Invalid clone result for List.of() and Set.of() objects on JDK15

[shm-dmitry]

In JDK15 changed default implementation for List.of() and Set.of() methods.
So, cloner returned invalid result.

Steps to reproduce
Execute this JAVA code on JDK14 and JDK15

public class Test1 {
	private static final Cloner CLONER = Cloner.standard();

	@Test
	public void testList() {
		Assert.assertEquals(0, CLONER.deepClone(List.of()).size());
		Assert.assertEquals(1, CLONER.deepClone(List.of(1)).size());
		Assert.assertEquals(2, CLONER.deepClone(List.of(1, 2)).size());
		Assert.assertEquals(3, CLONER.deepClone(List.of(1, 2, 3)).size());
	}

	@Test
	public void testSet() {
		Assert.assertEquals(0, CLONER.deepClone(Set.of()).size());
		Assert.assertEquals(1, CLONER.deepClone(Set.of(1)).size());
		Assert.assertEquals(2, CLONER.deepClone(Set.of(1, 2)).size());
		Assert.assertEquals(3, CLONER.deepClone(Set.of(1, 2, 3)).size());
	}
}

In JDK

    private static final Object EMPTY;

    static <E> List<E> of(E e1) {
        return new ImmutableCollections.List12<>(e1);
    }

....

        List12(E e0) {
            this.e0 = Objects.requireNonNull(e0);
            // Use EMPTY as a sentinel for an unused element: not using null
            // enable constant folding optimizations over single-element lists
            this.e1 = EMPTY;
        }

...

        public int size() {
            return e1 != EMPTY ? 2 : 1;
        }

Problem in internal EMPTY value - library cloned it in e1 field.

[shm-dmitry]

Workaround:

	private static final Cloner CLONER = Cloner.standard();

	static {
		try {
			var list = List.of(1);

			var field = list.getClass().getDeclaredField("e1");
			field.setAccessible(true);
			var value = field.get(list);

			if (value != null) {
				CLONER.registerConstant(value);
			}
		} catch (Exception e) {
			// ignore
		}
	}

[tweimer]

I can confirm. It should rather be fixed by adding a custom IFastCloner, like in FastClonerArrayList.
In Cloner.registerFastCloners(), you can register those custom cloner.