variadic template usage of 6 dots in libstdc++

while reading through libstdc++'s implementation of std::function and i saw

template<typename _Res, typename... _ArgTypes>
struct _Weak_result_type_impl<_Res(_ArgTypes......)>
  { typedef _Res result_type; };

what i don't understand in this is the ......, i think it is two ... operators but even that wouldn't really make sense.

can someone explain me what it is ?