bug#33163: Adding a " in c++-mode fills the whole file with red

[Alan Mackenzie <acm@muc.de>]

In article <mailman.2795.1540562775.1284.bug-gnu-emacs@gnu.org> you wrote:
> This long-standing problem appears occasionally, and only disappears if
> you kill the buffer completely, then reopen the file. I never got to
> report it because I never could find steps to reproduce it… until now
> :) Nvm the non-sense code, I reduced testcase as far as I managed.

> # Steps to reproduce:

> 1. Save the following text in `test.cpp`
>     int main() {
>             puts(It's");
>         auto foo = []()-> void {  };
>             puts("It's");
>     }
> 2. Open `emacs -Q ./test.cpp`
> 3. Now, the text lacks a quote in the first `puts(It's");`. Add it so
> it looks like `puts("It's");`. Essentially, if you'd compile it with
> g++, it lacks any syntax error besides `puts` being undeclared.

> # Expected

> The line with `auto foo…` is not red.

> # Actual

> The line with `auto foo…` (in particular) becomes red.

Thanks for taking the trouble to report this bug.  Thanks even more for
condensing it down to a minimal test case.

The following patch should fix it.  Would you please apply the patch,
recompile .../lisp/progmodes/cc-mode.el, load the new file in to your
Emacs (or restart Emacs), then try it out on your real C++ code.  Then
please let me know whether it's working or not, and if not, how it's
going wrong.  Thanks!

> # Additional information

> Emacs version GNU Emacs 27.0.50 (build 1, x86_64-pc-linux-gnu, GTK+
> Version 3.22.30) of 2018-10-01, build with -flto.



diff -r 2f7029fa229d cc-mode.el
--- a/cc-mode.el	Sun Oct 14 20:01:07 2018 +0000
+++ b/cc-mode.el	Sun Oct 28 10:20:38 2018 +0000
@@ -1216,21 +1216,21 @@
       (if (eq beg-literal-type 'string)
 	  (setq c-new-BEG (min (car beg-limits) c-new-BEG))))
 
-     ((< c-new-END (point-max))
-      (goto-char (1+ c-new-END))	; might be a newline.
+     ((< end (point-max))
+      (goto-char (1+ end))	; might be a newline.
       ;; In the following regexp, the initial \n caters for a newline getting
       ;; joined to a preceding \ by the removal of what comes between.
       (re-search-forward "[\n\r]?\\(\\\\\\(.\\|\n\\|\r\\)\\|[^\\\n\r]\\)*"
 			 nil t)
       ;; We're at an EOLL or point-max.
-      (setq c-new-END (min (1+ (point)) (point-max)))
-      (goto-char c-new-END)
-      (if (equal (c-get-char-property (1- (point)) 'syntax-table) '(15))
-	  (if (memq (char-before) '(?\n ?\r))
+      (setq c-new-END (max c-new-END (min (1+ (point)) (point-max))))
+      (if (equal (c-get-char-property (point) 'syntax-table) '(15))
+	  (if (memq (char-after) '(?\n ?\r))
 	      ;; Normally terminated invalid string.
-	      (progn
+	      (let ((eoll-1 (point)))
+		(forward-char)
 		(backward-sexp)
-		(c-clear-char-property (1- c-new-END) 'syntax-table)
+		(c-clear-char-property eoll-1 'syntax-table)
 		(c-clear-char-property (point) 'syntax-table))
 	    ;; Opening " at EOB.
 	    (c-clear-char-property (1- (point)) 'syntax-table))
@@ -1238,7 +1238,7 @@
 	    ;; Opening " on last line of text (without EOL).
 	    (c-clear-char-property (point) 'syntax-table))))
 
-     (t (goto-char c-new-END)
+     (t (goto-char end)			; point-max
 	(if (c-search-backward-char-property 'syntax-table '(15) c-new-BEG)
 	    (c-clear-char-property (point) 'syntax-table))))
 
@@ -1327,9 +1327,9 @@
 	     (while (progn
 		      (setq s (parse-partial-sexp (point) c-new-END nil
 						  nil s 'syntax-table))
-		      (and (not (nth 3 s))
-			   (< (point) c-new-END)
-			   (not (memq (char-before) c-string-delims)))))
+                      (and (< (point) c-new-END)
+                           (or (not (nth 3 s))
+                               (not (memq (char-before) c-string-delims))))))
 	     ;; We're at the start of a string.
 	     (memq (char-before) c-string-delims)))
 	(if (c-unescaped-nls-in-string-p (1- (point)))


-- 
Alan Mackenzie (Nuremberg, Germany).