Taken from the convo @ https://twitter.com/mattbrowndev/status/1367965245084491783 (sorry, I just happen to review too much code :| )

<?php

(function () : void {
    echo (fn(string $b) => "$b - $undefined")("hello");
})();

https://psalm.dev/r/d5662fa596

It should report $undefined as undefined, but it doesn't.

See https://3v4l.org/D8tJR for runtime verification.