help and version show in Positional arguments on subcommands if there's no other options

[daenney]

When you define a struct which only has positional arguments, the built in --help and --version are printed as part of the positional arguments, instead of the options on subcommands.

On the "root", it works as expected:

type root struct {
	Test string `arg:"positional"`
}

func main() {
	arg.MustParse(&root{})
}

$ go run main.go --help
Usage: main [TEST]

Positional arguments:
  TEST

Options:
  --help, -h             display this help and exit
  --version              display version and exit

But with a subcommand (with 1 or multiple positionals):

type root struct {
	PosOnly *PosOnly `arg:"subcommand:posonly"`
}

type PosOnly struct {
	INPUT  string `arg:"positional"`
	OUTPUT string `arg:"positional"`
}

func main() {
	arg.MustParse(&root{})
}

$ go run main.go --help
Usage: main <command> [<args>]

Options:
  --help, -h             display this help and exit
  --version              display version and exit

Commands:
  posonly

$ go run main.go posonly --help
Usage: main posonly [INPUT [OUTPUT]]

Positional arguments:
  INPUT
  OUTPUT
  --help, -h             display this help and exit
  --version              display version and exit

But add an option:

type root struct {
	PosOnly *PosOnly `arg:"subcommand:posonly"`
}

type PosOnly struct {
	INPUT  string `arg:"positional"`
	OUTPUT string `arg:"positional"`
	Test   string `arg:"-t" default:"test"`
}

func main() {
	arg.MustParse(&root{})
}

$ go run main.go posonly --help
Usage: main posonly [--test TEST] [INPUT [OUTPUT]]

Positional arguments:
  INPUT
  OUTPUT

Options:
  --test TEST, -t TEST [default: test]
  --help, -h             display this help and exit
  --version              display version and exit