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trait Covariant[+A]
trait Contra[-A] { def accept(p: A): Unit }
trait Invariant[A] extends Covariant[A] with Contra[A]
case class Unravel[A](m: Contra[A], msg: A)
object Test extends Covariant[Any] {
def g(m: Contra[Any]): Unit = m accept 5
def f(x: Any): Unit = x match {
case Unravel(m, msg) => g(m)
case _ =>
}
def main(args: Array[String]) {
f(Unravel[String](new Contra[String] { def accept(x: String) = x.length }, ""))
}
}
// java.lang.ClassCastException: java.lang.Integer cannot be cast to java.lang.String
// at Test$$anon$1.accept(a.scala:18)
// at Test$.g(a.scala:13)
// at Test$.f(a.scala:15)
// at Test$.main(a.scala:18)
// at Test.main(a.scala)
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