bug#60643: 29.0.50; set-buffer-major-mode resets buffer local variables

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

Joseph Turner via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org> writes:

> Hello!
>
> It appears that set-buffer-major-mode resets buffer local variables.
>
> To reproduce with `emacs -Q`:
>
> (defvar-local test-var nil)
> (setq test-var t)
> (message "%s" test-var) ;; t
> ;; (set-auto-mode)
> (set-buffer-major-mode (current-buffer))
> (message "%s" test-var) ;; nil
>
> Is this behavior expected?
>
> Thank you!!
>
> Joseph
>
> In GNU Emacs 29.0.50 (build 1, x86_64-pc-linux-gnu, GTK+ Version
> 3.24.30, cairo version 1.16.0)
> Windowing system distributor 'The X.Org Foundation', version 11.0.12011000
> System Description: Debian GNU/Linux 11 (bullseye)
>
> Configured using:
>  'configure
>  CONFIG_SHELL=/gnu/store/4y5m9lb8k3qkb1y9m02sw9w9a6hacd16-bash-minimal-5.1.8/bin/bash
>  SHELL=/gnu/store/4y5m9lb8k3qkb1y9m02sw9w9a6hacd16-bash-minimal-5.1.8/bin/bash
>  --prefix=/gnu/store/4mnib031vflf88ms8w7kyfahcbv1k9vc-emacs-next-29.0.50-3.22e8a77
>  --enable-fast-install --with-modules --with-cairo
>  --with-native-compilation --disable-build-details'
>
> 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 SQLITE3
> THREADS TIFF TOOLKIT_SCROLL_BARS X11 XDBE XIM XINPUT2 XPM GTK3 ZLIB

There is another concept called "permanent local variables" which you
might be thinking about when looking at "local variables", see info node
`(elisp) Standard Properties' for more information.

Best,


RY