Re: Error in C++ mode with Emacs 27.0.90

[Alan Mackenzie]

Hello, Dmitry.

On Sun, Mar 29, 2020 at 01:18:13 +0200, Dmitry Gutov wrote:
> Hi Alan,

> On 28.03.2020 22:10, Alan Mackenzie wrote:
> > I think the following patch fixes the trouble.  Please try it out and
> > report on this list how well it works

> I wonder if we could rewrite the related code such as not to depend on 
> whether a list has one element or several. That sounds ugly. Though the 
> change would likely take a fair amount of rework.

It seems unlikely that we can get a nested list (in imenu--index-alist)
with only one element.  I have tried creating a C++ buffer with 25
functions in it.  This gives a simple list of 25 elements.  With 26
functions, I get a nested alist with two elements.

However, in an emacs-lisp-mode buffer, we get things like "Variables"
and "*Rescan*", but I've not yet succeeded in causing a nested list with
just one element.  I'm not convinced enough that it couldn't happen,
though.

The latter part of the function looks like this (before my patch):

        (let* ((menu (imenu--split-menu index-alist
                                        (buffer-name)))
               (menu1 (imenu--create-keymap (car menu)
========>                                   (cdr (if (< 1 (length (cdr menu)))
                                                     menu
                                                   (car (cdr menu))))
                                            'imenu--menubar-select)))
          (setcdr imenu--menubar-keymap (cdr menu1)))))))

That `if' form has been there since imenu-update-menubar was first
written by Karl Heuer in 1997 (commit 0a8e8bc63e3).  Presumably, it
really was needed back then.

> In any case, imenu--create-keymap is also called from imenu--mouse-menu, 
> which probably needs the same kind of fix.

Possibly.  I've not managed to create the same error in
imenu--mouse-menu, but perhaps it could do with the same correction,
just in case.  What do you think?

Incidentally, that patch from last night was a bit untidy.  I think it
needs cleaning up a bit.

-- 
Alan Mackenzie (Nuremberg, Germany).