SC2317: false-positive with EXIT trap in subshell?

[pschyska]

For bugs

• Rule Id (if any, e.g. SC1000): SC2317, SC2317, SC2317, SC2317
• My shellcheck version (shellcheck --version or 'online'): online
• [x] I tried on shellcheck.net and verified that this is still a problem on the latest commit
• [ ] It's not reproducible on shellcheck.net, but I think that's because it's an OS, configuration or encoding issue

For new checks and feature suggestions

• [x] 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:

#!/usr/bin/env bash

set -o errexit
set -o nounset
set -o pipefail

(
	tmp=$(mktemp -d)
	cleanup() {
		if [[ -d $tmp ]]; then
			rm -rf "$tmp"
		fi
		printf "removed tmp: %s\n" "$tmp"
	}
	trap cleanup EXIT

	printf "tmp is %s\n" "$tmp"
)
printf "After subshell\n"

Here's what shellcheck currently says:

Line 10	SC2317: Command appears to be unreachable. Check usage (or ignore if invoked indirectly).
Line 10	SC2317: Command appears to be unreachable. Check usage (or ignore if invoked indirectly).
Line 11	SC2317: Command appears to be unreachable. Check usage (or ignore if invoked indirectly).
Line 13	SC2317: Command appears to be unreachable. Check usage (or ignore if invoked indirectly).

Here's what I wanted or expected to see:

This looks like a false positive, because running that script gives the expected outcome:

tmp is /tmp/tmp.coTHszefaD
removed tmp: /tmp/tmp.coTHszefaD
After subshell

I don't think that case is covered in the other bug reports.

The following, in this case equivalent, script doesn't trigger the warning:

#!/usr/bin/env bash

set -o errexit
set -o nounset
set -o pipefail

tmp=$(mktemp -d)
cleanup() {
	if [[ -d $tmp ]]; then
		rm -rf "$tmp"
	fi
	printf "removed tmp: %s\n" "$tmp"
}
(
	trap cleanup EXIT
	printf "tmp is %s\n" "$tmp"
)
printf "After subshell\n"