bug#3688: Also need (auto-fill-mode nil) to trigger this bug

[Alan Mackenzie]

Hi, Kees!

On Fri, Jun 26, 2009 at 01:56:48PM +0200, Kees Bakker wrote:
> Hi,

> In my report I forgot to run emacs with -Q, but the problem is still
> present with -Q. But here is extra information to reproduce the error.

> After entering the suggested C text (and emacs is in a basic C mode)
> you have to evaluate the following before the error can be reproduced.
>    (auto-fill-mode nil)

> In the status bar it shows "*C/l Abbrev Fill)". In the buffer you'll
> have the following.

>    /*type a space here=>
>    /* 3 tabs following after this=>                   */

> (Notice that there are 3 TABs before the closing C comment.) If you
> type a space on the first comment line, you'll see that it jumps back
> 21 positions.  (The 21 appears to be equal to NumOfTabs * (8-1)).

Yes, that's exactly where the 21 comes from.  :-)

> If you evaluate this again, the problem goes away.
>    (auto-fill-mode nil)
> Now, the "Fill" flag is gone from the status bar.

The mechanism for the bug is thus: when doing filling, c-mask-paragraph
temporarily replaces the whitespace before "*/" with a string of "x"s, so
that Emacs's low level filling routine can't put "*/" on a line of its
own.  At the end of the routine, the calculation to restore point does a
spurious correction to correct for an imagined difference between the
size of the WS (24 columns) and the number of bytes (3 tab characters).

Removing this "uncorrection" fixes things.  Please test out the patch
below.

Yidong and Stefan: would it be OK to install this patch for Emacs 23?
The original coding error was my own, made on 2005-08-18 in version 5.314
of cc-cmds.el at SourceForge
(http://cc-mode.cvs.sourceforge.net/viewvc/cc-mode/cc-mode/cc-cmds.el).



2009-06-26  Alan Mackenzie  <acm@muc.de>

        * progmodes/cc-cmds.el (c-mask-paragraph): Remove a spurious
        correction between the visible width of TABs and their number of
        bytes.

Index: cc-cmds.el
===================================================================
RCS file: /cvsroot/emacs/emacs/lisp/progmodes/cc-cmds.el,v
retrieving revision 1.82
diff -c -r1.82 cc-cmds.el
*** cc-cmds.el  13 Feb 2009 14:54:27 -0000      1.82
--- cc-cmds.el  26 Jun 2009 20:54:39 -0000
***************
*** 4202,4209 ****
          (forward-char (- hang-ender-stuck))
          (if (or fill-paragraph (not auto-fill-spaces))
              (insert-char ?\  hang-ender-stuck t)
!           (insert auto-fill-spaces)
!           (setq here (- here (- hang-ender-stuck (length auto-fill-spaces)))))
          (delete-char hang-ender-stuck)
          (goto-char here))
        (set-marker tmp-post nil))
--- 4202,4208 ----
          (forward-char (- hang-ender-stuck))
          (if (or fill-paragraph (not auto-fill-spaces))
              (insert-char ?\  hang-ender-stuck t)
!           (insert auto-fill-spaces))
          (delete-char hang-ender-stuck)
          (goto-char here))
        (set-marker tmp-post nil))

> Kind regards,
> Kees Bakker

-- 
Alan Mackenzie (Nuremberg, Germany).