https://gerrit.wikimedia.org/r/c/mediawiki/core/+/447099 changes the signature of ContentHandler::getUndoContent from <Revision, Revision, Revision> to <Revision|Content, Revision|Content, Revision|Content[, bool]> (with Revision soft-deprecated). This is a breaking change as ContentHandler is meant to be subclassed, and any subclass that typehints Revision will raise a warning at compile-time, and a fatal when called with Content.
Searching Gerrit, Github and Google only yielded a single instance of a subclass overriding the method (which is being fixed) and providing backwards compatibility in a robust way does not seem feasible, so we are changing the method signature without a deprecation period (in the sense of subclass compatibility; with respect to calling the method with Revision, we follow the normal deprecation process).
For more information on the MediaWiki deprecation policy see https://www.mediawiki.org/wiki/Deprecation_policy#Removal_without_deprecatio...
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