bug#20425: 24.5; completion-at-point not working with prefix starting with '(:'

[Alexander Miller]

The issue can best be demonstrated as follows:

* Start vanilla emacs via emacs -q
* Switch to scratch buffer
* Enter the string '(:backg'
* Toggle completion-at-point (using C-M-i and M-x makes no difference)
* emacs will incorrectly report that no matches have been found

What should happen is that the prefix is completed to ':background'.
Removing the open paren and trying to complete with ':backg' as prefix
leads to the correct behaviour.

The above is just a specific example. In general completion-at-point is
not correctly completing for all of emacs' keywords which start with a
colon if the colon is preceded by '('.


In GNU Emacs 24.5.1 (x86_64-unknown-linux-gnu, GTK+ Version 3.16.2)
of 2015-04-20 on bitzer.hoetzel.info
Windowing system distributor `The X.Org Foundation', version 11.0.11701000
Configured using:
`configure --prefix=/usr --sysconfdir=/etc --libexecdir=/usr/lib
--localstatedir=/var --with-x-toolkit=gtk3 --with-xft
'CFLAGS=-march=x86-64 -mtune=generic -O2 -pipe -fstack-protector-strong
--param=ssp-buffer-size=4' CPPFLAGS=-D_FORTIFY_SOURCE=2
LDFLAGS=-Wl,-O1,--sort-common,--as-needed,-z,relro'

Important settings:
value of $LANG: de_DE.UTF-8
locale-coding-system: utf-8-unix

Major mode: Lisp Interaction

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

Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.
No match [2 times]
You can run the command `completion-at-point' with C-M-i
No match [4 times]
Making completion list...

Load-path shadows:
None found.

Features:
(shadow sort gnus-util mail-extr emacsbug message idna format-spec
rfc822 mml mml-sec mm-decode mm-bodies mm-encode mail-parse rfc2231
mailabbrev gmm-utils mailheader sendmail rfc2047 rfc2045 ietf-drums
mm-util help-fns mail-prsvr mail-utils help-mode easymenu time-date
tooltip electric uniquify ediff-hook vc-hooks lisp-float-type mwheel
x-win x-dnd tool-bar dnd fontset image regexp-opt fringe tabulated-list
newcomment lisp-mode prog-mode register page menu-bar rfn-eshadow timer
select scroll-bar 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 minibuffer nadvice loaddefs button faces cus-face macroexp
files text-properties overlay sha1 md5 base64 format env code-pages mule
custom widget hashtable-print-readable backquote make-network-process
dbusbind gfilenotify dynamic-setting system-font-setting
font-render-setting move-toolbar gtk x-toolkit x multi-tty emacs)

Memory information:
((conses 16 74736 8772)
(symbols 48 17662 0)
(miscs 40 42 206)
(strings 32 9306 4675)
(string-bytes 1 253406)
(vectors 16 8988)
(vector-slots 8 383795 17139)
(floats 8 64 248)
(intervals 56 221 24)
(buffers 960 13)
(heap 1024 33123 898))