bug#37580: 26.3; setting buffer as unibyte temporarily may change buffer contents

[ynyaaa]

If a multibyte buffer contains eight-bit character sequences,
evaluating the form
 (progn (set-buffer-multibyte nil) (set-buffer-multibyte t))
may convert them to multibyte characters.

Afterwards, buffer-undo-list may be inappropriate.
Undo in the form below changes the position of character '1'.

(with-temp-buffer
  (insert "一123")
  (encode-coding-region 1 2 'utf-8)
  (buffer-enable-undo)
  (undo-boundary)
  (progn (goto-char (point-min))
         (search-forward "1")
         (delete-char -1))
  (undo-boundary)
  (progn (set-buffer-multibyte nil)
         (set-buffer-multibyte t))
  (undo)
  (buffer-string))
=>"一231"


In GNU Emacs 26.3 (build 1, x86_64-w64-mingw32)
 of 2019-08-29 built on CIRROCUMULUS
Repository revision: 96dd0196c28bc36779584e47fffcca433c9309cd
Windowing system distributor 'Microsoft Corp.', version 6.3.9600
Recent messages:

Configured using:
 'configure --without-dbus --host=x86_64-w64-mingw32
 --without-compress-install 'CFLAGS=-O2 -static -g3''

Configured features:
XPM JPEG TIFF GIF PNG RSVG SOUND NOTIFY ACL GNUTLS LIBXML2 ZLIB
TOOLKIT_SCROLL_BARS THREADS LCMS2

Important settings:
  value of $LANG: JPN
  locale-coding-system: cp932

Major mode: Lisp Interaction

Minor modes in effect:
  tooltip-mode: t
  global-eldoc-mode: t
  eldoc-mode: t
  electric-indent-mode: t
  mouse-wheel-mode: t
  tool-bar-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  line-number-mode: t

Load-path shadows:
None found.

Features:
(network-stream nsm starttls tls gnutls mailalias smtpmail auth-source
cl-seq eieio eieio-core cl-macs eieio-loaddefs misearch multi-isearch pp
shadow sort mail-extr emacsbug message rmc puny seq byte-opt gv bytecomp
byte-compile cconv dired dired-loaddefs format-spec rfc822 mml mml-sec
password-cache epa derived epg epg-config gnus-util rmail rmail-loaddefs
mm-decode mm-bodies mm-encode mail-parse rfc2231 mailabbrev gmm-utils
mailheader sendmail rfc2047 rfc2045 ietf-drums mm-util mail-prsvr
mail-utils cl-extra thingatpt help-fns radix-tree help-mode cl-loaddefs
cl-lib image-mode easymenu elec-pair time-date mule-util japan-util
tooltip eldoc electric uniquify ediff-hook vc-hooks lisp-float-type
mwheel dos-w32 ls-lisp disp-table term/w32-win w32-win w32-vars
term/common-win tool-bar dnd fontset image regexp-opt fringe
tabulated-list replace newcomment text-mode elisp-mode lisp-mode
prog-mode register page menu-bar rfn-eshadow isearch timer select
scroll-bar mouse jit-lock font-lock syntax facemenu font-core
term/tty-colors frame cl-generic 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 charscript charprop case-table epa-hook jka-cmpr-hook
help simple abbrev obarray minibuffer cl-preloaded nadvice loaddefs
button faces cus-face macroexp files text-properties overlay sha1 md5
base64 format env code-pages mule custom widget hashtable-print-readable
backquote threads w32notify w32 lcms2 multi-tty make-network-process
emacs)

Memory information:
((conses 16 120602 43655)
 (symbols 48 21333 2)
 (miscs 40 80 287)
 (strings 32 35067 1048)
 (string-bytes 1 899321)
 (vectors 16 16930)
 (vector-slots 8 597369 14750)
 (floats 8 64 249)
 (intervals 56 848 3)
 (buffers 992 21))