type ArgError struct {
    arg int
    message string
}

func (e *ArgError) Error() string {
    return fmt.Sprintf("%d - %s", e.arg, e.message)
}

func f(arg int) (int, error) {
    if arg == 42 {

        return -1, &ArgError{arg, "can't work with it"}
    }
    return arg + 3, nil
}

func main() {

    _, err := f(42)
    var ae *ArgError
    if errors.As(err, &ae) {
        fmt.Println(ae.arg)
        fmt.Println(ae.message)
    } else {
        fmt.Println("err doesn't match ArgError")
    }
}