Diff module support broken

Pushed to an MR.

This will fix 2.x support and not break 8.x-1.x support (since adding return types on child classes does not break compatibility).

Makes sense, merged.

Also verified that return type mixed works all the way back to PHP 7.0, so not an issue with older PHP versions like union types or so.

Automatically closed - issue fixed for 2 weeks with no activity.