Bug: Iterating preg_match_all() matches no longer allowed in pure functions

[colinodell]

Iterating over preg_match_all() matches within a pure function used to be allowed but now it causes an ImpureMethodCall error: Cannot call a possibly-mutating iterator from a pure context

Example: https://psalm.dev/r/f170f02c7e

I would expect this example to be allowed from within a pure function since it matches the definition of a pure function and the value of $matches can never be modified outside of that function.

The first time I noticed this error was in version 3.15. This error was not raised in 3.14.2.

I believe this might be related to #4064 but I'm not entirely sure.

[psalm-github-bot]

I found these snippets:

https://psalm.dev/r/f170f02c7e

<?php

/**
 * @psalm-pure
 *
 * @return string[]
 */
function extractUsernames(string $input): array
{
    preg_match_all('/@[a-z\d](?:[a-z\d]|-(?=[a-z\d])){0,38}(?!\w)/', $input, $matches);
    
    $usernames = [];
    foreach ($matches[0] as $username) {
        $usernames[] = ltrim($username, '@');
    }
    
    return $usernames;
}

var_dump(extractUsernames('This issue was reported by @colinodell'));

Psalm output (using commit 66251d8):

INFO: PossiblyUndefinedIntArrayOffset - 13:14 - Possibly undefined array offset 'int(0)' is risky given expected type 'array-key'. Consider using isset beforehand.

ERROR: ImpureMethodCall - 13:5 - Cannot call a possibly-mutating iterator from a pure context

INFO: MixedAssignment - 13:29 - Unable to determine the type that $username is being assigned to

INFO: MixedArgument - 14:30 - Argument 1 of ltrim cannot be mixed, expecting string

ERROR: ForbiddenCode - 20:1 - Unsafe var_dump

[weirdan]

Simplified: https://psalm.dev/r/aa99f2c9fc

[psalm-github-bot]

I found these snippets:

https://psalm.dev/r/aa99f2c9fc

<?php

/**
 * @psalm-pure
 *
 * @param mixed $input
 */
function extractUsernames($input): void
{
  foreach ($input as $_v) {}  
}

Psalm output (using commit 66251d8):

ERROR: ImpureMethodCall - 10:3 - Cannot call a possibly-mutating iterator from a pure context

[muglug]

Yeah, what really needs to happen here is that Psalm understands the type of $matches much better

[weirdan]

Shouldn't there be a separate issue type, something like MixedIteration? Then ImpureMethodCall could be made more specific by only emitting it when iterating over a known Traversable?

[muglug]

Yeah, you're right about that too