Subject: 31.0.50; completion styles substring and flex are broken

From: Stephen Berman <[email-masked]>
Date: Fri, 11 Apr 2025 00:22:01 +0200

0. Start Emacs like this:

   emacs -Q --eval "(custom-set-variables '(completion-category-overrides '((file (styles substring)))))"

   or like this:

   emacs -Q --eval "(custom-set-variables '(completion-category-overrides '((file (styles flex)))))"

1. Type `C-x d' and at the prompt enter: `/usr/ TAB'.
=> Now the text following the prompt is this: "/usr//" and "/usr/" is
fontified with face file-name-shadow.

2. Type `TAB' again.
=> Now the text following the prompt is this: "//" and the first forward
slash is fontified with face file-name-shadow.

3. Type `TAB' again.
=> Now a *Completions* buffer pops up listing all the directories
directly under root (dev/, etc/, home/ and so on).

This only happens with the substring and flex completion styles; with
any of the other completion styles (basic, partial-completion, emacs22,
initials), or if Emacs is started just with -Q, then after step 1 as
expected a *Completions* buffer pops up listing the directories under
/usr (bin/, include/, lib/ and so on).

The broken behavior with the substring and flex styles is due to the
change in completion-pcm--merge-completions in this commit:

commit 0fbba16387513e7692b46885833e4a9c218251f0
Author:     Spencer Baugh <[email-masked]>
Commit:     Stefan Monnier <[email-masked]>
CommitDate: Tue Apr 8 14:36:30 2025 -0400

  Preserve an explicit * in pcm-try-completion
  [...]
  * lisp/minibuffer.el (completion-pcm--merge-completions): Don't drop
  important wildcards. (bug#74420)

In GNU Emacs 31.0.50 (build 3, x86_64-pc-linux-gnu, GTK+ Version
 3.24.49, cairo version 1.18.4) of 2025-04-10 built on strobelfs2
Repository revision: c0ea954d0f650227dc518f02a292daeb27cf0c37
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version 11.0.12101016
System Description: Linux From Scratch r12.3-20

Configured using:
 'configure -C 'CFLAGS=-Og -g3' PKG_CONFIG_PATH=/opt/qt6/lib/pkgconfig'

Configured features:
ACL CAIRO DBUS FREETYPE GIF GLIB GMP GNUTLS GPM GSETTINGS HARFBUZZ JPEG
LCMS2 LIBSYSTEMD LIBXML2 MODULES NATIVE_COMP NOTIFY INOTIFY PDUMPER PNG
RSVG SECCOMP SOUND SQLITE3 THREADS TIFF TOOLKIT_SCROLL_BARS TREE_SITTER
WEBP X11 XDBE XIM XINERAMA XINPUT2 XPM XRANDR GTK3 ZLIB