bug#71934: comp--spill-lap-function and closure (wad: bug#71934: 31.0.50; edebug--called-interactively-skip vs. new fun objects)

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

> Not sure what you mean by "no such thing as a form ... like a closure".

A form that starts with `closure` is not a valid form because there is
no definition for `closure`: (fboundp 'closure) => nil.

> I bumped into one last summer.
>
> In particular (in my development repo fixing bug #64646) I put this into
> *scratch*:
>
>     (defconst foo (lambda (baz) (car baz)))
>
> , evaluated it with C-x C-e and then M-: (native-compile foo).  This
> threw the error "Cannot native-compile, form is not a lambda".

That error seems right according to the docstring:

   (defun native-compile (function-or-file &optional output)
     "Compile FUNCTION-OR-FILE into native code.
   This is the synchronous entry-point for the Emacs Lisp native
   compiler.  FUNCTION-OR-FILE is a function symbol, a form, or the
   filename of an Emacs Lisp source file.  If OUTPUT is non-nil, use
   it as the filename for the compiled object.  If FUNCTION-OR-FILE
   is a filename, if the compilation was successful return the
   filename of the compiled object.  If FUNCTION-OR-FILE is a
   function symbol or a form, if the compilation was successful
   return the compiled function."

(closure ...) is not a function symbol nor a valid form.  Instead it's
a function value and the docstring doesn't say such are
a valid arguments to `native-compile`.

Admittedly, maybe we should extend `native-compile` to accept function
values, just like `byte-compile`.

> The value of foo had been turned into a form (closure ....).

No, "form" is a specific term which denotes a piece of source code (I
usually used the term "expression" for that, but IIUC in the Lisp world
the more common term for that is "form").

(closure ....) is not a *form* code any more than (1 foo) or (+ . 4)
[ except to the extent that we *could* make it into a valid form by
  providing a definition for `closure` (as a macro, function, or
  special form).  ]

> So I fixed
> comp--spill-lap-function (form version) so as to compile that form.

Why `comp--spill-lap-function` specifically (instead of
`native-compile`, for example)?

> I've no idea how Emacs would handle that defconst now.

Hmm... AFAICT your example doesn't relate to `defconst`.
You'd get the same result with

    M-: (native-compile (lambda (baz) (car baz))) RET


- Stefan