TypeInType.scala

sealed trait Data { self =>
  type Type >: self.type <: Data
  type Value
  
  implicitly[Type <:< Type#Type]
  implicitly[self.type <:< Type]
}
sealed trait DataF[T <: DataF[T]] extends Data { self : T =>
  // type Type >: self.type <: DataF[T]
  type Type = T
  
  implicitly[T <:< T#Type]
  implicitly[Type <:< Type#Type]
  implicitly[self.type <:< Type]
  implicitly[self.type <:< T]
}

type Same[A <: Data] = A#Type { type Type = A#Type }

////////////////////////////////////////////////////////////////////////////

sealed trait Nat extends DataF[Nat] { type Value = BigInt }
final class Z private () extends Nat { override type Type = Nat }
final class S[N <: Nat] private () extends Nat { override  type Type = Nat }

implicitly[Nat <:< Nat#Type]

def f[A <: Nat]: Unit = {
  implicitly[A <:< A#Type]
}
f[Nat]
f[Z]
f[S[Z]]

def g[A <: Data]: Unit = {
  implicitly[A <:< A#Type]
}
g[Nat]
g[Z]
g[S[Z]]

def h[A <: Data, B <: Same[A]]: Unit = {
  implicitly[A <:< A#Type]
  implicitly[B <:< B#Type]
  
  implicitly[B <:< A#Type]
  implicitly[A <:< B#Type]
  
  implicitly[A#Type <:< B#Type]
  implicitly[B#Type <:< A#Type]
  implicitly[A#Type =:= B#Type]
}

h[Nat, Nat]
h[Nat, Z]
h[Z, Nat]
h[Z, Z]