Avoiding temporary fails with xt::concatenate #2819

When using -DXTENSOR_FORCE_TEMPORARY_MEMORY_IN_ASSIGNMENTS=Off, the following simple test fails:

    TEST(xbuilder, concatenate_assign_to_argument)
    {
        xt::xarray<int> a = xt::ones<int>({ 4 });
        xt::xarray<int> b = xt::ones<int>({ 3 });

        a = xt::concatenate(xt::xtuple(a, b));
        ASSERT_EQ(a, xt::ones<int>({7}));
    }

Instead of containing 7 ones, a will contain garbage.

I have searched for the root cause of the issue, and found the following:

• Changing overlapping_memory_checker_traits for xgenerator, so check_overlap returns true instead of false, solves the issue. The change is in line 90 of xgenerator.hpp.

• The problem occurs because of an assumption that an xgenerator always generates fresh values using some generator function. In this case, memory overlap indeed never occurs.

• xt::concatenate creates an xgenerator that reads values from the arrays in its input tuple, which may overlap with the output array.

The following happens in the test:

1. xgenerator::assign_to resizes a, so it can hold seven elements. It re-allocates memory for a, without initializing the elements.
2. The function in the xgenerator, which has type concatenate_access, still wants to concatenate a and b. It uses the new value for a.
3. Since a now has length 7, concatenate_access::access only uses values from a. It thus copies the uninitialized values from a to a.

Although patching xgenerator works around the issue, a proper fix would be calling check_overlap on the functor inside the xgenerator, and then calling check_overlap on the tuples inside the functor, in case the functor holds a tuple.