Cannot Define Recursive MatchTypes at the Toplevel

[odersky]

Minimized example

trait HList
case object HNil extends HList
case class HCons[H, T <: HList](h: H, t: T) extends HList

  type Concat[Xs, Ys <: HList] <: HList = Xs match
    case HNil.type => Ys
    case HCons[x, xs1] => HCons[x, Concat[xs1, Ys]]

Output

5 |type Concat[Xs <: HList, Ys <: HList] <: HList = Xs match
  |^
  |Cyclic reference involving type Concat

Expectation

Should compile. It works when I place Concat in an object.

[OlivierBlanvillain]

Duplicate of #6362