Class-Wide `using` alias as return type issuing compiler error

Question

In order to keep some code readable and avoid typos, I'm using the following statement in the public section of a class definition in a header file:

using Assembly_Tuple = std::tuple <std::vector <std::string>, Trigger_Map, Attribute_Map>;

I'm declaring a function in the header file with Assembly_Tuple as a return type:

Assembly_Tuple merge_node_attributes(const std::string& node_name, std::string tmpl_name="");

And I'm defining the function in the source file:

Widget_Server_Interface::Assembly_Tuple 
Widget_Server_Interface::merge_node_attributes(const std::string& n, const std::string& t)
{
    //...
}

But when I try to compile, I get the following error:

src/WidgetServer/WidgetServerInterface.cpp:31:1: error: ‘Assembly_Tuple’ in ‘class Widget_Server_Interface’ does not name a type

However, inside definitions, there aren't any problems.

If I change that line to the egregious:

std::tuple<std::vector<std::string>, Trigger_Map, std::map<int,Node_Value>>
Widget_Server_Interface::merge_node_attributes (...) {...}

it's fine. Clearly the problem is using the alias outside of scope, even though it's public and I'm explicitly calling on the class namespace.

I looked in Bjarne's book but he doesn't mention anywhere whether or not this is legal.

This is using gcc 4.7.

Mostly, I would just like to know why this isn't valid.