bug#49848: 27.2.50; map-merge plist return alist

[Lars Ingebrigtsen <larsi@gnus.org>]

"Basil L. Contovounesios" <contovob@tcd.ie> writes:

> In the specific case of merging maps into a desired type, we can simply
> be more careful in such ambiguous cases.  The attached patch does that,
> while also avoiding the quadratic lookup behaviour for lists.

I think that's the correct (and indeed only) solution here when dealing
with alist/plist ambiguities (if we don't want to say explicitly for
every map* call what the type actually is meant to be).

> (progn
>   (require 'map)
>   (map-merge-with 'list (lambda (a b) (message ">>> %s %s" a b) b)
>                   `((,(string ?a) . 1))
>                   `((,(string ?a) . 2))))
>
> In Emacs 26, it returns (("a" . 2) ("a" . 1)) without printing.
> In Emacs 27 and 28, it returns (("a" . 2)) without printing.
>
> Do we consider this a regression in Emacs 27 and try to fix it in 28
> (keeping in mind that map.el is also a GNU ELPA package)?

I think the current result value is the one that makes sense...

> Or do we drop the eql guarantee in the docstring, and call the function
> argument whenever two keys are merged, as in the attached patch?

I think that makes sense.

-- 
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