[Question] Why are the options that start with `--no` ignored?

[noraworld]

Problem

I realized the option that starts with --no (maybe --without too?) is just ignored.

Reproduction

I tested this by making a tiny change from example.sh in README.md.

The sample code that reproduces this:

#!/bin/sh

set -eu

# @getoptions
parser_definition() {
  setup   REST help:usage -- "Usage: example.sh [options]... [arguments]..." ''
  msg -- 'Options:'
  flag    FLAG    -f --no-flag              -- "takes no arguments"
  param   PARAM   -p --param                -- "takes one argument"
  option  OPTION  -o --option on:"default"  -- "takes one optional argument"
  disp    :usage  -h --help
  disp    VERSION    --version
}
# @end

# @gengetoptions parser -i parser_definition parse
# Generated by getoptions (BEGIN)
# URL: https://github.com/ko1nksm/getoptions (v3.3.2)
FLAG=''
PARAM=''
OPTION=''
REST=''
parse() {
  OPTIND=$(($#+1))
  while OPTARG= && [ "${REST}" != x ] && [ $# -gt 0 ]; do
    case $1 in
      --?*=*) OPTARG=$1; shift
        eval 'set -- "${OPTARG%%\=*}" "${OPTARG#*\=}"' ${1+'"$@"'}
        ;;
      --no-*|--without-*) unset OPTARG ;;
      -[po]?*) OPTARG=$1; shift
        eval 'set -- "${OPTARG%"${OPTARG#??}"}" "${OPTARG#??}"' ${1+'"$@"'}
        ;;
      -[fh]?*) OPTARG=$1; shift
        eval 'set -- "${OPTARG%"${OPTARG#??}"}" -"${OPTARG#??}"' ${1+'"$@"'}
        case $2 in --*) set -- "$1" unknown "$2" && REST=x; esac;OPTARG= ;;
    esac
    case $1 in
      '-f'|'--no-flag')
        [ "${OPTARG:-}" ] && OPTARG=${OPTARG#*\=} && set "noarg" "$1" && break
        eval '[ ${OPTARG+x} ] &&:' && OPTARG='1' || OPTARG=''
        FLAG="$OPTARG"
        ;;
      '-p'|'--param')
        [ $# -le 1 ] && set "required" "$1" && break
        OPTARG=$2
        PARAM="$OPTARG"
        shift ;;
      '-o'|'--option')
        set -- "$1" "$@"
        [ ${OPTARG+x} ] && {
          case $1 in --no-*|--without-*) set "noarg" "${1%%\=*}"; break; esac
          [ "${OPTARG:-}" ] && { shift; OPTARG=$2; } || OPTARG='default'
        } || OPTARG=''
        OPTION="$OPTARG"
        shift ;;
      '-h'|'--help')
        usage
        exit 0 ;;
      '--version')
        echo "${VERSION}"
        exit 0 ;;
      --)
        shift
        while [ $# -gt 0 ]; do
          REST="${REST} \"\${$(($OPTIND-$#))}\""
          shift
        done
        break ;;
      [-]?*) set "unknown" "$1"; break ;;
      *)
        REST="${REST} \"\${$(($OPTIND-$#))}\""
    esac
    shift
  done
  [ $# -eq 0 ] && { OPTIND=1; unset OPTARG; return 0; }
  case $1 in
    unknown) set "Unrecognized option: $2" "$@" ;;
    noarg) set "Does not allow an argument: $2" "$@" ;;
    required) set "Requires an argument: $2" "$@" ;;
    pattern:*) set "Does not match the pattern (${1#*:}): $2" "$@" ;;
    notcmd) set "Not a command: $2" "$@" ;;
    *) set "Validation error ($1): $2" "$@"
  esac
  echo "$1" >&2
  exit 1
}
usage() {
cat<<'GETOPTIONSHERE'
Usage: example.sh [options]... [arguments]...

Options:
  -f, --no-flag               takes no arguments
  -p, --param PARAM           takes one argument
  -o, --option[=OPTION]       takes one optional argument
  -h, --help                  
      --version               
GETOPTIONSHERE
}
# Generated by getoptions (END)
# @end

parse "$@"
eval "set -- $REST"

echo "FLAG: $FLAG, PARAM: $PARAM, OPTION: $OPTION"
printf '%s\n' "$@" # rest arguments

The diff between them is:

@@ -6,7 +6,7 @@
 parser_definition() {
   setup   REST help:usage -- "Usage: example.sh [options]... [arguments]..." ''
   msg -- 'Options:'
-  flag    FLAG    -f --no-flag              -- "takes no arguments"
+  flag    FLAG    -f --flag                 -- "takes no arguments"
   param   PARAM   -p --param                -- "takes one argument"
   option  OPTION  -o --option on:"default"  -- "takes one optional argument"
   disp    :usage  -h --help
@@ -37,7 +37,7 @@
         case $2 in --*) set -- "$1" unknown "$2" && REST=x; esac;OPTARG= ;;
     esac
     case $1 in
-      '-f'|'--no-flag')
+      '-f'|'--flag')
         [ "${OPTARG:-}" ] && OPTARG=${OPTARG#*\=} && set "noarg" "$1" && break
         eval '[ ${OPTARG+x} ] &&:' && OPTARG='1' || OPTARG=''
         FLAG="$OPTARG"
@@ -91,7 +91,7 @@
 Usage: example.sh [options]... [arguments]...
 
 Options:
-  -f, --no-flag               takes no arguments
+  -f, --flag                  takes no arguments
   -p, --param PARAM           takes one argument
   -o, --option[=OPTION]       takes one optional argument
   -h, --help

The result is:

> ./example.sh --no-flag --param foo --option
FLAG: , PARAM: foo, OPTION: default
^----^ $FLAG not set

It turns out the following line causes this:

--no-*|--without-*) unset OPTARG ;;

Question

If this is just a specification, I would like to know why it was made this way.

Environments

> cat /etc/lsb-release
DISTRIB_ID=Ubuntu
DISTRIB_RELEASE=22.04
DISTRIB_CODENAME=jammy
DISTRIB_DESCRIPTION="Ubuntu 22.04.2 LTS"

> ls -l /bin/sh
lrwxrwxrwx 1 root root 4 Mar 23  2022 /bin/sh -> dash

> dpkg -s dash | grep "^Version: "
Version: 0.5.11+git20210903+057cd650a4ed-3build1

[vabatta]

This is how you do it: https://github.com/ko1nksm/getoptions/blob/master/examples/advanced.sh#L23-L24