Double coalescing confuses array analyser

[someniatko]

https://psalm.dev/r/fa647867eb
Upd: reproducer without using explode: https://psalm.dev/r/97e3b85fac

Should report no issue in both functions.

The bug has been introduced sometime after 5.4.0.
Upd: the explode types refinement has been introduced in 5.5.0 by #9016 which exposed the bug, but the bug itself most probably existed way before the 5.4.0.

[psalm-github-bot]

I found these snippets:

https://psalm.dev/r/fa647867eb

<?php

function doubleCoalesce(?string $param): string {
    return explode(':', $param ?? '', 2)[1] ?? '';
}

function singleCoalesce(string $param): string {
    return explode(':', $param, 2)[1] ?? '';
}

Psalm output (using commit e784128):

ERROR: PossiblyUndefinedArrayOffset - 4:12 - Possibly undefined array key  on list{0: string, 1?: string}

[Boldewyn]

I’ve been biten by this bug, too. Upgrading from 5.4.0 to 5.6.0 left me with a PossiblyUndefinedArrayOffset for code that looks like this:

<?php

function test(string $input) : void {
    if (strpos($input, '=') === false) {
        return;
    }
    list($key, $value) = explode('=', $input, 2); # ! here I know that explode returns
                                                  # an array of length 2, but Psalm reports an error
}

test('a=b');

[weirdan]

@Boldewyn that's beyond Psalm's understanding, so you need to help it a bit: https://psalm.dev/r/3140be0736

It doesn't seem to have anything to do with the original report though.

[psalm-github-bot]

I found these snippets:

https://psalm.dev/r/3140be0736

<?php

function test(string $input) : void {
    if (strpos($input, '=') === false) {
        return;
    }
    $v = explode('=', $input, 2); 
    assert(count($v) === 2);
    list($_key, $_value) = $v;
}

test('a=b');

Psalm output (using commit aec0edc):

No issues!

[Boldewyn]

OK, understood! Thank you!

It just happens that psalm started to report these lines in versions > 5.4.0, and the snippet in the original post seemed to match somewhat the case I saw.

[someniatko]

@Boldewyn It seems the thing which is introduced in 5.5.0 is not the bug itself, but rather the new feature - better types of explode function, done in #9016

And that feature probably just exposed the already existing bug I'm reporting here. And this is also the reason why you too get the error which you didn't earlier.

[someniatko]

I have updated the issue title and the first message description. Provided there a reproducer which does not rely on explode types.

[orklah]

Hum, weird, doc says that null coalescing should suppress the error: https://psalm.dev/docs/running_psalm/issues/PossiblyUndefinedArrayOffset/

https://psalm.dev/r/97e3b85fac (I posted your snippet again for better triage)

[psalm-github-bot]

I found these snippets:

https://psalm.dev/r/97e3b85fac

<?php

/** @return list{0: string, 1?: string} */
function returnOneOrTwoVals(string $param): array {
    return $param === 'two' ? [ $param, $param ] : [ $param ] ;
}

function doubleCoalesce(?string $param): string {
    return returnOneOrTwoVals($param ?? '')[1] ?? '';
}

function singleCoalesce(string $param): string {
    return returnOneOrTwoVals($param)[1] ?? '';
}

Psalm output (using commit d7e9cbc):

ERROR: PossiblyUndefinedArrayOffset - 9:12 - Possibly undefined array key  on list{0: string, 1?: string}