Invalid unused variable inside closure

[jpresutti]

Using Psalm/Phar 4.7

ERROR: UnusedVariable - Traits/Collection.php:120:25 - $sortA is never referenced or the value is not used (see https://psalm.dev/077)
$sortA .= $a->$k;

$sortA is definitely used in the strcmp

usort(
            $array,
            function (object $a, object $b) use ($key) {
                if (is_array($key)) {
                    $sortA = '';
                    $sortB = '';
                    foreach ($key as $k) {
                        $sortA .= $a->$k;
                        $sortB .= $b->$k;
                    }

                    return strcmp($sortA, $sortB);
                }

                return strcmp((string)$a->$key, (string)$b->$key);
            }
        );

[psalm-github-bot]

Hey @jpresutti, can you reproduce the issue on https://psalm.dev ?

[muglug]

Concatenation with unknown object property is cause of the bug: https://psalm.dev/r/e5c59c7b9b

[psalm-github-bot]

I found these snippets:

https://psalm.dev/r/e5c59c7b9b

<?php

/** @param array<string> $key */
function foo(object $a, string $k) : string {
	$sortA = '';
    
    $sortA .= $a->$k;

    return $sortA;
}

Psalm output (using commit 3ce41d7):

INFO: UnusedVariable - 7:5 - $sortA is never referenced or the value is not used