cpp/test/mesh/generation.cpp: build_tet cell-vertex order doesn't match hand-written expected tables

[jhale]

Summary

cpp/test/mesh/generation.cpp has a CHECK_adjacency_list_equal test helper whose per-row structural comparison is commented out:

template <typename T>
void CHECK_adjacency_list_equal(
    const dolfinx::graph::AdjacencyList<T>& adj_list,
    const std::vector<std::vector<T>>& expected_list)
{
  REQUIRE(static_cast<std::size_t>(adj_list.num_nodes())
          == expected_list.size());

  // for (T i = 0; i < adj_list.num_nodes(); i++)
  // {
  //   CHECK_THAT(adj_list.links(i),
  //              Catch::Matchers::RangeEquals(expected_list[i]));
  // }
}

So every one of its ~9 call sites across the file (interval, triangle left/right/crossed diagonals, hexahedron, tetrahedron) currently only checks num_nodes() — the number of rows — never the actual vertex lists. The hand-written expected tables (e.g. the tet Kuhn-decomposition cell-to-vertex list in the "Box tetrahedron mesh" case) are dead text and give no structural coverage.

Reproduction

Re-enable the loop (with std::int32_t i instead of T i, which doesn't compile as written) and rebuild:

  for (std::int32_t i = 0; i < adj_list.num_nodes(); i++)
  {
    CHECK_THAT(adj_list.links(i),
               Catch::Matchers::RangeEquals(expected_list[i]));
  }

The "Box tetrahedron mesh" case then fails on the cell-to-vertex (connectivity(3, 0)) check, e.g.:

CHECK_THAT( adj_list.links(i), Catch::Matchers::RangeEquals(expected_list[i]) )
with expansion:
  { 0, 4, 1, 3 } elements are { 0, 1, 3, 4 }

CHECK_THAT( adj_list.links(i), Catch::Matchers::RangeEquals(expected_list[i]) )
with expansion:
  { 0, 5, 3, 2 } elements are { 0, 2, 5, 3 }

CHECK_THAT( adj_list.links(i), Catch::Matchers::RangeEquals(expected_list[i]) )
with expansion:
  { 0, 6, 4, 3 } elements are { 0, 4, 3, 6 }

CHECK_THAT( adj_list.links(i), Catch::Matchers::RangeEquals(expected_list[i]) )
with expansion:
  { 0, 7, 3, 5 } elements are { 0, 5, 7, 3 }

Each failure is a permutation of the same 4 vertices — never a wrong vertex set — for both float and double. The edge (connectivity(1,0)) and face (connectivity(2,0)) checks earlier in the same test case pass.

Why I didn't just fix it

Vertex order within a cell is geometrically meaningful (orientation), so this isn't obviously safe to paper over by rewriting the expected tables to match current output. But I also couldn't confirm it's a genuine build_tet bug, because:

• Topology construction re-numbers/re-orders cell vertices during distribution (even for a single-rank MPI_COMM_SELF mesh in this test), and
• no ordering contract is documented for connectivity(tdim, 0) anywhere I could find.

So this could be either:

1. A real bug in build_tet's Kuhn decomposition (cpp/dolfinx/mesh/generation.h), or
2. Expected-value tables that were written against raw build_tet output and never updated after a later, legitimate reordering elsewhere in the topology-construction pipeline, or
3. Working as intended, with the row order simply never having been a documented guarantee — in which case the test itself is asserting something that shouldn't be asserted, and should compare vertex sets per row (or an orientation-aware equality) rather than exact sequences.

Where this came from

Found while working through the cpp/dolfinx/mesh/ code review test-gap appendix (re-enabling this exact check was one of its proposed items, F13-9). Verified against commit 44891b763603cf2c6b038597b031f2cb29155ce1 on main — build_tet and this test file are unmodified relative to that commit, so the repro applies directly to main.