Widening the types of parameters is allowed by the stable interface policy, as it it does not break any callers. Example:
- public function addLink( Title $title, $id = null ) {..}
+ public function addLink( LinkTarget $link, $id = null ) {..}
This can pose an issue for developers of extensions and skins, who want to maintain compatibility for multiple versions of MediaWiki core: when refactoring or writing new code, we may accidentally use LinkTarget (with the above example) even though it will not work with older versions of MediaWiki. Detecting these issues is difficult due to low(er) testing with older versions of MediaWiki compared to the latest version.
For other things, that may be backwards incompatible, we can check the @since tags for the relevant classes/methods/constants/etc. For this kind of compatibility issue there is currently no standard way to check other than reading the source of each supported MediaWiki core version.
I propose that we mention these kind of type changes in the parameter documentation. IDEs like PhpStorm will show this documentation when writing code, which makes it easier to avoid introducing these kind of breakages accidentally.