Function should return int|(T of int|null) but returns T (which is int|null).

[rx80]

Bug report

Maybe i am missing some detail, but i think this code shouldn't report a type mismatch:

/**
 * @template T of int|null
 * @phpstan-param T $b
 * @phpstan-return int|T
 */
function a(?int $b): ?int
{
    if ($b === null) {
        return $b; // phpstan reports: Function a() should return int|(T of int|null) but returns T.
    }

    return $b;
}

Code snippet that reproduces the problem

https://phpstan.org/r/fbce81ec-7a7c-46e0-afb0-44d01d9e69b5

Expected output

Expected: no type error

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No response

[ondrejmirtes]

PHPStan is confused here because of:

 * @template T of int|null
 * @phpstan-param T $b
 * @phpstan-return int|T

Since T can already be int, there's no point in doing @phpstan-return int|T. Something like this works better: https://phpstan.org/r/c4e7d18d-3efe-4590-972f-823c6ae55a47

If you provide an actual implementation of your function and this signature doesn't work for you, I can help you find a better one.

[rx80]

There are a few issues here.

The first one is that in your example, now i have 2 places instead of 1 in which i constrain the argument.

The second one is that now i have less readable (or more convoluted code) to express the same thing. In my example it's easy to read: the function returns: "int or whatever the type of $b is".

The third issue is that if this is done through a second argument it works fine:

/**
 * @template T of int|null
 * @phpstan-param T $b
 * @phpstan-return int|T
 */
function a(int $a, ?int $b): ?int
{
    if ($a < 6) {
        return $b;
    }

    return $a;
}

and the type of a(1,1) is correctly deduced to be not null: https://phpstan.org/r/08af0c6c-92a1-4fcd-8dc3-462d5bbe9053

If my original report is marked "wontfix", would it be appropriate for me to try and find the issue in phpstan and attempt to fix/improve, if it doesn't interfere with other type deduction stuff?

EDIT:

A fourth problem is demonstrated here: https://phpstan.org/r/a8e6eabb-dc6f-40f6-9bb7-f79cdc562277
The problem only arises when null is involved, for some reason.

[ondrejmirtes]

I'd say the proper return type for your function is int|null. No generics involved. You don't need to get more precise than that.

If you want to be more precise, you need to mark both parameters as generic and then return A|B: https://phpstan.org/r/6ff4e0fd-d5db-4eee-8c2e-675ebb413082

Because PHPStan reports an error for a legitimate return statement, I'm reopening this.

[ondrejmirtes]

As a workaround, if you don't care about the specific integer values, you can do this: https://phpstan.org/r/f767bbc9-946d-4ea5-9a4f-6aa2a8c03803

[rx80]

As a workaround, if you don't care about the specific integer values, you can do this: https://phpstan.org/r/f767bbc9-946d-4ea5-9a4f-6aa2a8c03803

Thank you for your reply, and i understand that it is possible to get around the issue by writing code, but since it's possible to operate purely on types in all other cases, it would be nice if it would also work for the NULL case.

Probably my last example (https://phpstan.org/r/a8e6eabb-dc6f-40f6-9bb7-f79cdc562277) demonstrates it best, my original post probably didn't hit the problem exactly.

[ondrejmirtes]

Fixed phpstan/phpstan-src#4267