bug#68514: 30.0.50; minibuffer-choose-completion + elisp-c-a-p delete next sexp when completing after open paren

[Spencer Baugh <sbaugh@janestreet.com>]

In an Elisp buffer, if you trigger completion after an open paren then
use minibuffer-choose-completion to choose a completion,
minibuffer-choose-completion will delete the next sexp before inserting
the completion.

Step by step:
1. emacs -Q
2. In an Elisp buffer, type in:
(

(progn (some) (big) (expression))
3. Move point to the end of the initial "(" and hit M-TAB
4. Use M-down and M-RET to select some completion e.g. "current-buffer".
5. When the completion is inserted, "(progn (some) (big) (expression))"
is deleted, so the buffer contains just e.g. "(current-buffer"

If at step 2, the user types any characters after the "(", the progn
won't be deleted; for example, typing instead:
(a

(progn (some) (big) (expression))
and doing completion and using M-RET to pick a completion doesn't delete
the progn sexp.

This is because elisp-completion-at-point returns a completion region
covering the entire sexp after the start of the region.  That sexp is
"(progn (some) (big) (expression))" in the first case and "a" in the
second case.

I'm not sure how to fix this.  I don't know all the features of
elisp-completion-at-point, but maybe it shouldn't use (forward-sexp 1)
when finding the end of the completion region.  Maybe it should be doing
something like (skip-syntax-forward "w_") to determine the end of the
region?

BTW, this completion region returned by elisp-completion-at-point also
causes a bug in company-mode: since company-capf stops completion when
the end of the completion region is after point, it's not possible to do
Elisp company completion immediately after an open paren no matter how
company is configured.


In GNU Emacs 30.0.50 (build 20, x86_64-pc-linux-gnu, X toolkit, cairo
 version 1.15.12, Xaw scroll bars) of 2023-12-27 built on
 igm-qws-u22796a
Repository revision: b9c0ce0a400f18e1bba4e3491c94994d73ef9405
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version 11.0.12011000
System Description: Rocky Linux 8.9 (Green Obsidian)

Configured using:
 'configure -C 'CFLAGS=-O0 -g3' --with-gif=ifavailable
 --with-x-toolkit=lucid'

Configured features:
CAIRO DBUS FREETYPE GLIB GMP GNUTLS GSETTINGS HARFBUZZ JPEG JSON
LIBSELINUX LIBSYSTEMD LIBXML2 MODULES NOTIFY INOTIFY PDUMPER PNG RSVG
SECCOMP SOUND SQLITE3 THREADS TIFF TOOLKIT_SCROLL_BARS X11 XDBE XIM
XINPUT2 XPM LUCID ZLIB

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

Major mode: Lisp Interaction

Minor modes in effect:
  tooltip-mode: t
  global-eldoc-mode: t
  eldoc-mode: t
  show-paren-mode: t
  electric-indent-mode: t
  mouse-wheel-mode: t
  tool-bar-mode: t
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  global-font-lock-mode: t
  font-lock-mode: t
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  minibuffer-regexp-mode: t
  line-number-mode: t
  indent-tabs-mode: t
  transient-mark-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t

Load-path shadows:
None found.

Features:
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Memory information:
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