LibJS: Add the Object.prototype.__proto__ native accessor property

This is part of the Annex B extension of the specification.

Userland/Libraries/LibJS/Runtime/CommonPropertyNames.h

@@ -12,6 +12,7 @@
 namespace JS {
 
 #define ENUMERATE_STANDARD_PROPERTY_NAMES(P) \
+    P(__proto__)                             \
     P(BYTES_PER_ELEMENT)                     \
     P(BigInt)                                \
     P(Boolean)                               \

Userland/Libraries/LibJS/Runtime/ObjectPrototype.cpp

@@ -35,6 +35,9 @@ void ObjectPrototype::initialize(GlobalObject& global_object)
     define_native_function(vm.names.valueOf, value_of, 0, attr);
     define_native_function(vm.names.propertyIsEnumerable, property_is_enumerable, 1, attr);
     define_native_function(vm.names.isPrototypeOf, is_prototype_of, 1, attr);
+
+    // Annex B
+    define_native_accessor(vm.names.__proto__, proto_getter, proto_setter, Attribute::Configurable);
 }
 
 ObjectPrototype::~ObjectPrototype()
@@ -144,4 +147,40 @@ JS_DEFINE_NATIVE_FUNCTION(ObjectPrototype::is_prototype_of)
     }
 }
 
+// B.2.2.1.1 get Object.prototype.__proto__, https://tc39.es/ecma262/#sec-get-object.prototype.__proto__
+JS_DEFINE_NATIVE_FUNCTION(ObjectPrototype::proto_getter)
+{
+    auto object = vm.this_value(global_object).to_object(global_object);
+    if (vm.exception())
+        return {};
+    auto proto = object->prototype();
+    if (vm.exception())
+        return {};
+    return proto;
+}
+
+// B.2.2.1.2 set Object.prototype.__proto__, https://tc39.es/ecma262/#sec-set-object.prototype.__proto__
+JS_DEFINE_NATIVE_FUNCTION(ObjectPrototype::proto_setter)
+{
+    auto object = require_object_coercible(global_object, vm.this_value(global_object));
+    if (vm.exception())
+        return {};
+
+    auto proto = vm.argument(0);
+    if (!proto.is_object() && !proto.is_null())
+        return js_undefined();
+
+    if (!object.is_object())
+        return js_undefined();
+
+    auto status = object.as_object().set_prototype(proto.is_object() ? &proto.as_object() : nullptr);
+    if (vm.exception())
+        return {};
+    if (!status) {
+        vm.throw_exception<TypeError>(global_object, ErrorType::ObjectSetPrototypeOfReturnedFalse);
+        return {};
+    }
+    return js_undefined();
+}
+
 }

Userland/Libraries/LibJS/Runtime/ObjectPrototype.h

@@ -27,6 +27,8 @@ private:
     JS_DECLARE_NATIVE_FUNCTION(value_of);
     JS_DECLARE_NATIVE_FUNCTION(property_is_enumerable);
     JS_DECLARE_NATIVE_FUNCTION(is_prototype_of);
+    JS_DECLARE_NATIVE_FUNCTION(proto_getter);
+    JS_DECLARE_NATIVE_FUNCTION(proto_setter);
 };
 
 }