Why does 2.10 insist on specifying type parameter bounds (worked fine in 2.9)?

Question

I have the following case class:

case class Alert[T <: Transport](destination: Destination[T], message: Message[T])

In Scala 2.9.2, the following method signature has compiled fine:

def send(notification: Alert[_]) {
  notification match {
    ...
  }
}

Now in Scala 2.10.1 it fails to compile with the following error:

type arguments [_$1] do not conform to class Alert's type parameter bounds [T <: code.notifications.Transport]

Why is this? How can I fix the error? Simply giving the same type bounds to send results in a lot more compile errors...

Update: Looking at SIP-18, I don't think the reason is that I don't have existential types enabled, as SIP-18 says it's only needed for non-wildcard types, which is exactly what I do have here.

Answer 1

There error seems to be saying that the existential type "_" is not constrained to be a subtype of Transport. This may be the preferred solution,

trait Transport
trait Destination[T]
trait Message[T]
case class Alert[T <: Transport](destination: Destination[T], message: Message[T])

def send[T <: Transport](notification: Alert[T]) {
  notification match {
    case _ => ()
  }
}

This also seems to work,

def send(notification: Alert[_ <: Transport])

but I think it's preferable not to use existential types.