For bugs

• Rule Id: SC2154/check-unassigned-uppercase
• My shellcheck version: 0.10.0
• The rule's wiki page does not already cover this (e.g. https://shellcheck.net/wiki/SC2086)
• I tried on https://www.shellcheck.net/ and verified that this is still a problem on the latest commit

For new checks and feature suggestions

• https://www.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/bash
if [[ -n "$4" ]]
then
    echo "$5"
fi

Here's what shellcheck currently says:

No error

Here's what I wanted or expected to see:

5 is referenced but not assigned

The global variables $1, $2,... (not $0) should be checked like any other uppercase variable when check-unassigned-uppercase is used.
The numbers are easily fatfingered and this is an easy mistake to make when restructuring code too.