bug#30092: Compose key behaviour is incorrect when locale is set to Esperanto

[Carmen Bianca Bakker <carmen@carmenbianca.eu>]

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Dear Emacs maintainer(s),

I am running into an unusual bug of sorts.  That is:  The behaviour of the
compose key is incorrect when the locale is set to "eo" (Esperanto).  The exact
behaviour is as follows:

- When running `XMODIFIERS="" emacs`, `<MULTI_KEY> a e` does not produce æ.  It
  simply produces nothing.  `<MULTI_KEY> ' e` correctly produces é, however.

- When running `XMODIFIERS=@im=ibus emacs` (my default), simply pressing the
  compose key results in `<MULTI_KEY> is undefined`.

Emacs is (almost) the only program where this happens. The only other program
that reliably fails on the same sequences is xterm.  xterm results in the same
behaviour for both `XMODIFIERS` values.  All other programs (Firefox,
LibreOffice, gedit, Yakuake, etc.) function correctly.

But, when I run `LC_ALL=en_GB.UTF-8 emacs`, the behaviour is as expected:  All
compose key sequences just work.  The same is true for running xterm with the
en_GB.UTF-8 locale.

I am running Debian Buster with GNOME on Wayland.  The bug occurs with a vanilla
.emacs.d/ folder.  I have ibus installed and running, but also without ibus
running, I run into the first of the two items.

Because it happens on both Emacs and xterm, I suspect that this bug may not be
exclusive to Emacs, but I am hoping that someone with a better understanding
than I could point me in the right direction.

Thank you very much.

Yours sincerely,
Carmen

In GNU Emacs 25.2.2 (x86_64-pc-linux-gnu, GTK+ Version 3.22.20)
 of 2017-09-12, modified by Debian built on trouble
Windowing system distributor 'The X.Org Foundation', version 11.0.11905000
System Description:	Debian GNU/Linux testing (buster)

Configured using:
 'configure --build x86_64-linux-gnu --prefix=/usr
 --sharedstatedir=/var/lib --libexecdir=/usr/lib
 --localstatedir=/var/lib --infodir=/usr/share/info
 --mandir=/usr/share/man --with-pop=yes
 --enable-locallisppath=/etc/emacs25:/etc/emacs:/usr/local/share/emacs/25.2/site-lisp:/usr/local/share/emacs/site-lisp:/usr/share/emacs/25.2/site-lisp:/usr/share/emacs/site-lisp
 --with-sound=alsa --without-gconf --build x86_64-linux-gnu
 --prefix=/usr --sharedstatedir=/var/lib --libexecdir=/usr/lib
 --localstatedir=/var/lib --infodir=/usr/share/info
 --mandir=/usr/share/man --with-pop=yes
 --enable-locallisppath=/etc/emacs25:/etc/emacs:/usr/local/share/emacs/25.2/site-lisp:/usr/local/share/emacs/site-lisp:/usr/share/emacs/25.2/site-lisp:/usr/share/emacs/site-lisp
 --with-sound=alsa --without-gconf --with-x=yes --with-x-toolkit=gtk3
 --with-toolkit-scroll-bars 'CFLAGS=-g -O2
 -fdebug-prefix-map=/build/emacs25-XrMyQe/emacs25-25.2+1=. -fstack-protector-strong
 -Wformat -Werror=format-security -Wall' 'CPPFLAGS=-Wdate-time
 -D_FORTIFY_SOURCE=2' LDFLAGS=-Wl,-z,relro'

Configured features:
XPM JPEG TIFF GIF PNG RSVG IMAGEMAGICK SOUND GPM DBUS GSETTINGS NOTIFY
ACL LIBSELINUX GNUTLS LIBXML2 FREETYPE M17N_FLT LIBOTF XFT ZLIB
TOOLKIT_SCROLL_BARS GTK3 X11

Important settings:
  value of $LANG: eo
  value of $XMODIFIERS: @im=ibus
  locale-coding-system: iso-latin-3-unix

Major mode: Fundamental

Minor modes in effect:
  tooltip-mode: t
  global-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
  blink-cursor-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  buffer-read-only: t
  line-number-mode: t
  transient-mark-mode: t

Recent messages:
Loading debian-ispell...done
Loading /var/cache/dictionaries-common/emacsen-ispell-dicts.el (source)...done
Loading /etc/emacs/site-start.d/50dictionaries-common.el (source)...done
Loading /etc/emacs/site-start.d/50latex-cjk-common.el (source)...
Loading cjk-enc...done
Loading /etc/emacs/site-start.d/50latex-cjk-common.el (source)...done
Loading /etc/emacs/site-start.d/50latex-cjk-thai.el (source)...done
Loading /etc/emacs/site-start.d/50mu4e.el (source)...done
Loading /etc/emacs/site-start.d/50texlive-lang-english.el (source)...done
For information about GNU Emacs and the GNU system, type C-h C-a.
scroll-up-command: End of buffer [2 times]

Load-path shadows:
/usr/share/emacs25/site-lisp/cmake-data/cmake-mode hides /usr/share/emacs/site-lisp/cmake-mode
/usr/share/emacs/25.2/site-lisp/debian-startup hides /usr/share/emacs/site-lisp/debian-startup
/usr/share/emacs25/site-lisp/latex-cjk-thai/thai-word hides /usr/share/emacs/25.2/lisp/language/thai-word

Features:
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mwheel x-win term/common-win x-dnd tool-bar dnd fontset image regexp-opt
fringe tabulated-list newcomment elisp-mode lisp-mode prog-mode register
page menu-bar rfn-eshadow timer select scroll-bar mouse jit-lock
font-lock syntax facemenu font-core 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 charscript case-table epa-hook
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hashtable-print-readable backquote dbusbind inotify dynamic-setting
system-font-setting font-render-setting move-toolbar gtk x-toolkit x
multi-tty make-network-process emacs)

Memory information:
((conses 16 99825 4821)
 (symbols 48 21198 0)
 (miscs 40 92 117)
 (strings 32 19586 4935)
 (string-bytes 1 552873)
 (vectors 16 14014)
 (vector-slots 8 455925 4133)
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