bug#25929: 25.2; map-delete doesn't delete permanently 1st alist elt

[Lars Ingebrigtsen <larsi@gnus.org>]

Michael Heerdegen <michael_heerdegen@web.de> writes:

> That's a relatively small gain, and some people might prefer the
> explicit form (2) for readability.

If we don't do this, (map-delete foo 'bar) has surprising effects in
this one corner case.

That is, if foo is a hash table, then it's fine to say
(map-delete foo 'bar).  If you change foo to be an alist, you then have
to transform all the calls to be (setq foo (map-delete foo 'bar))
instead, which kind of breaks the abstraction that map-delete tried to
offer, I think?

Making map-delete signal an error in the
alist-where-bar-is-the-first-element sounds even worse to me, since the
user may already be writing this stuff as (setq foo (map-delete foo
'bar)).

(I think this was what Stefan suggested...)

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