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

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

okamsn@protonmail.com writes:

> My purpose with this patch and for bug#68863 regarding `seq-subseq`
> (which does not currently support `setf`, and I think should allow
> subplaces like `substring` claims to)

The `substring' gv-setter doesn't need a loop however, it creates a new
string using `concat'.  Your patch would probably "work" ok in this
case, but I'm not convinced that this would be an improvement, still for
the same reasons.

> was for destructuring as `setf`-able places, like in cl-loop's `for
> VAR in-ref LIST`. I have implemented that for my Emacs Lisp package
> (https://github.com/okamsn/loopy), but not all of the `setf`-able
> destructuring constructs support sub-places in the expected way, due
> to how some of the GV expansions are defined.

But if loopy would base on an inefficient implementations this would not
be useful.

Are there examples where your patch is really a clear improvement?


Michael.