[SOLVED] Why does Kotlin produce an Unchecked Cast warning when casting a covariant type parameter to invariant type parameter?

Why does Kotlin produce an Unchecked Cast warning when casting a covariant type parameter to invariant type parameter?

Suppose I have a class Message and a class Channel<T : Message>.

Now, how come I can't cast Channel<out Message> to Channel<Message> without an Unchecked Cast warning?

Shouldn't this cast always be safe since Channel<out Message can only contain objects of type Message or subclasses of type Message?

Solution

No, and here is proof.

class Channel<T : Message> {
  fun send(t: T)
  fun receive(): T
}
class Apple : Message()
class AppleChannel : Channel<Apple>() {
  ...
}

val appleChannel = AppleChannel()
val outMessageChannel: Channel<out Message> = appleChannel // MUST WORK
val messageChannel: Channel<Message> = outMessageChannel 
   // you say this should work, but...
messageChannel.send(Orange()) 
   // sending orange on a channel that only knows how to send apples!  
   // Error!

The only consistent thing to do is to not allow the conversion of Channel<out Message> to Channel<Message> in this scenario.

If it were defined as Channel<out T: Message>, then this would be safe, but trying to define send would cause an error.