Index: sys/sys/cdefs.h
===================================================================
RCS file: /cvsroot/src/sys/sys/cdefs.h,v
retrieving revision 1.144
diff -u -r1.144 cdefs.h
--- sys/sys/cdefs.h	18 Sep 2019 15:06:03 -0000	1.144
+++ sys/sys/cdefs.h	19 Sep 2019 11:39:04 -0000
@@ -172,8 +172,15 @@
 #define	__CTASSERT99(x, a, b)	__CTASSERT0(x, __CONCAT(__ctassert,a), \
 					       __CONCAT(_,b))
 #endif
-#define	__CTASSERT0(x, y, z)	__CTASSERT1(x, y, z) 
-#define	__CTASSERT1(x, y, z)	typedef char y ## z[/*CONSTCOND*/(x) ? 1 : -1] __unused
+#define	__CTASSERT0(x, y, z)	__CTASSERT1(x, y, z)
+/*
+ * Compilers accept 'typedef char x[n]' where n is not constant
+ * at compile time. Use bit-field to enforce compile-time constant.
+ */
+#define	__CTASSERT1(x, y, z)	\
+	typedef struct { \
+		unsigned int y ## z : /*CONSTCOND*/(x) ? 1 : -1; \
+	} y ## z ## _struct __unused
 
 /*
  * The following macro is used to remove const cast-away warnings