(defun flatten (array)
  (make-array (array-total-size array) :displaced-to array))

(defun test ()
  (let ((a (make-array '(2 2) :initial-contents '((1 2) (3 4)))))
    (flatten a)))

;; CL-USER> (load "/home/death/tmp/foo.lisp")
;; T
;; CL-USER> (test)
;; #(1 2 3 4)
;; CL-USER> (load (compile-file "/home/death/tmp/foo.lisp"))
;; ; compiling file "/home/death/tmp/foo.lisp" (written 05 MAR 2024 01:57:09 PM):
;;
;; ; wrote /home/death/tmp/foo.fasl
;; ; compilation finished in 0:00:00.010
;; T
;; CL-USER> (test)
;; ; Evaluation aborted on #<TYPE-ERROR expected-type: (VECTOR T) datum: #<(VECTOR T 4) {10069BDF4F}>>.