<mdspan>: Implement missing precondition in layout_stride::mapping<E>'s constructor (#5505)

JMazurkiewicz · web-flow · commit ad4bf41e9395 · 2025-05-16T21:55:52.000-07:00
diff --git a/stl/inc/mdspan b/stl/inc/mdspan
@@ -826,8 +826,9 @@ public:
     using layout_type  = layout_stride;
 
 private:
-    using _Extents_base = _Maybe_fully_static_extents<extents_type>;
-    using _Strides_base = _Maybe_empty_array<index_type, _Extents::rank()>;
+    using _Extents_base       = _Maybe_fully_static_extents<extents_type>;
+    using _Strides_base       = _Maybe_empty_array<index_type, _Extents::rank()>;
+    using _Stride_extent_pair = pair<rank_type, rank_type>;
 
     static_assert(_Is_extents<extents_type>,
         "Extents must be a specialization of std::extents (N4950 [mdspan.layout.stride.overview]/2).");
@@ -847,6 +848,7 @@ private:
             bool _Found_zero          = false;
             bool _Overflow            = false;
             index_type _Req_span_size = 0;
+            array<_Stride_extent_pair, extents_type::rank()> _Pairs;
             for (rank_type _Idx = 0; _Idx < extents_type::_Rank; ++_Idx) {
                 const index_type _Stride = this->_Array[_Idx];
                 _STL_VERIFY(_Stride > 0, "Value of s[i] must be greater than 0 for all i in the range [0, rank_) "
@@ -861,9 +863,22 @@ private:
                     _Overflow = _Mul_overflow(static_cast<index_type>(_Ext - 1), _Stride, _Prod)
                              || _Add_overflow(_Req_span_size, _Prod, _Req_span_size);
                 }
+
+                _Pairs[_Idx] = {static_cast<rank_type>(_Stride), static_cast<rank_type>(_Ext)};
             }
             _STL_VERIFY(_Found_zero || !_Overflow, "REQUIRED-SPAN-SIZE(e, s) must be representable as a value of type "
                                                    "index_type (N4950 [mdspan.layout.stride.cons]/4.2).");
+
+            if (!_Found_zero) {
+                _RANGES sort(_Pairs);
+                for (rank_type _Idx = 1; _Idx < extents_type::_Rank; ++_Idx) {
+                    _STL_VERIFY(_Pairs[_Idx].first >= _Pairs[_Idx - 1].first * _Pairs[_Idx - 1].second,
+                        "If rank_ is greater than 0, then there must exist a permutation P of the integers in the "
+                        "range [0, rank_), such that 's[p(i)] >= s[p(i-1)] * e.extent(p(i-1))' is true for all i in "
+                        "the range [1, rank_), where p(i) is the i-th element of P (N5008 "
+                        "[mdspan.layout.stride.cons]/4.3).");
+                }
+            }
         }
 #endif // _ITERATOR_DEBUG_LEVEL != 0
     }
@@ -1111,7 +1126,6 @@ private:
     }
 
     _NODISCARD constexpr bool _Is_exhaustive_special_case() const noexcept {
-        using _Stride_extent_pair = pair<rank_type, rank_type>;
         array<_Stride_extent_pair, extents_type::rank()> _Pairs;
         for (rank_type _Idx = 0; _Idx < extents_type::_Rank; ++_Idx) {
             rank_type _Ext = static_cast<rank_type>(this->_Exts.extent(_Idx));
diff --git a/tests/std/tests/P0009R18_mdspan_layout_stride/test.cpp b/tests/std/tests/P0009R18_mdspan_layout_stride/test.cpp
@@ -386,7 +386,7 @@ constexpr void check_required_span_size() {
         using M1 = layout_stride::mapping<extents<int, 3, 3, 0, 3>>;
         static_assert(M1{}.required_span_size() == 0);
 
-        layout_stride::mapping<dextents<int, 4>> m2{dextents<int, 4>{3, 0, 3, 3}, array{1, 3, 1, 1}};
+        layout_stride::mapping<dextents<int, 4>> m2{dextents<int, 4>{3, 0, 3, 3}, array{1, 3, 1, 3}};
         assert(m2.required_span_size() == 0);
     }
 
@@ -800,37 +800,37 @@ constexpr void check_correctness() {
 
     { // 3x2x2 tensor
         using E = extents<int, 3, 2, 2>;
-        const array vals{0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23};
-        layout_stride::mapping<E> m{E{}, array{8, 1, 6}}; // non-exhaustive mapping
+        const array vals{0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18};
+        layout_stride::mapping<E> m{E{}, array{7, 1, 3}}; // non-exhaustive mapping
         assert(!m.is_exhaustive());
         mdspan<const int, E, layout_stride> tensor{vals.data(), m};
 
 #ifdef __cpp_multidimensional_subscript // TRANSITION, P2128R6
         assert((tensor[0, 0, 0] == 0));
-        assert((tensor[0, 0, 1] == 6));
+        assert((tensor[0, 0, 1] == 3));
         assert((tensor[0, 1, 0] == 1));
-        assert((tensor[0, 1, 1] == 7));
-        assert((tensor[1, 0, 0] == 8));
-        assert((tensor[1, 0, 1] == 14));
-        assert((tensor[1, 1, 0] == 9));
-        assert((tensor[1, 1, 1] == 15));
-        assert((tensor[2, 0, 0] == 16));
-        assert((tensor[2, 0, 1] == 22));
-        assert((tensor[2, 1, 0] == 17));
-        assert((tensor[2, 1, 1] == 23));
+        assert((tensor[0, 1, 1] == 4));
+        assert((tensor[1, 0, 0] == 7));
+        assert((tensor[1, 0, 1] == 10));
+        assert((tensor[1, 1, 0] == 8));
+        assert((tensor[1, 1, 1] == 11));
+        assert((tensor[2, 0, 0] == 14));
+        assert((tensor[2, 0, 1] == 17));
+        assert((tensor[2, 1, 0] == 15));
+        assert((tensor[2, 1, 1] == 18));
 #else // ^^^ defined(__cpp_multidimensional_subscript) / !defined(__cpp_multidimensional_subscript) vvv
         assert((tensor[array{0, 0, 0}] == 0));
-        assert((tensor[array{0, 0, 1}] == 6));
+        assert((tensor[array{0, 0, 1}] == 3));
         assert((tensor[array{0, 1, 0}] == 1));
-        assert((tensor[array{0, 1, 1}] == 7));
-        assert((tensor[array{1, 0, 0}] == 8));
-        assert((tensor[array{1, 0, 1}] == 14));
-        assert((tensor[array{1, 1, 0}] == 9));
-        assert((tensor[array{1, 1, 1}] == 15));
-        assert((tensor[array{2, 0, 0}] == 16));
-        assert((tensor[array{2, 0, 1}] == 22));
-        assert((tensor[array{2, 1, 0}] == 17));
-        assert((tensor[array{2, 1, 1}] == 23));
+        assert((tensor[array{0, 1, 1}] == 4));
+        assert((tensor[array{1, 0, 0}] == 7));
+        assert((tensor[array{1, 0, 1}] == 10));
+        assert((tensor[array{1, 1, 0}] == 8));
+        assert((tensor[array{1, 1, 1}] == 11));
+        assert((tensor[array{2, 0, 0}] == 14));
+        assert((tensor[array{2, 0, 1}] == 17));
+        assert((tensor[array{2, 1, 0}] == 15));
+        assert((tensor[array{2, 1, 1}] == 18));
 #endif // ^^^ !defined(__cpp_multidimensional_subscript) ^^^
     }
 
diff --git a/tests/std/tests/P0009R18_mdspan_layout_stride_death/test.cpp b/tests/std/tests/P0009R18_mdspan_layout_stride_death/test.cpp
@@ -29,6 +29,12 @@ void test_construction_from_extents_and_array_2() {
     [[maybe_unused]] layout_stride::mapping<Ext> m{Ext{}, array<int, 1>{2}};
 }
 
+void test_construction_from_extents_and_array_3() {
+    using Ext = extents<short, 2, 3, 5>;
+    const array<short, 3> a{29, 2, 6};
+    // Incorrect strides
+    [[maybe_unused]] layout_stride::mapping<Ext> m{Ext{}, a};
+}
 
 void test_construction_from_extents_and_span_1() {
     array<int, 1> a{-1};
@@ -44,6 +50,14 @@ void test_construction_from_extents_and_span_2() {
     [[maybe_unused]] layout_stride::mapping<Ext> m{Ext{}, s};
 }
 
+void test_construction_from_extents_and_span_3() {
+    using Ext = extents<unsigned char, 3, 3, 1, 1>;
+    array<long long, 4> a{3, 1, 8, 3};
+    const span s{a};
+    // Incorrect strides
+    [[maybe_unused]] layout_stride::mapping<Ext> m{Ext{}, s};
+}
+
 void test_construction_from_strided_layout_mapping() {
     layout_right::mapping<extents<int, 256>> m1;
     // Value of other.required_span_size() must be representable as a value of type index_type
@@ -73,8 +87,10 @@ int main(int argc, char* argv[]) {
         test_default_construction,
         test_construction_from_extents_and_array_1,
         test_construction_from_extents_and_array_2,
+        test_construction_from_extents_and_array_3,
         test_construction_from_extents_and_span_1,
         test_construction_from_extents_and_span_2,
+        test_construction_from_extents_and_span_3,
         test_construction_from_strided_layout_mapping,
         test_call_operator,
         test_stride_with_empty_extents,
