Last modified: 2009-02-09 07:43:02 UTC
It is clearly not supposed to be possible to diff to a deleted revision that you can't view the content of, since the radio buttons on the history page are greyed out, as are the 'cur' / 'prev' links as applicable. However, it is still possible to get a diff in a number of ways. Firstly, if the second-most-recent revision of a page is concealed, the radio button marker is still positioned on it by default (although if you move it, you can't put it back). It should be a simple check on data that's already present to see if the second revision is deleted (the first can't be) and move the radio button default down one if so. Secondly, if you diff to the revision before a deleted one, the "next diff" link is still available, and takes you to the diff with the deleted revision. The problem with diffing to a deleted version is that the diff function silently returns a 'nothing changed' diff just like when you diff to a 'null' revision (eg a "this page was moved/protected" rev). In this case, that's disingenous: it should not be allowed to present a diff involving content the user can't see, and an explanation of the issue should be provided. It seems fairly pointless to allow the diff but not allow it to contain any useful information.
Fixed in r47025