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From: Eli Zaretskii <eliz@gnu.org>
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Subject: Re: Arbitrary function: find the number(s) of expected arguments
Date: Sat, 19 Mar 2016 15:10:41 +0200
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To: Paul Pogonyshev <pogonyshev@gmail.com>
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> Date: Sat, 19 Mar 2016 13:26:47 +0100
> From: Paul Pogonyshev <pogonyshev@gmail.com>
> Cc: emacs-devel@gnu.org
> 
> What if I use `subr-arity' for whatever reason in a case where it works, but in the next version Emacs moves
> the function from `.c' to `.el' and it ceases to be a built-in?

You need to guard the calls to subr-arity with subrp, and then your
code will be future-proof.

Patches to add func-arity to Emacs are welcome.