Sometimes when threading is enabled, Gnus displays the messages in
a thread in a bizarre order in the *Summary* buffer, with an older
message shown as a reply to a newer message. This cannot be right
unless the author of one of the messages has a time machine.

Attached are two screen shots showing this problem, one with a
thread from the Emacs development list and one with a thread from
this list.

I would estimate that the frequency of the problem is about once
in around 1000 messages.

The problem isn't new, but I do not think it has always happened.
It's been happening for maybe a year or two I think.

I have the following settings for threading:

  (setq gnus-article-sort-functions '((not gnus-article-sort-by-number)))
  (setq gnus-thread-sort-functions '((not gnus-thread-sort-by-most-recent-number)))
  (setq gnus-subthread-sort-functions '((not gnus-thread-sort-by-most-recent-number)))

. Changing these to:

  (setq gnus-article-sort-functions '((not gnus-article-sort-by-date)))
  (setq gnus-thread-sort-functions '((not gnus-thread-sort-by-most-recent-date)))
  (setq gnus-subthread-sort-functions '((not gnus-thread-sort-by-most-recent-date)))

does not solve the problem.

I thought perhaps that the `gnus-sort-gathered-threads-function'
might be the solution here, but so far I have had no success with
it. If I try to do

  (setq gnus-sort-gathered-threads-function '((not gnus-thread-sort-by-date)))

, when the *Summary* buffer opens I get the error message

  sort: Invalid function: ((not gnus-thread-sort-by-date))

.

It almost seems that "a list containing functions and ‘(not
some-function)’ elements" is not supported here?

To try to work around this, I tried

  (defun nlj-thread-sort-by-date-descending (h1 h2)
    "Sort threads in the opposite order to `gnus-thread-sort-by-date'."
    (not (gnus-thread-sort-by-date h1 h2)))

  (setq gnus-sort-gathered-threads-function 'nlj-thread-sort-by-date-descending)

which avoids the error message but produces an outlandish sorting
order.