Steps to reproduce:
1) Launch emacs -Q
2) M-x set-input-method german-prefix
3) Type " (double quote) followed by S (capital s)

Expected behavior:
Emacs should insert ẞ, that is, a capital sharp S (which is an official
letter in Germany
as of 2017, see https://en.wikipedia.org/wiki/Capital_%E1%BA%9E )

Actual behavior:
Emacs inserts the text literally.



In GNU Emacs 25.3.1 (x86_64-pc-linux-gnu, GTK+ Version 3.22.26)
of 2018-02-09 built on bisson
Windowing system distributor 'The X.Org Foundation', version 11.0.11906000
Configured using:
'configure --prefix=/usr --sysconfdir=/etc --libexecdir=/usr/lib
--localstatedir=/var --with-x-toolkit=gtk3 --with-xft --with-modules
'CFLAGS=-march=x86-64 -mtune=generic -O2 -pipe -fstack-protector-strong
-fno-plt' CPPFLAGS=-D_FORTIFY_SOURCE=2
LDFLAGS=-Wl,-O1,--sort-common,--as-needed,-z,relro,-z,now'

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

Important settings:
value of $LC_ALL: en_US.UTF-8
value of $LC_COLLATE: C
value of $LANG: en_US.UTF-8
value of $XMODIFIERS: @im=xim
locale-coding-system: utf-8-unix

Major mode: Lisp Interaction

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
font-lock-mode: t
blink-cursor-mode: t
auto-composition-mode: t
auto-encryption-mode: t
auto-compression-mode: t
line-number-mode: t
transient-mark-mode: t

Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.
Making completion list...

Load-path shadows:
None found.

Features:
(shadow sort mail-extr emacsbug message dired format-spec rfc822 mml
mml-sec password-cache epg epg-config gnus-util mm-decode mm-bodies
mm-encode mail-parse rfc2231 mailabbrev gmm-utils mailheader sendmail
rfc2047 rfc2045 ietf-drums mm-util help-fns help-mode easymenu
cl-loaddefs pcase cl-lib mail-prsvr mail-utils time-date mule-util
tooltip eldoc electric uniquify ediff-hook vc-hooks lisp-float-type
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
jka-cmpr-hook help simple abbrev 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 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 86355 6655)
(symbols 48 19767 0)
(miscs 40 50 121)
(strings 32 14299 4549)
(string-bytes 1 407478)
(vectors 16 11747)
(vector-slots 8 430579 4683)
(floats 8 167 83)
(intervals 56 224 0)
(buffers 976 19))