bug#28623: 27.0.50; lisp/progmodes/cc-engine.el incorrect indentation of C++14 curly-brace initializer list

[Tadeus Prastowo]

The following C++14 source code, which is also attached as `mwe.cpp',
shows the indentation problem:

--8<------------------------------
#include <vector>
#include <iostream>

static std::vector<std::vector<unsigned>>
fn(std::vector<std::vector<unsigned>> data) {
  return {
          {1, 2, 3},
          {4, 5, 6},
          {7, 8, 9},
  };
}

static std::vector<std::vector<unsigned>>
fn(unsigned n, std::vector<std::vector<unsigned>> data) {
  return {
          {n + 1, n + 2, n + 3},
          {n + 4, n + 5, n + 6},
          {n + 7, n + 8, n + 9},
  };
}

int main() {
  /* Expected indentation */
  fn({
      {1, 2, 3},
      {3, 4, 5},
      {6, 7, 8},
    });
  for (const auto &v : fn({
                           {3, 4, 5},
                           {6, 7, 8},
                           {9, 10, 11},
      })) {
    for (const auto &a : v) {
      std::cout << a << '\n';
    }
  }
  /* End: Expected indentation */

  /* Problem */
  fn(20, {
      {1, 2, 3},
        {3, 4, 5},
          {6, 7, 8},
            });
  for (const auto &v : fn(20, {
        {3, 4, 5},
          {6, 7, 8},
            {9, 10, 11},
              })) {
    for (const auto &a : v) {
      std::cout << a << '\n';
    }
  }
  /* End: Problem */
}
--8<------------------------------

To fix the problem, I make the following patch:
--8<------------------------------
diff --git a/lisp/progmodes/cc-engine.el b/lisp/progmodes/cc-engine.el
index 05b391a..077e9c9 100644
--- a/lisp/progmodes/cc-engine.el
+++ b/lisp/progmodes/cc-engine.el
@@ -10387,6 +10387,7 @@ comment at the start of cc-engine.el for more info."
               (eq (char-after) ?\())
          (setq braceassignp 'c++-noassign))
         ((looking-at c-pre-id-bracelist-key))
+        ((looking-at ",\\s *"))
         ((looking-at c-return-key))
         ((and (looking-at c-symbol-start)
               (not (looking-at c-keywords-regexp)))
@@ -10398,6 +10399,7 @@ comment at the start of cc-engine.el for more info."
            (and (c-go-up-list-backward nil lim) ; FIXME!!! Check
`lim' 2016-07-12.
             (eq (char-after) ?\()))
           ((looking-at c-pre-id-bracelist-key))
+          ((looking-at ",\\s *"))
           ((looking-at c-return-key))
           (t (setq after-type-id-pos (point))
              nil))))
--8<------------------------------

Any better suggestion as to how to fix the problem?

Thanks.

In GNU Emacs 27.0.50 (build 2, x86_64-pc-linux-gnu, GTK+ Version 3.18.9)
 of 2017-09-27 built on lgw01-amd64-052
Windowing system distributor 'The X.Org Foundation', version 11.0.11804000
System Description:    Ubuntu 16.04.3 LTS

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

Configured using:
 'configure --build=x86_64-linux-gnu --prefix=/usr
 '--includedir=${prefix}/include' '--mandir=${prefix}/share/man'
 '--infodir=${prefix}/share/info' --sysconfdir=/etc --localstatedir=/var
 --disable-silent-rules '--libdir=${prefix}/lib/x86_64-linux-gnu'
 '--libexecdir=${prefix}/lib/x86_64-linux-gnu' --disable-maintainer-mode
 --disable-dependency-tracking --prefix=/usr --sharedstatedir=/var/lib
 --program-suffix=-snapshot --with-modules=yes --with-x=yes
 --with-x-toolkit=gtk3 --with-xwidgets=yes 'CFLAGS=-g -O2
 -fstack-protector-strong -Wformat -Werror=format-security'
 'CPPFLAGS=-Wdate-time -D_FORTIFY_SOURCE=2'
 'LDFLAGS=-Wl,-Bsymbolic-functions -Wl,-z,relro''

Configured features:
XPM JPEG TIFF GIF PNG RSVG IMAGEMAGICK SOUND GPM DBUS GSETTINGS NOTIFY
LIBSELINUX GNUTLS LIBXML2 FREETYPE M17N_FLT LIBOTF XFT ZLIB
TOOLKIT_SCROLL_BARS GTK3 X11 MODULES XWIDGETS LIBSYSTEMD LCMS2

Important settings:
  value of $LC_MONETARY: en_US.UTF-8
  value of $LC_NUMERIC: en_US.UTF-8
  value of $LC_TIME: en_US.UTF-8
  value of $LANG: en_US.UTF-8
  value of $XMODIFIERS: @im=ibus
  locale-coding-system: utf-8-unix

Major mode: C++//l

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
  abbrev-mode: t

Load-path shadows:
None found.

Features:
(shadow sort mail-extr emacsbug message subr-x puny seq byte-opt gv
bytecomp byte-compile cconv dired dired-loaddefs format-spec rfc822 mml
mml-sec password-cache epa derived epg epg-config gnus-util rmail
rmail-loaddefs mm-decode mm-bodies mm-encode mail-parse rfc2231
mailabbrev gmm-utils mailheader sendmail rfc2047 rfc2045 ietf-drums
mm-util mail-prsvr mail-utils cc-mode cc-fonts easymenu cc-guess
cc-menus cc-cmds cc-styles cc-align cc-engine cc-vars cc-defs
cl-loaddefs cl-lib elec-pair time-date mule-util 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 menu-bar rfn-eshadow isearch timer select
scroll-bar mouse jit-lock font-lock syntax facemenu font-core
term/tty-colors 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 composite charscript charprop case-table epa-hook jka-cmpr-hook
help simple abbrev obarray 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 lcms2 dynamic-setting system-font-setting
font-render-setting xwidget-internal move-toolbar gtk x-toolkit x
multi-tty make-network-process emacs)

Memory information:
((conses 16 115959 7786)
 (symbols 48 22648 1)
 (miscs 40 46 140)
 (strings 32 34019 980)
 (string-bytes 1 1013691)
 (vectors 16 17237)
 (vector-slots 8 521799 8627)
 (floats 8 50 155)
 (intervals 56 397 1)
 (buffers 992 13)
 (heap 1024 40263 1174))

--
Best regards,
Tadeus

mwe.cpp
Description: Text Data