g++ -g bug, C++ (gcc)

g++ -g bug

#include <vector>
#include <assert.h>

class Vector
{
  public:
    Vector(std::size_t n) : elems(n)
    {
    }

// construct vector using initializer list
    Vector(std::initializer_list<double> init)
    {
        for (auto i : init)
            elems.push_back(i);
    }

double &operator[](std::size_t i)
    {
        return elems[i];
    }

double operator[](std::size_t i) const
    {
        return elems[i];
    }

std::size_t size() const
    {
        return elems.size();
    }

private:
    std::vector<double> elems;
};

template <typename E1, typename E2>
class VectorSum
{
    const E1 &_u;
    const E2 &_v;

public:
    VectorSum(const E1 &u, const E2 &v) : _u(u), _v(v)
    {
        assert(u.size() == v.size());
    }

double operator[](std::size_t i) const
    {
        return _u[i] + _v[i];
    }

std::size_t size() const
    {
        return _v.size();
    }
};

template <typename E1, typename E2>
const VectorSum<E1, E2> operator + (const E1 &u, const E2 &v)
{
    return VectorSum<E1, E2>(u, v);
}

int main()
{
    Vector v0 = {23.4, 12.5, 144.56, 90.56};
    Vector v1 = {67.12, 34.8, 90.34, 89.30};
    Vector v2 = {34.90, 111.9, 45.12, 90.5};
    auto sum = v0 + v1 + v2;

Vector v3(v0.size());

v3[0] = sum[0];
}


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