Cannot access member functions of constexpr CRTP Class after casting - c++

Question

Building on the answer I got to my question Constexpr Class taking const references not compiling, I was trying go further (playing around with expression templates).
So as the answer suggested, I now use static variables as inputs, and everything compiles correctly (on MSVC 2019).
However, upon adding constexpr static auto val = expr[0] I get error C2131: expression did not evaluate to a constant. The issue seems to be with m_lhs[idx] and m_rhs[idx] as those do not seem to evaluate to a constant, despite the fact that i've marked all operator[] as constexpr.
Is it possible to fix this?
#include <cstddef>
template<class VecType>
struct Vector
{
constexpr const VecType& get() const noexcept
{
return static_cast<const VecType&>(*this);
}
};
template<size_t Size>
class FixedSizeVector : public Vector<FixedSizeVector<Size>>
{
public:
constexpr explicit FixedSizeVector(const int(&array)[Size]) noexcept
: m_vector()
{
for (size_t i = 0; i < Size; ++i)
m_vector[i] = array[i];
}
constexpr const int& operator[](size_t idx) const noexcept {
return m_vector[idx];
}
private:
int m_vector[Size];
};
template<class VecType1, class VecType2>
class ExpressionAdd : public Vector<ExpressionAdd<VecType1, VecType2>>
{
public:
constexpr ExpressionAdd(const VecType1& lhs, const VecType2& rhs) noexcept
: m_lhs(lhs), m_rhs(rhs) { }
constexpr int operator[](size_t idx) const noexcept {
return m_lhs[idx] + m_rhs[idx];
}
private:
const VecType1& m_lhs;
const VecType2& m_rhs;
};
template<class VecType1, class VecType2>
constexpr auto operator+(const Vector<VecType1>& lhs, const Vector<VecType2>& rhs)
{
return ExpressionAdd(lhs.get(), rhs.get());
}
int main()
{
constexpr static int arr[]{ 4,2,9 };
constexpr static FixedSizeVector a(arr);
constexpr static auto expr = a + a;
constexpr static auto val = expr[0]; /* <--- this line causes an error */
return 0;
}