bug#10650: view-mode inconsistencies wrt special mode-class

[Stefan Monnier <monnier@iro.umontreal.ca>]

>>> The Lisp manual (node Basic Major Modes) says:
>>> Special mode is a basic major mode for buffers containing text
>>> that is produced specially by Emacs, rather than from a file.
>> Then again, doc-view-mode is a special mode even though the PDF code is
>> rarely if ever generated by Emacs.
> I interpret special modes as being applicable to stuff where you won't
> ever want to create an empty buffer and just start writing said stuff.
> That covers PDFs, tar files, dired buffers, etc.

Yes, my understanding of it also revolves around this idea.  And I think
that's what:

   If the default-value of major-mode is nil, then creating a new buffer
   inherits the major-mode of the previously current buffer UNLESS that
   buffer has a special mode-class, in which case the new buffer gets
   fundamental mode. (Doesn't seem a very useful feature to me.)

is hinting at.  Still, this notion of "special mode" seems of little
practical value since it only seems to be usable in the above case of
setting up the default major mode of a newly created buffer.

The default global value of major-mode is not nil, so in 99% of the
cases whether a mode is special will have zero effect.  I really would
be happy to declare that a nil default value for `major-mode' is not
supported any more.  So we can simply get rid of this "special
mode-class" mess.

> Personally I think a separate variable controlling which modes are
> incompatible with view-mode would be better, but I'm not going to
> complain if the definition of special modes is extended to cover this.

I'm not sure exactly what's the idea with view-mode's use of special
mode class, but IIUC it's that we don't want to enable view-mode in
major modes that already have view-mode-like keybindings, which might
boil down to something along the lines of "major modes which use
suppress-keymap".


        Stefan