bug#45768: icomplete-mode does not provide in region completion for dabbrev-completion

[Simon Lang]

emacs -Q
in *scratch* evaluate
(setq icomplete-in-buffer t)
(icomplete-mode t)

type:
asdf1
asdf2
asdf followed by C-M-/ (dabbrev-completion) 
->  default completion is invoked, not icomplete-mode


In GNU Emacs 27.1 (build 1, x86_64-apple-darwin18.7.0, NS appkit-1671.60 
Version 10.14.6 (Build 18G95))
   of 2020-08-12 built on builder10-14.porkrind.org
Windowing system distributor 'Apple', version 10.3.1894
System Description:    Mac OS X 10.15.7

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

Configured using:
   'configure --with-ns '--enable-locallisppath=/Library/Application
   Support/Emacs/${version}/site-lisp:/Library/Application
   Support/Emacs/site-lisp' --with-modules'

Configured features:
NOTIFY KQUEUE ACL GNUTLS LIBXML2 ZLIB TOOLKIT_SCROLL_BARS NS MODULES
THREADS JSON PDUMPER

Important settings:
     value of $LANG: en_GB.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
     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

Load-path shadows:
None found.

Features:
(shadow sort mail-extr emacsbug message rmc puny dired dired-loaddefs
format-spec rfc822 mml easymenu mml-sec password-cache epa derived epg
epg-config gnus-util rmail rmail-loaddefs text-property-search time-date
subr-x seq byte-opt gv bytecomp byte-compile cconv mm-decode mm-bodies
mm-encode mail-parse rfc2231 mailabbrev gmm-utils mailheader cl-loaddefs
cl-lib sendmail rfc2047 rfc2045 ietf-drums mm-util mail-prsvr mail-utils
tooltip eldoc electric uniquify ediff-hook vc-hooks lisp-float-type
mwheel term/ns-win ns-win ucs-normalize mule-util 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
tab-bar menu-bar rfn-eshadow isearch timer select scroll-bar mouse
jit-lock font-lock syntax facemenu font-core term/tty-colors frame
minibuffer 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 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 kqueue cocoa ns multi-tty make-network-process emacs)

Memory information:
((conses 16 44811 6704)
   (symbols 48 5927 1)
   (strings 32 15254 1703)
   (string-bytes 1 504784)
   (vectors 16 10174)
   (vector-slots 8 126879 9518)
   (floats 8 19 39)
   (intervals 56 189 0)
   (buffers 1000 11))