IDL: Simplify [TypeChecking=Interface] code generation

Source/bindings/templates/attributes.cpp

 {##############################################################################}
 {% macro attribute_getter(attribute, world_suffix) %}
 {% filter conditional(attribute.conditional_string) %}
 static void {{attribute.name}}AttributeGetter{{world_suffix}}(
 {%- if attribute.is_expose_js_accessors %}
 const v8::FunctionCallbackInfo<v8::Value>& info
 {%- else %}
 const v8::PropertyCallbackInfo<v8::Value>& info
 {%- endif %})
 {
@@ skipping 199 matching lines @@
           and is_node and not attribute.is_implemented_in_private_script %}
     {% set cpp_class, v8_class = 'Element', 'V8Element' %}
     {% endif %}
     {# Local variables #}
     {% if not attribute.is_static %}
     v8::Handle<v8::Object> holder = info.Holder();
     {% endif %}
     {% if attribute.has_setter_exception_state %}
     ExceptionState exceptionState(ExceptionState::SetterContext, "{{attribute.name}}", "{{interface_name}}", holder, info.GetIsolate());
     {% endif %}
-    {# Type checking #}
-    {% if attribute.has_type_checking_interface %}
-    {# Type checking for interface types (if interface not implemented, throw
-       TypeError), per http://www.w3.org/TR/WebIDL/#es-interface #}
-    if ({% if attribute.is_nullable %}!isUndefinedOrNull(v8Value) && {% endif %}!V8{{attribute.idl_type}}::hasInstance(v8Value, info.GetIsolate())) {
-        exceptionState.throwTypeError("The provided value is not of type '{{attribute.idl_type}}'.");
-        exceptionState.throwIfNeeded();
-        return;
-    }
-    {% endif %}
     {% if attribute.use_output_parameter_for_result %}
     {{attribute.cpp_type}} cppValue;
     {% endif %}
     {# impl #}
     {% if attribute.put_forwards %}
     {{cpp_class}}* proxyImpl = {{v8_class}}::toImpl(holder);
     {{attribute.cpp_type}} impl = WTF::getPtr(proxyImpl->{{attribute.name}}());
     if (!impl)
         return;
     {% elif not attribute.is_static %}
@@ skipping 13 matching lines @@
        http://www.w3.org/TR/WebIDL/#es-type-mapping #}
     {% if attribute.has_type_checking_unrestricted %}
     {# Non-finite floating point values (NaN, +Infinity or −Infinity), per:
        http://heycam.github.io/webidl/#es-float
        http://heycam.github.io/webidl/#es-double #}
     if (!std::isfinite(cppValue)) {
         exceptionState.throwTypeError("The provided {{attribute.idl_type}} value is non-finite.");
         exceptionState.throwIfNeeded();
         return;
     }
+    {% elif attribute.has_type_checking_interface %}
+    {# Type checking for interface types (if interface not implemented, throw
+       TypeError), per http://www.w3.org/TR/WebIDL/#es-interface #}
+    if (!cppValue{% if attribute.is_nullable %} && !isUndefinedOrNull(v8Value){% endif %}) {
+        exceptionState.throwTypeError("The provided value is not of type '{{attribute.idl_type}}'.");
+        exceptionState.throwIfNeeded();
+        return;
+    }
     {% elif attribute.enum_validation_expression %}
     {# Setter ignores invalid enum values:
        http://www.w3.org/TR/WebIDL/#idl-enums #}
     String string = cppValue;
     if (!({{attribute.enum_validation_expression}}))
         return;
     {% endif %}
     {# Pre-set context #}
     {% if attribute.is_custom_element_callbacks or
           (attribute.is_reflect and
@@ skipping 161 matching lines @@
        access_control,
        property_attribute,
        only_exposed_to_private_script,
    ] %}
 {% if not attribute.is_expose_js_accessors %}
 {% set attribute_configuration_list = attribute_configuration_list
                                     + [on_prototype] %}
 {% endif %}
 {{'{'}}{{attribute_configuration_list | join(', ')}}{{'}'}}
 {%- endmacro %}