how could a symlink from one version to a different one be left in /opt/homebrew/opt

[190n]

Output of brew config

HOMEBREW_VERSION: 4.5.8
ORIGIN: https://github.com/Homebrew/brew
HEAD: c19f08b3e8ea46ba94bd9315c46c0dda10ead31c
Last commit: 2 weeks ago
Branch: stable
Core tap JSON: 07 Jul 20:46 UTC
Core cask tap JSON: 07 Jul 20:46 UTC
HOMEBREW_PREFIX: /opt/homebrew
HOMEBREW_CASK_OPTS: []
HOMEBREW_MAKE_JOBS: 24
Homebrew Ruby: 3.4.4 => /opt/homebrew/Library/Homebrew/vendor/portable-ruby/3.4.4/bin/ruby
CPU: 24-core 64-bit arm_blizzard_avalanche
Clang: 15.0.0 build 1500
Git: 2.39.3 => /Library/Developer/CommandLineTools/usr/bin/git
Curl: 8.4.0 => /usr/bin/curl
macOS: 14.4.1-arm64
CLT: 15.3.0.0.1.1708646388
Xcode: N/A
Rosetta 2: false

Output of brew doctor

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Warning: A newer Command Line Tools release is available.
Update them from Software Update in System Settings.

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  sudo rm -rf /Library/Developer/CommandLineTools
  sudo xcode-select --install

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Warning: Some installed formulae are deprecated or disabled.
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Warning: You have the following deprecated, official taps tapped:
  Homebrew/homebrew-services
Untap them with `brew untap`.

Warning: /usr/bin occurs before /opt/homebrew/bin in your PATH.
This means that system-provided programs will be used instead of those
provided by Homebrew. Consider setting your PATH so that
/opt/homebrew/bin occurs before /usr/bin. Here is a one-liner:
  echo 'export PATH="/opt/homebrew/bin:$PATH"' >> ~/.zshrc

The following tools exist at both paths:
  clang
  clang++
  clangd
  dsymutil
  lldb
  openssl
  pip3
  python3

Warning: Homebrew's "bin" was not found in your PATH.
Consider setting your PATH for example like so:
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Warning: Homebrew's "sbin" was not found in your PATH but you have installed
formulae that put executables in /opt/homebrew/sbin.
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Description of issue

I work for https://github.com/oven-sh where we have many Macs with Homebrew installed and use LLVM 19 to compile our C++ code. At various times on different machines, Homebrew has created a symlink from the llvm@19 prefix to the llvm@20 one:

% ls -ld /opt/homebrew/opt/llvm@19
lrwxr-xr-x  1 administrator  admin  21 Jul 10 00:10 /opt/homebrew/opt/llvm@19 -> ../Cellar/llvm/20.1.7

I feel pretty strongly that there is some Homebrew bug here -- it should never create a symlink like that. But so far I haven't been able to reproduce this situation starting from a fresh Homebrew install, so I don't know how to report it.

On this particular machine, I know that we previously had pinned llvm@19 and also had an old Rust version installed that depended on LLVM 19. Upgrading Rust was blocked because the new version would require the LLVM version be upgraded to 20. The last commands ran were:

brew unpin llvm
HOMEBREW_NO_AUTO_UPDATE=1 brew install rust

Unpinning LLVM turned out to be a mistake since we actually need to keep version 19 too. But that should have left us with /opt/homebrew/opt/llvm@19 deleted, not changed to point to version 20.

I did try to reproduce this situation on a fresh VM, by:

• tapping homebrew/core and reverting the repository to Homebrew/homebrew-core@2511b1a (when Rust 1.86, the last version to use LLVM 19, was added)
• installing Rust 1.86 and LLVM 19
• upgrading everything

But I always observed that /opt/homebrew/opt/llvm@19 was either preserved with the right version, or deleted. I couldn't recreate the situation where it is a symlink to the wrong version.

Has anyone else observed this issue, or does anyone have ideas for what commands might lead to this situation?