template <typename T, typename S>
struct Decl
{
};

template <>
struct Decl<int, int>
{
    typedef int Type;
};

template <>
struct Decl<double, int>
{
    typedef double Type;
};

template <>
struct Decl<int, double>
{
    typedef double Type;
};

template <>
struct Decl<double, double>
{
    typedef double Type;
};