Index: sys/ddb/db_expr.c
===================================================================
--- sys/ddb/db_expr.c
+++ sys/ddb/db_expr.c
@@ -34,6 +34,7 @@
 __FBSDID("$FreeBSD$");
 
 #include <sys/param.h>
+#include <sys/systm.h>
 
 #include <ddb/ddb.h>
 #include <ddb/db_lex.h>
Index: sys/dev/nvdimm/nvdimm.c
===================================================================
--- sys/dev/nvdimm/nvdimm.c
+++ sys/dev/nvdimm/nvdimm.c
@@ -35,6 +35,7 @@
 #include "opt_ddb.h"
 
 #include <sys/param.h>
+#include <sys/systm.h>
 #include <sys/bio.h>
 #include <sys/bitstring.h>
 #include <sys/bus.h>
Index: sys/sys/cdefs.h
===================================================================
--- sys/sys/cdefs.h
+++ sys/sys/cdefs.h
@@ -383,9 +383,13 @@
 #endif
 
 #if __GNUC_PREREQ__(4, 6) || __has_builtin(__builtin_unreachable)
-#define	__unreachable()	__builtin_unreachable()
+#define	__ct_unreachable()	__builtin_unreachable()
 #else
-#define	__unreachable()	((void)0)
+#define	__ct_unreachable()	((void)0)
+#endif
+/* _KERNEL version must be obtained from sys/systm.h. */
+#ifndef _KERNEL
+#define	__unreachable()		__ct_unreachable()
 #endif
 
 /* XXX: should use `#if __STDC_VERSION__ < 199901'. */
Index: sys/sys/systm.h
===================================================================
--- sys/sys/systm.h
+++ sys/sys/systm.h
@@ -117,6 +117,7 @@
 	VNASSERT(exp, vp, ("condition %s not met at %s:%d (%s)",	\
 	    _exp, __FILE__, __LINE__, __func__));			\
 } while (0)
+#define	__unreachable()	panic("Unreachable segment reached\n")
 #else
 #define	KASSERT(exp,msg) do { \
 } while (0)
@@ -125,6 +126,7 @@
 } while (0)
 #define	VNPASS(exp, vp) do { \
 } while (0)
+#define	__unreachable()	__ct_unreachable()
 #endif
 
 #ifndef CTASSERT	/* Allow lint to override */