bug#20718: 25.0.50; find-function fails on advised subrs (again)

[Dmitry Gutov <dgutov@yandex.ru>]

On 06/02/2015 03:49 PM, Michael Heerdegen wrote:

> It doesn't work because `find-function-library' performs the subrp test
> just on the symbol function of the given function name, which is the
> advice, not the original subr.

At a glance, the problem is in find-function-advised-original: it only 
supports the old-style advice, and returns nil for 'handle-switch-frame 
augmented with advice-add.

Something like the patch below should fix it, but then the function 
returns a #<subr> object, instead of function symbol, and several 
callers seem to depend on the return value being the latter (because 
they want to call symbol-function on it).

diff --git a/lisp/emacs-lisp/find-func.el b/lisp/emacs-lisp/find-func.el
index 7ea13d4..75fdfc7 100644
--- a/lisp/emacs-lisp/find-func.el
+++ b/lisp/emacs-lisp/find-func.el
@@ -192,9 +192,9 @@ defined in C.")
    "Return the original function symbol of an advised function FUNC.
  If FUNC is not the symbol of an advised function, just returns FUNC."
    (or (and (symbolp func)
-	   (featurep 'advice)
-	   (let ((ofunc (cdr (assq 'origname (ad-get-advice-info func)))))
-	     (and (fboundp ofunc) ofunc)))
+	   (featurep 'nadvice)
+	   (let ((ofunc (advice--symbol-function func)))
+	     (and (advice--p ofunc) (advice--cd*r ofunc))))
        func))

  (defun find-function-C-source (fun-or-var file type)