bug#31715: cl-incf and cl-decf error out when passed a nil-valued variable as 'offset'

[Noam Postavsky <npostavs@gmail.com>]

Clément Pit-Claudel <clement.pitclaudel@live.com> writes:

> The following works:
>   (let ((x 1)) (cl-incf x nil))
>
> … but following raises "setq: Wrong type argument: number-or-marker-p, nil":
>   (let ((x 1) (y nil)) (cl-incf x y))
>
> … yet the docs say this, which suggests that both should work:
>   (cl-incf PLACE &optional X)
>   Increment PLACE by X (1 by default).

X is an optional macro parameter, so the "optionalness" applies at
compile time.

> The issue comes from the expansion of cl-incf:
>
>     (defmacro cl-incf (place &optional x) …
>       (if (symbolp place)
>           (list 'setq place (if x (list '+ place x) (list '1+ place)))
>         (list 'cl-callf '+ place (or x 1))))
>
> Shouldn't that `if x' check be quoted?  Same for the second branch of the if (shouldn't the `(or x 1)' part be quoted, too?)

I think that would approximately double the cost of cl-incf in the
simple case.  And since you would expect cl-incf to be used in loops a
lot, that seems like a bad idea.