[Proxies] Support constructable proxy as new.target

src/runtime/runtime-object.cc

Index: src/runtime/runtime-object.cc
diff --git a/src/runtime/runtime-object.cc b/src/runtime/runtime-object.cc
index 3be9c0edc001fc41a32f538c8e3f87463289819c..5ebdd5e0a819917c04ffc88be3bc3611e5d65abe 100644
--- a/src/runtime/runtime-object.cc
+++ b/src/runtime/runtime-object.cc
@@ -1007,25 +1007,55 @@ RUNTIME_FUNCTION(Runtime_AllocateHeapNumber) {
 }
 
 
+static MaybeHandle<Map> GetDerivedMap(Isolate* isolate,
+                                      Handle<JSFunction> constructor,
+                                      Handle<JSReceiver> new_target) {
+  JSFunction::EnsureHasInitialMap(constructor);
+  DCHECK_NE(JS_FUNCTION_TYPE, constructor->initial_map()->instance_type());
+
+  if (new_target->IsJSProxy()) {
+    Handle<JSProxy> new_target_proxy = Handle<JSProxy>::cast(new_target);
+    Handle<Object> prototype;
+    Handle<String> prototype_string = isolate->factory()->prototype_string();
+    ASSIGN_RETURN_ON_EXCEPTION(
+        isolate, prototype,
+        JSReceiver::GetProperty(new_target_proxy, prototype_string), Map);
+    Handle<Map> constructor_initial_map(constructor->initial_map());
+    Handle<Map> map = Map::CopyInitialMap(constructor_initial_map);
+
+    if (!prototype->IsJSReceiver()) {
+      Handle<Context> context;
+      ASSIGN_RETURN_ON_EXCEPTION(
+          isolate, context, JSProxy::GetFunctionRealm(new_target_proxy), Map);
+      DCHECK(context->IsNativeContext());
+      // TODO(verwaest): Use the intrinsicDefaultProto instead.
+      prototype = handle(context->initial_object_prototype(), isolate);
+    }
+
+    if (map->prototype() != *prototype) {
+      Map::SetPrototype(map, prototype, FAST_PROTOTYPE);
+    }
+
+    map->SetConstructor(*constructor);
+    return map;
+  }
+
+  return JSFunction::EnsureDerivedHasInitialMap(
+      Handle<JSFunction>::cast(new_target), constructor);
+}
+
+
 static Object* Runtime_NewObjectHelper(Isolate* isolate,
                                        Handle<JSFunction> constructor,
                                        Handle<JSReceiver> new_target,
                                        Handle<AllocationSite> site) {
-  // TODO(verwaest): new_target could be a proxy. Read new.target.prototype in
-  // that case.
-  Handle<JSFunction> original_function = Handle<JSFunction>::cast(new_target);
-
-  // The function should be compiled for the optimization hints to be
+  // The constructor should be compiled for the optimization hints to be
   // available.
   Compiler::Compile(constructor, CLEAR_EXCEPTION);
 
-  JSFunction::EnsureHasInitialMap(constructor);
-  DCHECK_NE(JS_FUNCTION_TYPE, constructor->initial_map()->instance_type());
-
-  // TODO(verwaest): original_function could have non-instance-prototype
-  // (non-JSReceiver), requiring fallback to the intrinsicDefaultProto.
-  Handle<Map> initial_map =
-      JSFunction::EnsureDerivedHasInitialMap(original_function, constructor);
+  Handle<Map> initial_map;
+  ASSIGN_RETURN_FAILURE_ON_EXCEPTION(
+      isolate, initial_map, GetDerivedMap(isolate, constructor, new_target));
 
   Handle<JSObject> result =
       isolate->factory()->NewJSObjectFromMap(initial_map, NOT_TENURED, site);