wrong default value for ensure-generic-function's :generic-function-class argument

[Nikodemus Siivola]

    (reported by Bruno Haible)
  ANSI CL is silent on this, but the MOP's specification of ENSURE-GENERIC-FUNCTION says:
   "The remaining arguments are the complete set of keyword arguments
    received by ENSURE-GENERIC-FUNCTION."
  and the spec of ENSURE-GENERIC-FUNCTION-USING-CLASS:
   ":GENERIC-FUNCTION-CLASS - a class metaobject or a class name. If it is not
    supplied, it defaults to the class named STANDARD-GENERIC-FUNCTION."
  This is not the case in SBCL. Test case:
   (defclass my-generic-function (standard-generic-function)
     ()
     (:metaclass sb-pcl:funcallable-standard-class))
   (setf (fdefinition 'foo1)
         (make-instance 'my-generic-function :name 'foo1))
   (ensure-generic-function 'foo1
     :generic-function-class (find-class 'standard-generic-function))
   (class-of #'foo1)
   ; => #<SB-MOP:FUNCALLABLE-STANDARD-CLASS STANDARD-GENERIC-FUNCTION>
   (setf (fdefinition 'foo2)
         (make-instance 'my-generic-function :name 'foo2))
   (ensure-generic-function 'foo2)
   (class-of #'foo2)
  Expected: #<SB-MOP:FUNCALLABLE-STANDARD-CLASS STANDARD-GENERIC-FUNCTION>
  Got: #<SB-MOP:FUNCALLABLE-STANDARD-CLASS MY-GENERIC-FUNCTION>

As of 1.0.23 this test case behaves a bit differently, getting instead
  Cannot CHANGE-CLASS objects into GENERIC-FUNCTION metaobjects.
     [Condition of type SB-PCL::METAOBJECT-INITIALIZATION-VIOLATION]
  See also:
    The Art of the Metaobject Protocol, GENERIC-FUNCTION [:initialization]
on both ENSURE-GENERIC-FUNCTION calls. Need to figure out if this is righ, and if it is, if it
is possible to still provoke the original problem.

provoke a sim