bug#62020: Lisp reader: dotted pair notation not working when initial elements are omitted

[Eli Zaretskii]

> From: Federico Tedin <federicotedin@gmail.com>
> Date: Tue, 7 Mar 2023 02:14:02 +0100
> 
> According to the Elisp docs
> (https://www.gnu.org/software/emacs/manual/html_node/elisp/Dotted-Pair-Notation.html),
> one should be able to evaluate e.g.:
>     (. 1)
> to:
>     1
> 
> This works correctly in Emacs 28.1 However, in the emacs-29 branch
> (bd07cec) this results in:
>     *** Read error ***  Invalid read syntax: "."
> 
> Since the description in the docs are the same in the emacs-29 I
> assume this is a bug.
> I believe this may have been caused by the changes implementing the
> nonrecursive Lisp reader.
> I believe the fix would be roughly:
> - in read0 (lread.c), when handling c == '.', ensure we handle not
> only the case where the top of the read stack is RE_list but *also*
> RE_list_start.
> - if the top of the stack was effectively RE_list_start, then the top
> of the stack needs to be manipulated somehow so that at the end of
> read0 the correct value is returned, when emptying the stack.

Stefan and Mattias, can you please see how to fix this regression?