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

[Stefan Monnier via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

Package: Emacs
Version: 30.0.50


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?


        Stefan


diff --git a/lisp/emacs-lisp/find-func.el b/lisp/emacs-lisp/find-func.el
index 411602ef166..3458ace1c08 100644
--- a/lisp/emacs-lisp/find-func.el
+++ b/lisp/emacs-lisp/find-func.el
@@ -552,7 +552,7 @@ find-function-library
     (cons function
           (cond
            ((autoloadp def) (nth 1 def))
-           ((subr-primitive-p def)
+           ((or (subr-primitive-p def) (special-form-p def))
             (if lisp-only
                 (error "%s is a built-in function" function))
             (help-C-file-name def 'subr))
diff --git a/lisp/subr.el b/lisp/subr.el
index 38a3f6edb34..f403369f534 100644
--- a/lisp/subr.el
+++ b/lisp/subr.el
@@ -315,7 +315,8 @@ subr-primitive-p
   "Return t if OBJECT is a built-in primitive function."
   (declare (side-effect-free error-free))
   (and (subrp object)
-       (not (subr-native-elisp-p object))))
+       (not (or (subr-native-elisp-p object)
+                (special-form-p object)))))
 
 (defsubst xor (cond1 cond2)
   "Return the boolean exclusive-or of COND1 and COND2.