Hi,

I have found two related issues. Both are evident in csharp-mode, but
can easily be reproduced in c-mode.

Issue 1 (cc-mode.el):

  When c-multiline-string-start-char is set, font locking breaks after
  changes in string literals. The issue started after commit 2ca12bb3de:
  I believe that '(c-remove-string-fences (car end-limits))' should be
  '(c-remove-string-fences (1- (cdr end-limits)))'

  Steps to reproduce:
    1. Open a .c file;
    2. Eval '(setq c-multiline-string-start-char ?@)';
    3. Delete/insert double quotes.

  I think this issue is related:
  https://github.com/Fuco1/smartparens/issues/992


Issue 2 (maybe syntax.el):

  When syntax-propertize-function is set (to '(lambda (beg end) nil)',
  for example), a similar problem occurs if electric-pair-mode (or even
  smartparens) is enabled. The symptoms started after commit 585fb95739,
  but I don't know if it is related.

  Steps to reproduce:
    1. Open a .c file;
    2. Eval '(setq syntax-propertize-function (lambda (beg end) nil)';
    3. Enable electric-pair-mode;
    4. Insert/delete empty strings.




In GNU Emacs 27.1 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.22.30,
cairo version 1.16.0)
of 2020-08-06 built on gustavo-Lenovo-ideapad-330-15IKB
Windowing system distributor 'The X.Org Foundation', version 11.0.12008000
System Description: Ubuntu 18.04.5 LTS

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

Configured using:
'configure --with-x-toolkit=gtk3 --with-modules --with-sound=yes
--with-file-notification=yes --with-mailutils --with-cairo'

Configured features:
XPM JPEG TIFF GIF PNG RSVG CAIRO SOUND GPM DBUS GSETTINGS GLIB NOTIFY
INOTIFY ACL LIBSELINUX GNUTLS LIBXML2 FREETYPE HARFBUZZ M17N_FLT LIBOTF
ZLIB TOOLKIT_SCROLL_BARS GTK3 X11 XDBE XIM MODULES THREADS PDUMPER LCMS2
GMP

Important settings:
value of $LANG: en
value of $XMODIFIERS:
locale-coding-system: iso-latin-1-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
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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:
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Features:
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Memory information:
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(symbols 48 5980 1)
(strings 32 15464 904)
(string-bytes 1 505487)
(vectors 16 9275)
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