*     might be the keys' natural order)
     * @since 31.0
     */
    @Override
    @SuppressWarnings("unchecked") // see inline comments
    public ImmutableSortedMap<K, V> buildOrThrow() {
      switch (size) {
        case 0:
          return emptyMap(comparator);
        case 1:
          // We're careful to put only K and V instances in.