Different Behaviour

// MatrixMeta.hpp

#include <tuple>

template <size_t x = 0>
class M;

// Note, if this is removed the problem disappears
namespace meta
{

    inline auto f()
    {
        return std::make_tuple();
    }

    template <size_t x, typename ... Args>
    auto f(const M<x>& , Args && ... args)
    {
        return f(args...);
    }

    template <typename ... Args>
    auto f(const M<>& first, Args && ... args)
    {
            return std::tuple_cat(std::make_tuple(first), f(args...));
    }
}

// Matrix.hpp
template <size_t x>
class M{};

template <>
class M<0>{};

// main.cpp

int main(int, char* [])
{

    M<1> a;
    M<> b;

    return std::tuple_size<decltype(meta::f(a, b, b))>::value; // (0, 0)

}