[gouarin]

Checklist

• The title and commit message(s) are descriptive.
• Small commits made to fix your PR have been squashed to avoid history pollution.
• Tests have been added for new features or bug fixes.
• API of new functions and classes are documented (no public API changes).

Description

Like containers, builder expressions should accept fewer indices than dimensions (missing leading indices are 0) or more (extra leading ones are ignored). They did not:

xt::xarray<int> a = {{1, 2, 3}, {4, 5, 6}};
auto h = xt::hstack(xt::xtuple(a, a));
h(4);  // reads before the start of the index array

concatenate_access computed distance(first, last) - dimension() in std::size_t, which wraps around when there are fewer indices than dimensions: concatenate returned garbage and hstack crashed with a segmentation fault. stack_access ignored the number of indices, so stack and 1-D vstack returned wrong values in both directions.

Both now go through a detail::trailing_index helper that aligns the indices on the last dimensions, as containers do. This also removes the -Wsign-conversion warnings in xbuilder.hpp.

xbuilder.concatenate_stack_index_count checks concatenate, hstack, stack and vstack with fewer and more indices against the evaluated result. It fails on master and passes with the fix, also under AddressSanitizer and UBSan.

[codspeed]

Merging this PR will not alter performance

1 warning

⚠️ Unable to measure the execution of 1 benchmark

This benchmark measured no execution time, so its result is not comparable and counts as unchanged. This usually happens when the compiler removes the code under test.

Prevent compiler optimizations

See affected benchmarks

✅ 255 untouched benchmarks

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_{Comparing gouarin:fix-builder-index-count (4026ec1) with master (57f3d07)}