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
D. Malayeri. OOPSLA Companion '08: Companion to the 23rd ACM SIGPLAN conference on Object oriented programming systems languages and applications, page 923--924. New York, NY, USA, ACM, (2008)
N. Schärli, S. Ducasse, O. Nierstrasz, and A. Black. ECOOP 2003 -- Object-Oriented Programming, volume 2743/2003 of Lecture Notes in Computer Science, page 327--339. OGI School of Science & Engineering , Oregon Health and Science University, Springer, (November 2003)