bug#36227: 26.1; sgml-mode indents as comment inside of -- text blocks

[Akkana Peck]

In emacs -Q, edit a file in html-mode (derived from sgml-mode),
e.g. visit /tmp/foo.html.

M-x auto-fill-mode <RET>

Type some nonsense words with ' -- ' in the middle, like this:

asd shdf ladshjkl sjdk -- asdfh jklsdfh

and keep typing until it wraps.

sgml-mode will wrap adding unwanted extra dashes and indentations,
like this:

asd shdf ladshjkl sjdk -- asdfh jklsdfh jasdklf hjsdkl hjsdaklf sdf --
                       -- jkldsfhj kalshfjkasdlfh jaksd

This happens because sgml-mode is confused about what constitutes a
comment in sgml/html.

At some point, someone clearly agreed that this was a bug and tried to
fix it. If you look at the sgml.el source and search for --,
you'll see comments like:

> (defvar sgml-specials '(?\")
>   "List of characters that have a special meaning for SGML mode.
> This list is used when first loading the `sgml-mode' library.
> The supported characters and potential disadvantages are:
>
>   ?\\\"       Makes \" in text start a string.
>   ?\\='       Makes \\=' in text start a string.
>   ?-  Makes -- in text start a comment.
>
> When only one of ?\\\" or ?\\=' are included, \"\\='\" or \\='\"\\=', as can 
> be found in
> DTDs, start a string.  To partially avoid this problem this also makes these
> self insert as named entities depending on `sgml-quick-keys'.
>
> Including ?- has the problem of affecting dashes that have nothing to do
> with comments, so we normally turn it off.")

The dash is not included in sgml-specials by default, but the problem
affecting dashes that have nothing to do with comments happens anyway.
Setting sgml-specials to nil in the mode hook doesn't help either.

The problematic behavior seems to come from comment-line-break-function.
One workaround I've found is to add this in my html-mode and sgml-mode
hooks to prevent the special processing of line breaks inside comments:

    (kill-local-variable 'comment-line-break-function)

Maybe comment-line-break-function should check sgml-specials and return
without doing anything if - isn't one of the specials?



In GNU Emacs 26.1 (build 2, x86_64-pc-linux-gnu, GTK+ Version 3.24.4)
 of 2019-02-03, modified by Debian built on zam904
Windowing system distributor 'The X.Org Foundation', version 11.0.12004000
System Description:     Debian GNU/Linux 10 (buster)

Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.

Configured using:
 'configure --build x86_64-linux-gnu --prefix=/usr
 --sharedstatedir=/var/lib --libexecdir=/usr/lib
 --localstatedir=/var/lib --infodir=/usr/share/info
 --mandir=/usr/share/man --enable-libsystemd --with-pop=yes
 
--enable-locallisppath=/etc/emacs:/usr/local/share/emacs/26.1/site-lisp:/usr/local/share/emacs/site-lisp:/usr/share/emacs/26.1/site-lisp:/usr/share/emacs/site-lisp
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--enable-locallisppath=/etc/emacs:/usr/local/share/emacs/26.1/site-lisp:/usr/local/share/emacs/site-lisp:/usr/share/emacs/26.1/site-lisp:/usr/share/emacs/site-lisp
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Configured features:
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ACL LIBSELINUX GNUTLS LIBXML2 FREETYPE M17N_FLT LIBOTF XFT ZLIB
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Important settings:
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  value of $LANG: en_US.UTF-8
  locale-coding-system: utf-8-unix

Major mode: Lisp Interaction

Minor modes in effect:
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Load-path shadows:
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Memory information:
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