Unexpected "java.lang.ClassCastException: Cannot cast to scala.Nothing"

Compiler version

3.2.1 (as well as 3.1.0, 3.0.0, etc)

Minimized code

Scala3MatchCrash.scala

sealed trait Foo[A] {

  def last: Option[A] = this match {
    case _: FooEmpty.type => None
    case FooNode(_: FooEmpty.type, bar) => Some(bar)
    case FooNode(foo, _) => foo.last
  }
}

final case class FooNode[A](foo: Foo[A], bar: A) extends Foo[A]

case object FooEmpty extends Foo[Nothing]

object Scala3MatchCrash extends App {

  val foo0 = FooEmpty.asInstanceOf[Foo[Int]]
  val foo1 = FooNode(foo0, 1)

  println(foo1.last)
}

Output

Exception in thread "main" java.lang.ExceptionInInitializerError
	at Scala3MatchCrash.main(Scala3MatchCrash.scala)
Caused by: java.lang.ClassCastException: Cannot cast to scala.Nothing
	at Foo.last(Scala3MatchCrash.scala:5)
	at Foo.last$(Scala3MatchCrash.scala:1)
	at FooNode.last(Scala3MatchCrash.scala:10)
	at Scala3MatchCrash$.<clinit>(Scala3MatchCrash.scala:19)
	... 1 more

Expectation

Some(1)

Additional Info

This code snippet compiles and works as expected on previous Scala versions: 2.13.10, 2.13.0, 2.12.17, etc.

[Linyxus]

CCE in the second case happens because of the cast we insert for it, the typed tree looks like:

case _:FooEmpty.type =>
  None
case FooNode.unapply[A](_:FooEmpty.type, bar @ _):FooNode[A] =>
  Some.apply[Nothing](bar.$asInstanceOf[(bar : A) & Nothing])

In the second case, we have to use the GADT constraint A =:= Nothing to make the clause well-typed, so we insert the cast bar.$asInstanceOf[bar.type & Nothing]. The cast should be safe as long as the constraint A =:= Nothing is sound, which is not the case here.

For the first case, no casts are inserted because no GADT constraint is used to type None as Option[A].

[satorg]

I see, thank you all for looking into it and the detailed explanation.

I’m still wondering though how come the first case does not cause CCE, what is the difference between the first and the second one from the prospective of GADT?

    case _: FooEmpty.type => None // does not cause any CCE
    case FooNode(_: FooEmpty.type, bar) => Some(bar)

because if I change the println line to this one:

println(foo0.last)

then it simply prints None and no CCE gets raised.

Just FYI: the snippet above is actually an excerpt from Diet which I’m currently trying to massage up a little bit.