IDL: make optional arguments (without default) explicit sometimes

Source/bindings/scripts/v8_methods.py

Index: Source/bindings/scripts/v8_methods.py
diff --git a/Source/bindings/scripts/v8_methods.py b/Source/bindings/scripts/v8_methods.py
index 6f8378d24fd8ec381513a5af52b3d70b4707b6e9..4b79eb64dd696aa2cec205e46b5f2d2f1defa2d6 100644
--- a/Source/bindings/scripts/v8_methods.py
+++ b/Source/bindings/scripts/v8_methods.py
@@ -67,6 +67,21 @@ def argument_needs_try_catch(argument):
         (base_type == 'DOMString' and not argument.is_variadic))
 
 
+def argument_is_explicit_optional(method, index):
+    argument = method.arguments[index]
+    if not(argument.is_optional and
+           not argument.default_value and
+           not 'Default' in argument.extended_attributes):
+        # Argument itself is not optional, or has a default value.
+        return False
+    for following in method.arguments[index + 1:]:
+        if (following.default_value or
+            'Default' in following.extended_attributes):
+            # A following argument has a default value.
+            return True
+    return False
+
+
 def generate_method(interface, method):
     arguments = method.arguments
     extended_attributes = method.extended_attributes
@@ -203,6 +218,7 @@ def generate_argument(interface, method, argument, index):
         'index': index,
         'is_clamp': 'Clamp' in extended_attributes,
         'is_callback_interface': idl_type.is_callback_interface,
+        'is_explicit_optional': argument_is_explicit_optional(method, index),
         'is_nullable': idl_type.is_nullable,
         'is_optional': argument.is_optional,
         'is_variadic_wrapper_type': is_variadic_wrapper_type,
@@ -220,7 +236,8 @@ def generate_argument(interface, method, argument, index):
 ################################################################################
 
 def cpp_value(interface, method, number_of_arguments):
-    def cpp_argument(argument):
+    def cpp_argument(index):
+        argument = method.arguments[index]
         idl_type = argument.idl_type
         if idl_type.name == 'EventListener':
             if (interface.name == 'EventTarget' and
@@ -233,10 +250,11 @@ def cpp_value(interface, method, number_of_arguments):
             idl_type.name in ['NodeFilter', 'XPathNSResolver']):
             # FIXME: remove this special case
             return '%s.release()' % argument.name
+        if argument_is_explicit_optional(method, index):
+            return 'makeOptional({name}, {name}Missing)'.format(
+                name=argument.name)
         return argument.name
 
-    # Truncate omitted optional arguments
-    arguments = method.arguments[:number_of_arguments]
     cpp_arguments = v8_utilities.call_with_arguments(method)
     # Members of IDL partial interface definitions are implemented in C++ as
     # static member functions, which for instance members (non-static members)
@@ -244,7 +262,8 @@ def cpp_value(interface, method, number_of_arguments):
     if ('PartialInterfaceImplementedAs' in method.extended_attributes and
         not method.is_static):
         cpp_arguments.append('*impl')
-    cpp_arguments.extend(cpp_argument(argument) for argument in arguments)
+    cpp_arguments.extend(cpp_argument(index)
+                         for index in range(number_of_arguments))
     this_union_arguments = method.idl_type and method.idl_type.union_arguments
     if this_union_arguments:
         cpp_arguments.extend(this_union_arguments)