diff --git a/include/stddef.h b/include/stddef.h
--- a/include/stddef.h
+++ b/include/stddef.h
@@ -61,7 +61,7 @@
 #endif
 #endif
 
-#if __ISO_C_VISIBLE >= 2023
+#if __ISO_C_VISIBLE >= 2023 && !defined(__cplusplus)
 #define unreachable(x) __unreachable(x)
 #endif