Subject: bug#36492: c-mode fails with errors during fontification

From: Alan Mackenzie <[email-masked]>
Date: 6 Jul 2019 14:27:48 -0000

In article <[email-masked]> you wrote:
> [-- text/plain, encoding 7bit, charset: US-ASCII, 6 lines --]

>> Opening this message in Gnus fails with the following backtrace
>> because c-mode fails to fontify the attachment.

> Actually, with the attachment in this message the bug in c-mode
> font-lock is much easier to reproduce.

That is one ugly piece of erroneous C.  ;-)

What triggers the bug is the ) without a preceding (.  A variable
recording the position of the opening ( is still set to nil, and we
tried to use it in the given buffer with the unbalanced ).

> [-- text/x-csrc, encoding 7bit, charset: US-ASCII, 3 lines, name: test.c --]

The following patch fixes it, I hope.  Would you please do the usual,
and confirm that it does indeed fix the bug (or tell me what's still
wrong).

Thanks!

diff -r eb4ee9bb0682 cc-engine.el
--- a/cc-engine.el	Thu Jul 04 13:01:08 2019 +0000
+++ b/cc-engine.el	Sat Jul 06 14:16:43 2019 +0000
@@ -9480,6 +9480,7 @@
 			   (not got-prefix)
 			   (or (eq context 'top) make-top)
 			   (eq (char-after) ?\))
+			   after-paren-pos
 			   (or (memq at-type '(nil maybe))
 			       (not got-identifier)
 			       (save-excursion
@@ -9516,7 +9517,7 @@
 	    ;; (con|de)structors in C++ and `c-typeless-decl-kwds'
 	    ;; style declarations.  That isn't applicable in an
 	    ;; arglist context, though.
-	    (when (and (> paren-depth 0)
+	    (when (and (> paren-depth 0) ; ensures `after-paren-pos' is non-nil
 		       (not got-prefix-before-parens)
 		       (not (eq at-type t))
 		       (or backup-at-type

-- 
Alan Mackenzie (Nuremberg, Germany).