bug#69832: 30.0.50; Should `subr-primitive-p` apply to special-forms?

[Eli Zaretskii <eliz@gnu.org>]

> Cc: monnier@iro.umontreal.ca
> Date: Sat, 16 Mar 2024 15:30:01 -0400
> From:  Stefan Monnier via "Bug reports for GNU Emacs,
>  the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>
> 
> Currently (subr-primitive-p (symbol-function 'if)) returns t.
> Its docstring disagrees:
> 
>     Return t if OBJECT is a built-in primitive function.
> 
> because `if` is indeed a "built-in primitive" but not a "function" (you
> can't `funcall` it and it is rejected by `functionp`: it's a special
> form instead).
> 
> For ELisp's type hierarchy/DAG we need a type for "a built-in primitive
> which is also a function".  Originally, based on the docstring,
> I thought I could use `subr-primitive`.
> But it turns out that the code doesn't quite match the docstring.
> 
> I can see two ways to fix that:
> 
> - Introduce a new type, says `subr-function(-p)` which returns non-nil
>   if and only if the argument is a built-in primitive *and* a function.
> 
> - Change the implementation of `subr-primitive-p` to match its docstring.
> 
> The patch below does the second (including changing the only place
> I found that wants the current semantics.
> 
> Comments?  Objections?

Why take the path of a breaking change instead of the non-breaking
alternative?