Here is a simplified testcase:
trap ' trap - HUP EXIT || exit "$?" kill -s HUP "$$" || exit "$?" ' HUP INT kill -HUP $$
$ dash dash-bug dash-bug: 3: /home2/ahq/dash-bug: Syntax error: EOF in backquote substitution
Modifying the length or content of the quoted code may either "fix" the bug or produce different and interesting corruptions, with random bytes appearing out of nowhere.
This was caused by a use-after-free bug which was fixed since dash 0.5.9, but which is still present in the dash 0.5.8 from Debian 9.8 stable (stretch), Ubuntu 18.04 (bionic) and Ubuntu 18.10 (cosmic).
This is the commit which fixed it:
commit 6c3f73bc536082fec38bd36e6c8a121033c68835 Author: Herbert Xu <firstname.lastname@example.org> Date: Thu Oct 2 08:26:06 2014 +0800 [EVAL] Fix use-after-free in dotrap/evalstring The function dotrap calls evalstring using the stored trap string. If evalstring then unsets that exact trap string then we will end up using freed memory. This patch fixes it by making evalstring always duplicate the string before using it. Signed-off-by: Herbert Xu <email@example.com>