CLJS-2799: Handle nth on seqables with negative indexes

This adds a negative-index check to nth, prior to calling
linear-traversal-nth. (The check could concievably be done inside
linear-traversal-nth, but that would be less efficient as that function
recurs on itself and the check would then be made on each loop.)

Author: mfikes

src/main/cljs/cljs/core.cljs

@@ -1866,7 +1866,9 @@ reduces them without incurring seq initialization"
 
       (or (implements? ISeq coll)
           (implements? ISequential coll))
-      (linear-traversal-nth coll n)
+      (if (neg? n)
+        (throw (js/Error. "Index out of bounds"))
+        (linear-traversal-nth coll n))
 
       (native-satisfies? IIndexed coll)
       (-nth coll n)
@@ -1897,7 +1899,9 @@ reduces them without incurring seq initialization"
 
       (or (implements? ISeq coll)
           (implements? ISequential coll))
-      (linear-traversal-nth coll n not-found)
+      (if (neg? n)
+        not-found
+        (linear-traversal-nth coll n not-found))
 
       (native-satisfies? IIndexed coll)
       (-nth coll n not-found)

src/test/cljs/cljs/core_test.cljs

@@ -1566,6 +1566,10 @@
   (is (thrown-with-msg? js/Error #".*Invalid arity: 0" ({})))
   (is (thrown-with-msg? js/Error #".*Invalid arity: 3" ({} 1 2 3))))
 
+(deftest test-cljs-2799
+  (is (thrown? js/Error (nth (repeat :x) -1)))
+  (is (= ::not-found (nth (repeat :x) -1 ::not-found))))
+
 (comment
   ;; ObjMap
   ;; (let [ks (map (partial str "foo") (range 500))