SC2154 not reported if referenced in arbitrary condition

[kkmuffme]

For bugs

• Rule Id: SC2154
• My shellcheck version (shellcheck --version or "online"): online
• 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 "$x" ]]
then
        echo "$bar"
fi

echo "$bar"

Also for easy-to-make mistakes when refactoring:

if [[ -z "${bar+x}" ]]
then
    # bar is not defined
    echo "$bar"
fi

Here's what shellcheck currently says:

No issues detected!

Here's what I wanted or expected to see:

Line 8 SC2154 (warning): bar is referenced but not assigned.