[Clang Frontend Potentially Breaking Changes](https://releases.llvm.org/18.1.6/tools/clang/docs/ReleaseNotes.html#id208)
Target OS macros extension A new Clang extension (see [here](https://releases.llvm.org/18.1.6/tools/clang/docs/ReleaseNotes.html#target-os-detail)) is enabled for Darwin (Apple platform) targets. Clang now defines TARGET_OS_* macros for these targets, which could break existing code bases with improper checks for the TARGET_OS_ macros. For example, existing checks might fail to include the TargetConditionals.h header from Apple SDKs and therefore leaving the macros undefined and guarded code unexercised.

Affected code should be checked to see if it’s still intended for the specific target and fixed accordingly.

The extension can be turned off by the option -fno-define-target-os-macros as a workaround.