llvm@19 links against wrong libc++ library version

[ibis-hdl]

Output of brew config

HOMEBREW_VERSION: 4.4.15
ORIGIN: https://github.com/Homebrew/brew
HEAD: b6fafba4864d65acabf966415e14b2dd86d81e1a
Last commit: 33 hours ago
Branch: stable
Core tap JSON: 06 Jan 11:30 UTC
Core cask tap JSON: 06 Jan 11:30 UTC
HOMEBREW_PREFIX: /opt/homebrew
HOMEBREW_CASK_OPTS: ["--no-quarantine"]
HOMEBREW_COLOR: set
HOMEBREW_MAKE_JOBS: 3
HOMEBREW_NO_AUTO_UPDATE: set
HOMEBREW_NO_INSTALL_CLEANUP: set
Homebrew Ruby: 3.3.6 => /opt/homebrew/Library/Homebrew/vendor/portable-ruby/3.3.6/bin/ruby
CPU: 3-core 64-bit dunno
Clang: 16.0.0 build 1600
Git: 2.47.1 => /opt/homebrew/bin/git
Curl: 8.7.1 => /usr/bin/curl
macOS: 15.2-arm64
CLT: 16.2.0.0.1.1733547573
Xcode: 16.0 => /Applications/Xcode_16.app/Contents/Developer
Rosetta 2: false

Output of brew doctor

Your system is ready to brew.

Description of issue

What you were trying to do (and why)

I'm using clang-19 on Github's macos-15 runner, installed by homebrew by:

  brew update
  brew install ninja llvm@19
  echo "$(brew --prefix llvm@19)/bin" >> $GITHUB_PATH

The image has pre-installed clang-18, also via homebrew, see Readme. My CMake Project uses clang++ 19 and is linked against libc++, but if I check the linked lib I see:

What happened (include command output)

$ otool -L .../gha-clang19
...
    /usr/lib/libc++.1.dylib (compatibility version 1.0.0, current version 1800.101.0)
    /usr/lib/libSystem.B.dylib (compatibility version 1.0.0, current version 1351.0.0)

I would expect libc++.1.dylib (... current version 190106). Did something go wrong at CMake's configure time?

What you expected to happen

Linked against the right libc++

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

Compile main.cpp with e.g.

std::cout << std::format("LibC++ {:d}\n", _LIBCPP_VERSION);

and run the binary, which shows

LibC++ 190106

So obviously the header version is correct, but isn't linked correctly.

I added ~/.config/clang file with:

# See note on brew install llvm@19
-I/opt/homebrew/opt/llvm/include
-L/opt/homebrew/opt/llvm/lib/c++
-L/opt/homebrew/opt/llvm/lib/unwind -lunwind

On CMake's side, additionally I configured as shown from brew install llvm@19, using CMake's inherited preset

        {
            "name": "settings-stdlib-libc++",
            "cacheVariables": {
                "CMAKE_CXX_FLAGS_INIT": "-stdlib=libc++ -I/opt/homebrew/opt/llvm/include",
                "CMAKE_EXE_LINKER_FLAGS_INIT": "-stdlib=libc++ -L/opt/homebrew/opt/llvm/lib",
                "CMAKE_SHARED_LINKER_FLAGS_INIT": "-stdlib=libc++ -L/opt/homebrew/opt/llvm/lib"
            }
        },
...
        {
            "name": "compiler-clang-libc++",
            "inherits": [ "settings-stdlib-libc++" ],
            "cacheVariables": {
                "CMAKE_C_COMPILER": "clang",
                "CMAKE_CXX_COMPILER": "clang++"
            }
        }
...
        {
            "name": "clang-libc++",
            "inherits": [
                "compiler-clang-libc++",
                "condition-linux-or-macos",
                ...
            ]
        },

The complete project (to clang-19 reduced/related; also devcontainer / Linux) can be found here.