Upgrade detection on HEAD-only cask not working

[aWZHY0yQH81uOYvH]

Output of brew config

HOMEBREW_VERSION: 3.6.16-97-ge76c55e
ORIGIN: https://github.com/Homebrew/brew
HEAD: e76c55e18434c870cca1de7aadf9d5218271fe2d
Last commit: 2 days ago
Core tap ORIGIN: https://github.com/Homebrew/homebrew-core
Core tap HEAD: d56813baa5270d5e333708cabf85fa17c900bc0b
Core tap last commit: 7 hours ago
Core tap branch: master
HOMEBREW_PREFIX: /usr/local
HOMEBREW_CASK_OPTS: []
HOMEBREW_DISPLAY: /private/tmp/com.apple.launchd.SXSxd919Ch/org.xquartz:0
HOMEBREW_MAKE_JOBS: 12
Homebrew Ruby: 2.6.10 => /System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/bin/ruby
CPU: dodeca-core 64-bit kabylake
Clang: 14.0.0 build 1400
Git: 2.39.0 => /usr/local/bin/git
Curl: 7.85.0 => /usr/bin/curl
macOS: 13.1-x86_64
CLT: 14.2.0.0.1.1668646533
Xcode: 14.2

Description of issue

I'm working on updating a simple HEAD-only formula. The formula installs correctly, but it doesn't detect updates.

% brew upgrade openems
Running `brew update --auto-update`...
==> Auto-updated Homebrew!
Updated 1 tap (thliebig/openems).
No changes to formulae.

Warning: thliebig/openems/openems HEAD-a37a29b already installed

% brew livecheck --debug openems
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::FormulaLoader): loading /usr/local/Homebrew/Library/Taps/thliebig/homebrew-openems/openEMS.rb

git --git-dir /Users/pleiades/Library/Caches/Homebrew/openEMS--git/.git status -s
git config remote.origin.url https://github.com/thliebig/openEMS-Project.git
git config remote.origin.fetch \+refs/heads/master:refs/remotes/origin/master
git config remote.origin.tagOpt --no-tags
git config advice.detachedHead false
git fetch origin
git checkout -f master --
git reset --hard origin/master --
git submodule foreach --recursive git\ submodule\ sync
git submodule update --init --recursive
git submodule --quiet foreach --recursive pwd
git --git-dir /Users/pleiades/Library/Caches/Homebrew/openEMS--git/.git rev-parse --short=7 HEAD
git --git-dir /Users/pleiades/Library/Caches/Homebrew/openEMS--git/.git rev-parse --short=7 HEAD
Error: openEMS: undefined method `null?' for "a37a29b":String
/usr/local/Homebrew/Library/Homebrew/livecheck/livecheck_version.rb:54:in `<=>'
/usr/local/Homebrew/Library/Homebrew/livecheck/livecheck.rb:342:in `=='
/usr/local/Homebrew/Library/Homebrew/livecheck/livecheck.rb:342:in `!='
/usr/local/Homebrew/Library/Homebrew/livecheck/livecheck.rb:342:in `block in run_checks'
/usr/local/Homebrew/Library/Homebrew/livecheck/livecheck.rb:221:in `map'
/usr/local/Homebrew/Library/Homebrew/livecheck/livecheck.rb:221:in `with_index'
/usr/local/Homebrew/Library/Homebrew/livecheck/livecheck.rb:221:in `run_checks'
/usr/local/Homebrew/Library/Homebrew/dev-cmd/livecheck.rb:124:in `livecheck'
/usr/local/Homebrew/Library/Homebrew/brew.rb:97:in `<main>'

It fails when doing its livecheck, and doesn't detect that the newest commit hash is 1c9cb1d instead of a37a29b, which is presumably why it says a37a29b is already installed. (?) If so, it isn't telling the user about the failure. Am I doing something wrong in writing the formula code, or is there a problem in the way livecheck is handling the commit hashes? This comment leads me to believe it is trying to compare the commit hashes as I would expect.

[SMillerDev]

Livecheck is not meant to compare commits. It can only work with actual versions.

[aWZHY0yQH81uOYvH]

Ok it sounds like --fetch-HEAD is what's required to get upgrade to keep a HEAD-only formula up to date, given there aren't any version tags.