bug#635: Adding font-lock keywords results in no font-lock at all

[Klaus Zeitler <kzeitler@alcatel-lucent.com>]

For many years I had the following statement in my .emacs file:

(defun show-tabs () "Show tabs with a slightly changed background"
  (font-lock-add-keywords nil '(("\t"  (0 'tab-face t))) t))
(add-hook 'font-lock-mode-hook 'show-tabs)

This has worked without any problems up to 22.1.
In version 22.2 and 23 I do have problems with syntax highlighting due
to these few lines above in some modes, e.g. when I try M-x list-faces-display
all colors are gone.

To reproduce:

1. start "emacs -Q"
2. in scratch buffer evaluate
(defun show-tabs () "Show tabs with a slightly changed background"
  (font-lock-add-keywords nil '(("\t"  (0 'font-lock-constant-face t))) t))
(add-hook 'font-lock-mode-hook 'show-tabs)
3. M-x list-faces-display
=> everything's displayed in default face



In GNU Emacs 22.2.2 (sparc-sun-solaris2.8, Motif Version 2.1.0)
 of 2008-04-09 on sfsws4
Windowing system distributor `Sun Microsystems, Inc.', version 11.0.6410
configured using `configure  '--with-gcc' '--prefix=/vol/freeware/SunOS-5.8/emacs/22.2' '--sharedstatedir=/vol/freeware/SunOS-5.8/emacs/22.2/share' '--with-x-toolkit=motif' '--x-includes=/usr/openwin/include:/usr/local/gnu/include:/opt/exp/gnu/include:/opt/exp/include:/opt/exp/lib/xpm/include' '--x-libraries=/usr/openwin/lib:/usr/local/gnu/lib:/opt/exp/gnu/lib:/opt/exp/lib:/opt/exp/lib/xpm/lib' '--with-pop' 'CC=gcc' 'XMKMF=/usr/openwin/bin/xmkmf''

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: C
  locale-coding-system: nil
  default-enable-multibyte-characters: t

Major mode: Lisp Interaction

Minor modes in effect:
  tooltip-mode: t
  tool-bar-mode: t
  mouse-wheel-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  blink-cursor-mode: t
  unify-8859-on-encoding-mode: t
  utf-translate-cjk-mode: t
  auto-compression-mode: t
  line-number-mode: t

Recent input:
<help-echo> <help-echo> <menu-bar> <help-menu> <se
nd-emacs-bug-report>

Recent messages:
("emacs" "-Q")
For information about GNU Emacs and the GNU system, type C-h C-a.
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