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

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

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