When I attempt to submit an update to a differential revision which has been tagged with a milestone, arcanist isn't happy.
arc diff --trace
EXCEPTION: (ArcanistDifferentialCommitMessageParserException) Error parsing field "Tags": The objects you have listed include objects which do not exist (Release-Engineering-Team, (Long-Lived-Branches)). at [<arcanist>/src/differential/ArcanistDifferentialCommitMessage.php:54] arcanist(), phutil() #0 ArcanistDifferentialCommitMessage::pullDataFromConduit(ConduitClient) called at [<arcanist>/src/workflow/ArcanistDiffWorkflow.php:1795] #1 ArcanistDiffWorkflow::getCommitMessageFromRevision(string) called at [<arcanist>/src/workflow/ArcanistDiffWorkflow.php:1527] #2 ArcanistDiffWorkflow::buildCommitMessage() called at [<arcanist>/src/workflow/ArcanistDiffWorkflow.php:469] #3 ArcanistDiffWorkflow::run() called at [<arcanist>/scripts/arcanist.php:394]
Subpport sub-commands on cli.Application methods Tags: Release-Engineering-Team (Long-Lived-Branches), #scap3 Maniphest Tasks: T142880 Summary: ...
Notice the Tags: line has a milestone formatted as Parent-project (milestone) and it isn't using the #long-lived-branches hashtag.
I added the milestone tag via the differential web ui. I'm not even sure if I could have added it to the commit message, most likely that would have also caused arcanist to complain.
I tried amending the commit message, changing tags to refer to the milestone's hashtag, as follows:
Tags: #long-lived-branches, #scap3
Unfortunately, arc diff amends it again, putting it back (to Tags: Release-Engineering-Team (Long-Lived-Branches), #scap3) then aborts with the exception I pasted at the top of this report.