bug#21848: Bug in emacs: filling a certain region with (fill-region) causes a certain character to disapear

bug#21848: Bug in emacs: filling a certain region with (fill-region) causes a certain character to disapear

[Ruy Exel]

[-- Attachment #1: Type: text/plain, Size: 5158 bytes --]

Hi Folks,

Sorry I cannot send e-mail from Emacs but I have tried to adhere to
the message it generated as best as I could.

Please describe exactly what actions triggered the bug
and the precise symptoms of the bug.  If you can, give
a recipe starting from `emacs -Q':

1) Load the attached text file (bug-in-gnu-emacs.txt)
2) Mark a region consisting of the first two lines, namely

| a |
| a

3) Observe that these two lines contain *three* vertical bars
4) Run the command "ESC-x fill-region"
5) Observe that thare are only *two* vertical bars now!

I also tested this in a much more recent Emacs build:
GNU Emacs 24.4.1 (x86_64-pc-linux-gnu, GTK+ Version 3.14.5) of
2015-03-07 on trouble, modified by Debian

In GNU Emacs 23.4.1 (i686-pc-linux-gnu, GTK+ Version 2.24.21)
 of 2013-10-25 on akateko, modified by Debian
Windowing system distributor `The X.Org Foundation', version 11.0.11501000
configured using `configure  '--build' 'i686-linux-gnu' '--build'
'i686-linux-gnu' '--prefix=/usr' '--sharedstatedir=/var/lib'
'--libexecdir=/usr/lib' '--localstatedir=/var/lib'
'--infodir=/usr/share/info' '--mandir=/usr/share/man' '--with-pop=yes'
'--enable-locallisppath=/etc/emacs23:/etc/emacs:/usr/local/share/emacs/23.4/site-lisp:/usr/local/share/emacs/site-lisp:/usr/share/emacs/23.4/site-lisp:/usr/share/emacs/site-lisp'
'--with-crt-dir=/usr/lib/i386-linux-gnu' '--with-x=yes'
'--with-x-toolkit=gtk' '--with-toolkit-scroll-bars'
'build_alias=i686-linux-gnu' 'CFLAGS=-g -O2 -fstack-protector
--param=ssp-buffer-size=4 -Wformat -Werror=format-security -Wall
-DDEBIAN -O2' 'CPPFLAGS=-D_FORTIFY_SOURCE=2''

Important settings:
  value of $LC_ALL:
  value of $LC_COLLATE: C
  value of $LC_CTYPE: en_US.utf8
  value of $LC_MESSAGES: en_US.utf8
  value of $LC_MONETARY: en_US.utf8
  value of $LC_NUMERIC: en_US.utf8
  value of $LC_TIME: en_US.utf8
  value of $LANG: en_US.utf8
  value of $XMODIFIERS: @im=ibus
  locale-coding-system: utf-8-unix
  default enable-multibyte-characters: t

Major mode: Text

Minor modes in effect:
  display-time-mode: t
  shell-dirtrack-mode: t
  tooltip-mode: t
  mouse-wheel-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  blink-cursor-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  column-number-mode: t
  line-number-mode: t


Load-path shadows:
/usr/share/emacs/23.4/site-lisp/debian-startup hides
/usr/share/emacs/site-lisp/debian-startup
/usr/share/emacs23/site-lisp/flim/sha1 hides /usr/share/emacs/23.4/lisp/sha1
/usr/share/emacs23/site-lisp/flim/hex-util hides
/usr/share/emacs/23.4/lisp/hex-util
/usr/share/emacs23/site-lisp/flim/md4 hides /usr/share/emacs/23.4/lisp/md4
/usr/share/emacs23/site-lisp/dictionaries-common/flyspell hides
/usr/share/emacs/23.4/lisp/textmodes/flyspell
/usr/share/emacs23/site-lisp/dictionaries-common/ispell hides
/usr/share/emacs/23.4/lisp/textmodes/ispell
/usr/share/emacs23/site-lisp/flim/hmac-md5 hides
/usr/share/emacs/23.4/lisp/net/hmac-md5
/usr/share/emacs23/site-lisp/flim/sasl-ntlm hides
/usr/share/emacs/23.4/lisp/net/sasl-ntlm
/usr/share/emacs23/site-lisp/flim/hmac-def hides
/usr/share/emacs/23.4/lisp/net/hmac-def
/usr/share/emacs23/site-lisp/flim/sasl-digest hides
/usr/share/emacs/23.4/lisp/net/sasl-digest
/usr/share/emacs23/site-lisp/flim/ntlm hides /usr/share/emacs/23.4/lisp/net/ntlm
/usr/share/emacs23/site-lisp/flim/sasl-cram hides
/usr/share/emacs/23.4/lisp/net/sasl-cram
/usr/share/emacs23/site-lisp/flim/sasl hides /usr/share/emacs/23.4/lisp/net/sasl
/usr/share/emacs23/site-lisp/wl/rfc2368 hides
/usr/share/emacs/23.4/lisp/mail/rfc2368
/usr/share/emacs23/site-lisp/latex-cjk-thai/thai-word hides
/usr/share/emacs/23.4/lisp/language/thai-word

Features:
(shadow sort mail-extr message sendmail ecomplete rfc822 mml mml-sec
password-cache mm-decode mm-bodies mm-encode mailcap mail-parse rfc2231
rfc2047 rfc2045 qp ietf-drums mailabbrev nnheader gnus-util netrc
time-date mm-util mail-prsvr gmm-utils mailheader canlock sha1 sha1-el
hex-util hashcash mail-utils emacsbug debug etags latexenc dired-x
face-remap cc-awk cc-mode cc-fonts cc-menus cc-cmds cc-styles cc-align
cc-engine cc-vars cc-defs dired-aux tabify man assoc help-fns newcomment
tex-mode compile descr-text skeleton help-mode view multi-isearch
dabbrev ansi-color compare-w sh-script executable easy-mmode time
cus-edit easymenu cus-start cus-load wid-edit iso-transl cl cl-19
edmacro kmacro shell comint ring dired regexp-opt tooltip ediff-hook
vc-hooks lisp-float-type mwheel x-win x-dnd font-setting tool-bar dnd
fontset image fringe lisp-mode register page menu-bar rfn-eshadow timer
select scroll-bar mldrag mouse jit-lock font-lock syntax facemenu
font-core frame cham georgian utf-8-lang misc-lang vietnamese tibetan
thai tai-viet lao korean japanese hebrew greek romanian slovak czech
european ethiopic indian cyrillic chinese case-table epa-hook
jka-cmpr-hook help simple abbrev loaddefs button minibuffer faces
cus-face files text-properties overlay md5 base64 format env code-pages
mule custom widget hashtable-print-readable backquote
make-network-process dbusbind system-font-setting font-render-setting
gtk x-toolkit x multi-tty emacs)

[-- Attachment #2: bug-in-gnu-emacs.txt --]
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| a |
| a

How many vertical bars do you see above?  Three, for sure!  Now run
the following commands (that is, put the cursor where indicated below
and press "Control-X Control-E"):

(progn
  (goto-char 1)
  (set-mark-command nil)
  (forward-line 2)
  (fill-region (region-beginning) (region-end))
  )                    <-- PUT CURSOR RIGHT AFTER THIS CLOSING PARENTHESIS AND PRESS "Control-X Control-E

Now you see only *two* vertical bars!  Where did the third one go?

Tested on

GNU Emacs 23.4.1 (i686-pc-linux-gnu, GTK+ Version 2.24.21) of 2013-10-25 on akateko, modified by Debian
GNU Emacs 24.4.1 (x86_64-pc-linux-gnu, GTK+ Version 3.14.5) of 2015-03-07 on trouble, modified by Debian

bug#21848: Bug in emacs: filling a certain region with (fill-region) causes a certain character to disapear

[Glenn Morris]

Ruy Exel wrote:

> 2) Mark a region consisting of the first two lines, namely
>
> | a |
> | a
>
> 3) Observe that these two lines contain *three* vertical bars
> 4) Run the command "ESC-x fill-region"
> 5) Observe that thare are only *two* vertical bars now!


This is not a bug, but rather "adaptive filling" behaving as designed.
Compare the result if you first: (setq adaptive-fill-mode nil).

bug#21848: Bug in emacs: filling a certain region with (fill-region) causes a certain character to disapear

[Ruy Exel]

Hi Glenn,

Thanks for your reply.  The great thing abour Emacs is that, even if
you've been using it almost daily for the past 25 years, there is
always a lot to learn!

Best,
Ruy

On Sat, Nov 7, 2015 at 3:20 AM, Glenn Morris <rgm@gnu.org> wrote:
> Ruy Exel wrote:
>
>> 2) Mark a region consisting of the first two lines, namely
>>
>> | a |
>> | a
>>
>> 3) Observe that these two lines contain *three* vertical bars
>> 4) Run the command "ESC-x fill-region"
>> 5) Observe that thare are only *two* vertical bars now!
>
>
> This is not a bug, but rather "adaptive filling" behaving as designed.
> Compare the result if you first: (setq adaptive-fill-mode nil).

bug#21848: Bug in emacs: filling a certain region with (fill-region) causes a certain character to disapear

[Xue Fuqiao]

Closing.