bug#20698: 25.0.50; REGRESSION in `transpose-sexps' introduced in Emacs 24.4

[Drew Adams <drew.adams@oracle.com>]

I say regression, but perhaps there was a good reason for the change in
behavior.  I couldn't find anything about it, checking NEWS and looking
through some bug reports.

In all Emacs versions prior to 24.4 (back through Emacs 20, at least),
when you use a negative prefix arg with `transpose-sexps' point ends up
after the sexp that moved.  Starting with 24.4, point does not move.

For example:

((s1) (s2) (s3) (s4) | (s5))

With point at |, `C-- C-M-t' gives you this prior to 24.4:

((s1) (s2) (s4) | (s3) (s5))

For 24.4+ it gives you this:

((s1) (s2) (s4) (s3) | (s5))

With `C-- 2', you get this:

< 24.4:

((s1) (s4) | (s2) (s3) (s5))

24.4+:

((s1) (s4) (s2) (s3) | (s5))

Is this by design or a mistake (regression)?  There is no change in the
code of `transpose-sexps' itself.  Perhaps that is a sign that this is
an unplanned side effect (bug)?  Or perhaps it was planned (why?).

It would be good, I think, if the doc mentioned what happens to point.

It would be good, I think - unless there is some great reason for this
change in behavior, to fix this bug.

As an example, the bug interferes with using repetition with a negative
prefix arg.  With the change (bug), repeating a negative prefix arg just
reverses the prefix transposition.

E.g., prior to this bug, you can effectively use `C-- C-M-t' repeatedly
to move a sexp backward:

((s1) (s2) (s3) (s4) | (s5))

C-- C-M-t

((s1) (s2) (s4) | (s3) (s5))

C-- C-M-t

((s1) (s4) | (s2) (s3) (s5))

After this bug:

((s1) (s2) (s3) (s4) | (s5))

C-- C-M-t

((s1) (s2) (s4) (s3) | (s5))

C-- C-M-t  ; Back to where we started.

((s1) (s2) (s3) (s4) | (s5))

Here is a command I use, for example, that lets you repeatedly
transpose and use a negative arg to reverse the direction.
Note the hack to compensate for this bug (I don't claim it is
the best hack for this).

(defun reversible-transpose-sexps (arg)
  "Reversible and repeatable `transpose-sexp'.
Like `transpose-sexps', but:
 1. Leaves point after the moved sexp.
 2. When repeated, a negative prefix arg flips the direction."
  (interactive "p")
  (when (eq last-command 'rev-transp-sexps-back) (setq arg  (- arg)))
  (transpose-sexps arg)
  (unless (natnump arg)
    (when (or (> emacs-major-version 24)
              (and (= emacs-major-version 24)  (> emacs-minor-version 3))) ; Emacs 24.4+
      (backward-sexp (abs arg))
      (skip-syntax-backward " ."))
    (setq this-command 'rev-transp-sexps-back)))


In GNU Emacs 25.0.50.1 (i686-pc-mingw32)
 of 2014-10-20 on LEG570
Bzr revision: 118168 rgm@gnu.org-20141020195941-icp42t8ttcnud09g
Windowing system distributor `Microsoft Corp.', version 6.1.7601
Configured using:
 `configure --enable-checking=yes,glyphs CPPFLAGS=-DGLYPH_DEBUG=1'