Unsound deserialization involving AnyVal on scala 3

[martijnhoekstra]

json4s version

4.0.7

scala version

3.4.1, 3.3.3

jdk version

8, 11, 17, 21

see https://scastie.scala-lang.org/kNYNFfjiS32A8oLJ7VR9pA

case class Example(elem: String) extends AnyVal

case object ExampleSerializer
  extends CustomSerializer[Example](_ =>
    (
      { case JString(s) => Example(s) },
      { case Example(s) => JString(s) },
    ),
  )

case class Outer(examples: List[Example])

implicit val formats: Formats = DefaultFormats + ExampleSerializer

val in = read[Outer](write(o))
val firstExample: Example = in.examples.head // class java.lang.String cannot be cast to class Example

Removing the AnyVal makes this work again.