bug#57325: 27.1; functions in ff-other-file-alist

[Felician Nemeth]

>> Also, when the function returns an absolute file name, then the
>> returned file should be already opened, otherwise ff-find-other-file
>> cannot find it.
>
> Hmm... not sure how this is relevant.  The function should return a
> list of extensions, not a file name.  What am I missing?

In my case, the related file of foo.toml is bar.json.  From an extension
it is not possible to guess whether the related file is bar.json or
baz.json.  find-file.el of Emacs 27.1 has this in the Commentary
section:

;; These functions must return a list consisting of the possible names of the
;; corresponding file, with or without path.

That's why I thought my function could return an absolute file name.