Building xtensor with c++20 and tests enabled fails

[vakokako]

In c++20 std::allocator::reference was removed, but the xt::tracking_allocator uses it with using reference = typename A::reference; which results in error:

.../xtensor/xutils.hpp:731:39: error: no type named 'reference' in 'std::allocator<double>'
        using reference = typename A::reference;
                          ~~~~~~~~~~~~^~~~~~~~~

[ImJustAskingDude]

xt::tile fails with bad_alloc too, though U am not sure what the source of this problem is, just throwing it out there

[spectre-ns]

Looks like a duplicate of #2504. C++20 is not supported.