Before d473e2e0e8 (diff.c: convert -U|--unified, 2019-01-27), -U and --unified are implemented with a custom parser opt_arg() in diff.c. I didn't check this code carefully and not realize that it's the equivalent of PARSE_OPT_NONEG | PARSE_OPT_OPTARG. In other words, if -U is specified without any argument, the option should be accepted, and the default value should be used. Without PARSE_OPT_OPTARG, parse_options() will reject this case and cause a regression. Reported-by: Bryan Turner <bturner@atlassian.com> Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>