exec() incorrectly gives TypeDoesNotContainType error when checking for false return

[bitwise-operators]

The php exec() function has return type 'string|false', like so many PHP functions.

However, when you check for this return value, Psalm (tested on v4.8) gives
TypeDoesNotContainType - string does not contain false

Demo here:
https://psalm.dev/r/b0213d4c41

[psalm-github-bot]

I found these snippets:

https://psalm.dev/r/b0213d4c41

<?php
if (false === exec('ls')) {
    echo 'failed to execute';
}

Psalm output (using commit 7aa5564):

ERROR: TypeDoesNotContainType - 2:5 - string does not contain false