--- llvm-18.1.8.src/tools/clang/lib/Basic/Targets/OSTargets.h.orig
+++ llvm-18.1.8.src/tools/clang/lib/Basic/Targets/OSTargets.h
@@ -590,6 +590,7 @@
   void getOSDefines(const LangOptions &Opts, const llvm::Triple &Triple,
                     MacroBuilder &Builder) const override {
     DefineStd(Builder, "sun", Opts);
+    Builder.defineMacro("__illumos__");
     DefineStd(Builder, "unix", Opts);
     Builder.defineMacro("__svr4__");
     Builder.defineMacro("__SVR4");