**List of steps to reproduce** (step by step, including full links if applicable):
* using MediaWiki 1.35 LTS (but looking at the source I guess this exists in master as well)
* Set up a primary wiki and a secondary wiki with shared user and user_properties tables but SEPARATE actor tables (default for wikis set up before the actor table existed).
* Create user on secondary wiki.
* Observe the user getting created in the primary/shared wiki user table, actor getting created in secondary table (as expected).
* Observe logging in on the secondary wiki works fine.
* Observe logging in on the primary wiki FAILS - no actor entry gets created in the primary actor table.
Error message: [[ https://github.com/wikimedia/mediawiki/blob/747b5c037f7af9dfcc6a0a8cde03181422367d36/includes/auth/AuthManager.php#L640 | authmanager-authn-autocreate-failed ]] / userexists
I take it this happens because the [[ https://github.com/wikimedia/mediawiki/blob/747b5c037f7af9dfcc6a0a8cde03181422367d36/includes/auth/AuthManager.php#L625 | call to User::newFromName() ]] / the later User.getId() call fails to get the user in the primary wiki since [[ https://github.com/wikimedia/mediawiki/blob/747b5c037f7af9dfcc6a0a8cde03181422367d36/includes/user/User.php#L320 | User.load() ]] seems to consider the actor table only when fetching users. Since the corresponding actor entry does not exist (yet) in the primary wiki, that fails.
Fix would be to create the actor entry on the fly if it doesn't exist yet.
Related to https://phabricator.wikimedia.org/T243276#6620681