I think I've come across a bug with arcanist when running "arc diff" without an active branch. Trace:
>>>  (+9,233) <exec> $ git symbolic-ref --quiet HEAD <<<  (+9,235) <exec> 1,791 us [2023-05-22 10:35:55] EXCEPTION: (RuntimeException) strlen(): Passing null to parameter #1 ($string) of type string is deprecated at [<arcanist>/src/error/PhutilErrorHandler.php:261] arcanist(), tmphutil() #0 PhutilErrorHandler::handleError(integer, string, string, integer) called at [<arcanist>/src/workflow/ArcanistDiffWorkflow.php:2484] #1 ArcanistDiffWorkflow::getDiffOntoTargets() called at [<arcanist>/src/workflow/ArcanistDiffWorkflow.php:2462] #2 ArcanistDiffWorkflow::updateOntoDiffProperty() called at [<arcanist>/src/workflow/ArcanistDiffWorkflow.php:435] #3 ArcanistDiffWorkflow::run() called at [<arcanist>/scripts/arcanist.php:427] <<<  (+9,273) <exec> 9,273,390 us
I've found a fix in the phabricator repo and I think the same should be applied here: https://secure.phabricator.com/rARCc39ab20eb3717a15aed2467842bd77d9addce96a
As you can see from the git history of Phorge we are really hammering fixes for PHP 8.1 but this is really not ready for production as avivey said. So your report is really appreciated, but I suggest PHP 7.4 at the moment.