arena: preserve rank>0 zero-volume ranges; fix stale view comment

ArenaToTBuilder::emplace, given a zero-volume range, used to leave the
cell default/null. For an owning (non-view) inner that drops the range
metadata -- arena_outer_init keeps a rank>0 zero-volume range as an
empty-but-ranked tensor and only collapses a rank-0 range to null. Since
make_nested_tile now routes through the builder, mirror that handling so
a TA::Tensor inner with e.g. Range{0} stays an empty rank-1 tensor.
Arena view inners (which cannot carry a standalone range) still go null.
Adds a regression test.

Also drops a stale type_traits.h comment that listed TensorInterface as
an is_tensor_view specialization -- it is deliberately not a view.

Author: evaleev

src/TiledArray/tensor/arena_kernels.h

@@ -188,14 +188,23 @@ class ArenaToTBuilder {
 
   /// Size and bind the inner cell at outer cell ordinal `ord` to
   /// `inner_range`, returning a reference to the bound cell for the caller to
-  /// fill. A zero-volume range leaves the cell null. Outer element indices
-  /// translate via `outer_range().ordinal(idx)`.
+  /// fill. A zero-volume range yields an empty cell: an owning inner keeps a
+  /// rank>0 range (a rank-0 range stays null), a view inner stays null.
+  /// Outer element indices translate via `outer_range().ordinal(idx)`.
   inner_t& emplace(std::size_t ord, inner_range_t inner_range) {
     TA_ASSERT(ord < n_cells_);
     inner_t& cell = data_[ord];
-    const std::size_t vol = inner_range.volume();
-    if (vol == 0) return cell;  // stays null
     constexpr bool arena = is_arena_tensor_v<inner_t>;
+    const std::size_t vol = inner_range.volume();
+    if (vol == 0) {
+      // Mirror arena_outer_init: an owning (non-view) inner preserves a
+      // rank>0 zero-volume range as an empty-but-ranked tensor; a rank-0
+      // range -- and any arena view inner -- leaves the cell default/null.
+      if constexpr (!arena) {
+        if (inner_range.rank() != 0) cell = inner_t(std::move(inner_range));
+      }
+      return cell;
+    }
     std::size_t stride;
     std::size_t bytes;
     if constexpr (arena) {

src/TiledArray/tensor/type_traits.h

@@ -120,9 +120,11 @@ inline constexpr const bool is_nested_tensor_v = is_nested_tensor<Ts...>::value;
 }  // namespace detail
 
 /// Forward decl for the tensor-view predicate. Specializations live in
-/// `tensor/arena_tensor.h` (`ArenaTensor`, `detail::TensorInterface`) and
-/// `external/btas.h` (`btas::TensorView`). Declared here so the operator-body
-/// predicates below can consult it without including arena_tensor.h.
+/// `tensor/arena_tensor.h` (`ArenaTensor`) and `external/btas.h`
+/// (`btas::TensorView`). Declared here so the operator-body predicates below
+/// can consult it without including arena_tensor.h. Note `TensorInterface` /
+/// `TensorMap` is deliberately *not* a view here -- it has value-returning
+/// member arithmetic (see arena_tensor.h).
 template <typename T>
 struct is_tensor_view : std::false_type {};
 template <typename T>

tests/arena_kernels.cpp

@@ -199,6 +199,21 @@ BOOST_AUTO_TEST_CASE(builder_single_cell_uses_one_exact_page) {
     BOOST_CHECK_EQUAL(built.data()->at_ordinal(i), double(i));
 }
 
+BOOST_AUTO_TEST_CASE(builder_zero_volume_nonscalar_range_keeps_rank) {
+  // an owning inner given a zero-volume but rank>0 range keeps that range
+  // (mirrors arena_outer_init): the rank-1 range is preserved rather than
+  // collapsed to a rank-0 null cell
+  TA::detail::ArenaToTBuilder<outer_t> b(TA::Range{2l});
+  inner_t& c0 = b.emplace(0, TA::Range{0l});  // rank 1, extent 0, volume 0
+  inner_t& c1 = b.emplace(1, TA::Range{3l});
+  BOOST_CHECK_EQUAL(c0.range().rank(), 1u);
+  BOOST_CHECK_EQUAL(c0.range().volume(), 0u);
+  BOOST_CHECK(!c1.empty());
+  outer_t built = std::move(b).finish();
+  BOOST_CHECK_EQUAL(built.data()[0].range().rank(), 1u);
+  BOOST_CHECK_EQUAL(built.data()[1].range().volume(), 3u);
+}
+
 BOOST_AUTO_TEST_CASE(compact_coalesces_a_multipage_tile) {
   const std::size_t N = 16;
   TA::detail::ArenaToTBuilder<outer_t> b(TA::Range{static_cast<long>(N)}, 1,