programmable auto completion with embedded ":" not working?

[fabien]

Configuration Information [Automatically generated, do not change]:
Machine: i486
OS: linux-gnu
Compiler: gcc
Compilation CFLAGS:  -DPROGRAM='bash' -DCONF_HOSTTYPE='i486' 
-DCONF_OSTYPE='linux-gnu' -DCONF_MACHTYPE='i486-pc-linux-gnu' 
-DCONF_VENDOR='pc' -DLOCALEDIR='/usr/share/locale' -DPACKAGE='bash' -DSHELL 
-DHAVE_CONFIG_H  -I.  -I../bash -I../bash/include -I../bash/lib   -g -O2
uname output: Linux briare.cri.ensmp.fr 2.6.12.5 #1 Mon Aug 22 20:07:17 CEST 
2005 i686 GNU/Linux
Machine Type: i486-pc-linux-gnu

Bash Version: 3.0
Patch Level: 16
Release Status: release

Description:
        when programming an auto-completion with embedded ":" in the
        complete names, the auto-completion does not look good.

Repeat-By:
        function _xxx() 
        {
                COMPREPLY=( $( compgen -W "x:yyy x:yzz" -- 
${COMP_WORDS[COMP_CWORD]} ) ) ;
                return 0;
        }
        complete -F _xxx -o default xxx
        xxx <TAB>
        xxx x:y<TAB AGAIN>
        xxx x:x:y

        the second auto completion result does not look good. it should have
        displayed the list of choices. It seems that the substitution stuff 
        does not want to go before ":"? It is also possible that I do not
        understand something. If a "-" is put instead of ":", it works 
        properly. Maybe a side effect of url-looking substitutions?