The MediaWiki OAuth extension allows a MediaWiki wiki to act as an OAuth provider, which can be used by third-party websites to use that wiki for identifying users: the user is sent to the wiki, where they can grant permission to the third-party website through a confirmation dialog to read their identity.
If a user is sent to the confirmation page but they are not logged into the wiki, they are redirected to the wiki's login page, and then back to the confirmation dialog. This might be confusing - users might not understand why they need to login to a completely different site then where they originally were.
This task is about adding an informational message to the wiki login page, which shortly explains why the user needs to log in here. This can be done by implementing the LoginFormValidErrorMessages hook in the OAuth extension.
Extension repository: https://gerrit.wikimedia.org/g/mediawiki/extensions/OAuth
Learn more about Git, Gerrit and how to submit a patch: https://www.mediawiki.org/wiki/Gerrit/Tutorial