diff --git a/blink/checked.h b/blink/checked.h
index 8bd9134b7..93ece01e8 100644
--- a/blink/checked.h
+++ b/blink/checked.h
@@ -1,23 +1,41 @@
 #ifndef EASYCKDINT_H_
 #define EASYCKDINT_H_
-#if defined(__has_include) && __has_include(<stdckdint.h>)
+
+#ifdef __has_include
+#if __has_include(<stdckdint.h>)
 #include <stdckdint.h>
-#else
+#ifndef ckd_add
+#error "<stdckdint.h> lacks ckd_add() macro?"
+#endif /* !ckd_add */
+#endif /* <stdckdint.h> */
+#endif /* __has_include */
+
+#ifndef ckd_add
 
 #define __STDC_VERSION_STDCKDINT_H__ 202311L
 
-#if ((defined(__GNUC__) && __GNUC__ >= 5 && !defined(__ICC)) ||           \
-     (defined(__has_builtin) && (__has_builtin(__builtin_add_overflow) && \
-                                 __has_builtin(__builtin_sub_overflow) && \
-                                 __has_builtin(__builtin_mul_overflow))))
+#if defined(__GNUC__) && __GNUC__ >= 5 && !defined(__ICC)
+#define ckd_add(res, x, y) __builtin_add_overflow((x), (y), (res))
+#define ckd_sub(res, x, y) __builtin_sub_overflow((x), (y), (res))
+#define ckd_mul(res, x, y) __builtin_mul_overflow((x), (y), (res))
+#endif /* __GNUC__ >= 5 && !__ICC */
+
+#if !defined(ckd_add) && defined(__has_builtin)
+#if __has_builtin(__builtin_add_overflow) && \
+    __has_builtin(__builtin_sub_overflow) && \
+    __has_builtin(__builtin_mul_overflow)
 #define ckd_add(res, x, y) __builtin_add_overflow((x), (y), (res))
 #define ckd_sub(res, x, y) __builtin_sub_overflow((x), (y), (res))
 #define ckd_mul(res, x, y) __builtin_mul_overflow((x), (y), (res))
-#else
+#endif /* __builtin_..._overflow */
+#endif /* !ckd_add && __has_builtin */
+
+#ifndef ckd_add /* FIXME */
 #define ckd_add(res, x, y) (*(res) = (x) + (y), 0)
 #define ckd_sub(res, x, y) (*(res) = (x) - (y), 0)
 #define ckd_mul(res, x, y) (*(res) = (x) * (y), 0)
-#endif
+#endif /* !ckd_add */
+
+#endif /* !ckd_add */
 
-#endif /* <stdckdint.h> */
 #endif /* EASYCKDINT_H_ */