bug#12812: defsetf with third argument = t

[Glenn Morris <rgm@gnu.org>]

Package: emacs
Version: 24.2.50

In Emacs 24.2 and earlier, defsetf was documented [1] to accept a third
argument = t, with the same meaning as gv-define-simple-setter's
optional FIX-RETURN argument. This behaviour no longer works.

Here is an artificial example in `emacs -Q -l cl':

(defun foo (x)
  (car x))

(defun foo-set (x y)
  (setcar x y)
  nil)

(defsetf foo foo-set t)

(setq bar '(0 2 3))

(setf (foo bar) 1)   ; -> nil
bar                  ; -> (1 2 3)


The setf call correctly returns 1 in 24.2.


[1] (info "(cl)Customizing Setf")
   [...]
   As a special (non-Common-Lisp) extension, a third argument of `t'
   to `defsetf' says...