bug#70524: [PATCH] Fix `map-elt` with `setf` for subplaces

[Stefan Monnier via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

>> Currently, the use
>>
>>      (let ((arr (vector 0 1 2 3 4 5 6)))
>>        (setf (map-elt (cl-subseq arr 3) 0)
>>              27)
>>        arr)
>>
>> expands to [...]
>
> But... I must admit I'm not really convinced that this has to be
> changed.

I'm also unconvinced.  AFAICT the same problem occurs with

    (setf (aref (cl-subseq arr 3) 0) 27)

and I can't think of a good reason why `map-elt` should behave differently.
Furthermore, the change would also fundamentally change the way
`map-elt` can be used as a gv-place, in the sense that

    (setf (map-elt (funcall foo bar) 0) 27)

would signal an error during macroexpansion because (funcall foo bar)
is not a valid gv-place.

> But second - doesn't your patch lead to very inefficient code in this
> example, where nearly all elements of the original sequence get replaced
> by themselves in a loop (through the setter of `cl-subseq')?

Yes, that's another issue.  I think if we want to support such `setf` in
a way that is good enough to that we can recommend its use, we'd need to
turn it into something that behaves more or less like:

    (map-set! arr (+ 3 0) 27)

But I must admit that I don't know how to get there.


        Stefan