[libcxx-commits] [libcxx] [libc++][NFC] Refactor _LIBCPP_AVAILABILITY_HAS_* macros to always be defined (PR #71002)

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wang-bin wrote:

how to disable _LIBCPP_AVAILABILITY_HAS_ADDITIONAL_IOSTREAM_EXPLICIT_INSTANTIATIONS_1? I use the latest llvm toolchain on ubuntu to cross build linux programs, and expect they can run on OSes with old system libc++ version. in llvm-17 the produced binaries are compatible with old libc++ because `_LIBCPP_ABI_ENABLE_ADDITIONAL_IOSTREAM_EXPLICIT_INSTANTIATIONS_1` is not defined by default, but in llvm-18 `_LIBCPP_AVAILABILITY_HAS_ADDITIONAL_IOSTREAM_EXPLICIT_INSTANTIATIONS_1` is always 1 and breaks abi

https://github.com/llvm/llvm-project/pull/71002