CLJS-1284: IndexedSeq -seq implementation incorrect for i >= alength of internal array

for performance reasons we sometimes inline calls to the IndexedSeq constructor,
however this bypasses the validation offered by prim-seq and array-seq.

Fix IndexedSeq -seq so that it checks current index to length of array.

Fix IndexedSeq -count so that negative values are never returned.

Add tests.

Author: dnolen

src/main/cljs/cljs/core.cljs

@@ -1256,7 +1256,9 @@ reduces them without incurring seq initialization"
   (-clone [_] (IndexedSeq. arr i))
 
   ISeqable
-  (-seq [this] this)
+  (-seq [this]
+    (when (< i (alength arr))
+      this))
 
   ASeq
   ISeq
@@ -1271,7 +1273,8 @@ reduces them without incurring seq initialization"
                nil))
 
   ICounted
-  (-count [_] (- (alength arr) i))
+  (-count [_]
+    (max 0 (- (alength arr) i)))
 
   IIndexed
   (-nth [coll n]

src/test/cljs/cljs/core_test.cljs

@@ -2833,6 +2833,22 @@
   (is (= (get #js [\h \i] 1.7) \i))
   (is (= (get "hi" 1.7) \i)))
 
+(defn foo-1284
+  ([] nil)
+  ([x0 & xs] [x0 xs]))
+
+(deftest test-cljs-1284
+  (let [xs (IndexedSeq. #js [] 0)
+        ys (IndexedSeq. #js [1] 3)]
+    (is (nil? (first xs)))
+    (is (nil? (seq xs)))
+    (is (= (rest xs) ()))
+    (is (= (pr-str xs) "()"))
+    (is (= (foo-1284 0) [0 ()]))
+    (is (= (pr-str (foo-1284 0)) "[0 ()]"))
+    (is (zero? (count ys)))
+    (is (= (transduce (map inc) conj [] ys) []))))
+
 (comment
   ;; ObjMap
   ;; (let [ks (map (partial str "foo") (range 500))