getopts: detect when OPTARG is used without colon in optstring

[wwuck]

For new checks and feature suggestions

• shellcheck.net (i.e. the latest commit) currently gives no useful warnings about this
• I searched through https://github.com/koalaman/shellcheck/issues and didn't find anything related

Here's a snippet or screenshot that shows the problem:

#!/bin/sh

usage() {
        echo "$0 [-a text] [-b]"
        exit 1
}

while getopts "ab" opt; do
        case "$opt" in
                a) foo=$OPTARG ;;
                b) alias bar=true ;;
                *) usage ;;
        esac
 done

echo "$foo"

Here's what shellcheck currently says:

No issues detected!

Here's what I wanted or expected to see:

Something similar to OPTARG used in option case a but no colon specified after a in optstring.

Shellcheck should warn that foo is being assigned to $OPTARG when there is no colon after a in the optstring. This could lead to the user expecting foo to get a value but no value being passed to the script command line arguments.