CLJ-2822: Add RT.nthImpl

* Add `RT.nthImpl` which accepts an `Object n` instead of `int n`, and throws an `UnsupportedOperationException` before casting.
* Change `clojure.core/nth` to call `RT.nthImpl`.

src/clj/clojure/core.clj

@@ -893,11 +893,11 @@
   bounds, nth throws an exception unless not-found is supplied.  nth
   also works for strings, Java arrays, regex Matchers and Lists, and,
   in O(n) time, for sequences."
-  {:inline (fn  [c i & nf] `(. clojure.lang.RT (nth ~c ~i ~@nf)))
+  {:inline (fn  [c i & nf] `(. clojure.lang.RT (nthImpl ~c ~i ~@nf)))
    :inline-arities #{2 3}
    :added "1.0"}
-  ([coll index] (. clojure.lang.RT (nth coll index)))
-  ([coll index not-found] (. clojure.lang.RT (nth coll index not-found))))
+  ([coll index] (. clojure.lang.RT (nthImpl coll index)))
+  ([coll index not-found] (. clojure.lang.RT (nthImpl coll index not-found))))
 
 (defn <
   "Returns non-nil if nums are in monotonically increasing order,

src/jvm/clojure/lang/RT.java

@@ -917,6 +917,24 @@ static public Object nth(Object coll, int n){
 	return nthFrom(Util.ret1(coll, coll = null), n);
 }
 
+static int castNthIdx(Object o) {
+	if (o instanceof Number) {
+		return intCast(o);
+	}
+	throw new UnsupportedOperationException(
+			"nth idx not supported on this type: " + o.getClass().getSimpleName());
+}
+
+/**
+ * Only for use in `clojure.core/nth`.
+ */
+static public Object nthImpl(Object coll, Object n){
+	int idx = castNthIdx(n);
+	if(coll instanceof Indexed)
+		return ((Indexed) coll).nth(idx);
+	return nthFrom(Util.ret1(coll, coll = null), idx);
+}
+
 static Object nthFrom(Object coll, int n){
 	if(coll == null)
 		return null;
@@ -960,6 +978,18 @@ static public Object nth(Object coll, int n, Object notFound){
 	return nthFrom(coll, n, notFound);
 }
 
+/**
+ * Only for use in `clojure.core/nth`.
+ */
+static public Object nthImpl(Object coll, Object n, Object notFound){
+	int idx = castNthIdx(n);
+	if(coll instanceof Indexed) {
+		Indexed v = (Indexed) coll;
+		return v.nth(idx, notFound);
+	}
+	return nthFrom(coll, idx, notFound);
+}
+
 static Object nthFrom(Object coll, int n, Object notFound){
 	if(coll == null)
 		return notFound;

test/clojure/test_clojure/sequences.clj

@@ -641,6 +641,16 @@
   (is (thrown? IndexOutOfBoundsException (nth (java.util.ArrayList. []) -1)))       ; ???
   (is (thrown? IndexOutOfBoundsException (nth (java.util.ArrayList. [1 2 3]) -1)))  ; ???
 
+  ;; CLJ-2822
+  (is (thrown? UnsupportedOperationException (nth 1 [1 2 3])))
+  (is (try ^{:line 12345} (nth 1 [1 2 3])
+           (catch UnsupportedOperationException e
+             (let [line (->> (.getStackTrace e)
+                             (drop-while #(= "clojure.lang.RT" (.getClassName ^StackTraceElement %)))
+                             (first)
+                             (#(.getLineNumber ^StackTraceElement %)))]
+               (= 12345 line)))))
+
   (are [x y] (= x y)
       (nth '(1) 0) 1
       (nth '(1 2 3) 0) 1