boost::hof::is_invocable is always creating std::true_type causing this code to not to compile:

#include <boost/variant.hpp>
#include <string>
#include <set>
#include <type_traits>
#include <boost/hof/is_invocable.hpp>


struct direct
{
};
template<typename T>
struct CDep
{   template<typename T1>
    CDep operator-(const CDep<T1>&) const
    {   static_assert(
            !std::is_same<T, std::string>::value
                && !std::is_same<T1, std::string>::value,
            "must not be a string!"
        );
        return {};
    }
};
typedef boost::variant<
    CDep<double>,
    CDep<int>,
    CDep<std::string>
> values;


template<typename T0>
struct second
{   const T0&m_r;
    second(const T0&_r)
        :m_r(_r)
    {
    }
    template<typename T>
    values operator()(const T&_r1)
    {   return m_r - _r1;
    }
    template<typename T>
    values operator()(const T&_r1, const std::true_type&) const
    {   return (*this)(_r1);
    }
    template<typename T>
    values operator()(const T&, const std::false_type&) const
    {   throw std::logic_error("does not compile!");
    }
    template<typename T>
    values operator()(const T&_r0) const
    {   return (*this)(
            _r0,
            std::integral_constant<
                bool,
                (boost::hof::is_invocable<second<T0>, T>())
            >()
        );
    }
};
struct first
{   const values&m_r;
    first(const values&_r)
        :m_r(_r)
    {
    }
    template<typename T>
    values operator()(const T&_r0) const
    {   return boost::apply_visitor(second<T>(_r0), m_r);
    }
};
values operator-(const values&_r0, const values&_r1)
{   return boost::apply_visitor(first(_r1), _r0);
}


int main(int, char**)
{   values(CDep<int>()) - values(CDep<int>());
    values(CDep<std::string>()) - values(CDep<std::string>());
}

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