After had been fixed and the bugfix backported to 10.0, upgrade from MySQL 5.6 to 10.0 on Sid, Vivid, Wily is able to proceed further: the packages are installed all right, but the server does not start after upgrade.
If /etc/init.d/mysql start is run manually, it exits with zero code immediately without any messages, MariaDB is not started. Nothing in the logs.
As far as I could tell, the following happens.
/etc/init.d/mysql at the very beginning calls /lib/lsb/init-functions
/lib/lsb/init-functions triggers some hooks, one of which is /lib/lsb/init-functions.d/40-systemd
/lib/lsb/init-functions.d/40-systemd checks the status for service mysql.service, and if it finds LoadState=masked, it exits, silently and without errors.
Apparently, the status is exactly that after MySQL 5.6 is uninstalled.
I don't really know what can be done about it. If we don't support crossgrade from MySQL 5.6 to 10.0, so be it – it's not so very important since we already have 10.1 and it works; but then we should make the decision and disable the test in buildbot.