Re: Arbitrary function: find the number(s) of expected arguments

[Davis Herring <herring@lanl.gov>]

[I'm back from vacation; a few already-old threads caught my eye.]

> Any existing code that depends on an error being raised by
> `subr-arity' would break.  E.g.,
>
> (condition-case err
>      (subr-arity 'foo)
>    (error (do-something)))

This issue is that of undefined behavior: is (subr-arity 0) UB or is it 
defined to be an error?  The doc string says simply "SUBR must be a 
built-in function." which is language typical of UB in some contexts, 
but Emacs doesn't explicitly have that notion to my knowledge.

If it's UB, then your example is wrong already and its breakage is 
irrelevant.  If the error is a defined part of the interface, then yes 
the change is incompatible.  All kinds of interfaces are thoroughly 
defined only for a subset of cases, and it's as much as anything a 
cultural issue whether the (historical) implementation's behavior 
outside that set is sacrosanct or not.

Davis

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