template< int size, typename Type >
class Vector;

template< int size, typename LeftType, typename RightType >
Vector<size, ?> operator+( const Vector<size, LeftType>& left, const Vector<size, RightType>& right )
    {...}

#include <iostream>
#include <type_traits>
#include <vector>

template< int size, typename Type >
struct Vector
{
    std::vector<Type> v;
};

template< int size, typename LeftType, typename RightType >
Vector<size, typename std::common_type<LeftType, RightType>::type>
operator +( const Vector<size, LeftType>& left, const Vector<size, RightType>& right )
{
    Vector<size, typename std::common_type<LeftType, RightType>::type> v;
    v.v.resize( size );

    for ( int i = 0; i < size; i++ ) v.v[i] = left.v[i] + right.v[i];

    return v;
}

int main()
{
    Vector<5, int> left = { { 1, 2, 3, 4, 5 } };
    Vector<5, double> right = { { 0.1, 0.2, 0.3, 0.4, 0.5 } };

    auto v = left + right;

    for ( int i = 0; i < 5; i++ ) std::cout << v.v[i] << ' ';
    std::cout << std::endl;
}

1.1 2.2 3.3 4.4 5.5

#include<utility>

template< int size, typename LeftType, typename RightType >
Vector<size, decltype(std::declval<LeftType>() + std::declval<RightType>())> ...