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

From: Konstantin Kharlamov
Subject: bug#33163: Adding a " in c++-mode fills the whole file with red
Date: Sun, 28 Oct 2018 14:21:11 +0300
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.0

On 10/28/18 1:49 PM, Alan Mackenzie wrote:
In article <address@hidden> 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() {
         auto foo = []()-> void {  };
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!

Thank you for the quick patch! I tried to do some editing and navigating, and all seems well. More real testing I can only do on Monday, so I guess I'll reply somewhere on Monday evening.

# 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)
-               (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)))

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