binding: Indexed property returns undefined if out-of-range.

third_party/WebKit/Source/bindings/templates/interface.cpp

 {% extends 'interface_base.cpp' %}
 
 {% set has_prepare_prototype_and_interface_object =
     unscopeables or has_conditional_attributes_on_prototype or
     methods | conditionally_exposed(is_partial) %}
 {% set prepare_prototype_and_interface_object_func =
     '%s::preparePrototypeAndInterfaceObject' % v8_class
     if has_prepare_prototype_and_interface_object
     else 'nullptr' %}
 
 
 {##############################################################################}
 {% block indexed_property_getter %}
 {% if indexed_property_getter and not indexed_property_getter.is_custom %}
 {% set getter = indexed_property_getter %}
 static void indexedPropertyGetter(uint32_t index, const v8::PropertyCallbackInfo<v8::Value>& info)
 {
     {% if getter.is_raises_exception %}
     ExceptionState exceptionState(info.GetIsolate(), ExceptionState::IndexedGetterContext, "{{interface_name}}");
     {% endif %}
 
     {{cpp_class}}* impl = {{v8_class}}::toImpl(info.Holder());
 
+    // We assume that all the implementations support length() method, although

[peria, 2016/09/14 15:25:03]

Please use "{#...#}" style comment not to generate code with comments.

[Yuki, 2016/09/15 09:49:17]

I intentionally made this comment C++ comment so that it's visible to Blink
developers.

If someone implements a new interface (or updates an existing interface) and
hits a compile error that class T does not declare length(), this comment will
be useful for Blink developers to understand why the compile error happens.

+    // the spec doesn't require that length() must exist.  It's okay that
+    // the interface does not have length attribute as long as the
+    // implementation supports length() member function.
+    if (index >= impl->length())
+        return;  // Returns undefined due to out-of-range.
+
     {% set getter_name = getter.name or 'anonymousIndexedGetter' %}
     {% set getter_arguments = ['index'] %}
     {% if getter.is_call_with_script_state %}
     ScriptState* scriptState = ScriptState::forReceiverObject(info);
     {% set getter_arguments = ['scriptState'] + getter_arguments %}
     {% endif %}
     {% if getter.is_raises_exception %}
     {% set getter_arguments = getter_arguments + ['exceptionState'] %}
     {% endif %}
     {{getter.cpp_type}} result = impl->{{getter_name}}({{getter_arguments | join(', ')}});
-    if ({{getter.is_null_expression}})
-        return;
     {{getter.v8_set_return_value}};
 }
 
 {% endif %}
 {% endblock %}
 
 
 {##############################################################################}
 {% block indexed_property_getter_callback %}
 {% if indexed_property_getter or named_property_getter %}
@@ skipping 939 matching lines @@
 
 {% for method in methods if method.overloads and method.overloads.has_partial_overloads %}
 void {{v8_class}}::register{{method.name | blink_capitalize}}MethodForPartialInterface(void (*method)(const v8::FunctionCallbackInfo<v8::Value>&))
 {
     {{cpp_class}}V8Internal::{{method.name}}MethodForPartialInterface = method;
 }
 
 {% endfor %}
 {% endif %}
 {% endblock %}