Collection<Integer> reserializedValues = reserialize(map.values());
    assertEquals(Lists.newArrayList(map.values()), Lists.newArrayList(reserializedValues));
    assertTrue(reserializedValues instanceof ImmutableCollection);
  }

  @J2ktIncompatible
  @GwtIncompatible // SerializableTester
  public void testKeySetIsSerializable_regularImmutableMap() {
    class NonSerializableClass {}

import javax.annotation.CheckForNull;
import org.checkerframework.checker.nullness.qual.Nullable;

/**
 * A {@link NavigableSet} whose contents will never change, with many other important properties
 * detailed at {@link ImmutableCollection}.
 *
 * <p><b>Warning:</b> as with any sorted collection, you are strongly advised not to use a {@link
 * Comparator} or {@link Comparable} type whose comparison behavior is <i>inconsistent with

   * @since 12.0
   */
  public static <E extends @Nullable Object> NavigableSet<E> unmodifiableNavigableSet(
      NavigableSet<E> set) {
    if (set instanceof ImmutableCollection || set instanceof UnmodifiableNavigableSet) {
      return set;
    }
    return new UnmodifiableNavigableSet<>(set);
  }

  static final class UnmodifiableNavigableSet<E extends @Nullable Object>