* must work on any type of {@code K}.
   *
   * @param map the navigable map for which an unmodifiable view is to be returned
   * @return an unmodifiable view of the specified navigable map
   * @since 12.0
   */
  @GwtIncompatible // NavigableMap
  public static <K extends @Nullable Object, V extends @Nullable Object>
      NavigableMap<K, V> unmodifiableNavigableMap(NavigableMap<K, ? extends V> map) {