using MyNS;

// ...

public void load_from_uri (string uri, MyClass encoding = MyClass.utf8) throws GLib.Error {
// ...
}

/* valac compiles this code without problems. but namespace is missing from generated vapi :
** (valac:23817): CRITICAL **: vala_data_type_compatible: assertion 'self != NULL' failed
** (valac:23817): CRITICAL **: vala_code_node_to_string: assertion 'self != NULL' failed
mee-json-2.0.vapi:164.72-164.84: error: Cannot convert from `(null)' to `MyNS.Encoding'
            public void load_from_uri (string uri, MyNS.MyClass encoding = MyClass.utf8) throws GLib.Error {
*/