Impure type arguments in user-written types is not boxed during CC setup

[Linyxus]

I checked the issue again and it turns out that actually the recent reproduction is completely a new one with a different cause. Another minimisation that is more salient:

def runOps(ops: List[() => Unit]): () -> Unit =
  () =>
    ops.foreach: op =>
      op()

Originally posted by @Linyxus in #16725 (comment)