#include <vector>
#include <type_traits>
 
using namespace std;
 
template <class S, class T> vector<typename std::common_type<S, T>::type> operator+(const vector<S> &a, const vector<T> &b)
{
    vector<typename std::common_type<S, T>::type> result;
    result.reserve(a.size());
    for(unsigned int i = 0; i < a.size();++i){
        result[i] = a[i] + b[i];
    }
    return result;
}
 
int main() {
    vector<double> bla;
    bla = bla + bla;
    return 0;
}

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