Type bounds for unapplied type parameters

Question

How can I specify the bounds of an unapplied higher order type parameter, such that the following compiles:

trait Declr[Impl[_]]   // need to specify constraints for Impl's type parameter

trait Sys[S <: Sys[S]]

object X extends Declr[X]  // doesn't compile, because S is more strict
trait X[S <: Sys[S]]

Without adding more type parameters to Declr. Note that a solution is allowed to assume that the bound of Impl's parameter is always S <: Sys[S] (i.e that is the bound for any type that can go into Declr).

I think I can write the constraint with a type member like this:

trait Declr {
  type Impl[S <: Sys[S]]
}

object X extends Declr {
  type Impl[S <: Sys[S]] = X[S]
}
trait X[S <: Sys[S]]

But how to do this as a type parameter (as the restatement of type Impl is very noisy, and I want to have object X as clean as possible)?

Answer 1

Why not

trait Declr[Impl[S <: Sys[S]]]
trait Sys[ S <: Sys[ S ]]

object X extends Declr[ X ]  
trait X[ S <: Sys[ S ]]