fix bug with arguments enumerator in JSON.parse

Author: MikeHolman

lib/Runtime/Library/ArgumentsObjectEnumerator.h

@@ -21,6 +21,7 @@ namespace Js
         ArgumentsObjectPrefixEnumerator(ArgumentsObject* argumentsObject, EnumeratorFlags flags, ScriptContext* requestContext);
         virtual void Reset() override;
         virtual JavascriptString * MoveAndGetNext(PropertyId& propertyId, PropertyAttributes* attributes = nullptr) override;
+        virtual uint32 GetCurrentItemIndex()  override { return formalArgIndex; }
     };
 
     class ES5ArgumentsObjectEnumerator : public ArgumentsObjectPrefixEnumerator

test/JSON/arguments.js

@@ -0,0 +1,16 @@
+//-------------------------------------------------------------------------------------------------------
+// Copyright (C) Microsoft. All rights reserved.
+// Licensed under the MIT license. See LICENSE.txt file in the project root for full license information.
+//-------------------------------------------------------------------------------------------------------
+
+let set = false;
+let a = JSON.parse('["first", null]', function (key, value) {                                    
+  if(!set) {
+    this[1] = arguments;
+    set = true;
+  }
+  return value;
+});
+let passed = JSON.stringify(a) === `["first",{"0":"0","1":"first"}]`;
+
+print(passed ? "Pass" : "Fail")

test/JSON/rlexe.xml

@@ -27,6 +27,11 @@
       <tags>exclude_test,Slow</tags>
     </default>
   </test>
+  <test>
+    <default>
+      <files>arguments.js</files>
+    </default>
+  </test>
   <test>
     <default>
       <files>replacerFunction.js</files>