add test (closes #4757)

tests/unit/src/unit/issues/Issue4757.hx

@@ -0,0 +1,15 @@
+package unit.issues;
+
+class Issue4757 extends Test {
+
+	function test() {
+		eq('ca: 2, cb: 1', cunion(1, 2));
+	}
+
+	static function cunion(ca:Int, cb:Int) {
+		var tmp = ca;
+		ca = cb;
+		cb = tmp;
+		return 'ca: $ca, cb: $cb';
+	}
+}