Stefan Monnier <monnier@iro.umontreal.ca> schrieb am So., 12. Apr. 2015 um
05:48 Uhr:

> The number of arguments passed to a function is something that can be
> easily checked by the function, but checking the number of arguments
> accepted by a function is generally impossible (think of a function like
>
>    (lambda (&rest args)
>      (if (> (length args) (if (halting-p (car args)) 2 3))
>          (signal 'wrong-number-of-arguments 'foo (length args)))
>      ...)
>

But most functions are well-behaved and don't impose additional
restrictions over those expressed in their parameter declaration; in these
cases inspecting the argument list using help-function-arglist should work
well enough.
Agreed that using this as safe predicate is a security hole, the choices
should just be normal symbols that are not used as functions.