To clarify (@DannyS712 you might be able to correct me if I'm wrong) without this, I don't think there's a reliable way to get watchlist expiry status for a given page? T263796 helps but wouldn't cover it.

@MusikAnimal Hello! How many points would you estimate this ticket to be?

Change 649736 had a related patch set uploaded (by MusikAnimal; owner: MusikAnimal):
[mediawiki/core@master] [WIP] ApiQueryInfo: Return watchlist expiry instead of true

https://gerrit.wikimedia.org/r/649736

Change 649736 merged by jenkins-bot:
[mediawiki/core@master] ApiQueryInfo: Return watchlist expiry when applicable for inprop=watched

https://gerrit.wikimedia.org/r/649736

When looking up information for a page via the api, you can now return the watch expiry.

For example, https://en.wikipedia.beta.wmflabs.org/wiki/Special:ApiSandbox#action=query&format=json&prop=info&titles=Foobar&inprop=watched

The response if the page is temporarily watched is something like:

{
    "title": <title>,
    ...
    "watched": "",
    "watchlistexpiry": "2021-07-11T08:59:22Z"
}

If it is watched permanently, the response is like:

{
    "title": <title>,
    ...
    "watched": ""
}

This change slightly re-wrote a function used elsewhere. I tested other parts of the code which use this function. I did not find any problems.

Test Environment https://en.wikipedia.beta.wmflabs.org MediaWiki 1.36.0-alpha (90996aa) 07:32, 11 January 2021.

Just to confirm @MusikAnimal, am I right that this change has also tweaked (corrected?) the return value of watched when using format=json&formatversion=2? Without this change, a title I'm not watching returns "watched": false while after this change, it does not return watched. That's seems more correct, but just wanted to confirm since it wasn't mentioned.

In T268834#6739312, @Amorymeltzer wrote:

Just to confirm @MusikAnimal, am I right that this change has also tweaked (corrected?) the return value of watched when using format=json&formatversion=2? Without this change, a title I'm not watching returns "watched": false while after this change, it does not return watched. That's seems more correct, but just wanted to confirm since it wasn't mentioned.

The intention was to not change any long-standing behaviour. But it looks the fix at https://gerrit.wikimedia.org/r/c/mediawiki/core/+/655665 restored the "watched":false, so we're good :) With this API, you have to pass in inprop=watched to get any watch info, so I think it's appropriate to always return a value for watched (since you're explicitly asking for it).

Already in use, thanks all!