diff --git a/include/misc.h b/include/misc.h
index 29a3671c..3217c39a 100644
--- a/include/misc.h
+++ b/include/misc.h
@@ -82,6 +82,13 @@ extern void (*oops_handler)(void);
 #define BUILD_ASSERT_ONE(cond) \
     (sizeof(char[1 - 2 * !(cond)]))
 
+/*
+ * Assert constant expression @cond.
+ * If @cond is zero, force a compilation error.
+ */
+#define BUILD_ASSERT(cond) \
+    ((void)BUILD_ASSERT_ONE(cond))
+
 void exit_nomem(void) ATTRIBUTE((noreturn));
 
 	/* return codes from command routines */
diff --git a/src/lib/subs/show.c b/src/lib/subs/show.c
index b214af15..0ec50156 100644
--- a/src/lib/subs/show.c
+++ b/src/lib/subs/show.c
@@ -516,7 +516,7 @@ show_product(int tlev)
 	   pp->p_sname,
 	   pp->p_type < 0 ? ' ' : ichr[pp->p_type].i_mnem,
 	   pp->p_cost);
-	(void)CANT_HAPPEN(MAXPRCON > 3); /* output has only three columns */
+	BUILD_ASSERT(MAXPRCON <= 3); /* output has only three columns */
 	for (i = 0; i < 3; i++) {
 	    if (i < MAXPRCON && pp->p_camt[i]
 		&& pp->p_ctype[i] > I_NONE && pp->p_ctype[i] <= I_MAX)