@SuppressWarnings("unchecked") // implementation is "fully variant"
    SupplierFunction<T> sf = (SupplierFunction<T>) SupplierFunctionImpl.INSTANCE;
    return sf;
  }

  private interface SupplierFunction<T extends @Nullable Object> extends Function<Supplier<T>, T> {}

  private enum SupplierFunctionImpl implements SupplierFunction<@Nullable Object> {
    INSTANCE;

  }

  public void testSupplierFunction() {
    Supplier<Integer> supplier = Suppliers.ofInstance(14);
    Function<Supplier<Integer>, Integer> supplierFunction = Suppliers.supplierFunction();

    assertEquals(14, (int) supplierFunction.apply(supplier));
  }

  @J2ktIncompatible
  @GwtIncompatible // SerializationTester
  @SuppressWarnings("DoNotCall")
  public void testSerialization() {

      Supplier<T> supplier) {
    return new SupplierFunction<>(supplier);
  }

  /**
   * @see Functions#forSupplier
   */
  private static final class SupplierFunction<
          F extends @Nullable Object, T extends @Nullable Object>
      implements Function<F, T>, Serializable {

    private final Supplier<T> supplier;

    private SupplierFunction(Supplier<T> supplier) {

    @SuppressWarnings("unchecked") // implementation is "fully variant"
    SupplierFunction<T> sf = (SupplierFunction<T>) SupplierFunctionImpl.INSTANCE;
    return sf;
  }

  private interface SupplierFunction<T extends @Nullable Object> extends Function<Supplier<T>, T> {}

  private enum SupplierFunctionImpl implements SupplierFunction<@Nullable Object> {
    INSTANCE;