bug#40570: [PATCH] Alias cl-subseq to seq-subseq, define gv-setter in the latter

[Stefan Monnier]

> The definition was moved in
>
> 2019-10-27T13:25:00-04:00!monnier@iro.umontreal.ca
> 0e4dd67aae (* lisp/emacs-lisp/seq.el: Don't require cl-lib.)
>
> already, but not the gv-setter declaration, so 'setf' worked with
> 'cl-subseq', but not with 'seq-subseq'.

Indeed, when I made the move I just wanted to change the implementation
but not the featureset (AFAIK seq-subseq never supported `setf`).

So this bug report is fundamentally a feature request: make `seq-subseq`
into a (gv) generalized variable.

> --- a/lisp/emacs-lisp/seq.el
> +++ b/lisp/emacs-lisp/seq.el
> @@ -154,6 +154,11 @@ seq-subseq
>  START or END is negative, it counts from the end.  Signal an
>  error if START or END are outside of the sequence (i.e too large
>  if positive or too small if negative)."
> +  (declare (gv-setter
> +            (lambda (new)
> +              (macroexp-let2 nil new new
> +             `(progn (cl-replace ,sequence ,new :start1 ,start :end1 ,end)
> +                     ,new)))))

The main purpose of the move was to reverse the order of dependency so
that `cl-lib` would depend on `seq` rather than the reverse.
This implies that `seq` shouldn't use `cl-lib`.  The above `cl-replace`
is hence problematic.

Another issue is that `seq-subseq` is a generic function, so its
gv-setter should also use generic functions so that it can also be made
to work on other sequence types than the predefined ones.

IOW we should probably introduce a new `seq` generic function which does
something similar to `cl-replace`, then make `seq-subseq` use it in its
gv-setter, and ideally also make `cl-replace` use it ;-)


        Stefan