Instead, it should show a warning that neither the title nor the diff revision ID is provided.

why?

That is a good question, and possibly the reason this task may be declined (if declined).

My reasoning is this: https://en.wikipedia.org/w/index.php?diff= is just an incomplete URL from which a human would not understand which page or which revision is intended, so choosing both a page and a revision arbitrarily is just too many assumptions by the code.

Isn't this expected behavior? /w/index.php is the default entry point, but wmf rewrites article paths to look pretty. /w/index.php?diff= does just what I'd expect with an empty or missing parameter diff parameter: it takes you to the latest diff of the page, which, when unprovided, defaults to the main page.

Like I said, this task may be declined for that reason. I just feel like "expected" behavior is a bit subjective, and the fact that two assumptions are being made (both about the page and revision) is too liberal.

Change 982437 had a related patch set uploaded (by Novem Linguae; author: Novem Linguae):

[mediawiki/core@master] Invalid ?diff= now displays an error

https://gerrit.wikimedia.org/r/982437

Change 985107 had a related patch set uploaded (by Novem Linguae; author: Novem Linguae):

[mediawiki/core@master] Add tests for includes/MediaWiki.php

https://gerrit.wikimedia.org/r/985107

Change 985107 merged by jenkins-bot:

[mediawiki/core@master] Add tests for includes/MediaWiki.php

https://gerrit.wikimedia.org/r/985107

The behavior I expect is diff= has no effect, i.e. https://en.wikipedia.org/w/index.php?title=Foobar&diff= is equivalent to https://en.wikipedia.org/w/index.php?title=Foobar.