Type parameter defines type member under inheritance with Scala 3.2.2 ?

[devlaam]

Observation

trait Trait :
  type Y
  var list: List[Y] = Nil

class Class[Y] extends Trait :
  def add(elm: Y): Unit = list = elm :: list

object Object extends Class[Int] :
  add(42)

println(Object.list)

compiles under 3.2.2 but not under 3.2.1. The latter gives the error:

Found:    (elm : Y)
Required: Class.this.Y²

where:    Y  is a type in class Class
          Y² is a type in trait Trait

So it looks like under 3.2.2 it is possible to define the type member from Trait with the type parameter from Class. That would be nice if this would fail to compile:

trait Trait :
  type Y <: String
  var list: List[Y] = Nil

class Class[Y] extends Trait :
  def add(elm: Y): Unit = list = elm :: list

object Object extends Class[Int] :
  add(42)

println(Object.list)

where I changed type Y to type Y <: String but it does not. This still produces List(42).

Expectation

Compiler error on the latter code OR the same behaviour as in Scala 3.2.1 of Scala 3.2.2 on the former code .

Discussion

I observed this behaviour on the Jan, 13 and we discussed it om the scala users forum under the title:
Int <: String in Scala 3.2.2?. I had to wait though until Scastie with 3.2.2 was released for easy reference. I tested the latter code also under Scala 3.3.0-RC2 and 3.3.1-RC1-bin-20230204-a356581-NIGHTLY and they behave as 3.2.2 does.

[jchyb]

Thank you for submitting. This appears to also be a pretty severe soundness issue, as I try to show in the reproduction below:

trait Trait:
  type Y <: String
  var list: List[Y] = Nil
  def listSubstr() =
    list.map(_.substring(0))

class Class[Y] extends Trait :
  def add(elm: Y): Unit = list = elm :: list

object Object extends Class[Int]:
  add(42)
  listSubstr()

@main def main() =
  println(Object.list)

compiles "successfully" on main (21729d2) and 3.2.2, but when run will error out with a ClassCastException:

Exception in thread "main" java.lang.ExceptionInInitializerError
        at main$package$.main(main.scala:15)
        at main.main(main.scala:14)
Caused by: java.lang.ClassCastException: class java.lang.Integer cannot be cast to class java.lang.String (java.lang.Integer and java.lang.String are in module java.base of loader 'bootstrap')
        at scala.collection.immutable.List.map(List.scala:246)
        at Trait.listSubstr(main.scala:5)
        at Trait.listSubstr$(main.scala:1)
        at Class.listSubstr(main.scala:7)
        at Object$.<clinit>(main.scala:12)
        ... 2 more

[jchyb]

Bisect shows 66bdfb8 as "the first bad commit"