False-positive for "unused" variable in try-catch

[zerkms]

https://psalm.dev/r/585b7b7e67

[psalm-github-bot]

I found these snippets:

https://psalm.dev/r/585b7b7e67

<?php

/**
 * @throws Exception
 */
function mayThrow(): void {}

$step = 'zero';

try {
    $step = 'one';
    mayThrow();
    
    $step = 'two';
    mayThrow();
} catch (Throwable) {
    echo $step;
}

Psalm output (using commit 9d8b6d6):

INFO: UnusedVariable - 11:5 - $step is never referenced or the value is not used

[weirdan]

I would expect $step = 'zero'; to be marked as unused.

[zerkms]

That too: without zero $step is thought to be possibly undefined.

[AndrolGenhald]

I would expect $step = 'zero'; to be marked as unused.

I think Psalm just assumes it's possible that an exception is thrown immediately upon entering a try block, and makes no attempt to understand that $step = 'one' will always happen.

I might take a look at this tomorrow if no one else grabs it, I'm hoping it won't be too hard, but we'll see. I'm not sure if we still have a way to get at the $step = 'one' node to mark it as used by the time we analyze the catch. If we do it should be simple enough to mark every assignment in the try as used when a variable is referenced in the catch or finally.

[AndrolGenhald]

Took me longer than I'd hoped to get around to it, but I managed to find an alternate fix.