Scala lower type bound 'is a subtype' restriction on method arguments

Question

In Scala, why does setting lower type bounds on a method type parameter not enforce the "is a super-type" restriction on the method arguements ?

object TypeBounds extends App {
  class MotorVehicle
  class Truck extends MotorVehicle
  class Car extends MotorVehicle
  class Saloon extends Car
  class HatchBackSaloon extends Saloon

  def lowerTypeBound[C >: Car](c: C): C = c

  def upperTypeBound[C <: Car](c: C): C = c

  // Works. HatchBackSaloon is a sub class of Car
  println(upperTypeBound(new HatchBackSaloon())) 

  // as expected doesn't compile. Truck is not a subclass of Car    
  println(upperTypeBound( new Truck()))  

  // Compiles and runs, but why ?  HatchBackSaloon is not a super class of Car.
  println(lowerTypeBound(new HatchBackSaloon())) 

}

Answer 1

C in your example is materialized as Car , not HatchbackSaloon .

A function looking like def lowerTypeBound(c: Car): Car can accept arguments of type HatchbackSaloon , it is not surprising, right?

Try something like this:

val result: HatchBackSaloon = lowerTypeBound(new HatchBackSaloon)

This will not compile, because it would require C to be HatchbackSaloon , which is not a superclass of Car . But this will work:

val result: MotorVehicle = lowerTypeBound(new HatchbackSaloon)

because C is MotorVehicle here, and that is allowed.