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Although the false positives that caused it to be disabled originally may have been fixed in GCC 12, GCC also suffers from a problem where -Wuninitialized may cause the build to hang on some platforms (GCC #120729). This has been fixed in GCC 15, so turn on -Wuninitialized for GCC 15+ instead of GCC 12+.
See docs/CMake.html for instructions on how to build LLVM with CMake.