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

[Clément Pit-Claudel]

On 2018-06-05 18:53, Noam Postavsky wrote:
> Both SBCL and CLisp throw an error for (let ((x 1) (y nil)) (incf x y)),
> although I can't see anything in the Common Lisp spec to decide either
> way.  E.g., cltl2 says:
> 
>     If delta is not supplied, then the number in place is changed by 1.
> 
> https://www.cs.cmu.edu/Groups/AI/html/cltl/clm/node125.html

Thanks for looking up the spec and testing other implementations.  I'm not sure 
what to make of the result with SBCL and CLisp, since (incf x nil) also fails 
in both of them (whereas it works for us, since we can't distinguish nil and 
unspecified).

>> In the rest of Emacs there are lots of other examples.  Many of them
>> (for example semantic-find-tags-by-name or calendar-increment-month)
>> seem to work when passed a nil-valued variable, but many others behave
>> like cl-incf (for example gnus-summary-article-score).
> 
> I don't think those are great examples of macros to emulate.

Agreed, I was just collecting other examples, both in support and against my 
point.

Clément.

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