Eigen Types + variant + holds_alternative<> + immer::flex_vector segfault in opt builds.

[asa]

Using types like Eigen::Vector3d in structs in variants in flex vectors cause a crash in immer when I

a variant like:

struct a_t{
        Eigen::Vector3d position;
};
struct b_t{
        double x;
};
using nifty_variant = std::variant<a_t, b_t>;

and I try and filter out some of these structs with:

template<typename T>
inline auto filter(const immer::flex_vector<nifty_variant>& buf) {
    immer::flex_vector<T> out{};
    immer::for_each(buf.begin(), buf.end(), [&out](auto a) {
        if (std::holds_alternative<T>(a)) {
            out = move(out).push_back(std::get<T>(a));
        }
    });
    return out;
}

I get segfaults in opt builds, but not in debug builds like:

#0  0x0000000000476478 in immer::detail::rbts::rrbtree< nifty_variant, immer::memory_policy<immer::free_list_heap_policy<immer::cpp_heap, 1024ul>, immer::refcount_policy, immer::spinlock_policy, immer::no_transience_policy, false, true>, 5u, 1u>::push_back_mut(immer::no_transience_policy::apply<immer::free_list_heap_policy<immer::cpp_heap, 1024ul> >::type::edit, nifty_variant) ()

[arximboldi]

Thanks for the report!