How should I extend a case class if a derived class is meant to have the same parameter and shouldn't override?

case class Message(xml : Node) {
  def toXML : Node = xml
}

case class开发者_运维知识库 ReqValidationMessage (xml : Node) extends Message(xml){
  // ...
}

This causes a property naming conflict as Scala tries to create a second property named xml in ReqValidationMessage case class. But I want both constructors (of Message and ReqValidationMessage) to have the same argumentation. What should I do?

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The short answer is: You should not extend a case class — case class inheritance is now deprecated.

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Instead of subclassing case classes, why not use mixins to replicate common features:

trait XMLConvertible {
  def xml: Node
  def toXML = xml
}

case class Message(xml : Node) extends XMLConvertible

case class ReqValidationMessage(xml : Node) extends XMLConvertible {
  //...
}

Then if you want to use directly XMLConvertible for pattern matching add a companion object:

object XMLConvertible {
   def unapply( xc: XMLConvertible ) = Some( xc.xml )
}

Which allows you to write:

case XMLConvertible(xml) => println( xml )

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If you want to stick with the scheme you presented, then you can just change the name of argument in second constructor - to something like xml2. Then you will have no naming conflict, and everything will work.