Weak comparison must handle recursion

[mvorisek]

Description

The following code:

<?php

class Node {
    public $next;
}

$first = new Node();
$first->next = $first;

$new = new Node();
$new->next = $new;

var_dump($first == $new);

Resulted in this output:

Fatal error: Nesting level too deep - recursive dependency? in /in/qWtA7 on line 13

But I expected this output instead:

bool(true)

online repro: https://3v4l.org/AZV6v and https://3v4l.org/SttGY

The issue is present also for other "weak comparison operators" like <.

PHP Version

any

[iluuu1994]

I'd like to ask for a use-case. Implementing this is not trivial and may get philosophical, and adding it for the sake of perfectionism doesn't seem worth it.

[mvorisek]

I came across this when comparing some cache content.

I do not have to super pursue this, but on the other hand == (and > ...) is very basic operator and I would expect this to be handled natively, especially when == between recursive data structures (object. array with references) is legit operation.

[iluuu1994]

Ok, thanks. However, I think this will be essentially impossible to handle correctly. Consider these two graphs:

flowchart TD
    A --> B
    B --> C
    C --> A

Loading

flowchart TD
    A --> B
    B --> C
    C --> B

Loading

Are they equivalent? Assuming the elements in the graph don't have other properties, this becomes quite philosophical. There's likely no right answer, and it would also be quite hard to implement either way. Hence I'm closing this as a #wontfix. If you have a concrete idea for an algorithm, feel free to share or send a PR.

[mvorisek]

In sense of weak comparison, they are equal.