cakephp/tests/test_app/TestApp/Collection/TestCollection.php

<?php
declare(strict_types=1);

namespace TestApp\Collection;

use ArrayIterator;
use Cake\Collection\CollectionInterface;
use Cake\Collection\CollectionTrait;
use InvalidArgumentException;
use IteratorIterator;
use Traversable;

class TestCollection extends IteratorIterator implements CollectionInterface
{
    use CollectionTrait;

    /**
     * @param iterable $items
     * @throws \InvalidArgumentException
     */
    public function __construct(iterable $items)
    {
        if (is_array($items)) {
            $items = new ArrayIterator($items);
        }

        if (!($items instanceof Traversable)) {
            $msg = 'Only an array or \Traversable is allowed for Collection';
            throw new InvalidArgumentException($msg);
        }

        parent::__construct($items);
    }
}