}

  private static final class NonSerializableMemoizingSupplier<T extends @Nullable Object>
      implements Supplier<T> {
    private final Object lock = new Object();

    @SuppressWarnings("UnnecessaryLambda") // Must be a fixed singleton object
    private static final Supplier<@Nullable Void> SUCCESSFULLY_COMPUTED =
        () -> {
          throw new IllegalStateException(); // Should never get called.
        };