Linking a C++ project fails after upgrading to llvm 21

[ojrosten]

Output of brew config

HOMEBREW_VERSION: 4.6.9
ORIGIN: https://github.com/Homebrew/brew
HEAD: 1c959c22cee9eb333130633b30d380766e2dd09b
Last commit: 4 days ago
Branch: stable
Core tap JSON: 07 Sep 09:57 UTC
Core cask tap JSON: 07 Sep 09:57 UTC
HOMEBREW_PREFIX: /opt/homebrew
HOMEBREW_CASK_OPTS: []
HOMEBREW_FORBID_PACKAGES_FROM_PATHS: set
HOMEBREW_MAKE_JOBS: 14
Homebrew Ruby: 3.4.5 => /opt/homebrew/Library/Homebrew/vendor/portable-ruby/3.4.5/bin/ruby
CPU: 14-core 64-bit arm_brava
Clang: 17.0.0 build 1700
Git: 2.39.5 => /Applications/Xcode.app/Contents/Developer/usr/bin/git
Curl: 8.7.1 => /usr/bin/curl
macOS: 15.6.1-arm64
CLT: 16.4.0.0.1.1747106510
Xcode: 16.4
Rosetta 2: false

Output of brew doctor

Your system is ready to brew.

Description of issue

I just upgraded to llvm 21 and a project ceases to link with the following error:

Undefined symbols for architecture arm64:
"std::__1::__hash_memory(void const*, unsigned long)", referenced from:
unsigned long std::__1::__do_string_hash[abi:ne210100]<char const*>(char const*, char const*) in libavocet.a4

The project is built with cmake and the make output of the step which fails is:

//opt/homebrew/bin/cmake -E cmake_link_script CMakeFiles/TestAll.dir/link.txt --verbose=1
/opt/homebrew/opt/llvm/bin/clang++ -arch arm64 -Wl,-search_paths_first -Wl,-headerpad_max_install_names
.......
-Wl,-rpath,/opt/homebrew/lib curlew/libcurlew.a curlew/glfw/src/libglfw3.a -framework Cocoa -framework IOKit -framework CoreFoundation -framework QuartzCore curlew/avocet/libavocet.a curlew/avocet/glad/libglad.a curlew/avocet/sequoia/libsequoia.a /opt/homebrew/lib/libtbb.12.16.dylib

If anyone can shed any light on this I'd be very grateful!

[CraigHutchinson]

Hmm, reminds me of when a function is moved from being template/header code to being part of the std library but you're not actually linking it.

There looks to be some workaround in the repo that maybe is relevant to your issue. alive2: revision bump (llvm 21.1.0) · Homebrew/homebrew-core@e7425f0

The commit it links ([libc++] Instantiate hash function externally (#127040) · llvm/llvm-project@17d0569) peels back the change causing your issue which is what you'd expect really. Looks like the definition should be there though if built correctly.