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

[Clément Pit-Claudel]

On 2018-06-04 18:58, Noam Postavsky wrote:
> X is an optional macro parameter, so the "optionalness" applies at
> compile time.

I think I see what you mean, but I'm not entirely convinced (in part because 
the docstring doesn't say so, and in part because it doesn't seem worth it to 
break referential transparency: if we accept nil, we should also accept a 
variable that evaluates to nil).

> 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.

I think we could still optimize the case in which we get an explicit nil.

Clément.

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