Unbox requires a boxed type, but got B.

The following program:

```
type Status[B] {
Done(value: Int)
More(state: B)
}

def main(): Unit = {
Done(123) match {
case Done(value) => value
case More(task) => task()
}
()
}
```

yields the error message `Unbox requires a boxed type, but got B.`.

Desired behavior is either to accept the program and infer `B` to be instantiated with `() => Int at {}`, or reject the program with an error message like "could not infer type argument for `B` in `Done(123)`".