(defmulti obj-type ::obj-type)
(defmethod obj-type :a [_]
  ::base-obj)
(defmethod obj-type :b [_]
  ::base-obj)

(s/def ::obj (s/multi-spec obj-type ::obj-type))

(deftest multi-spec-override
         (is (every? (fn [m] (= :a (::obj-type m))) (gen/generate (s/gen (s/coll-of ::obj :min-count 1)
                                                                         {::obj-type #(gen/return :a)})))))