Index: head/bin/expr/Makefile
===================================================================
--- head/bin/expr/Makefile	(revision 280356)
+++ head/bin/expr/Makefile	(revision 280357)
@@ -1,15 +1,18 @@
 # $FreeBSD$
 
 .include <src.opts.mk>
 
 PROG=	expr
 SRCS=	expr.y
 YFLAGS=
 
+# expr relies on signed integer wrapping
+CFLAGS+= -fwrapv
+
 NO_WMISSING_VARIABLE_DECLARATIONS=
 
 .if ${MK_TESTS} != "no"
 SUBDIR+=	tests
 .endif
 
 .include <bsd.prog.mk>