The current release of LLVM21 available on brew fails to build Yosys with the following error:
Undefined symbols for architecture arm64:
"std::__1::__hash_memory(void const*, unsigned long)"
This is apparently due to something in the .dylib files causing a conflict with the latest versions of LLVM/clang and is affecting both arm64 and x86-64 macOS. LLVM are aware of the issue, and it may also be affecting GCC-15.
For now, specifying llvm@20 (or earlier) when installing from brew is sufficient for continuing to build Yosys, and the Brewfile included in the git repository is being updated to match until this can be resolved.