Another question about this example is how do we actually instantiate such collections:

import language.experimental.captureChecking
import caps.*

trait Collection[T] extends Mutable:
  update def add(elem: T): Unit
  update def remove(elem: T): Unit
  def get(index: Int): Option[T]

object Collection:
  def empty[T]: Collection[T] = ???

trait Foo:
  val thunks: Collection[() => Unit] // that's fine

object FooImpl1 extends Foo:
  val thunks: Collection[() => Unit] = Collection.empty // error

Looks like a bug?

-- [E007] Type Mismatch Error: local/i21675.scala:17:39 ------------------------
16 |  val thunks: Collection[() => Unit] = Collection.empty
   |                                       ^^^^^^^^^^^^^^^^
   |Found:    Collection[() ->? Unit]^{cap.rd}
   |Required: Collection[() => Unit]^{cap².rd}
   |
   |where:    cap  is a fresh root capability in the result type of method empty
   |          cap² is a fresh root capability in the type of value thunks
   |
   | longer explanation available when compiling with `-explain`

Originally posted by @bracevac in #21675