#include <tuple>

template <typename T>
auto
get_cost(const T &);

template <typename Graph>
using Vertex = typename Graph::vertex_descriptor;

template <typename Graph>
using Label = std::tuple<Vertex<Graph>>;

template <typename Graph>
struct Label1: std::tuple<Vertex<Graph>>
{
};

template <typename Graph>
struct Label2
{
  std::tuple<Vertex<Graph>> t;
};

template <typename Graph>
struct Label3
{
  Vertex<Graph> m_v;
};

template <typename Graph>
auto
get_cost(const Label<Graph> &l)
{
  return std::get<0>(l);
}

template <typename Graph>
auto
get_cost(const Label1<Graph> &l)
{
  return std::get<0>(l);
}

template <typename Graph>
auto
get_cost(const Label2<Graph> &l)
{
  return std::get<0>(l.t);
}

template <typename Graph>
auto
get_cost(const Label3<Graph> &l)
{
  return l.m_v;
}

struct graph
{
  using vertex_descriptor = unsigned;
};

template<typename T>
void
callme(const T &t)
{
  get_cost(t);
}

template<>
auto
get_cost(const Label<graph> &l)
{
  return std::get<0>(l);
}

// auto
// get_cost(const Label<graph> &l)
// {
//   return std::get<0>(l);
// }

int
main()
{
  Label<graph> l;
  Label1<graph> l1;
  Label2<graph> l2;
  Label3<graph> l3;
  get_cost(l);
  get_cost(l1);
  get_cost(l2);
  get_cost(l3);
  callme(l);
  callme(l1);
  callme(l2);
  callme(l3);
}