bug#42270: 28.0.50; cc-mode indentation issue with attributes

[Ergus via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

Hi:

Working in C++ I am getting this indentation difference when arguments
has attributes or not:

TaskDataAccesses(TaskDataAccessesInfo taskAccessInfo)
	: _lock(),
	  _accesses(),
	  _accessFragments(), _taskwaitFragments()
{
}


TaskDataAccesses(__attribute__((unused)) TaskDataAccessesInfo taskAccessInfo)
: _lock(),
_accesses(),
_accessFragments(), _taskwaitFragments()
{
}

The problem seems to be that in the first case the `:` indentation
symbol (C-c C-o) is recognised as `member-init-intro` (correct) but in
the second one it is detected as a `topmost-intro-cont` which is
actually wrong.

Best,
Ergus

In GNU Emacs 28.0.50 (build 10, x86_64-pc-linux-gnu, X toolkit, cairo version 1.16.0, Xaw scroll bars)
 of 2020-07-07 built on ergus
Repository revision: df3ece9d2ed61c9526dbf718e3c96d72bd53dccb
Repository branch: master
System Description: Debian GNU/Linux 10 (buster)

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

Configured using:
 'configure --prefix=/home/ergus/.local/ --with-mailutils'

Configured features:
XPM JPEG TIFF GIF PNG CAIRO SOUND DBUS GSETTINGS GLIB NOTIFY INOTIFY
LIBSELINUX GNUTLS LIBXML2 FREETYPE HARFBUZZ ZLIB TOOLKIT_SCROLL_BARS
LUCID X11 XDBE XIM MODULES THREADS PDUMPER GMP

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
  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
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
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 term/tmux term/xterm xterm byte-opt gv
bytecomp byte-compile cconv tooltip eldoc electric uniquify ediff-hook
vc-hooks lisp-float-type 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 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 dbusbind inotify dynamic-setting system-font-setting
font-render-setting cairo x-toolkit x multi-tty make-network-process
emacs)

Memory information:
((conses 16 48788 5097)
 (symbols 48 6074 1)
 (strings 32 15630 1699)
 (string-bytes 1 506737)
 (vectors 16 7254)
 (vector-slots 8 75610 8410)
 (floats 8 22 231)
 (intervals 56 180 0)
 (buffers 992 10))