expand-to-foreign-dyn ignored when structures are passed by value

[3b]

If structures are passed to/from a structure by value, it falls back to TRANSLATE-INTO-FOREIGN-MEMORY, even for other arguments.

(cffi:define-foreign-type a1 ()
  ()
  (:actual-type :pointer)
  (:simple-parser a1))

(cffi:defcstruct (a2 :class a2))

(defmethod expand-to-foreign-dyn (value var body (type a1))
  `(with-special-allocator (,var)
     (setf (cffi:mem-ref ,var a1) ,value)
     ,@body))

(third (macroexpand-1 '(cffi:defcfun "foo1" :void (a a1))))
;; =>
(DEFUN FOO1 (A)
  (WITH-SPECIAL-ALLOCATOR (#:G913)
    (SETF (MEM-REF #:G913 A1) A)
    (CFFI-SYS:%FOREIGN-FUNCALL "foo1" (:POINTER #:G913 :VOID) :CONVENTION
                               :CDECL :LIBRARY :DEFAULT)))

(macroexpand-1 (fourth (third (macroexpand-1 '(cffi:defcfun "foo2" :void (a a1) (b (:struct a2)))))))
;; =>
(CFFI:WITH-FOREIGN-OBJECTS ((CFFI::ARGVALUES :POINTER 2))
  (CFFI:WITH-FOREIGN-OBJECT (#:G643 'A1)
    (CFFI:TRANSLATE-INTO-FOREIGN-MEMORY A #<A1 {10150DC203}> #:G643)
    (CFFI:WITH-FOREIGN-OBJECT (#:G644 '(:STRUCT A2))
      (CFFI:TRANSLATE-INTO-FOREIGN-MEMORY B #<A2 A2> #:G644)
      (PROGN
       (LOOP :FOR CFFI::ARG :IN (LIST #:G643 #:G644)
             :FOR
             COUNT :FROM 0
             :DO (SETF (CFFI:MEM-AREF CFFI::ARGVALUES :POINTER COUNT)
                         CFFI::ARG))
       (CFFI::CALL
        (CFFI::PREPARE-FUNCTION "foo2" ':VOID '(:POINTER (:STRUCT A2))
                                ':DEFAULT-ABI)
        (CFFI:FOREIGN-SYMBOL-POINTER "foo2") (CFFI-SYS:NULL-POINTER)
        CFFI::ARGVALUES)
       (VALUES)))))

expected A1 to use WITH-SPECIAL-ALLOCATOR instead of TRANSLATE-INTO-FOREIGN-MEMORY

tested on sbcl win/linux x8664, cffi v0.15