brew link running for formula marked as keg_only

[nwhetsell]

It seems like there has been a recent to change to how brew handles formulae marked as keg_only. I maintain a tap for the unstable version of LilyPond; the stable version of LilyPond is in Homebrew/core. My formula for LilyPond is marked as keg_only so that it can coexist with LilyPond from core, but running—

brew install lilypond
brew install install nwhetsell/lilypond/lilypond-unstable

—started producing errors within the last few weeks:

✔︎ JSON API formula.jws.json                          Downloaded   31.8MB/ 31.8MB
✔︎ JSON API cask.jws.json                             Downloaded   15.4MB/ 15.4MB
==> Fetching downloads for: lilypond-unstable
✔︎ Bottle lilypond-unstable (2.25.35)                 Downloaded    8.3MB/  8.3MB
==> Installing lilypond-unstable from nwhetsell/lilypond
==> Pouring lilypond-unstable-2.25.35.arm64_sequoia.bottle.tar.gz
Error: The `brew link` step did not complete successfully
The formula built, but is not symlinked into /opt/homebrew
Could not symlink bin/abc2ly
Target /opt/homebrew/bin/abc2ly
is a symlink belonging to lilypond. You can unlink it:
  brew unlink lilypond

To force the link and overwrite all conflicting files:
  brew link --overwrite lilypond-unstable

To list all files that would be deleted:
  brew link --overwrite lilypond-unstable --dry-run

Possible conflicting files are:
/opt/homebrew/bin/abc2ly -> /opt/homebrew/Cellar/lilypond/2.24.4/bin/abc2ly
/opt/homebrew/bin/convert-ly -> /opt/homebrew/Cellar/lilypond/2.24.4/bin/convert-ly
/opt/homebrew/bin/etf2ly -> /opt/homebrew/Cellar/lilypond/2.24.4/bin/etf2ly
/opt/homebrew/bin/lilymidi -> /opt/homebrew/Cellar/lilypond/2.24.4/bin/lilymidi
/opt/homebrew/bin/lilypond -> /opt/homebrew/Cellar/lilypond/2.24.4/bin/lilypond
/opt/homebrew/bin/lilypond-book -> /opt/homebrew/Cellar/lilypond/2.24.4/bin/lilypond-book
/opt/homebrew/bin/lilypond-invoke-editor -> /opt/homebrew/Cellar/lilypond/2.24.4/bin/lilypond-invoke-editor
/opt/homebrew/bin/midi2ly -> /opt/homebrew/Cellar/lilypond/2.24.4/bin/midi2ly
/opt/homebrew/bin/musicxml2ly -> /opt/homebrew/Cellar/lilypond/2.24.4/bin/musicxml2ly
Error: Could not symlink share/lilypond/2.24.4/fonts/00-lilypond-fonts.conf
Target /opt/homebrew/share/lilypond/2.24.4/fonts/00-lilypond-fonts.conf
is a symlink belonging to lilypond. You can unlink it:
  brew unlink lilypond

To force the link and overwrite all conflicting files:
  brew link --overwrite lilypond

To list all files that would be deleted:
  brew link --overwrite lilypond --dry-run

~ took 5.1s 

This used to install without errors and (if I remember correctly) output a caveat about the installation being keg-only:

lilypond-unstable is keg-only, which means it was not symlinked into /opt/homebrew,
because this is an alternate version of another formula.

If you need to have lilypond-unstable first in your PATH, run:
  echo 'export PATH="/opt/homebrew/opt/lilypond-unstable/bin:$PATH"' >> ~/.zshrc

For compilers to find lilypond-unstable you may need to set:
  export LDFLAGS="-L/opt/homebrew/opt/lilypond-unstable/lib"

I believe this change occurred sometime after 21 Feb.

I have a feeling that I’m actually somewhat abusing the use of keg_only :versioned_formula in my tap, but it still seems unexpected that a formula marked as keg_only is not actually keg_only. Is there another way to avoid running brew link for a formula?

brew doctor output:

Please note that these warnings are just used to help the Homebrew maintainers
with debugging if you file an issue. If everything you use Homebrew for is
working fine: please don't worry or file an issue; just ignore this. Thanks!

Warning: Some installed casks are deprecated or disabled.
You should find replacements for the following casks:
  vmware-fusion

Warning: Unbrewed dylibs were found in /usr/local/lib.
If you didn't put them there on purpose they could cause problems when
building Homebrew formulae and may need to be deleted.

Unexpected dylibs:
  /usr/local/lib/libASAF.dylib
  /usr/local/lib/libcpsrt.dylib

~ took 5.4s 

brew config output:

HOMEBREW_VERSION: 5.0.16-82-gaa4eedb
ORIGIN: https://github.com/Homebrew/brew
HEAD: aa4eedbaf2b7b22bd73aad9e8b00fe77a1bb3341
Last commit: 3 hours ago
Branch: main
Core tap HEAD: 92dcec80508325a285156e7c77d2cda8f2e7b9d9
Core tap last commit: 43 minutes ago
Core tap JSON: 07 Mar 13:34 UTC
Core cask tap HEAD: 541d99fddf3c8daaeeea752b77e11031a43ee716
Core cask tap last commit: 2 hours ago
Core cask tap JSON: 07 Mar 13:34 UTC
HOMEBREW_PREFIX: /opt/homebrew
HOMEBREW_CASK_OPTS: []
HOMEBREW_DOWNLOAD_CONCURRENCY: 24
HOMEBREW_FORBID_PACKAGES_FROM_PATHS: set
HOMEBREW_MAKE_JOBS: 12
HOMEBREW_SORBET_RUNTIME: set
Homebrew Ruby: 4.0.1 => /opt/homebrew/Library/Homebrew/vendor/portable-ruby/4.0.1/bin/ruby
CPU: dodeca-core 64-bit arm_blizzard_avalanche
Clang: 17.0.0 build 1700
Git: 2.53.0 => /opt/homebrew/bin/git
Curl: 8.7.1 => /usr/bin/curl
macOS: 15.7.4-arm64
CLT: 26.2.0.0.1.1764812424
Xcode: 26.3
Metal Toolchain: N/A
Rosetta 2: false

[daeho-ro]

Can you try with any other reason of keg_only? For example, keg_only "something" not just a versioned_formula.

[daeho-ro]

This is a related PR for recent change

• Revert "Revert "Link versioned keg-only formulae by default"" brew#21684

[nwhetsell]

Using a string for the keg_only reason seems to fix the issue.