llvm --HEAD should be @20

[FabioFracassi]

brew gist-logs <formula> link OR brew config AND brew doctor output

$ brew doctor
Your system is ready to brew.

brew config
HOMEBREW_VERSION: 4.4.4-41-g451c3a8
ORIGIN: https://github.com/Homebrew/brew
HEAD: 451c3a8735eb9b3050e40802a771eddf7c23e568
Last commit: 8 hours ago
Core tap HEAD: 18d1256005fe2919416cc21b14197e851b050835
Core tap last commit: 22 minutes ago
Core tap JSON: 06 Nov 22:28 UTC
Core cask tap HEAD: fc825e5702a5ee354e2c8b4f4ac36aac9f552419
Core cask tap last commit: 8 minutes ago
Core cask tap JSON: 06 Nov 22:28 UTC
HOMEBREW_PREFIX: /opt/homebrew
HOMEBREW_CASK_OPTS: []
HOMEBREW_MAKE_JOBS: 10
HOMEBREW_SORBET_RUNTIME: set
Homebrew Ruby: 3.3.6 => /opt/homebrew/Library/Homebrew/vendor/portable-ruby/3.3.6/bin/ruby
CPU: 10-core 64-bit arm_firestorm_icestorm
Clang: 16.0.0 build 1600
Git: 2.39.5 => /Applications/Xcode.app/Contents/Developer/usr/bin/git
Curl: 8.6.0 => /usr/bin/curl
macOS: 14.5-arm64
CLT: 16.0.0.0.1.1724870825
Xcode: 16.1
Rosetta 2: false

Verification

• My brew doctor output says Your system is ready to brew. and am still able to reproduce my issue.
• I ran brew update and am still able to reproduce my issue.
• I have resolved all warnings from brew doctor and that did not fix my problem.
• I searched for recent similar issues at https://github.com/Homebrew/homebrew-core/issues?q=is%3Aissue and found no duplicates.

What were you trying to do (and why)?

Install llvm@19 alongside llvm --HEAD (which resolves to llvm/clang --version 20.x.x)

I already have llvm --HEAD and llvm@18 installed

What happened (include all command output)?

brew install llvm@19

==> Downloading https://formulae.brew.sh/api/formula.jws.json

==> Downloading https://formulae.brew.sh/api/cask.jws.json

Warning: llvm 19.1.3 is already installed and up-to-date.
To reinstall 19.1.3, run:
  brew reinstall llvm

What did you expect to happen?

Not exactly expect, but I would wish that it installed in the same way llvm@18 did

Step-by-step reproduction instructions (by running brew commands)

- `brew install llvm --HEAD`  (works, takes a while -> build from source as expected
                               llvm/bin/clang --version returns 20.x.x
                              )
- `brew install llvm@18`      (works, as expected llvm@18/bin/clang --version returns 18.x.x.)
- `brew install llvm@19`      (warning already installed)
- `brew reinstall llvm@19`    (works, as expected llvm@19/bin/clang --version returns 19.x.x.
                               but llvm/bin/clang --version returns 19.x.x now as well
                              )