java.util.Optional.or, orElse and orElseGet should really have their signatures changed so they can widen the type. For example, `Optional<T> or​(Supplier<? extends Optional<? extends T>> supplier)` should really be `<U, S super T & U> Optional<S> or​(Supplier<? extends Optional<? extends U>> supplier)`

Then we could eliminate the following unchecked cast:
```
public static double getOrNan(Optional<? extends Number> optionalNumber) {
return ((Optional<Number>)optional).getOrElse(Double.NaN).doubleValue();
}
```