GNU bug report logs - #80103
31.0.50; Subtle test-completion issue with duplicates

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Message #5 received at submit <at> debbugs.gnu.org (full text, mbox):

From: Daniel Mendler <mail <at> daniel-mendler.de>
To: bug-gnu-emacs <at> gnu.org
Subject: 31.0.50; Subtle test-completion issue with duplicates
Date: Wed, 31 Dec 2025 16:14:40 +0100

When duplicate candidates occur in an alist, the predicate of
`test-completion' is checked too late. Compare:

(defvar table '(("x" . 1) ("x" . 2)))
(test-completion "x" table (lambda (x) (= (cdr x) 1))) => t
(test-completion "x" table (lambda (x) (= (cdr x) 2))) => nil

In a strict sense this counts as a bug. On the other hand duplicate
candidates in completion tables are a gray zone. I would like to hear
Stefan's opinion on this (cc'ed).

Original discussion at https://github.com/minad/vertico/discussions/646

In GNU Emacs 31.0.50 (build 1, x86_64-pc-linux-gnu, GTK+ Version
 3.24.49, cairo version 1.18.4) of 2025-12-27
Windowing system distributor 'The X.Org Foundation', version 11.0.12101016
System Description: Debian GNU/Linux 13 (trixie)

Configured using:
 'configure --prefix=$HOME/.local/share/emacs
 --without-compress-install --with-tree-sitter --with-native-compilation
 --with-dbus --without-selinux --without-threads --disable-gc-mark-trace
 --without-gsettings --without-gpm --with-cairo --with-cairo-xcb
 --with-xinput2 --with-x-toolkit=gtk3 --without-toolkit-scroll-bars
 'CFLAGS=-O3 -mtune=native -march=native''

Configured features:
CAIRO DBUS FREETYPE GIF GLIB GMP GNUTLS HARFBUZZ JPEG LIBOTF LIBSYSTEMD
LIBXML2 MODULES NATIVE_COMP NOTIFY INOTIFY PDUMPER PNG RSVG SECCOMP
SOUND SQLITE3 TIFF TREE_SITTER WEBP X11 XDBE XIM XINERAMA XINPUT2 XPM
XRANDR GTK3 ZLIB

Message #8 received at 80103 <at> debbugs.gnu.org (full text, mbox):

From: Stefan Monnier <monnier <at> iro.umontreal.ca>
To: Daniel Mendler <mail <at> daniel-mendler.de>
Cc: 80103 <at> debbugs.gnu.org
Subject: Re: bug#80103: 31.0.50; Subtle test-completion issue with duplicates
Date: Wed, 31 Dec 2025 12:39:42 -0500

> When duplicate candidates occur in an alist, the predicate of
> `test-completion' is checked too late. Compare:
>
> (defvar table '(("x" . 1) ("x" . 2)))
> (test-completion "x" table (lambda (x) (= (cdr x) 1))) => t
> (test-completion "x" table (lambda (x) (= (cdr x) 2))) => nil
>
> In a strict sense this counts as a bug. On the other hand duplicate
> candidates in completion tables are a gray zone. I would like to hear
> Stefan's opinion on this (cc'ed).

I'd count it as a bug, yes.

Duplicates are indeed somewhat problematic because of the kinds of UIs
we want to support, but I think here The Right Thing to do is pretty clear.


        Stefan

Message #11 received at 80103 <at> debbugs.gnu.org (full text, mbox):

From: Daniel Mendler <mail <at> daniel-mendler.de>
To: Stefan Monnier <monnier <at> iro.umontreal.ca>
Cc: 80103 <at> debbugs.gnu.org
Subject: Re: bug#80103: 31.0.50; Subtle test-completion issue with duplicates
Date: Tue, 06 Jan 2026 16:21:13 +0100

Stefan Monnier <monnier <at> iro.umontreal.ca> writes:

>> When duplicate candidates occur in an alist, the predicate of
>> `test-completion' is checked too late. Compare:
>>
>> (defvar table '(("x" . 1) ("x" . 2)))
>> (test-completion "x" table (lambda (x) (= (cdr x) 1))) => t
>> (test-completion "x" table (lambda (x) (= (cdr x) 2))) => nil
>>
>> In a strict sense this counts as a bug. On the other hand duplicate
>> candidates in completion tables are a gray zone. I would like to hear
>> Stefan's opinion on this (cc'ed).
>
> I'd count it as a bug, yes.
>
> Duplicates are indeed somewhat problematic because of the kinds of UIs
> we want to support, but I think here The Right Thing to do is pretty clear.

Thanks. I consider this a low priority issue, given the lack of proper
support for duplicates, but I can prepare a patch if you think we should
fix this.

Daniel

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