[proxies] Recognize arraylike proxies in Object.prototype.toString.

We must print "[object Array]" for proxies that satisfy Array.isArray.

Cosmetic change on the side: move ObjectProtoToString from JSObject to Object
since it deals with arbitrary objects.

R=adamk@chromium.org, verwaest@chromium.org
BUG=v8:1543
LOG=n

Review URL: https://codereview.chromium.org/1526023002

Cr-Commit-Position: refs/heads/master@{#32902}

src/objects.cc

-// Copyright 2013 the V8 project authors. All rights reserved.
+// Copyright 2015 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
@@ -2333,6 +2333,18 @@ String* JSReceiver::class_name() {
 }
 
 
+MaybeHandle<String> JSReceiver::BuiltinStringTag(Handle<JSReceiver> object) {
+  Maybe<bool> is_array = Object::IsArray(object);
+  MAYBE_RETURN(is_array, MaybeHandle<String>());
+  if (is_array.FromJust()) {
+    return object->GetIsolate()->factory()->Array_string();
+  }
+  // TODO(adamk): class_name() is expensive, replace with instance type
+  // checks where possible.
+  return handle(object->class_name());
+}
+
+
 // static
 Handle<String> JSReceiver::GetConstructorName(Handle<JSReceiver> receiver) {
   Isolate* isolate = receiver->GetIsolate();
@@ -16267,8 +16279,8 @@ int JSObject::GetOwnElementKeys(FixedArray* storage, PropertyFilter filter) {
 }
 
 
-MaybeHandle<String> JSObject::ObjectProtoToString(Isolate* isolate,
-                                                  Handle<Object> object) {
+MaybeHandle<String> Object::ObjectProtoToString(Isolate* isolate,
+                                                Handle<Object> object) {
   if (object->IsUndefined()) return isolate->factory()->undefined_to_string();
   if (object->IsNull()) return isolate->factory()->null_to_string();
 
@@ -16288,9 +16300,8 @@ MaybeHandle<String> JSObject::ObjectProtoToString(Isolate* isolate,
   }
 
   if (tag.is_null()) {
-    // TODO(adamk): class_name() is expensive, replace with instance type
-    // checks where possible.
-    tag = handle(receiver->class_name(), isolate);
+    ASSIGN_RETURN_ON_EXCEPTION(isolate, tag,
+                               JSReceiver::BuiltinStringTag(receiver), String);
   }
 
   IncrementalStringBuilder builder(isolate);

src/objects.h

-// Copyright 2012 the V8 project authors. All rights reserved.
+// Copyright 2015 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
@@ -1359,6 +1359,10 @@ class Object {
 
   inline void VerifyApiCallResultType();
 
+  // ES6 19.1.3.6 Object.prototype.toString
+  MUST_USE_RESULT static MaybeHandle<String> ObjectProtoToString(
+      Isolate* isolate, Handle<Object> object);
+
   // Prints this object without details.
   void ShortPrint(FILE* out = stdout);
 
@@ -1887,6 +1891,10 @@ class JSReceiver: public HeapObject {
   // Returns the class name ([[Class]] property in the specification).
   String* class_name();
 
+  // Returns the builtin string tag used in Object.prototype.toString.
+  MUST_USE_RESULT static MaybeHandle<String> BuiltinStringTag(
+      Handle<JSReceiver> object);
+
   // Returns the constructor name (the name (possibly, inferred name) of the
   // function that was used to instantiate the object).
   static Handle<String> GetConstructorName(Handle<JSReceiver> receiver);
@@ -2472,10 +2480,6 @@ class JSObject: public JSReceiver {
   static bool AllCanRead(LookupIterator* it);
   static bool AllCanWrite(LookupIterator* it);
 
-  // ES6 19.1.3.6 Object.prototype.toString
-  MUST_USE_RESULT static MaybeHandle<String> ObjectProtoToString(
-      Isolate* isolate, Handle<Object> object);
-
  private:
   friend class JSReceiver;
   friend class Object;

test/mjsunit/es6/object-tostring.js

-// Copyright 2014 the V8 project authors. All rights reserved.
+// Copyright 2015 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// Flags: --harmony-tostring
+// Flags: --harmony-tostring --harmony-proxies
 
 var global = this;
 
@@ -137,3 +137,25 @@ function testObjectToStringOwnNonStringValue() {
   assertEquals("[object Object]", ({}).toString.call(obj));
 }
 testObjectToStringOwnNonStringValue();
+
+
+// Proxies
+
+function assertTag(tag, obj) {
+  assertEquals("[object " + tag + "]", Object.prototype.toString.call(obj));
+}
+
+assertTag("Object", new Proxy({}, {}));
+assertTag("Array", new Proxy([], {}));
+assertTag("Function", new Proxy(() => 42, {}));
+assertTag("Foo", new Proxy(() => 42, {get() {return "Foo"}}));
+assertTag("Function", new Proxy(() => 42, {get() {return 666}}));
+
+revocable = Proxy.revocable([], {});
+revocable.revoke();
+assertThrows(() => Object.prototype.toString.call(revocable.proxy), TypeError);
+
+handler = {};
+revocable = Proxy.revocable([], handler);
+handler.get = () => revocable.revoke();
+assertThrows(() => Object.prototype.toString.call(revocable.proxy), TypeError);