libxsimd-dev 10 triggers an error in xtensor tests on arm64
xsimd passes its own tests, and xtensor passes on other arches (except
armel which has known issues)

The error output from
https://ci.debian.net/data/autopkgtest/unstable/arm64/x/xtensor/38197207/log.gz
is

127s [ 26%] Building CXX object CMakeFiles/test_xtensor_lib.dir/test_xoptional_assembly.cpp.o
127s /usr/bin/g++ -DXSIMD_ENABLE_XTL_COMPLEX -DXTENSOR_USE_XSIMD  -DXSIMD_ENABLE_XTL_COMPLEX=1 -march=native -std=c++14 -Wunused-parameter -Wextra -Wreorder -Wconversion -Wno-sign-conversion  -Wold-style-cast -Wunused-variable -ftemplate-backtrace-limit=0 -O3 -DNDEBUG -MD -MT CMakeFiles/test_xtensor_lib.dir/test_xoptional_assembly.cpp.o -MF CMakeFiles/test_xtensor_lib.dir/test_xoptional_assembly.cpp.o.d -o CMakeFiles/test_xtensor_lib.dir/test_xoptional_assembly.cpp.o -c /tmp/autopkgtest-lxc.1p570fd4/downtmp/autopkgtest_tmp/test_xoptional_assembly.cpp
131s In file included from /usr/include/xsimd/types/xsimd_batch.hpp:493,
131s                  from /usr/include/xsimd/xsimd.hpp:61,
131s                  from /usr/include/xtensor/xtensor_config.hpp:61,
131s                  from /usr/include/xtensor/xexception.hpp:24,
131s                  from /usr/include/xtensor/xstorage.hpp:21,
131s                  from /usr/include/xtensor/xbuffer_adaptor.hpp:21,
131s                  from /usr/include/xtensor/xarray.hpp:19,
131s                  from /tmp/autopkgtest-lxc.1p570fd4/downtmp/autopkgtest_tmp/test_xoperation.cpp:13:
131s /usr/include/xsimd/types/xsimd_traits.hpp: In instantiation of ‘struct xsimd::detail::static_check_supported_config_emitter<bool, xsimd::neon64>’:
131s /usr/include/xsimd/types/xsimd_traits.hpp:84:19:   required from ‘void xsimd::detail::static_check_supported_config() [with T = bool; A = xsimd::neon64]’
131s /usr/include/xsimd/types/xsimd_api.hpp:437:55:   required from ‘xsimd::simd_return_type<From, To, A> xsimd::broadcast_as(From) [with To = int; A = neon64; From = bool; simd_return_type<From, To, A> = batch_bool<int, neon64>]’
131s /usr/include/xtensor/xscalar.hpp:952:53:   required from ‘xt_simd::simd_return_type<typename xt::xcontainer_inner_types<xt::xscalar<CT> >::value_type, requested_type> xt::xscalar<CT>::load_simd(size_type) const [with align = xt::inner_aligned_mode; requested_type = int; long unsigned int N = 4; CT = bool; xt_simd::simd_return_type<typename xt::xcontainer_inner_types<xt::xscalar<CT> >::value_type, requested_type> = xsimd::batch_bool<int, xsimd::neon64>; typename xt::xcontainer_inner_types<xt::xscalar<CT> >::value_type = bool; size_type = long unsigned int]’
...
131s /usr/include/xtensor/xarray.hpp:510:30:   required from ‘xt::xarray_container<EC, L, SC, Tag>::xarray_container(const xt::xexpression<E>&) [with E = xt::xfunction<xt::detail::logical_and, const xt::xarray_container<xt::uvector<bool, xsimd::aligned_allocator<bool, 16> >, xt::layout_type::row_major, xt::svector<long unsigned int, 4, std::allocator<long unsigned int>, true>, xt::xtensor_expression_tag>&, xt::xscalar<bool> >; EC = xt::uvector<bool, xsimd::aligned_allocator<bool, 16> >; xt::layout_type L = xt::layout_type::row_major; SC = xt::svector<long unsigned int, 4, std::allocator<long unsigned int>, true>; Tag = xt::xtensor_expression_tag]’
131s /tmp/autopkgtest-lxc.1p570fd4/downtmp/autopkgtest_tmp/test_xoperation.cpp:378:28:   required from ‘void xt::DOCTEST_ANON_SUITE_131::DOCTEST_ANON_TMP_150() [with TypeParam = xt::xarray_container<xt::uvector<double, xsimd::aligned_allocator<double, 16> >, xt::layout_type::row_major, xt::svector<long unsigned int, 4, std::allocator<long unsigned int>, true>, xt::xtensor_expression_tag>]’
131s /tmp/autopkgtest-lxc.1p570fd4/downtmp/autopkgtest_tmp/test_xoperation.cpp:372:9:   required from ‘xt::DOCTEST_ANON_SUITE_131::{anonymous}::DOCTEST_ANON_TMP_150ITERATOR<std::tuple<_El0, _El ...> >::DOCTEST_ANON_TMP_150ITERATOR(const char*, unsigned int, int) [with Type = xt::xarray_container<xt::uvector<double, xsimd::aligned_allocator<double, 16> >, xt::layout_type::row_major, xt::svector<long unsigned int, 4, std::allocator<long unsigned int>, true>, xt::xtensor_expression_tag>; Rest = {xt::xtensor_container<xt::uvector<double, xsimd::aligned_allocator<double, 16> >, 2, xt::layout_type::row_major, xt::xtensor_expression_tag>}]’
131s /tmp/autopkgtest-lxc.1p570fd4/downtmp/autopkgtest_tmp/test_xoperation.cpp:372:9:   required from here
131s /usr/include/xsimd/types/xsimd_traits.hpp:64:43: error: static assertion failed: usage of batch type with unsupported type
131s    64 |             static_assert(!A::supported() || xsimd::has_simd_register<T, A>::value,
131s       |                           ~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
131s /usr/include/xsimd/types/xsimd_traits.hpp:64:43: note: ‘((! xsimd::neon64::supported()) || ((bool)std::integral_constant<bool, false>::value))’ evaluates to false


Since xsimd passes it's own test, this might be a bug in xtensor.
Filing against libxsimd-dev to pause migration to testing.

xsimd upstream https://github.com/xtensor-stack/xsimd/issues/945
identified the problem that neon64 does not support bool.

A minimal test case using xtensor only was found:

#include <xtensor/xtensor.hpp>

int main()
{
    {
        xt::xtensor<double, 1> a{0., 1.};
        xt::xtensor<double, 1> b = a && 0.;
    }

    {
        xt::xtensor<bool, 1> a{false, true};
        //xt::xtensor<bool, 1> b = a && false;
    }

    return 0;
}


That contains the problem inside xtensor, so transferring this bug to
the xtensor package.

xsimd support is optional for xtensor.

Use of batch<bool> is apparently not expected to be routinely used
anyway. See further discussion at
https://github.com/xtensor-stack/xsimd/issues/945

In summary, xtensor can be run on arm64 (and armel) without xsimd,
and batch<bool> support should not be considered release-critical.

For these reasons I'm downgrading this bug to Severity: important.
It wouldn't be constructive to remove xtensor from testing.