Maintain loop start val after increment/decrement

[robchett]

Fixes #10213

[orklah]

(Note that I've not commented on this yet because there were errors on unit tests and I assumed you were still working on it :) )

[robchett]

Yes, two steps forwards, one step back. Still working out the intricacies of the loopAnalyzer

[robchett]

@orklah the lower bound is being properly detected.

I had to modify a test tests/Loop/ForTest.php
as it now shows an error which was a false-negative before. The continue statement was incorrectly suppressing an list-key offset if the form was changed very slightly. https://psalm.dev/r/303d48a67b shows it allowing list access via a negative key. It's not 100% as it hasn't reconciled the $i > 0 and $i-- to int<0, 20> before checking the array offset. I think the false positive is better than the false negative?

[psalm-github-bot]

I found these snippets:

https://psalm.dev/r/303d48a67b

<?php
/**
 * @param list<int> $arr
 */
function cartesianProduct(array $arr) : void {
    for ($i = 20; $arr[$i] === 5 && $i >= -10; $i--) {
        /** @psalm-trace $i */
    }
}

Psalm output (using commit 147505c):

INFO: Trace - 7:0 - $i: int<-10, max>

[orklah]

Thanks! Seems like an improvement :)