*
   * <pre>{@code
   * Optional<Integer> optionalInt = getSomeOptionalInt();
   * Number value = optionalInt.or(0.5); // error
   *
   * FluentIterable<? extends Number> numbers = getSomeNumbers();
   * Optional<? extends Number> first = numbers.first();
   * Number value = first.or(0.5); // error
   * }</pre>
   *
   * <p>As a workaround, it is always safe to cast an {@code Optional<? extends T>} to {@code