CLJS-2113: nth function produces different results from clojure when using a negative index on a sequence

Fix range’s implementation of IIndexed for negative values of n

Author: mfikes

src/main/cljs/cljs/core.cljs

@@ -9174,15 +9174,15 @@ reduces them without incurring seq initialization"
 
   IIndexed
   (-nth [rng n]
-    (if (< n (-count rng))
+    (if (and (<= 0 n) (< n (-count rng)))
       (+ start (* n step))
-      (if (and (> start end) (zero? step))
+      (if (and (<= 0 n) (> start end) (zero? step))
         start
         (throw (js/Error. "Index out of bounds")))))
   (-nth [rng n not-found]
-    (if (< n (-count rng))
+    (if (and (<= 0 n) (< n (-count rng)))
       (+ start (* n step))
-      (if (and (> start end) (zero? step))
+      (if (and (<= 0 n) (> start end) (zero? step))
         start
         not-found)))
 

src/test/cljs/cljs/core_test.cljs

@@ -1337,6 +1337,12 @@
              "unreachable")
           "cljs-2104"))))
 
+(deftest test-cljs-2113
+  (is (thrown? js/Error (nth (range 2) -2)))
+  (is (thrown? js/Error (nth (range 2 1 0) -2)))
+  (is (= ::not-found (nth (range 2) -2 ::not-found)))
+  (is (= ::not-found (nth (range 2 1 0) -2 ::not-found))))
+
 (comment
   ;; ObjMap
   ;; (let [ks (map (partial str "foo") (range 500))