Возврат auto значения шаблона

Как вернуть неизвестный заранее тип переменной шаблона при помощи ключевого слова auto ?

error C2672: 'GetValue': no matching overloaded function found
error C2783: '_Out GetValue(_In &)': could not deduce template argument for '_Out'
note: see declaration of 'GetValue'
error C2672: 'GetValue': no matching overloaded function found
error C2783: '_Out GetValue(_In &)': could not deduce template argument for '_Out'
note: see declaration of 'GetValue'

Код:

const unsigned short V_INT = 0;
const unsigned short V_DOUBLE = 1;
typedef unsigned short TYPE;
struct ESAMPLE
{
    union
    {
        TYPE type;
        union
        {
            int _INT;
            double _DOUBLE;
        };
    };
};
template<typename _Out, typename _In>
inline _Out GetValue(_In& v)
{
    switch (v.type)
    {
    case V_INT:
        return v._INT;
    case V_DOUBLE:
        return v._DOUBLE;
    }
}
int main()
{
    ESAMPLE e;
    e.type = V_INT;
    e._INT = -128;
    auto x = GetValue(e); // must return 'int'
    ESAMPLE e2;
    e.type = V_DOUBLE;
    e._DOUBLE = 128;
    auto x2 = GetValue(e2); // must return 'double'
}