Demonstrates an issue with `\iter\isEmpty()` for objects implementing `\IteratorAggregate` where `getIterator()` is impure

isEmptyIssue.php

<?php

/**
 * Copied directly from https://github.com/nikic/iter/blob/master/src/iter.php#L1090
 * on commit d9f88bc
 */
function isEmpty($iterable): bool {
    if (\is_array($iterable) || $iterable instanceof \Countable) {
        return count($iterable) == 0;
    }

    if ($iterable instanceof \Iterator) {
        return !$iterable->valid();
    } else if ($iterable instanceof \IteratorAggregate) {
        $inner = $iterable->getIterator();

        if ($inner instanceof \Iterator) {
            return !$inner->valid();
        } else if ($inner !== $iterable) {
            return isEmpty($inner);
        } else {
            throw new \InvalidArgumentException(
                'Argument must not be recursively defined');
        }
    } else {
        throw new \InvalidArgumentException(
            'Argument must be iterable or implement Countable');
    }
}

class MyIter implements \IteratorAggregate
{
    /**
     * Simulates some stateful generator which cannot be safely restarted
     * or rewound
     */
    private array $inner;

    public function __construct() {
        $this->inner = [1, 2, 3, 4, 5];
    }

    public function getIterator(): \Traversable
    {
        while (!empty($this->inner)) {
            yield array_shift($this->inner);
        }
    }
}

$gen = new MyIter();

echo "gen isEmpty? " . (isEmpty($gen) ? "yes" : "no") . "\n";
// prints: no

foreach ($gen as $value) {
    echo "got $value\n";
}
// prints:
//
// > got 2
// > got 3
// > got 4
// > got 5
//
// instead of expected
//
// > got 1
// > got 2
// > got 3
// > got 4
// > got 5