Composer 2 has been released and Wikibase installations should support it. In particular, when we install Wikibase on Travis CI, we are running the composer self-update command which updates Composer to v2. That caused all our Travis CI jobs to fail, because the Wikimedia composer-merge-plugin isn't compatible with Composer 2 yet.
- The Secondary CI Wikibase CI jobs are using Composer 2
How to start working on it once it unstalled:
- run composer self-update locally (may require using composer as phar, depending on your system) and then check that a composer update still works
- The relevant code for Travis CI is in https://github.com/wikimedia/mediawiki-extensions-Wikibase/blob/master/build/travis/install.sh