SC2154 false positive in single/double quoted vars/aliases and inconsistencies

[pstueck]

For bugs

• Rule Id (if any, e.g. SC1000): SC2154
• My shellcheck version (shellcheck --version or "online"): 0.9.0, "online", VSCode Extension 0.35.0
• [X] The rule's wiki page does not already cover this (e.g. https://shellcheck.net/wiki/SC2086)
• [X] 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
• [X] 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
alias newalias='_ugs=5 _ug=0 ; echo $_ugs $_ug'

Here's what shellcheck currently says:

SC2154 (warning): _ug is referenced but not assigned.
SC2154 (warning): _ugs is referenced but not assigned.

Here's what I wanted or expected to see:

no warning

---

On the other hand …

#!/bin/bash
_ugs=9
alias newalias='_ugs=5 ; echo $_ugs $((_ug=_ug+1))'

… is accepted, although I think, _ug should have caused an SC2154

[skull-squadron]

Similar false positives with lower-cased variables in a trap statement despite being referenced. It doesn't happen with upper-cased variable names.

In files/base/installer/texlive line 9:
trap 'e=$?; trap - EXIT; rm -rf "$TEXLIVE_TMP_DIR"; exit $e' EXIT
     ^-- SC2154 (warning): e is referenced but not assigned.