bug#60712: 28.2; customize-set-variable won't let you change the type of a choice

[Omar Antolín Camarena]

If you declare a user option as follows:

(defcustom bool-or-alist '((a . 4) (b . 7))
  "A made up option to illustrate a problem."
  :type '(choice boolean (alist :key-type symbol :value-type integer)))

And then run M-x customize-set-variable to change the value to t the
change fails with the message:

widget-prompt-value: Value does not match alist type

That :type specification works just fine with the customize UI using
customize-variable, it's just a problem with customize-set-variable.

I think the problem is that widget-choice-prompt-value figures which
branch of the choice the variable currenty has and then expects the new
type to be the same.

If you tag everything, then customize-set-variable works just fine:

(defcustom bool-or-alist '((a . 4) (b . 7))
  "A made up option to illustrate a problem."
  :type '(choice (const :tag "Yes" t)
                 (const :tag "No" nil)
                 (alist :tag "An alist"
                        :key-type symbol :value-type integer)))


In GNU Emacs 28.2 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.24.34, cairo 
version 1.16.0)
 of 2022-10-25, modified by Debian built on x86-conova-01
Windowing system distributor 'The X.Org Foundation', version 11.0.12008000
System Description: Debian GNU/Linux 11 (bullseye)

Configured using:
 'configure --build x86_64-linux-gnu --prefix=/usr
 --sharedstatedir=/var/lib --libexecdir=/usr/libexec
 --localstatedir=/var/lib --infodir=/usr/share/info
 --mandir=/usr/share/man --with-libsystemd --with-pop=yes
 
--enable-locallisppath=/etc/emacs:/usr/local/share/emacs/28.2/site-lisp:/usr/local/share/emacs/site-lisp:/usr/share/emacs/28.2/site-lisp:/usr/share/emacs/site-lisp
 --with-sound=alsa --without-gconf --with-mailutils
 --with-native-compilation --build x86_64-linux-gnu --prefix=/usr
 --sharedstatedir=/var/lib --libexecdir=/usr/libexec
 --localstatedir=/var/lib --infodir=/usr/share/info
 --mandir=/usr/share/man --with-libsystemd --with-pop=yes
 
--enable-locallisppath=/etc/emacs:/usr/local/share/emacs/28.2/site-lisp:/usr/local/share/emacs/site-lisp:/usr/share/emacs/28.2/site-lisp:/usr/share/emacs/site-lisp
 --with-sound=alsa --without-gconf --with-mailutils
 --with-native-compilation --with-cairo --with-x=yes
 --with-x-toolkit=gtk3 --with-toolkit-scroll-bars 'CFLAGS=-g -O2
 -ffile-prefix-map=/build/emacs-mlMHQs/emacs-28.2+1=.
 -fstack-protector-strong -Wformat -Werror=format-security -Wall'
 'CPPFLAGS=-Wdate-time -D_FORTIFY_SOURCE=2' LDFLAGS=-Wl,-z,relro'

Configured features:
ACL CAIRO DBUS FREETYPE GIF GLIB GMP GNUTLS GPM GSETTINGS HARFBUZZ JPEG
JSON LCMS2 LIBOTF LIBSELINUX LIBSYSTEMD LIBXML2 M17N_FLT MODULES
NATIVE_COMP NOTIFY INOTIFY PDUMPER PNG RSVG SECCOMP SOUND THREADS TIFF
TOOLKIT_SCROLL_BARS X11 XDBE XIM XPM GTK3 ZLIB

Important settings:
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Major mode: ELisp/l

Minor modes in effect:
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  font-lock-mode: t
  blink-cursor-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  line-number-mode: t
  transient-mark-mode: t

Load-path shadows:
/home/omarantolin/.emacs.d/elpa/transient-20230107.1528/transient hides 
/usr/share/emacs/28.2/lisp/transient

Features:
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Memory information:
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 (symbols 48 38966 5)
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 (string-bytes 1 6784094)
 (vectors 16 68620)
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