Arithmetic warning thrown for impossible condition

Question

The following code triggers an arithmetic warning even though the condition it's warning about is impossible to reach. I tested this code in Clojure and it did not generate a warning. I would guess that the CLJS compiler doesn't take note of the (js/Error) in the same way that the Clojure compiler treats (Error.)

The exact warning triggered is below, followed by the code.


WARNING: cljs.core/+, all arguments must be numbers, got [number clj-nil] instead. at line 22 src\spurious_arithmetic\core.cljs



(def x [0 1 2 3 4 nil])
(def index (atom -1))
(defn take-value []
  (->> (swap! index inc)
       (nth x)))

(-> (loop [result (take-value)
           prev nil]
      (if (= nil result prev) (throw (js/Error. "This condition prevents nil arithmetic.")))
      (if (some? result)
        (recur (take-value) result)
        (+ 1 prev)))                                        ; this triggers the [number cljs-nil] warning
    (print)) ; 5

Answer 1

Comment made by: mfikes

The type inference logic in the compiler expects that the type of a loop local is static. In fact, somewhat the opposite of this ticket is being done in CLJS-1561.

Answer 2

Reference: https://clojure.atlassian.net/browse/CLJS-1450 (reported by alex+import)