[Bug] Psalm thinks that DateTimeImmutable::modify() can return false, if the passed argument contains a function call

[ThomasLandauer]

See https://psalm.dev/r/2105756a05

According to a related PR in PHPStan phpstan/phpstan-src#1267, in PHP 8 instead of returning false, an exception is thrown.

[psalm-github-bot]

I found these snippets:

https://psalm.dev/r/2105756a05

<?php

$datetime = new DateTimeImmutable();
// This raises a PossiblyFalseArgument error - apparently due to the function call inside `->modify()`:
$foo = $datetime->modify('+' . getInteger() . ' day');
new Foo($foo);

// This doesn't:
$foo2 = $datetime->modify('+1 day');
new Foo($foo2);

function getInteger(): int
{
    return 1;
}

class Foo
{
    public function __construct(DateTimeImmutable $datetime)
    {
    }
}

Psalm output (using commit 16b24bd):

ERROR: PossiblyFalseArgument - 6:9 - Argument 1 of Foo::__construct cannot be false, possibly DateTimeImmutable value expected