bug#74339: 30.0.92; CC Mode stomps C TS Mode

[Stefan Monnier via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

> One can write, in the Local Variables: section
>
>     mode: c-ts

FWIW, unless this file is personal, this is very bad practice, and not
only because `c-ts-mode` may not be available (in older Emacsen or if
the user didn't build Emacs with tree-sitter support or doesn't have the
appropriate grammar installed): the choice between `c-mode` and
`c-ts-mode` is a personal preference, so it's not the file's business to
tell the user which mode to use, just like it's not the file's business
to tell the user which editor to use.

There might be special situations where such a setting might be
warranted, but they're sufficiently hypothetical or rare that they
should not drive our decisions, as long as the users can still get the
behavior they want in such cases (which they can, regardless of what we
do with `major-mode-remap-defaults`).

> ..  This will cause the buffer to start in c-ts-mode regardless of any
> other current settings.

Actually, no: `major-mode-remap-*` can also remap `c-ts-mode` to some
other mode, such as `c-mode`.

> ..  When that line is followed by setting other CC Mode variables (as is
> surely common for any such use of the major mode setting) that will
> signal some sort of error on opening the buffer, should c-mode have been
> "remapped" to c-ts-mode.  Or if it doesn't do that, those local variables
> will be disregarded.  This is a Bad Thing.

If you really insist, we can add

    (add-to-list 'major-mode-remap-defaults '(c-ts-mode . c-mode))
    (add-to-list 'major-mode-remap-defaults '(c++-ts-mode . c++-mode))

to your `cc-mode.el` together with corresponding removals in
`c-ts-mode.el`, even though in my book file-local settings of
`mode: c-ts-mode` in non-personal files are just bugs.

> ... there will be no way for a user to specify c-mode unambiguously ...

Where/when?  AFAICT the only case where this might be true are the cases
where it does not do any damage.

> OK, thanks.  He is proposing that the meaning of -*- c -*-, `c-mode'
> as used in normal-mode, etc., should, from the user's point of view,
> be changed in an opt-out fashion.  He is proposing that there be no
> way to specify C Mode in a local variables section.  Such changes
> should be opt-in, not opt-out.  They would certainly need an entry in
> NEWS.  if there's not already one there.

It is supposed to opt-in, indeed.


        Stefan