diff --git a/sys/sys/cdefs.h b/sys/sys/cdefs.h
--- a/sys/sys/cdefs.h
+++ b/sys/sys/cdefs.h
@@ -326,6 +326,17 @@
 #if __GNUC_PREREQ__(3, 4)
 #define	__fastcall	__attribute__((__fastcall__))
 #define	__result_use_check	__attribute__((__warn_unused_result__))
+#ifdef __clang__
+/*
+ * clang and gcc have different semantics for __warn_unused_result__: the latter
+ * does not permit the use of a void cast to suppress the warning.  Use
+ * __result_use_or_ignore_check in places where a void cast is acceptable.
+ * This can be implemented by [[nodiscard]] from C23.
+ */
+#define	__result_use_or_ignore_check	__result_use_check
+#else
+#define	__result_use_or_ignore_check
+#endif /* !__clang__ */
 #else
 #define	__fastcall
 #define	__result_use_check
diff --git a/sys/sys/systm.h b/sys/sys/systm.h
--- a/sys/sys/systm.h
+++ b/sys/sys/systm.h
@@ -299,10 +299,11 @@
     void * _Nonnull __restrict kaddr, size_t len);
 int __result_use_check copyin_nofault(const void * __restrict udaddr,
     void * _Nonnull __restrict kaddr, size_t len);
-int copyout(const void * _Nonnull __restrict kaddr,
-    void * __restrict udaddr, size_t len);
-int copyout_nofault(const void * _Nonnull __restrict kaddr,
+int __result_use_or_ignore_check copyout(const void * _Nonnull __restrict kaddr,
     void * __restrict udaddr, size_t len);
+int __result_use_or_ignore_check copyout_nofault(
+    const void * _Nonnull __restrict kaddr, void * __restrict udaddr,
+    size_t len);
 
 #ifdef SAN_NEEDS_INTERCEPTORS
 int	SAN_INTERCEPTOR(copyin)(const void *, void *, size_t);
@@ -323,11 +324,11 @@
 int __result_use_check fueword(volatile const void *base, long *val);
 int __result_use_check fueword32(volatile const void *base, int32_t *val);
 int __result_use_check fueword64(volatile const void *base, int64_t *val);
-int subyte(volatile void *base, int byte);
-int suword(volatile void *base, long word);
-int suword16(volatile void *base, int word);
-int suword32(volatile void *base, int32_t word);
-int suword64(volatile void *base, int64_t word);
+int __result_use_or_ignore_check subyte(volatile void *base, int byte);
+int __result_use_or_ignore_check suword(volatile void *base, long word);
+int __result_use_or_ignore_check suword16(volatile void *base, int word);
+int __result_use_or_ignore_check suword32(volatile void *base, int32_t word);
+int __result_use_or_ignore_check suword64(volatile void *base, int64_t word);
 uint32_t casuword32(volatile uint32_t *base, uint32_t oldval, uint32_t newval);
 u_long	casuword(volatile u_long *p, u_long oldval, u_long newval);
 int	casueword32(volatile uint32_t *base, uint32_t oldval, uint32_t *oldvalp,