Comment 2 for bug 839937

One thing I have found is *setf-fdefinition-hook*. I have not yet found how one could copy the inferred ftype, but as a first fix, it might be sufficient to push a (lambda (function-name new-value) (declare (ignore new-value)) (clear-info :function :type function-name)) to *setf-fdefinition-hook*.

(Sorry for the ramblings; this is just my first peek into SBCL internals.)