I made this extension to the gitlab-ci.yml file to be able to automatically test for issues like this in the future: https://salsa.debian.org/mariadb-team/mariadb-10.3/commit/0ec004b4e4fc7c98a7b16cbe1a821be12ace1110
Ideally it would actually also attempt to build something first with MySQL and then with MariaDB and verify it actually works, but I don't know any good example to do it with.
