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

[Eli Zaretskii]

> Cc: Alan Mackenzie <acm@muc.de>
> Date: Wed, 13 Nov 2024 16:00:58 +0200
> From: Eli Zaretskii <eliz@gnu.org>
> 
> To reproduce:
> 
>   emacs -Q
>   M-x load-library RET c-ts-mode RET
>   C-x C-f src/buffer.c
>   M-: major-mode RET
>    => c-ts-mode
> 
> So far, so good: the user loads c-ts-mode, which means she prefers
> C/C++ TS Mode for C and C++ files, so visiting a C file turns on
> c-ts-mode instead of the default CC Mode.
> 
> But:
> 
>   emacs -Q
>   C-x C-f src/dispnew.c RET
>   M-x load-library RET c-ts-mode RET
>   C-x C-f src/buffer.c
>   M-: major-mode RET
>    => c-mode
> 
> This is unexpected.  It means that if even a single file loads CC
> Mode, the user's preference of using C TS Mode is effectively ignored.
> 
> This seems to happen due to this code in cc-mode.el:
> 
>   ;; Make entries in `major-mode-remap-defaults' to ensure that when CC
>   ;; Mode has been loaded, the symbols `c-mode' etc., will call CC Mode's
>   ;; modes rather than c-ts-mode etc..
>   (when (boundp 'major-mode-remap-defaults)
>     (add-to-list 'major-mode-remap-defaults '(c++-mode . c++-ts-mode))
>     (add-to-list 'major-mode-remap-defaults '(c-mode . c-ts-mode))
>     (add-to-list 'major-mode-remap-defaults '(c-or-c++-mode . 
> c-or-c++-ts-mode))
>     (let (entry)
>       (dolist (mode '(c-mode c++-mode c-or-c++-mode))
>       (if (and (setq entry (assq mode major-mode-remap-defaults))
>                (null (cdr entry)))
>           (setq major-mode-remap-defaults
>                 (delq entry major-mode-remap-defaults)))
>       (push (cons mode nil) major-mode-remap-defaults))))
> 
> Here's what major-mode-remap-defaults looks like when I load c-ts-mode
> into "emacs -Q":
> 
>   ((c-or-c++-mode . c-or-c++-ts-mode) (c-mode . c-ts-mode) (c++-mode . 
> c++-ts-mode) (LaTeX-mode . latex-mode) (plain-TeX-mode . plain-tex-mode) 
> (TeX-mode . tex-mode))
> 
> And here's how it looks if I first visit a C file (which loads
> cc-mode):
> 
>   ((c-or-c++-mode) (c++-mode) (c-mode) (c-or-c++-mode . c-or-c++-ts-mode) 
> (c-mode . c-ts-mode) (c++-mode . c++-ts-mode) (LaTeX-mode . latex-mode) 
> (plain-TeX-mode . plain-tex-mode) (TeX-mode . tex-mode))
> 
> The above snippet from cc-mode.el was installed this last May, with
> the following log message:
> 
>     In normal-mode, make c-mode call c-mode when CC Mode is loaded
> 
>     As regards which mode normal-mode calls for the symbols c-mode,
>     etc., the first of the following which applies holds:
>     (i) If the user has made a pertinent entry in
>     major-mode-remap-alist, this is used.
>     (ii) If CC Mode has been loaded, c-mode is called.
>     (iii) If library c-ts-mode has been loaded, c-ts-mode is
>     called.
>     (iv) Otherwise c-mode is called.
> 
>     * lisp/progmodes/cc-mode.el (top level): Add entries to
>     major-mode-remap-defaults to implement the above.
> 
> I don't quite understand the rationale (and even less the
> implementation), and don't recall any discussions of this; there's
> also no bug number for it.  But clearly the result is not acceptable,
> and I very much hope that there's some simple bug here that can be
> fixed real soon.  If the above is not a bug, but the intended (by you,
> Alan) behavior, then we need to talk about changing it, because this
> is not how user preferences in this regard are supposed to be heeded
> by Emacs.
> 
> The expected behavior is: as soon as the user loads c-ts-mode, all the
> subsequent C/C++ files are visited using C/C++ TS Mode.  To revert
> back to CC Mode, the user must load cc-mode again.

And adding Stefan to the discussion, since he wrote the
major-mode-remap-defaults stuff in c-ts-mode.el.