(back_comment): Detect cases where a comment-starter is

actually inside another comment as in:   /* a // b */ c // d \n.
Make it clear that `comstart_pos' is unused for nested comments.

1 files changed, 27 insertions, 16 deletions

diff --git a/src/syntax.c b/src/syntax.c
index 9e5247064ef..189e50d0963 100644
--- a/src/syntax.c
+++ b/src/syntax.c
@@ -469,12 +469,16 @@ back_comment (from, from_byte, stop, comnested, comstyle, charpos_ptr, bytepos_p
      PARITY is current parity of quotes from the comment end.  */
   int string_style = -1;        /* Presumed outside of any string. */
   int string_lossage = 0;
+  /* Not a real lossage: indicates that we have passed a matching comment
+     starter plus an non-matching comment-ender, meaning that any matching
+     comment-starter we might see later could be a false positive (hidden
+     inside another comment).
+     Test case:  { a (* b } c (* d *) */
+  int comment_lossage = 0;
   int comment_end = from;
   int comment_end_byte = from_byte;
   int comstart_pos = 0;
   int comstart_byte;
-  /* Value that PARITY had, when we reached the position
-     in COMSTART_POS.  */
   int scanstart = from - 1;
   /* Place where the containing defun starts,
      or 0 if we didn't come across it yet.  */
@@ -548,23 +552,24 @@ back_comment (from, from_byte, stop, comnested, comstyle, charpos_ptr, bytepos_p
           
         case Scomment:
           /* We've already checked that it is the relevant comstyle.  */
-          if (string_style != -1 || string_lossage)
+          if (string_style != -1 || comment_lossage || string_lossage)
             /* There are odd string quotes involved, so let's be careful.
                Test case in Pascal: " { " a { " } */
             goto lossage;
 
-          if (comnested && --nesting <= 0)
+          if (!comnested)
+            {
+              /* Record best comment-starter so far.  */
+              comstart_pos = from;
+              comstart_byte = from_byte;
+            }
+          else if (--nesting <= 0)
             /* nested comments have to be balanced, so we don't need to
                keep looking for earlier ones.  We use here the same (slightly
                incorrect) reasoning as below:  since it is followed by uniform
                paired string quotes, this comment-start has to be outside of
                strings, else the comment-end itself would be inside a string. */
             goto done;
-
-          /* Record comment-starters according to that
-             quote-parity to the comment-end.  */
-          comstart_pos = from;
-          comstart_byte = from_byte;
           break;
 
         case Sendcomment:
@@ -578,6 +583,15 @@ back_comment (from, from_byte, stop, comnested, comstyle, charpos_ptr, bytepos_p
                    this comment-ender rather than ours.  */
                 from = stop;    /* Break out of the loop.  */
             }
+          else if (comstart_pos != 0 || c != '\n')
+            /* We're mixing comment styles here, so we'd better be careful.
+               The (comstart_pos != 0 || c != '\n') check is not quite correct
+               (we should just always set comment_lossage), but removing it
+               would imply that any multiline comment in C would go through
+               lossage, which seems overkill.
+               The failure should only happen in the rare cases such as
+                 { (* } *)   */
+            comment_lossage = 1;
           break;
 
         case Sopen:
@@ -594,7 +608,7 @@ back_comment (from, from_byte, stop, comnested, comstyle, charpos_ptr, bytepos_p
           break;
 
         default:
-          continue;
+          break;
         }
     }
 
@@ -604,12 +618,9 @@ back_comment (from, from_byte, stop, comnested, comstyle, charpos_ptr, bytepos_p
       from_byte = comment_end_byte;
       UPDATE_SYNTAX_TABLE_FORWARD (comment_end - 1);
     }
-  /* If the earliest comment starter
-     is followed by uniform paired string quotes or none,
-     we know it can't be inside a string
-     since if it were then the comment ender would be inside one.
-     So it does start a comment.  Skip back to it.  */
-  else if (!comnested)
+  /* If comstart_pos is set and we get here (ie. didn't jump to `lossage'
+     or `done'), then we've found the beginning of the non-nested comment.  */
+  else if (1)   /* !comnested */
     {
       from = comstart_pos;
       from_byte = comstart_byte;