From e9155c4ae4fff20989e8ec0cfbe3c24036a897a3 Mon Sep 17 00:00:00 2001
From: Dmitry Gutov <dgutov@yandex.ru>
Date: Tue, 1 Oct 2013 04:13:48 +0300
Subject: [PATCH] * lisp/newcomment.el (comment-beginning): When
 `comment-use-syntax' is non-nil, use `syntax-ppss'.

Fixes: debbugs:15251
---
 lisp/ChangeLog     |  5 +++++
 lisp/newcomment.el | 54 +++++++++++++++++++++++++---------------------
 2 files changed, 35 insertions(+), 24 deletions(-)

diff --git a/lisp/ChangeLog b/lisp/ChangeLog
index 4eb0e7ec584..ab8c87fca97 100644
--- a/lisp/ChangeLog
+++ b/lisp/ChangeLog
@@ -1,3 +1,8 @@
+2013-10-01  Dmitry Gutov  <dgutov@yandex.ru>
+
+	* newcomment.el (comment-beginning): When `comment-use-syntax' is
+	non-nil, use `syntax-ppss' (Bug#15251).
+
 2013-09-30  Rüdiger Sonderfeld  <ruediger@c-plusplus.de>
 
 	* progmodes/octave.el (inferior-octave-startup-file): Prefer
diff --git a/lisp/newcomment.el b/lisp/newcomment.el
index 19a06bfe8e5..3702b55f0aa 100644
--- a/lisp/newcomment.el
+++ b/lisp/newcomment.el
@@ -515,30 +515,36 @@ Ensure that `comment-normalize-vars' has been called before you use this."
   "Find the beginning of the enclosing comment.
 Returns nil if not inside a comment, else moves point and returns
 the same as `comment-search-backward'."
-  ;; HACK ATTACK!
-  ;; We should really test `in-string-p' but that can be expensive.
-  (unless (eq (get-text-property (point) 'face) 'font-lock-string-face)
-    (let ((pt (point))
-	  (cs (comment-search-backward nil t)))
-      (when cs
-	(if (save-excursion
-	      (goto-char cs)
-	      (and
-	       ;; For modes where comment-start and comment-end are the same,
-	       ;; the search above may have found a `ce' rather than a `cs'.
-	       (or (if comment-end-skip (not (looking-at comment-end-skip)))
-		   ;; Maybe font-lock knows that it's a `cs'?
-		   (eq (get-text-property (match-end 0) 'face)
-		       'font-lock-comment-face)
-		   (unless (eq (get-text-property (point) 'face)
-			       'font-lock-comment-face)
-		     ;; Let's assume it's a `cs' if we're on the same line.
-		     (>= (line-end-position) pt)))
-	       ;; Make sure that PT is not past the end of the comment.
-	       (if (comment-forward 1) (> (point) pt) (eobp))))
-	    cs
-	  (goto-char pt)
-	  nil)))))
+  (if comment-use-syntax
+      (let ((state (syntax-ppss)))
+        (when (nth 4 state)
+          (goto-char (nth 8 state))
+          (prog1 (point)
+            (when (looking-at comment-start-skip)
+              (goto-char (match-end 0))))))
+    ;; Can't rely on the syntax table, let's guess based on font-lock.
+    (unless (eq (get-text-property (point) 'face) 'font-lock-string-face)
+      (let ((pt (point))
+            (cs (comment-search-backward nil t)))
+        (when cs
+          (if (save-excursion
+                (goto-char cs)
+                (and
+                 ;; For modes where comment-start and comment-end are the same,
+                 ;; the search above may have found a `ce' rather than a `cs'.
+                 (or (if comment-end-skip (not (looking-at comment-end-skip)))
+                     ;; Maybe font-lock knows that it's a `cs'?
+                     (eq (get-text-property (match-end 0) 'face)
+                         'font-lock-comment-face)
+                     (unless (eq (get-text-property (point) 'face)
+                                 'font-lock-comment-face)
+                       ;; Let's assume it's a `cs' if we're on the same line.
+                       (>= (line-end-position) pt)))
+                 ;; Make sure that PT is not past the end of the comment.
+                 (if (comment-forward 1) (> (point) pt) (eobp))))
+              cs
+            (goto-char pt)
+            nil))))))
 
 (defun comment-forward (&optional n)
   "Skip forward over N comments.
-- 
2.39.2