gistfile1.scala

class Student(val sex:String)
class Boy(val fightingInstinct:Int = 100) extends Student("male")
class Girl(val motherhood:Int = 100) extends Student("female")

class School[T <: Student](val students: Seq[T]){
  def studentsAsArray = students.toArray
  //  Error: No ClassTag available for T
  //  def studentsAsArray = students.toArray
  //  ^
  //  Error: not enough arguments for method toArray: (implicit evidence$1: scala.reflect.ClassTag[T])Array[T].
  //    Unspecified value parameter evidence$1.
  //  def studentsAsArray = students.toArray
  //  ^

  def studentsAsList = students.toList
  //  OK
}

object SchoolTest {
  val boysSchool = new School(Seq(new Boy, new Boy, new Boy))
  boysSchool.students.map(_.fightingInstinct).sum

  boysSchool.studentsAsArray // cannot compile
  boysSchool.studentsAsList // OK

  boysSchool.students.toArray // OK
}