bug#72156: 31.0.50; defcustom set issue

[Ergus via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

Hi:

I have this defcustom:

```
(defcustom gtags-mode-features '(control meta)
  "HELLO WORLD"
  :type '(set (const control)
              (const meta)
              (const shift)
              (const hyper)
              (const super)
              (const alt)))
```

When I try the customize-variable I get this:

```
Hide Gtags Mode Features:
Set:
[X] control
[X] meta
[ ] shift
[ ] hyper
[ ] super
[ ] alt
   [ State ]: EDITED, shown value does not take effect until you set or save it.
   HELLO WORLD
Groups: [Gtags Mode]
```

But, if I do the next change and save


```
Hide Gtags Mode Features:
Set:
[X] control
[X] meta
[X] shift
[ ] hyper
[ ] super
[ ] alt
   [ State ]: EDITED, shown value does not take effect until you set or save it.
   HELLO WORLD
Groups: [Gtags Mode]
```

The customize variable value saved is:

'(gtags-mode-features3 '(shift))

it only saved the "new" value and ignores the previous
existing ones.

When I restart and check the variable's value I get as expected '(shift)
and lost the control and meta.

Going to customize-variable again I get:

```
Hide Gtags Mode Features:
Set:
[ ] control
[ ] meta
[X] shift
[ ] hyper
[ ] super
[ ] alt
   [ State ]: EDITED, shown value does not take effect until you set or save it.
   HELLO WORLD
Groups: [Gtags Mode]
```

Consistent with the variable value, but inconsistent with what was
actually saved before.

IF instead of selecting a new value I un-select some, then variable
saves nil and will be nil.





In GNU Emacs 31.0.50 (build 13, x86_64-pc-linux-gnu, GTK+ Version
 3.24.43, cairo version 1.18.0) of 2024-07-17 built on RTX
Repository revision: 438a7da5530a32d20de769e3450e4fc3a74e9236
Repository branch: project
System Description: Arch Linux

Configured using:
 'configure --prefix=/home/ergo/.local/ --with-mailutils --with-pgtk
 --with-modules --with-cairo --with-harfbuzz
 --with-native-compilation=aot
 '--program-transform-name=s/^ctags$/ctags.emacs/''

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

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

Major mode: Custom

Minor modes in effect:
  fancy-compilation-mode: t
  global-auto-revert-mode: t
  diff-hl-margin-mode: t
  corfu-terminal-mode: t
  global-corfu-mode: t
  corfu-mode: t
  project-multi-mode: t
  gtags-mode: t
  repeat-mode: t
  xterm-mouse-mode: t
  xclip-mode: t
  override-global-mode: t
  winner-mode: t
  save-place-mode: t
  delete-selection-mode: t
  savehist-mode: t
  global-display-fill-column-indicator-mode: t
  display-fill-column-indicator-mode: t
  global-display-line-numbers-mode: t
  display-line-numbers-mode: t
  which-key-mode: t
  show-paren-mode: t
  mouse-wheel-mode: t
  file-name-shadow-mode: t
  context-menu-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  blink-cursor-mode: t
  minibuffer-regexp-mode: t
  size-indication-mode: t
  column-number-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:
/mnt/casa/gits/emacs_clones/cuda-mode/cuda-mode hides /home/ergo/.config/emacs/elpa/cuda-mode-20240716.1936/cuda-mode
/mnt/casa/gits/emacs_clones/gtags-mode/gtags-mode hides /home/ergo/.config/emacs/elpa/gtags-mode-1.8/gtags-mode
/home/ergo/.config/emacs/elpa/transient-20240713.2102/transient hides /home/ergo/.local/share/emacs/31.0.50/lisp/transient

Features:
(shadow sort fancy-compilation compile comint ansi-osc ansi-color
comp-run comp-common mail-extr emacsbug message mailcap yank-media puny
rfc822 mml mml-sec epa derived epg rfc6068 epg-config gnus-util
text-property-search mm-decode mm-bodies mm-encode mail-parse rfc2231
mailabbrev gmm-utils mailheader sendmail rfc2047 rfc2045 ietf-drums
mm-util mail-prsvr mail-utils time-date tabify help-fns radix-tree
cus-edit pp cus-start cus-load wid-edit find-func autorevert filenotify
ffap thingatpt url-parse auth-source eieio eieio-core icons
password-cache json map url-vars vc-git elec-pair whitespace
flyspell-correct flyspell ispell completion-preview diff-hl-margin
diff-hl-dired citre-lang-fileref citre-tags citre-ctags citre-readtags
citre-readtags-tables citre-backend-interface citre-common-tag rx
citre-common-util dired-x dired dired-loaddefs diff-hl log-view
pcvs-util vc-dir ewoc vc vc-dispatcher diff-mode track-changes
corfu-terminal popon corfu project-multi-mode gtags-mode cl-macs subr-x
files-x xref project term/tmux term/xterm xterm init repeat cape compat
use-package-ensure use-package-diminish xt-mouse xclip edmacro kmacro
byte-opt gv use-package-bind-key bind-key cl-extra help-mode
simple-16-theme winner ring saveplace delsel savehist easy-mmode
display-fill-column-indicator display-line-numbers diminish which-key
cl-seq use-package-core cl-loaddefs cl-lib bytecomp byte-compile
disp-table info ac-emoji-autoloads ac-haskell-process-autoloads
ac-html-autoloads arduino-cli-mode-autoloads auctex-autoloads tex-site
auto-complete-autoloads avy-zap-autoloads avy-autoloads
better-shell-autoloads caml-autoloads cape-autoloads citre-autoloads
clang-format-autoloads cobol-mode-autoloads compile-multi-autoloads
corfu-terminal-autoloads corfu-autoloads crdt-autoloads
csv-mode-autoloads cuda-mode-autoloads d-mode-autoloads
deadgrep-autoloads debbugs-autoloads diff-hl-autoloads
diminish-autoloads dired-sidebar-autoloads dired-subtree-autoloads
dired-hacks-utils-autoloads dumb-jump-autoloads e2ansi-autoloads
emamux-autoloads esup-autoloads evil-collection-autoloads
annalist-autoloads evil-leader-autoloads evil-autoloads
face-explorer-autoloads fancy-compilation-autoloads flx-autoloads
flycheck-julia-autoloads flycheck-rust-autoloads flycheck-autoloads
flymake-nasm-autoloads flymake-quickdef-autoloads
flyspell-correct-autoloads git-modes-autoloads git-timemachine-autoloads
gnuplot-autoloads google-c-style-autoloads goto-chg-autoloads
groovy-mode-autoloads gtags-mode-autoloads haskell-mode-autoloads
highlight-indent-guides-autoloads i3wm-config-mode-autoloads
ibuffer-sidebar-autoloads iedit-autoloads imenu-list-autoloads
julia-ts-mode-autoloads julia-mode-autoloads languagetool-autoloads
lice-autoloads lorem-ipsum-autoloads lua-mode-autoloads magit-autoloads
git-commit-autoloads magit-section-autoloads markdown-mode-autoloads
modern-cpp-font-lock-autoloads move-dup-autoloads
multiple-cursors-autoloads mutt-mode-autoloads nasm-mode-autoloads
neotree-autoloads nftables-mode-autoloads nginx-mode-autoloads
notmuch-autoloads objed-autoloads opencl-mode-autoloads
paradox-autoloads phi-search-autoloads pkg-info-autoloads epl-autoloads
pkgbuild-mode-autoloads platformio-mode-autoloads async-autoloads
popon-autoloads popup-autoloads projectile-autoloads
projection-autoloads protobuf-mode-autoloads protobuf-ts-mode-autoloads
ptemplate-templates-autoloads ptemplate-autoloads scopeline-autoloads
shell-command+-autoloads slime-autoloads macrostep-autoloads
sphinx-mode-autoloads f-autoloads dash-autoloads s-autoloads
spinner-autoloads ssh-config-mode-autoloads string-inflection-autoloads
sudo-edit-autoloads systemd-autoloads tmux-mode-autoloads
transient-autoloads tsc-autoloads urgrep-autoloads vdiff-autoloads
hydra-autoloads lv-autoloads vterm-toggle-autoloads vterm-autoloads
vundo-autoloads with-editor-autoloads xclip-autoloads
yasnippet-snippets-autoloads yasnippet-autoloads early-init rmc
iso-transl tooltip cconv eldoc paren electric uniquify ediff-hook
vc-hooks lisp-float-type elisp-mode mwheel term/pgtk-win pgtk-win
term/common-win touch-screen pgtk-dnd tool-bar dnd fontset image
regexp-opt fringe tabulated-list replace newcomment text-mode lisp-mode
prog-mode register page tab-bar menu-bar rfn-eshadow isearch easymenu
timer select scroll-bar mouse jit-lock font-lock syntax font-core
term/tty-colors frame minibuffer nadvice seq simple cl-generic
indonesian philippine cham georgian utf-8-lang misc-lang vietnamese
tibetan thai tai-viet lao korean japanese eucjp-ms cp51932 hebrew greek
romanian slovak czech european ethiopic indian cyrillic chinese
composite emoji-zwj charscript charprop case-table epa-hook
jka-cmpr-hook help abbrev obarray oclosure cl-preloaded button loaddefs
theme-loaddefs faces cus-face macroexp files window text-properties
overlay sha1 md5 base64 format env code-pages mule custom widget keymap
hashtable-print-readable backquote threads dbusbind inotify
dynamic-setting system-font-setting font-render-setting cairo gtk pgtk
lcms2 multi-tty move-toolbar make-network-process native-compile emacs)

Memory information:
((conses 16 218529 26550) (symbols 48 17046 3) (strings 32 56773 7397)
 (string-bytes 1 1882457) (vectors 16 20651)
 (vector-slots 8 239085 7886) (floats 8 102 151) (intervals 56 1573 0)
 (buffers 992 18))