[Boost-users] function template argument deduction with std::tuple

Hi, I have a problem related to function template argument deduction with std::tuple (with boost::tuple too). I attach a file with the complete source you can compile. Below I describe the code. I have a template function declaration:

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

Then I define my template types:

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

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

I define the specialization of the get_cost function for the Label type:

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

I can define alright a variable of type Label:

struct graph { using vertex_descriptor = unsigned; };

Label<graph> l;

But I cannot use the templated function:

get_cost(l);

I get this error:

error: use of 'auto get_cost(const T&) [with T = std::tuple<unsigned int>]' before deduction of 'auto'

I got this error with GCC HEAD 8.0.0 20171113 and Clang HEAD 6.0.0 (I used wandbox.org). Some other type definitions and their specializations of the get_cost function compile. For a struct derived from the std::tuple:

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

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

For a member of type std::tuple:

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

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

For a member of type Vertex<Graph>:

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

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

I would like to use the Label type, i.e., because it has the constructor and operators already defined, which I need. I would appreciate it if someone could explain what is going wrong here. Best, Irek