$ cat go.mod
module main

go 1.21.4

require github.com/alexflint/go-arg v1.4.3

require github.com/alexflint/go-scalar v1.1.0 // indirect

$ cat main.go
package main

import (
	arg "github.com/alexflint/go-arg"
)

type options struct {
	Sub *Sub `arg:"subcommand"`
}

type Sub struct {
	A string `arg:"positional"`
}

func main() {
	var opts options
	arg.MustParse(&opts)
}

$ go run . sub --help
Usage: main sub [A]

Positional arguments:
  A
  --help, -h             display this help and exit

$ go run . sub --help
Usage: main sub [A]

Positional arguments:
  A

Global options:
  --help, -h             display this help and exit