bug#66970: 29.1; Overwrite-mode is not working with Korean characters

[SUKBEOM KIM]

Hi,

I found a bug with typing Korean characters that "overwrite-mode" is not
actually working well. (For Japanese/Chinese characters, there is no
issue at all.)

aeoiu
=> あえu
=> 아에aeiou

In the thread, one of the Japanese users reported a bug but it is not
appropriate for Korean characters which requires a combination of
sub-characters to make a character unlike other Japanese or Chinese
characters.

- https://lists.nongnu.org/archive/html/emacs-devel/2015-05/msg00039.html

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

Major mode: Lisp Interaction

Minor modes in effect:
  tooltip-mode: t
  global-eldoc-mode: t
  eldoc-mode: t
  show-paren-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
  blink-cursor-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
  overwrite-mode: overwrite-mode-textual

Load-path shadows:
None found.

Features:
(shadow sort mail-extr emacsbug message mailcap yank-media puny dired
dired-loaddefs rfc822 mml mml-sec password-cache epa derived epg rfc6068
epg-config gnus-util text-property-search time-date mm-decode mm-bodies
mm-encode mail-parse rfc2231 mailabbrev gmm-utils mailheader sendmail
rfc2047 rfc2045 ietf-drums mm-util mail-prsvr mail-utils cus-start
cus-load hangul hanja-util quail comp comp-cstr warnings icons subr-x rx
cl-seq cl-macs gv cl-extra help-mode cl-loaddefs cl-lib bytecomp
byte-compile rmc iso-transl tooltip cconv eldoc paren electric uniquify
ediff-hook vc-hooks lisp-float-type elisp-mode mwheel term/x-win x-win
term/common-win x-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 lcms2 dynamic-setting system-font-setting
font-render-setting cairo move-toolbar gtk x-toolkit xinput2 x multi-tty
make-network-process native-compile emacs)

Memory information:
((conses 16 164922 8869)
 (symbols 48 8289 0)
 (strings 32 40752 3578)
 (string-bytes 1 1149899)
 (vectors 16 19161)
 (vector-slots 8 319338 11606)
 (floats 8 34 31)
 (intervals 56 269 0)
 (buffers 984 12))