] = Any
type Tuple = HList[Minimal]
type EmptyHuple = HNil.type
type *:[+H, +T <: Tuple] = HCons[Minimal, H, T]

or a Tuple where all items have an upper bound:

type Bounded[U, x <: HList[Bounded[U, *]]] = U
type BoundedTuple[U] = HList[Bounded[U, *]]
type EmptyBoundedTuple = HNil.type
type *<:[U, +H <: U, +T <: BoundedTuple[U]] = HCons[Bounded[U, *], H, T]

My further goal however was to support weird ways to restrict the head of an HList type based on the tail:

type FoodChain[x <: HList[FoodChain]] = x match {
  case HNil.type => Fruit | Vegetable
  case HCons[FoodChain, Fruit, t] => Fly | Pangolin
  case HCons[FoodChain, Fly, t] => Frog
}

summon[HNil.type <:< HList[FoodChain]]
summon[HCons[FoodChain, Fruit, HNil.type] <:< HList[FoodChain]]
summon[HCons[FoodChain, Frog, HCons[FoodChain, Fly, HCons[FoodChain, Fruit, HNil.type]]] <:< HList[FoodChain]]

Of course, Scala is not a fan of the weird recursion/inner-fbounding that I would do to make this work. (That is to say, the Scala compiler doesn't support this kind of typing.)

The question was brought up to me of whether or not this sort of recursive typing would still be sound. My assumption for why Scala doesn't support it was because it seems like an absolute pain to handle, but I don't know enough about F-bounds or related topics to even begin to figure out whether or not this would be sound or unsound.