Match types unable to match on `Nothing`

Apologies if this is expected behavior. I am trying to constrain the return type of Behavior Response to be Unit instead of Nothing for CounterMsg.Inc extends CounterMsg[Nothing] but it appears I can’t type match on Nothing?

The code works if I instead use the line // type ResponseOf[O] = O | Unit and comment the one with the match type. But it allows Unit type despite O isn’t necessarily Nothing

Compiler version

3.1.3

Minimized code

type ResponseOf[O] = O match
  case Nothing => Unit
  case _       => O

// type ResponseOf[O] = O | Unit

type Behavior[S, -I[_]] = [O] => ((S, I[O]) => (S, ResponseOf[O]))

enum CounterMsg[+Response]:
  case Inc
  case Get extends CounterMsg[Int]

val counterFsm: Behavior[Int, CounterMsg] =
  [O] =>
    (state: Int, msg: CounterMsg[O]) =>
      (msg match
        case CounterMsg.Inc => state + 1 -> ()
        case CounterMsg.Get => state -> state
      ): (Int, ResponseOf[O])

Scastie: https://scastie.scala-lang.org/KG1Q2sbVSUawd18xXk5kXQ

Output

Found:    (state : Int)
Required: Playground.ResponseOf[O]

where:    O is a type in method apply with bounds >: Int

Expectation

Compiles successfully

[odersky]

I think as long as we don’t have a clear issue we should close this.

[prolativ]

I think the compiler might be right to reject this code (although I’m not sure if the error messages could be more informative in such situations in general). If you define CounterMsg as covariant then Inc would be a subtype of both CounterMsg[Nothing] and CounterMsg[Int] so code like

counterFsm(1, CounterMsg.Inc: CounterMsg[Int])

should compile. But then in

[O] =>
    (state: Int, msg: CounterMsg[O]) =>
      (msg match
        case CounterMsg.Inc => state + 1 -> ()
        case CounterMsg.Get => state -> state
      ): (Int, ResponseOf[O])

O would get substituted with Int and while case CounterMsg.Inc pattern gets matched, the type of state + 1 -> () should be (Int, ResponseOf[O]), which should further expand to (Int, Int) but it’s actually (Int, Unit)