[HaraldNordgren]

I had my brew and homebrew-core checkouts on feature branches and ran brew update. It printed Updated Homebrew from fcdfa0476e to 1f55ebf835 and Updated 2 taps (homebrew/core and homebrew/cask), so I assumed both branches had been updated. Neither had, and the brew I was running was still my branch's code on top of the old main.

This happens in developer mode, which turns itself on the first time you run a developer command. update.sh then checks out the upstream branch, rebases it onto origin/main and checks the original branch out again. The summary lines describe the upstream branch and nothing says that another branch is checked out.

update-report now prints a warning under each summary line when the upstream branch moved but HEAD isn't where it ended up. Repositories on their default branch get no warning, so the output only changes for people working on a branch. brew doctor already flags taps that aren't on their default branch, but only if you think to run it.

==> Updated Homebrew from 411ad3ef42 to 1f55ebf835.
Warning: Homebrew/brew is on branch 'update-notice-non-default-branch'

To reproduce:

1. brew developer on
2. cd "$(brew --repository)" && git switch -c my-branch origin/main~10 && git branch -f main origin/main~10
3. brew update reports that Homebrew was updated, but my-branch is still checked out at origin/main~10.

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• I did not use AI/LLM to create this PR, or I disclosed the tool/model below and reviewed its output; I did not attribute commits to AI and will answer maintainer questions and review comments myself without AI/LLM.

Claude Code (Claude Opus 5.5) traced how brew update restores the checked-out branch, wrote the change and its specs and verified them. The new specs fail without the change and pass with it, and removing the HEAD check makes the "does not warn" spec fail. It ran brew lgtm --online, which passes, and the output above is from a real brew update with main rewound.

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[MikeMcQuaid]

Good idea, unsure about implementation as-is! Would be nice to only warn if there's upstream updates and you're on a non-default branch and the warning there is just about the branch you're on as concisely as possible (1 line max)

[coderabbitai]

Summary

Summary by CodeRabbit

• Bug Fixes
  • Update reports now warn when the updated revision differs from the current branch reference in the Homebrew repository or an installed Git tap with a recorded revision. Core taps are included, even when installing from the API.
  • No warning is shown when the updated branch is checked out, keeping reports clear when the repository and update are in sync.

The update report checks whether updated revisions match the current branch reference. It applies this check to Homebrew and installed Git taps. Tests cover warning and no-warning cases.

Changes: Branch update warning

Layer / File(s)	Summary
Branch check and reporting Library/Homebrew/cmd/update-report.rb, Library/Homebrew/test/cmd/update-report_spec.rb	The report compares updated revisions with the current branch reference and warns when they differ. It checks Homebrew and installed Git taps. Tests cover warning and no-warning cases.

Merge Risk: Low · up to cccac

The warning works on the covered paths, but a future caller-wiring regression could leave the helper specs green while silently disabling the warning.

Pre-merge checks: 4

Check name	Status	Explanation
Title check		The title states the main change: a warning is added when brew update leaves a branch unchanged. The raw summary and objectives both describe a warning shown when the upstream branch moves but HEAD do…
Description check		The description explains the problem, the cause in developer mode, the new warning behaviour, and reproduction steps. It also lists testing and checklist items. It relates directly to the changeset, s…
Linked Issues check		Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check		Check skipped because no linked issues were found for this pull request.

Full details: Title check

Explanation

The title states the main change: a warning is added when brew update leaves a branch unchanged. The raw summary and objectives both describe a warning shown when the upstream branch moves but HEAD does not. The title is specific and matches the changeset, so it passes.

Full details: Description check

Explanation

The description explains the problem, the cause in developer mode, the new warning behaviour, and reproduction steps. It also lists testing and checklist items. It relates directly to the changeset, so it passes.

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[coderabbitai]

Actionable comments posted: 1

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• Library/Homebrew/cmd/update-report.rb
• Library/Homebrew/test/cmd/update-report_spec.rb

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[coderabbitai]

Follow-up Comments

Keep installed Git tap warnings reachable when formula loading is disabled

296-298: Minor · Functional Correctness · Quick win

Keep installed Git tap warnings reachable when formula loading is disabled

When HOMEBREW_DISABLE_LOAD_FORMULA=1, brew update still updates Git taps, records their revisions, and runs brew update-report. The return at line 242 prevents the installed Git tap loop from running. Therefore, update-report omits the warning when an updated tap remains on its original branch.

Move this independent loop before the early return, or restructure the return so that it skips only formula-dependent reporting.

Suggested fix

         migrate_caskroom_caskfiles_to_json
+        Tap.installed.select(&:git?).each do |tap|
+          warn_if_branch_not_updated(tap.name, tap.git_repository, tap.repository_var_suffix)
+        end
+
         return if Homebrew::EnvConfig.disable_load_formula?
@@
-        Tap.installed.select(&:git?).each do |tap|
-          warn_if_branch_not_updated(tap.name, tap.git_repository, tap.repository_var_suffix)
-        end
-
         if updated

Open the source location

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[coderabbitai]

Nitpick comments: 1

Library/Homebrew/test/cmd/update-report_spec.rb: 1

147-168: Trivial · Maintainability & Code Quality · Quick win

Add a report-level example for the branch-warning call sites.

The new specs call warn_if_branch_not_updated directly and pass tap.git_head. They do not run UpdateReport#run. No spec therefore covers the call at update-report.rb:232 that passes HOMEBREW_UPDATE_AFTER, or the call at update-report.rb:313 that passes the suffixed tap revision. A removed call, or one that reads the wrong variable, would leave these specs passing.

The tap call sits after return if Homebrew::EnvConfig.disable_load_formula?. A report-level example must therefore leave that flag unset. It should set HOMEBREW_UPDATE_BEFORE<suffix> and HOMEBREW_UPDATE_AFTER<suffix> to different values while the tap has a non-updated branch checked out. It should then expect the warning on stderr. Stub the steps before the loop that the example does not need, as the existing report-level examples do.

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• Library/Homebrew/test/cmd/update-report_spec.rb

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[MikeMcQuaid]

Looks good now thanks!

[HaraldNordgren]

Looks good now thanks!

Thanks for the help here!