bindings: Throw a type error if converting an interface or callback function fails.

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

 {% extends 'interface_base.cpp.tmpl' %}
 
 {##############################################################################}
 {% 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 %}
@@ skipping 874 matching lines @@
   return hasInstance(value, isolate) ? toImpl(v8::Local<v8::Object>::Cast(value)) : nullptr;
 {% endif %}
 }
 
 {% endblock %}
 
 
 {##############################################################################}
 {% block native_value_traits %}
 {{cpp_class}}* NativeValueTraits<{{cpp_class}}>::nativeValue(v8::Isolate* isolate, v8::Local<v8::Value> value, ExceptionState& exceptionState) {
-  return {{v8_class}}::toImplWithTypeCheck(isolate, value);
+  {{cpp_class}}* nativeValue = {{v8_class}}::toImplWithTypeCheck(isolate, value);
+  if (!nativeValue)
+    exceptionState.throwTypeError("Unable to convert value to {{cpp_class}}.");

[Yuki, 2017/04/07 13:37:21]

nit: Can we use {{interface_name}} instead?
This is a user-visible message, so an internal C++ class' name is not preferred.

+  return nativeValue;
 }
 
 {% endblock %}
 
 
 {##############################################################################}
 {% block partial_interface %}
 {% if has_partial_interface %}
 {% if needs_runtime_enabled_installer %}
 InstallRuntimeEnabledFunction {{v8_class}}::install{{v8_class}}RuntimeEnabledFunction =
@@ skipping 23 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 %}