Unsafe typecast long to int:

public static <T> Node<T> flatten(Node<T> node, IntFunction<T[]> generator) {
    if (node.getChildCount() > 0) {
        T[] array = generator.apply((int) node.count());
    ..........
Same for:
public static Node.OfInt flattenInt(Node.OfInt node)
public static Node.OfLong flattenLong(Node.OfLong node)
public static Node.OfDouble flattenDouble(Node.OfDouble node)

static final class ConcNode<T>
    public void copyInto(T[] array, int offset) {
        .............
        right.copyInto(array, offset + (int) left.count());
    }
    private abstract static class OfPrimitive<E, T_CONS, T_ARR,
        .............
        public void copyInto(T_ARR array, int offset) {
            .............
            right.copyInto(array, offset + (int) left.count());
        }
        public T_ARR asPrimitiveArray() {
            T_ARR array = newArray((int) count());
            .............