False positive with template covariant

[enumag]

Bug report

I'm not 100% sure but I don't think there is much of a difference between toArray and ToPairs regarding type variance, right?

Or In case the error is correct then how should I fix it? 🤔

Code snippet that reproduces the problem

https://phpstan.org/r/ccc12749-6dfd-4e0d-abf5-54dcd11e6fd6

Expected output

no error

[ondrejmirtes]

Summoning @jiripudil to judge it here 😆

[jiripudil]

Hi, referencing phpstan/phpstan-src#2054 for context. I've stumbled upon a very similar error in phpstan's integration tests there. The issue is that the array shape / tuple type must be invariant because there's nothing that prevents you from writing into it:

/**
 * @param Map<int, Fruit>
 */
function doMagicWithFruit(Map $fruits)
{
    [$firstPair] = $fruits->toPairs();
    $firstPair[1] = new Apple();
}

The covariant template type makes it possible to pass a Map<int, Orange> into the magic function, but then, in the implementation, you're mixing apples and oranges. If there was a readonly-array shape type preventing the modification, it could safely be covariant.

Plain arrays (and, transitively, lists) do not behave so strictly type-wise (#8880 (comment)), and therefore do not need to be so strict variance-wise. I have my reservations about that (see my previous comment in the linked issue), but that's a different (and more complicated) discussion.

[ondrejmirtes]

Writing into arrays does not matter if they're not passed by reference, right? And references are they own beasts which are generally hands-off by the static analyser because they bring a lot of mess with themselves.

[jiripudil]

Writing into arrays does not matter if they're not passed by reference, right?

Oh, yeah, right! But in that case, I'd say this is indeed a bug. ConstantArrayType is still an array and has the same copy-on-write mechanism, so it too can safely be covariant in its keys and values because no change can modify the original value and its type 🤔

[ondrejmirtes]

I'd be really grateful if you could fix it and test it 😊 Thank you.

[ondrejmirtes]

Fixed: phpstan/phpstan-src#2464

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