SC2154 from strict bash mode example using trap

[peterjc]

For bugs

• Rule Id (if any, e.g. SC1000): 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
set -uo pipefail
trap 's=$?; echo "$0: Error on line "$LINENO": $BASH_COMMAND"; exit $s' ERR
IFS=$'\n\t'

echo "Example of strict bash mode from Michael Daffin, see"
echo "https://olivergondza.github.io/2019/10/01/bash-strict-mode.html"

Here's what shellcheck currently says:

[Line 3:](javascript:setPosition(3, 6))
trap 's=$?; echo "$0: Error on line "$LINENO": $BASH_COMMAND"; exit $s' ERR
     ^-- [SC2154](https://www.shellcheck.net/wiki/SC2154) (warning): s is referenced but not assigned.

Here's what I wanted or expected to see:

No error, my understanding is this does define s. Currently https://www.shellcheck.net/wiki/SC2154 currently says:

ShellCheck intentionally does not attempt to figure out runtime or dynamic assignments like with source "$(date +%F).sh" or eval var=value. See SC2034 for an extended discussion of why this is the case.

Is using trap equivalent, and if so it that worth adding to the documentation?

Is the recommendation as per the SC2034 documentation to use a throw-away variable _ like so:

#!/bin/bash
set -uo pipefail
trap '_=$?; echo "$0: Error on line "$LINENO": $BASH_COMMAND"; exit $_' ERR
IFS=$'\n\t'

echo "Example of strict bash mode from Michael Daffin, see"
echo "https://olivergondza.github.io/2019/10/01/bash-strict-mode.html"

This passes shellcheck 🚀