bug#39823: 26.3; update-directory-autoloads regression from Emacs 26 to Emacs 27

[Maxim Cournoyer <maxim.cournoyer@gmail.com>]

Hello again,

Eli Zaretskii <eliz@gnu.org> writes:

>> From: Noam Postavsky <npostavs@gmail.com>
>> Date: Thu, 12 Mar 2020 20:43:09 -0400
>> Cc: , 39823@debbugs.gnu.org
>> 
>> > Works on Emacs 26.3, but fails on Emacs 27.0.50, with the error message:
>> >
>> > Wrong type argument: stringp, nil
>> 
>> The difference is that --eval now evaluates with lexical-binding
>> enabled, so the let-binding of generated-autoload-file is lexical unless
>> autoload.el, with its (defvar generated-autoload-file nil ...), has been
>> loaded.
>> [...]
>> See also https://lists.gnu.org/r/emacs-devel/2020-03/msg00173.html for
>> (much) more discussion on this.
>
> The "wrap with eval" trick suggested by Stefan in the latter
> discussion does work here:
>
>  emacs --quick --batch --eval "(eval '(progn
>    (let ((backup-inhibited t)
>          (generated-autoload-file \"/tmp/toto\"))
>      (update-directory-autoloads \"/tmp\"))))"

Oh, that's an interesting trick.  I had not realized `eval' could be
switched to either lexical or dynamic (the default).

Thanks!

Maxim