A template metaprogramming value is a nullary template metafunction returning itself. For example:

struct void_
{
  using type = void_;
};

This template metaprogramming value is called void_. It is a nullary metafunction returning itself as its result. Because of this, it can be treated as a nullary metafunction and evaluated any number of times. For example void_::type::type::type is still void_.