sealed trait Key[A] { type Inner = A }
trait IntKey extends Key[Int]
trait StringKey extends Key[String]

// This should compile
object Key {
  def work[A](m: Key[A]): m.Inner = m match {
    case i: IntKey    => 1
    case s: StringKey => "1"
  }
}
// scala> Key work new IntKey{}
// res0: Int = 1

// scala> Key work new StringKey{}
// res1: String = 1