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>]

> Here is a section of a test file sent to bug-cc-mode some while ago:
>
> /*
> ** Local Variables:
> ** mode:c
> ** indent-tabs-mode:nil
> ** c-basic-offset:4
> ** End:
> */
>
> Here it is evident that by "mode:c" the OP means C Mode, not whatever
> random mode happens to have taken the symbol `c-mode'.

It is evident that the OP means CC-mode's C Mode because the bug report
is sent to `bug-cc-mode`, indeed.

But that doesn't mean these file-local settings intend to impose the use
of CC-mode to those users who prefer `c-ts-mode`.

>> Actually, no: `major-mode-remap-*` can also remap `c-ts-mode` to some
>> other mode, such as `c-mode`.
> Maybe I should have said "regardless of any non-crazy settings".

If something is "crazy" here it would be using a `mode: c-ts` cookie.
Adding such entries to `major-mode-remap-*` would just be the sane way
to work around that "crazy" part.  🙂

>> 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.
> The problem is that entries in major-mode-remap-defaults (and the other
> one) do not accurately represent user preferences.

Entries in `major-mode-remap-defaults` indeed do not reflect
a user preference, just a "circumstantial" preference.

[ Ideally, they shouldn't be set just by loading a file (whether
  `cc-mode.el` or `c-ts-mode.el`).  But after a long protracted
  discussion the Emacs maintainers decided that it was the least bad
  choice in Emacs-29.  This may be revisited in the future, e.g. for
  Emacs-31, but AFAICT it's too late to make such a change in
  Emacs-30.  ]

But entries in `major-mode-remap-alist` very much reflect a user preference.

> As a value for mode: in C files, we need three distinct settings: for
> C Mode, for c-ts-mode, and for any mode which handles C.

If someone sends me a file which says `mode: c-ts`, I will complain
and so should anyone else.  Whether you edit it with `c-mode` or with
`c-ts-mode` is not the file's business.

>> It is supposed to opt-in, indeed.
> So, for a user trying out Emacs 30, with a desktop file containing at
> least one file with major mode c-ts-mode; visiting a file with the above
> Local Variables section will indeed start the buffer with C Mode?

If you see a file that stipulates `c-ts-mode`, complain to the
file's author, not to us.

> I haven't looked just yet, but can I assume there's a recipe in the
> Emacs 30 NEWS file telling users how to switch between old behaviour
> (-*- c -*- always means C mode) and new behaviour (-*- c -*- can mean
> c-ts-mode depending on other settings)?

If anything, Emacs-30 makes it easier than Emacs-29:

    (add-to-list 'major-mode-remap-alist '(c-mode))

In Emacs-29, you had to go and undo the mess in `auto-mode-alist`.


        Stefan