indexcollection.php

См. документацию.

<?php
 
namespace Bitrix\Calendar\Core\Base;
 
abstract class IndexCollection extends Collection
{
    public function add($item, $index = null): IndexCollection
    {
        if (!$index)
        {
            throw new BaseException('you should pass index');
        }
 
        $this->collection[$index] = $item;
 
        return $this;
    }
}