I would strongly advise not to inherit from a case class. It has surprising effects on equals and hashCode, and has been deprecated in Scala 2.8.
Instead, define x in a trait or an abstract class.
scala> trait A { val x: Int }
defined trait A
scala> case class B(val x: Int, y: Int) extends A
defined class B
http://www.scala-lang.org/node/3289
http://www.scala-lang.org/node/1582
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