bug#1723: 23.0.60; Odd behaviour with #'check-coding-systems-region

[Aidan Kehoe <kehoea@parhasard.net>]

Hi!

Please write in English if possible, because the Emacs maintainers
usually do not have translators to read other languages for them.

Your bug report will be posted to the emacs-pretest-bug@gnu.org mailing list.

Please describe exactly what actions triggered the bug
and the precise symptoms of the bug:

The check-coding-systems-region docstring says this: 

  (check-coding-systems-region start end coding-system-list)
  
  Check if the region is encodable by coding systems.
  
  start and end are buffer positions specifying the region.
  coding-system-list is a list of coding systems to check.
  
  The value is an alist ((CODING-SYSTEM POS0 POS1 ...) ...), where
  CODING-SYSTEM is a member of coding-system-list and can't encode the
  whole region, POS0, POS1, ... are buffer positions where non-encodable
  characters are found.
  
  If all coding systems in coding-system-list can encode the region, the
  value is nil.

  [...]

It is unclear to me what "value" means there. My provisional
assumption was that it meant "return value", but that doesn't reflect
the behaviour of the function:

  (with-temp-buffer (insert "a") (check-coding-systems-region (point-min) (point-max) '(iso-8859-1)))
  => t

  ;; That would have been nil, not t, were my assumption correct. 
  
  (with-temp-buffer (insert "\u20ac") (check-coding-systems-region (point-min) (point-max) '(iso-8859-1)))
  => ((iso-8859-1 1))

Is the docstring wrong? Is the behaviour wrong? Is my understanding of the
docstring wrong? Whichever way, something needs to be fixed, I am a
generally- and technically-literate native speaker of English and if I have
misunderstood it, other people will too. 

A pre-emptive happy New Year to you all, 

Aidan

In GNU Emacs 23.0.60.1 (i386-apple-darwin8.11.1, NS apple-appkit-824.48)
 of 2008-12-27 on bonbon
Windowing system distributor `Apple', version 97.112.112.108.101.45.97.112.112.107.105.116.45.56.50.52.46.52.56
configured using `configure  '--with-ns' '--enable-asserts' '--enable-maintainer-mode' '--enable-cocoa-experimental-ctrl-g' '--with-xpm' '--with-jpeg' '--with-tiff' '--with-png' 'LDFLAGS=-L/opt/local/lib -L/usr/X11R6/lib -L/usr/local/lib' 'CPPFLAGS=-I/opt/local/include -I/usr/X11R6/include -I/usr/local/include''

Important settings:
  value of $LC_ALL: nil
  value of $LC_COLLATE: nil
  value of $LC_CTYPE: nil
  value of $LC_MESSAGES: nil
  value of $LC_MONETARY: nil
  value of $LC_NUMERIC: nil
  value of $LC_TIME: nil
  value of $LANG: de_DE.UTF-8
  value of $XMODIFIERS: nil
  locale-coding-system: utf-8-unix
  default-enable-multibyte-characters: t

Major mode: Lisp Interaction

Minor modes in effect:
  tooltip-mode: t
  mouse-wheel-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  global-auto-composition-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  line-number-mode: t
  transient-mark-mode: t

Recent input:
C-e C-j C-p C-p C-e C-b C-b C-p <return> <return> C-p 
M-4 SPC C-a C-k ? \ u 2 0 a c C-a C-d " C-e " C-j C-n 
C-e C-b C-b C-b C-k ( i s o - 8 8 5 9 - 1 ) ) C-j C-p 
C-p C-f C-f C-f C-f C-f C-f C-f C-f C-f C-f C-f C-f 
C-f C-f C-f C-f C-f C-f C-f C-f C-f C-f C-f C-f C-f 
C-f C-f C-f C-f C-d C-d C-d C-d C-d C-d C-d C-d C-d 
C-d C-d C-d SPC C-b C-p C-a C-p C-k C-n C-n C-e C-b 
C-b C-b C-b C-b C-b C-b C-b C-b C-b C-b C-b C-b C-b 
C-b C-b C-b C-b C-b C-b C-b C-b C-b C-b C-b C-b C-b 
C-b SPC C-y C-e C-j <down-mouse-1> <mouse-1> C-x o 
C-x b f d a s f d s a f d a s <return> <escape> : C-g 
M-4 SPC <backspace> <backspace> <backspace> <backspace> 
C-x o C-p C-p C-p C-p C-p C-p C-e C-n C-e C-j C-p C-d 
C-e <backspace> C-a C-k C-x o C-y C-x o C-n C-n C-n 
C-n C-f C-f C-f C-f C-f C-f C-f C-f C-f C-f C-f C-f 
C-f C-f C-f C-f C-f C-f C-f C-f C-f C-f C-f C-f C-f 
C-f C-f C-f C-f C-d C-d C-d C-d ( p o i n t - m i o 
n <backspace> <backspace> n ) C-a C-k C-x o <escape> 
: C-y <return> <help-echo> <down-mouse-1> <mouse-1> 
C-b C-u C-x = C-x C-o C-x o <down-mouse-1> <mouse-1> 
C-x 1 C-r f o n t C-s C-s C-r C-r C-p C-p C-p C-p C-p 
C-p C-p C-x b * s c <tab> <return> <escape> x r e p 
o r t - e m a c s - b u g <return>

Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.
Type C-x 1 to delete the help window, C-M-v to scroll help.
Mark set
Quit
Mark set
((iso-8859-1 1))
Type C-x 4 C-o RET to restore the other window.
Char: € (8364, #o20254, #x20ac, file ...) point=1 of 1 (0%) column=0
Mark saved where search started

-- 
¿Dónde estará ahora mi sobrino Yoghurtu Nghé, que tuvo que huir
precipitadamente de la aldea por culpa de la escasez de rinocerontes?