2.10.1+issue.scala

object Blah extends App {

  trait ArgProvider {
  }

  case class ColumnReference(value: String) extends ArgProvider


  abstract class Rule(name: String, val argProviders: ArgProvider*)

  case class InRule(inValue: ArgProvider) extends Rule("in", inValue) {

    def valid(): Boolean = {
      /*
      When compiled and ran with Scala 2.10.1
      this causes a ClassCastException...
      
      BUT NOT on Scala 2.10.0
      */
      println(inValue.getClass) 

      true
    }
  }

  val rule = InRule(ColumnReference("Column1"))
  rule.valid()

}

Slightly smaller repro

package test

object Blah /*extends App*/ {

  trait ArgProvider {
  }

  abstract class Rule(val argProviders: ArgProvider*)

  case class InRule(inValue: ArgProvider) extends Rule(inValue) {

    def valid(): Boolean = {
      /*
       When compiled and ran with Scala 2.10.1
       this causes a ClassCastException...
       BUT NOT on Scala 2.10.0
       */
      println(inValue.getClass)
      true
    }
  }

  val rule = InRule(new ArgProvider {})
  rule.valid()
}