From: Alan Mackenzie <acm@muc.de>
To: Michael Welsh Duggan <mwd@md5i.com>
Cc: 43558@debbugs.gnu.org, "Mattias Engdegård" <mattiase@acm.org>,
"Stefan Monnier" <monnier@iro.umontreal.ca>
Subject: bug#43558: [PATCH]: Fix (forward-comment 1) when end delimiter is escaped.
Date: Thu, 24 Sep 2020 19:57:42 +0000 [thread overview]
Message-ID: <20200924195742.GC4714@ACM> (raw)
In-Reply-To: <87v9g3yplr.fsf@md5i.com>
Hello, Michael.
On Thu, Sep 24, 2020 at 14:52:16 -0400, Michael Welsh Duggan wrote:
> Alan Mackenzie <acm@muc.de> writes:
> > On Wed, Sep 23, 2020 at 11:01:59 +0200, Mattias Engdegård wrote:
> >> Sorry if I misunderstood, but since when do backslashes escape */ in C?
> > Since forever, but only in the CC Mode test suite. :-(
> > I just tried it out with gcc, and it seems that \*/ does indeed end a
> > block comment. But an escaped newline doesn't end a line comment,
> > instead continuing it to the next line. So I got confused. Thanks for
> > pointing out the mistake.
> > It seems that as well as the existing variable
> > comment-end-can-be-escaped, we need a new one, say
> > line-comment-end-can-be-escaped, too. In C and C++ modes, these would
> > be nil and t respectively.
> But where does it say that backslashes escape */ in C++?
Nowhere. :-(
There has been a test in the CC Mode test suite for many years which
assumed this (but was disabled for existing (X)Emacs versions, waiting
for a new Emacs version to be "fixed").
> The C++ 14 standard (and it hasn't changed through C++ 20) says:
> 2.7 Comments [lex.comment]
> The characters /* start a comment, which terminates with the
> characters */. These comments do not nest. The characters // start
> a comment, which terminates immediately before the next new-line
> character.
For all the difference it makes, Emacs assumes the comment ends _after_
the NL.
> If there is a form-feed or a vertical-tab character in such a
> comment, only white-space characters shall appear between it and
> the new-line that terminates the comment; no diagnostic is
> required.
I didn't know that. Emacs/CC Mode doesn't code up this subtlety. It
probably isn't worth bothering about.
> [ Note: The comment characters //, /*, and */ have no special
> meaning within a // comment and are treated just like other
> characters. Similarly, the comment characters // and /* have no
> special meaning within a /* comment. — end note ]
Additionally, an escaped newline continues a comment onto the next line.
This happens, notionally, at a very early stage of compilation where a
backslash followed by NL anywhere get replaced by a space. I think that
even two backslashes followed by NL would get replaced by backslash,
space.
> --
> Michael Welsh Duggan
> (md5i@md5i.com)
--
Alan Mackenzie (Nuremberg, Germany).
next prev parent reply other threads:[~2020-09-24 19:57 UTC|newest]
Thread overview: 36+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-09-22 9:35 bug#43558: [PATCH]: Fix (forward-comment 1) when end delimiter is escaped Alan Mackenzie
2020-09-22 14:09 ` Stefan Monnier
2020-09-22 19:41 ` Alan Mackenzie
[not found] ` <handler.43558.B.160076736116422.ack@debbugs.gnu.org>
2020-09-23 8:57 ` Alan Mackenzie
2020-09-23 9:01 ` Mattias Engdegård
2020-09-23 14:48 ` Alan Mackenzie
2020-09-23 18:44 ` Stefan Monnier
2020-09-23 19:44 ` Alan Mackenzie
2020-09-23 20:02 ` Stefan Monnier
2020-09-24 10:20 ` Alan Mackenzie
2020-09-24 16:56 ` Stefan Monnier
2020-09-24 18:50 ` Alan Mackenzie
2020-09-24 22:43 ` Stefan Monnier
2020-11-19 21:18 ` Alan Mackenzie
2020-11-19 22:47 ` Stefan Monnier
2020-11-22 13:12 ` Alan Mackenzie
2020-11-22 15:20 ` Stefan Monnier
2020-11-22 17:08 ` Alan Mackenzie
2020-11-22 17:46 ` Dmitry Gutov
2020-11-22 18:19 ` Alan Mackenzie
2020-11-22 20:39 ` Dmitry Gutov
2020-11-22 21:13 ` Alan Mackenzie
2020-11-22 21:34 ` Dmitry Gutov
2020-11-22 22:01 ` Alan Mackenzie
2020-11-22 23:00 ` Stefan Monnier
2021-05-13 10:38 ` Lars Ingebrigtsen
2021-05-13 14:51 ` Alan Mackenzie
2021-05-16 13:53 ` Lars Ingebrigtsen
2022-04-28 11:17 ` Lars Ingebrigtsen
2022-04-28 18:52 ` Alan Mackenzie
2020-11-22 23:10 ` Stefan Monnier
2020-11-22 15:35 ` Eli Zaretskii
2020-11-22 17:03 ` Alan Mackenzie
2020-09-24 18:52 ` Michael Welsh Duggan
2020-09-24 19:57 ` Alan Mackenzie [this message]
2020-09-24 20:27 ` Michael Welsh Duggan
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