* java.util.function.Function) Collectors.toMap(Function, Function)}, which throws an {@code
   * IllegalStateException}.)
   *
   * @since 33.2.0 (available since 21.0 in guava-jre)
   */
  @IgnoreJRERequirement // Users will use this only if they're already using streams.
  public static <T extends @Nullable Object, K extends Enum<K>, V>
      Collector<T, ?, ImmutableMap<K, V>> toImmutableEnumMap(