Implement LWG-4266: layout_stride::mapping should treat empty mappings as exhaustive (#5880)

JMazurkiewicz · StephanTLavavej · web-flow · commit 5459853bf78c · 2025-11-19T14:25:40.000-08:00
Co-authored-by: Stephan T. Lavavej &lt;stl@microsoft.com&gt;
diff --git a/stl/inc/mdspan b/stl/inc/mdspan
@@ -316,7 +316,7 @@ public:
     }
 
     template <class _ExtentsT>
-    friend constexpr pair<size_t, size_t> _Count_dynamic_extents_equal_to_zero_or_one(
+    friend constexpr size_t _Count_dynamic_extents_equal_to_one(
         const _ExtentsT&) noexcept; // NB: used by 'layout_stride::mapping<E>::is_exhaustive'
 };
 
@@ -797,26 +797,10 @@ concept _Layout_mapping_alike = requires {
     bool_constant<_Mp::is_always_unique()>::value;
 };
 
-template <class _Extents, size_t _Val>
-constexpr size_t _Count_static_extents_equal_to = 0;
-
-template <class _IndexType, size_t... _Extents, size_t _Val>
-constexpr size_t _Count_static_extents_equal_to<extents<_IndexType, _Extents...>, _Val> =
-    (static_cast<size_t>(_Extents == _Val) + ... + 0);
-
 template <class _Extents>
-_NODISCARD constexpr pair<size_t, size_t> _Count_dynamic_extents_equal_to_zero_or_one(const _Extents& _Exts) noexcept {
+_NODISCARD constexpr size_t _Count_dynamic_extents_equal_to_one(const _Extents& _Exts) noexcept {
     _STL_INTERNAL_STATIC_ASSERT(_Is_extents<_Extents> && _Extents::rank_dynamic() != 0);
-    size_t _Zero_extents = 0;
-    size_t _One_extents  = 0;
-    for (const auto& _Ext : _Exts._Array) {
-        if (_Ext == 0) {
-            ++_Zero_extents;
-        } else if (_Ext == 1) {
-            ++_One_extents;
-        }
-    }
-    return {_Zero_extents, _One_extents};
+    return static_cast<size_t>(_RANGES count(_Exts._Array, static_cast<_Extents::index_type>(1)));
 }
 
 template <class _Extents>
@@ -833,7 +817,7 @@ public:
 private:
     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>;
+    using _Stride_extent_pair = pair<index_type, index_type>;
 
     static_assert(_Is_extents<extents_type>,
         "Extents must be a specialization of std::extents (N4950 [mdspan.layout.stride.overview]/2).");
@@ -869,7 +853,7 @@ private:
                              || _Add_overflow(_Req_span_size, _Prod, _Req_span_size);
                 }
 
-                _Pairs[_Idx] = {static_cast<rank_type>(_Stride), static_cast<rank_type>(_Ext)};
+                _Pairs[_Idx] = {_Stride, _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).");
@@ -998,7 +982,7 @@ public:
     }
 
     _NODISCARD static constexpr bool is_always_exhaustive() noexcept {
-        return false;
+        return extents_type::_Rank == 0 || extents_type::_Multidim_index_space_size_is_always_zero;
     }
 
     _NODISCARD static constexpr bool is_always_strided() noexcept {
@@ -1010,53 +994,28 @@ public:
     }
 
     _NODISCARD constexpr bool is_exhaustive() const noexcept {
-        constexpr size_t _Static_zero_extents = _Count_static_extents_equal_to<extents_type, 0>;
-        if constexpr (extents_type::rank() == 0) {
+        if constexpr (is_always_exhaustive()) {
             return true;
         } else if constexpr (extents_type::rank() == 1) {
-            return this->_Array[0] == 1;
-        } else if constexpr (_Static_zero_extents >= 2) {
-            // Per N5008 [mdspan.layout.stride.obs]/5.2, we are looking for a permutation P of integers in the range
-            // '[0, rank)' such that 'stride(p[i]) == stride(p[i-1])*extent(p[i-1])' is true for 'i' in the range
-            // '[1, rank)'. Knowing that at least two extents are equal to zero, we can deduce that such a permutation
-            // does not exist:
-            // - Some 'stride(p[j])' would have to be equal to 'stride(p[j-1])*extents(p[j-1]) = stride(p[j-1])*0 = 0'
-            //   which is not possible.
-            // - Only 'extent(p[rank-1])' can be equal to 0, because it's not required to satisfy the condition above.
-            //   Since we have two or more extents equal to 0 this is not possible either.
-            return false;
+            return this->_Array[0] == 1 || this->_Exts.extent(0) == 0;
         } else if constexpr (extents_type::rank() == 2) {
             return (this->_Array[0] == 1 && this->_Array[1] == this->_Exts.extent(0))
-                || (this->_Array[1] == 1 && this->_Array[0] == this->_Exts.extent(1));
-        } else {
-            // NB: Extents equal to 1 are problematic too - sometimes in such cases even when the mapping is exhaustive
+                || (this->_Array[1] == 1 && this->_Array[0] == this->_Exts.extent(1)) || this->_Exts.extent(0) == 0
+                || this->_Exts.extent(1) == 0;
+        } else if constexpr (_RANGES count(extents_type::_Static_extents, static_cast<rank_type>(1)) != 0) {
+            // NB: Extents equal to 1 are problematic - sometimes in such cases even when the mapping is exhaustive
             // this function should return false.
             // For example, when the extents are [2, 1, 2] and the strides are [1, 5, 2], the mapping is exhaustive
             // per N5008 [mdspan.layout.reqmts]/16 but not per N5008 [mdspan.layout.stride.obs]/5.2.
-            constexpr size_t _Static_zero_or_one_extents =
-                _Static_zero_extents + _Count_static_extents_equal_to<extents_type, 1>;
-
-            if constexpr (extents_type::rank_dynamic() != 0) {
-                const auto [_Dynamic_zero_extents, _Dynamic_one_extents] =
-                    _Count_dynamic_extents_equal_to_zero_or_one(this->_Exts);
-
-                const size_t _All_zero_extents = _Static_zero_extents + _Dynamic_zero_extents;
-                if (_All_zero_extents >= 2) {
-                    return false;
-                }
-
-                const size_t _All_zero_or_one_extents =
-                    _Static_zero_or_one_extents + _Dynamic_zero_extents + _Dynamic_one_extents;
-                if (_All_zero_or_one_extents == 0) {
-                    return _Is_exhaustive_common_case();
-                }
-
-                return _Is_exhaustive_special_case();
-            } else if constexpr (_Static_zero_or_one_extents == 0) {
-                return _Is_exhaustive_common_case();
-            } else {
+            return _Is_exhaustive_special_case();
+        } else if constexpr (extents_type::rank_dynamic() == 0) {
+            return _Is_exhaustive_common_case();
+        } else {
+            if (_Count_dynamic_extents_equal_to_one(this->_Exts) != 0) {
                 return _Is_exhaustive_special_case();
             }
+
+            return _Is_exhaustive_common_case();
         }
     }
 
@@ -1127,20 +1086,19 @@ private:
     }
 
     _NODISCARD constexpr bool _Is_exhaustive_common_case() const noexcept {
-        return required_span_size() == _Fwd_prod_of_extents<extents_type>::_Calculate(this->_Exts, extents_type::_Rank);
+        const index_type _Prod = _Fwd_prod_of_extents<extents_type>::_Calculate(this->_Exts, extents_type::_Rank);
+        return _Prod == required_span_size() || _Prod == 0;
     }
 
     _NODISCARD constexpr bool _Is_exhaustive_special_case() const noexcept {
         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));
+            const index_type _Ext = this->_Exts.extent(_Idx);
             if (_Ext == 0) {
-                // NB: _Ext equal to zero is special - we want it to end up as close to the end of the sorted range as
-                // possible, so we assign max value of rank_type to it.
-                _Ext = static_cast<rank_type>(-1);
+                return true;
             }
 
-            _Pairs[_Idx] = {static_cast<rank_type>(this->_Array[_Idx]), _Ext};
+            _Pairs[_Idx] = {this->_Array[_Idx], _Ext};
         }
 
         _RANGES sort(_Pairs);
diff --git a/tests/libcxx/expected_results.txt b/tests/libcxx/expected_results.txt
@@ -181,12 +181,13 @@ std/ranges/range.adaptors/range.lazy.split/range.lazy.split.outer.value/ctor.ite
 # libc++ doesn't implement LWG-4112
 std/ranges/range.adaptors/range.join/range.join.iterator/arrow.pass.cpp FAIL
 
+# libc++ doesn't implement LWG-4266 "`layout_stride::mapping` should treat empty mappings as exhaustive"
+std/containers/views/mdspan/layout_stride/is_exhaustive_corner_case.pass.cpp FAIL
+std/containers/views/mdspan/layout_stride/properties.pass.cpp FAIL
+
 # If any feature-test macro test is failing, this consolidated test will also fail.
 std/language.support/support.limits/support.limits.general/version.version.compile.pass.cpp FAIL
 
-# libc++ incorrectly implements `layout_stride::mapping<E>::is_exhaustive()`
-std/containers/views/mdspan/layout_stride/is_exhaustive_corner_case.pass.cpp FAIL
-
 
 # *** INTERACTIONS WITH MSVC THAT UPSTREAM LIKELY WON'T FIX ***
 # These tests set an allocator with a max_size() too small to default construct an unordered container
diff --git a/tests/std/tests/P0009R18_mdspan_layout_stride/test.cpp b/tests/std/tests/P0009R18_mdspan_layout_stride/test.cpp
@@ -199,7 +199,7 @@ constexpr void do_check_members(const extents<IndexType, Extents...>& ext,
 
     { // Check 'is_always_[unique/exhaustive/strided]' functions
         static_assert(Mapping::is_always_unique());
-        static_assert(!Mapping::is_always_exhaustive());
+        static_assert(Mapping::is_always_exhaustive() == (Ext::rank() == 0 || ((Extents == 0) || ...)));
         static_assert(Mapping::is_always_strided());
     }
 
@@ -410,7 +410,7 @@ constexpr void check_is_exhaustive() {
 
     // rank() is equal to 1
     check(extents<int, 0>{}, array{1}, true);
-    check(dextents<int, 1>{0}, array{2}, false);
+    check(dextents<int, 1>{0}, array{2}, true);
     check(extents<int, 1>{}, array{3}, false);
     check(dextents<int, 1>{2}, array{2}, false);
     check(extents<int, 3>{}, array{1}, true);
@@ -426,14 +426,14 @@ constexpr void check_is_exhaustive() {
     check(extents<int, dynamic_extent, 7>{5}, array{1, 8}, false);
     check(dextents<int, 2>{6, 5}, array{1, 10}, false);
     check(extents<int, 0, 3>{}, array{3, 1}, true);
-    check(extents<int, 0, 3>{}, array{6, 2}, false);
-    check(extents<int, dynamic_extent, 3>{0}, array{6, 1}, false);
-    check(extents<int, 0, dynamic_extent>{3}, array{6, 2}, false);
-    check(dextents<int, 2>{0, 3}, array{7, 2}, false);
-    check(extents<int, 0, 0>{}, array{1, 1}, false);
-    check(extents<int, 0, dynamic_extent>{0, 0}, array{1, 1}, false);
-    check(dextents<int, 2>{0, 0}, array{1, 2}, false);
-    check(extents<int, 1, dynamic_extent>{0}, array{1, 2}, false);
+    check(extents<int, 0, 3>{}, array{6, 2}, true);
+    check(extents<int, dynamic_extent, 3>{0}, array{6, 1}, true);
+    check(extents<int, 0, dynamic_extent>{3}, array{6, 2}, true);
+    check(dextents<int, 2>{0, 3}, array{7, 2}, true);
+    check(extents<int, 0, 0>{}, array{1, 1}, true);
+    check(extents<int, 0, dynamic_extent>{0, 0}, array{1, 1}, true);
+    check(dextents<int, 2>{0, 0}, array{1, 2}, true);
+    check(extents<int, 1, dynamic_extent>{0}, array{1, 2}, true);
 
     // rank() is greater than 2
     check(extents<int, 2, 3, 5>{}, array{1, 2, 6}, true);
@@ -449,20 +449,20 @@ constexpr void check_is_exhaustive() {
     // rank() is greater than 2 and some extents are equal to 0
     check(extents<int, 2, 0, 7>{}, array{7, 14, 1}, true);
     check(extents<int, dynamic_extent, 0, 7>{2}, array{1, 14, 2}, true);
-    check(extents<int, 2, dynamic_extent, 7>{0}, array{14, 28, 1}, false);
-    check(extents<int, 2, dynamic_extent, dynamic_extent>{0, 7}, array{1, 2, 2}, false);
-    check(dextents<int, 3>{2, 0, 7}, array{2, 28, 4}, false);
-    check(extents<int, 5, 0, 0>{}, array{3, 1, 1}, false);
-    check(extents<int, 5, dynamic_extent, 0>{0}, array{1, 5, 1}, false);
-    check(dextents<int, 3>{5, 0, 0}, array{2, 1, 10}, false);
-    check(extents<int, 0, 0, 0>{}, array{1, 1, 1}, false);
+    check(extents<int, 2, dynamic_extent, 7>{0}, array{14, 28, 1}, true);
+    check(extents<int, 2, dynamic_extent, dynamic_extent>{0, 7}, array{1, 2, 2}, true);
+    check(dextents<int, 3>{2, 0, 7}, array{2, 28, 4}, true);
+    check(extents<int, 5, 0, 0>{}, array{3, 1, 1}, true);
+    check(extents<int, 5, dynamic_extent, 0>{0}, array{1, 5, 1}, true);
+    check(dextents<int, 3>{5, 0, 0}, array{2, 1, 10}, true);
+    check(extents<int, 0, 0, 0>{}, array{1, 1, 1}, true);
     check(extents<int, 0, 1, 1>{}, array{1, 1, 1}, true);
 
     // rank() is greater than 2 - one extent is equal to 0 while others are equal to each other
     check(extents<int, 3, 0, 3>{}, array{1, 9, 3}, true);
     check(extents<int, dynamic_extent, 0, 3>{3}, array{3, 9, 1}, true);
-    check(extents<int, 3, dynamic_extent, dynamic_extent>{0, 3}, array{1, 3, 3}, false);
-    check(dextents<int, 3>{3, 0, 3}, array{1, 4, 8}, false);
+    check(extents<int, 3, dynamic_extent, dynamic_extent>{0, 3}, array{1, 3, 3}, true);
+    check(dextents<int, 3>{3, 0, 3}, array{1, 4, 8}, true);
     check(dextents<int, 3>{0, 1, 1}, array{1, 1, 1}, true);
 
     // required_span_size() is equal to 1
