Local fresh caps are ignored in classes

[Linyxus]

trait IO:
  def f(): Unit

case class BoxIO[+T <: IO^](value: T) extends IO:
  def f(): Unit = value.f()

def test(io: IO^): IO =
  val b: BoxIO[IO^] = BoxIO(io)
  val a: IO = b // leak
  a

Originally posted by @noti0na1 in #23746

[Linyxus]

I looked into it a bit. It turns out that a local fresh cap is charged to the environment by unboxing, which should be illegal (since this will not be charged to the class when a class is created).