[HaraldNordgren]

On my MacBook Air (macOS 12.7.6, Xcode 14.2, so no bottles), brew install nmap wanted to rebuild four dependencies from source to go from nmap 7.991 to 7.993:

==> Would upgrade 1 formula:
nmap 7.991 -> 7.993
==> Would upgrade 4 dependencies for nmap:
openssl@4          4.0.3  -> 4.1.0
sqlite             3.54.0 -> 3.54.1
python@3.15        3.15.0 -> 3.15.2
python-setuptools  84.0.0 -> 84.2.1

sqlite is a runtime dependency of python@3.15, and the installed 3.54.0 has the same compatibility_version as the current formula. By the Formula Cookbook's definition, that means nothing built against 3.54.0 needs rebuilding when it's upgraded, so upgrading it before building python@3.15 gains nothing. Since #24273, source builds already keep an outdated build dependency in this case, but runtime dependencies are still always upgraded.

This applies the same rule to runtime dependencies: when the dependent is built from source, keep an outdated runtime dependency whose installed compatibility_version matches, along with the dependencies it was built against. As with build dependencies, it's still upgraded if this install would incompatibly upgrade one of those. Pouring a bottle is unchanged, since #15927 already keeps outdated runtime dependencies that the bottle's tab says are new enough. Building a bottle still upgrades everything.

With this branch, the plan drops sqlite:

==> Would upgrade 1 formula:
nmap 7.991 -> 7.993
==> Would upgrade 3 dependencies for nmap:
openssl@4          4.0.3  -> 4.1.0
python@3.15        3.15.0 -> 3.15.2
python-setuptools  84.0.0 -> 84.2.1

The other three are still upgraded because their formulae don't set compatibility_version, so there's nothing to compare.

One trade-off: if a new version of a dependent needs something newer from a kept runtime dependency, its build fails, just as it would with a kept build dependency, and brew upgrade on that dependency fixes it.

Changes

• FormulaInstaller: when the dependent is built from source, an outdated runtime dependency now takes the same keep path as build dependencies. :no_linkage still only keeps build dependencies whatever their compatibility_version. kept_build_dependencies and keepable_build_dependencies are renamed to match.
• Specs: the example that expected an outdated runtime dependency with a matching compatibility_version to be upgraded now expects it to be kept, plus a new example for pouring a bottle. Three specs that stubbed install_bottle_for?: false only to simplify their setup now stub true, since they cover bottle tab metadata and satisfaction caching for runtime dependencies.

AI usage

Claude Code (Claude Opus 5.5) helped trace the dependency resolution and audit code and write the change and specs. I verified it by running brew install nmap on my macOS 12 machine with this branch, which dropped sqlite from the plan as shown above. brew style, brew typecheck and the formula_installer specs pass locally.

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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.

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[coderabbitai]

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📝 Summary

Summary by CodeRabbit

• Bug Fixes
  • Improved dependency handling during formula installation, retaining eligible installed dependencies and accounting for their own runtime dependencies.
  • Corrected handling of runtime dependencies with matching compatibility versions: compatible dependencies are retained during normal installation, while dependencies may be upgraded when their opt prefix is unavailable or a bottle is being poured.

Dependency expansion now retains eligible runtime dependencies as well as build dependencies. Bottle builds skip retention. Updated tests cover bottle-installation conditions and compatibility-version handling.

Changes: Dependency retention

Layer / File(s)	Summary
Dependency retention and re-expansion Library/Homebrew/formula_installer.rb, Library/Homebrew/test/formula_installer_spec.rb	The expansion method applies retention to build and runtime dependencies, subject to bottle-build and installed-runtime-dependency conditions. It records installed kegs’ runtime dependencies and excludes kept dependencies when those runtime dependencies require upgrades. Tests cover bottle metadata, dependency caching and compatibility-version cases.

Priority: Normal

Merge Risk: Moderate · previous review 3aa27

A source build can keep a dependency installed without options it requires and omit the needed rebuild. Check those options before merging.

Pre-merge checks: 4

Check name	Status	Explanation
Title check		The title clearly and concisely describes the main change: retaining compatible outdated runtime dependencies.
Description check		The description directly explains the problem, implementation, scope, tests, and verification for keeping compatible outdated runtime dependencies.
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.

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Actionable comments posted: 1

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[coderabbitai]

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[MikeMcQuaid]

Thanks!