Contravariance.scala

trait Item

trait PlasticItem extends Item

trait PlasticBottle extends PlasticItem

trait PaperItem extends Item

trait NewsPaper extends PaperItem

class GarbageCan[-A] {}

// PlasticItem <: Item, GarbageCan[Item] <: GarbageCan[PlasticItem] it has allowed because Function1[-T,+R]
val filter = new Function1[GarbageCan[PlasticItem], Boolean] {
  override def apply(v1: GarbageCan[PlasticItem]): Boolean = true
}
filter(new GarbageCan[Item])
filter(new GarbageCan[PlasticItem])
//filter(new GarbageCan[PlasticBottle]) // I know Why It is failing

List(new GarbageCan[PlasticItem], new GarbageCan[Item]).filter(filter) //works

// PlasticItem <: Item, GarbageCan[B] <: GarbageCan[A]
val filter1 = new Function1[PlasticItem, Boolean] {
  override def apply(v1: PlasticItem): Boolean = ???
}

//List(new PlasticItem {}, new Item {}).filter(filter1)