Need a way to specify a method's charging a capture set parameter

[natsukagami]

Minimized Example

//> using scala 3.6.0-RC1-bin-SNAPSHOT

import language.experimental.captureChecking

import scala.collection.mutable

abstract class Future[T, Cap^] :
  def complete(res: T): Unit // allow `complete` to charge `Cap` to the environment?

def f[T, Cap^](): Future[T, Cap]^{Cap^} = new Future[T, Cap]:
  val set: mutable.Set[(T -> Unit)^{Cap^}] = mutable.Set[(T -> Unit)^{Cap^}]()
  override def complete(res: T) = set.foreach(_(res))

complete cannot be overridden in f, since it charges the reach capability of set (which is Cap) to the environment.

12 |  override def complete(res: T) = set.foreach(_(res))
   |                                              ^^^^^^
   |         Local reach capability Cap leaks into capture scope of method f

While this is correct to be rejected, there is no way to relax this restriction; there are no syntax to specify in Future that complete may charge Cap.

[odersky]

Agreed that we need something here, but I don't yet have an idea how. Do people have suggestions how to do it?

[Linyxus]

def f[T, Cap^ @unbox]? :P

[odersky]

Yes, but @unbox is both needed a lot and very hard to explain. Can we come up with rules that make @unbox less needed and easier to explain? Maybe change the name, too?