What is the meaning of "... ..." token? i.e. double ellipsis operator on parameter pack

While browsing through gcc's current implementation of new C++11 headers, I stumbled upon "......" token. You can check, that the following code compiles fine [via godbolt.org].

template <typename T>
struct X
{ /* ... */ };

template <typename T, typename ... U>
struct X<T(U......)> // this line is the important one
{ /* ... */ };

So, what is the meaning of this token?

edit: Looks like SO trimmed "......" in question title to "...", I did really mean "......" . :)