xt::linalg::trace does not support complex arrays

[Yue-Zhengyuan]

The xt::linalg::trace cannot deal with complex arrays, which is reflected in this source code (line 1315):

template <class T>
    auto trace(const xexpression<T>& M, int offset = 0, int axis1 = 0, int axis2 = 1)
    {
        const auto& dM = M.derived_cast();
        auto d = xt::diagonal(dM, offset, std::size_t(axis1), std::size_t(axis2));
        
        std::size_t dim = d.dimension();
        if (dim == 1)
        {
            return xt::xarray<double>(xt::sum(d)());
        }
        else
        {
            return xt::xarray<double>(xt::sum(d, {dim - 1}));
        }
    }

I tried to modify this part directly by changing xarray<double> to xarray<std::complex<double>>. But after I do this, I get another error from xutils.hpp of xtensor(line 628):

template <class T>
    struct conditional_cast_functor<true, T>
    {
        template <class U>
        inline auto operator()(U&& u) const
        {
            return static_cast<T>(std::forward<U>(u));
        }
    };

Error:

Cannot convert 'const std::__1::complex<double>' to 'double' without a conversion operator

Although there is a workaround (tracing the real and the complex parts separately), this is not convenient. Could you please look into this problem and fix it? Thanks!