bug#36418: 26.2.90; mapconcat "SEPARATOR must be a string." is incorrect

[Stefan Kangas <stefan@marxist.se>]

Xu Chunyang <mail@xuchunyang.me> writes:

> C-h f mapconcat says
>
>> SEPARATOR must be a string.
>
> but nil works fine
>
>     (mapconcat #'identity '("a" "b") nil)
>     ;; => "ab"

This seems to have been added in commit aecbbd57, which fixes
Bug#35710:

   https://debbugs.gnu.org/cgi/bugreport.cgi?bug=35710

Perhaps Eli would want to weigh in on this?

> and `string-join` relies on this fact (by the way, string-join's
> docstring doesn't explain the case when separator is omitted or nil)
>
>     (defsubst string-join (strings &optional separator)
>       "Join all STRINGS using SEPARATOR."
>       (mapconcat 'identity strings separator))

This could be expanded, in my opinion.

> C-h S mapconcat says
>
>> SEPARATOR, which also must be a string, or a vector or list of
>> characters.
>
> C-h f concat says
>
>> Each argument may be a string or a list or vector of characters (integers).

In other words, since SEPARATOR can be a "list of characters", it
follows that it can also be nil (which is equivalent to the empty list).

Thanks,
Stefan Kangas