--- a/lisp/progmodes/cc-mode.el.orig	2020-07-29 23:40:41.000000000 +0200
+++ b/lisp/progmodes/cc-mode.el	2020-09-18 15:36:37.409779350 +0200
@@ -2007,6 +2007,10 @@
   ;; larger than (beg end).
   (setq c-new-END (- (+ c-new-END (- end beg)) old-len))
 
+  ;; Trying to fix bug#40317
+  (if (> c-new-END (point-max))
+         (error "c-new-END is too big! %d > %d" c-new-END (point-max)))
+
   (unless (c-called-from-text-property-change-p)
     (setq c-just-done-before-change nil)
     (c-save-buffer-state (case-fold-search)