gistfile1.scala

object Example {
  // Works
  sealed trait Command1[R] // R - return type
  case class IntCommand1() extends Command1[Int]
  case class StringCommand1() extends Command1[String]
  
  def execute[R](cmd: Command1[R]): R = cmd match {
    case IntCommand1() => 5
    case StringCommand1() => "x"
  }

  // Doesn't work; same as above, only difference is the variance annotation
  sealed trait Command2[+R] // R - return type
  case class IntCommand2() extends Command2[Int]
  case class StringCommand2() extends Command2[String]

  /*
  Compile errors:
  
  error: type mismatch;
  found   : Int(5)
  required: R
  case IntCommand2() => 5
   */
  def execute[R](cmd: Command2[R]): R = cmd match {
    case IntCommand2() => 5
    case StringCommand2() => "x"
  }
}