Check preconditions of mdspan::operator[] clarified by LWG-3974

microsoft · Nov 22, 2023 · a82957c · a82957c
1 parent 730af17
commit a82957c
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diff --git a/stl/inc/mdspan b/stl/inc/mdspan
@@ -1131,6 +1131,22 @@ struct _Mdspan_accessor_base<_AccessorPolicy> {
     static constexpr _AccessorPolicy _Acc{};
 };
 
+#if _CONTAINER_DEBUG_LEVEL > 0
+template <class _IndexType, class _OtherIndexType>
+_NODISCARD constexpr _IndexType _Mdspan_checked_index_cast(_OtherIndexType&& _Idx) noexcept(
+    is_nothrow_constructible_v<_IndexType, _OtherIndexType>) {
+    _STL_INTERNAL_STATIC_ASSERT(is_integral_v<_IndexType> && is_constructible_v<_IndexType, _OtherIndexType>);
+
+    using _Arg_value_t = remove_cvref_t<_OtherIndexType>;
+    if constexpr (is_integral_v<_Arg_value_t> && !is_same_v<_Arg_value_t, bool>) {
+        _STL_VERIFY(_STD in_range<_IndexType>(_Idx),
+            "The index must be representable by index_type in order to be in a valid "
+            "multidimensional index (N4964 [mdspan.mdspan.members]/3).");
+    }
+    return static_cast<_IndexType>(_STD forward<_OtherIndexType>(_Idx));
+}
+#endif // _CONTAINER_DEBUG_LEVEL > 0
+
 _EXPORT_STD template <class _ElementType, class _Extents, class _LayoutPolicy = layout_right,
     class _AccessorPolicy = default_accessor<_ElementType>>
 class __declspec(empty_bases) mdspan : private _Mdspan_mapping_base<_Extents, _LayoutPolicy>,
@@ -1278,7 +1294,11 @@ public:
               && (sizeof...(_OtherIndexTypes) == rank())
     _NODISCARD constexpr reference operator[](_OtherIndexTypes... _Indices) const
         noexcept(noexcept(_Access_impl(static_cast<index_type>(_STD move(_Indices))...))) /* strengthened */ {
+#if _CONTAINER_DEBUG_LEVEL > 0
+        return _Access_impl(_STD _Mdspan_checked_index_cast<index_type>(_STD move(_Indices))...);
+#else // ^^^ _CONTAINER_DEBUG_LEVEL > 0 / _CONTAINER_DEBUG_LEVEL <= 0 vvv
         return _Access_impl(static_cast<index_type>(_STD move(_Indices))...);
+#endif // ^^^ _CONTAINER_DEBUG_LEVEL <= 0 ^^^
     }
 #endif // ^^^ defined(__cpp_multidimensional_subscript) ^^^
 
@@ -1287,7 +1307,11 @@ private:
     _NODISCARD constexpr reference _Multidimensional_subscript(
         span<_OtherIndexType, rank()> _Indices, index_sequence<_Seq...>) const
         noexcept(noexcept(_Access_impl(static_cast<index_type>(_STD as_const(_Indices[_Seq]))...))) {
+#if _CONTAINER_DEBUG_LEVEL > 0
+        return _Access_impl(_STD _Mdspan_checked_index_cast<index_type>(_STD as_const(_Indices[_Seq]))...);
+#else // ^^^ _CONTAINER_DEBUG_LEVEL > 0 / _CONTAINER_DEBUG_LEVEL <= 0 vvv
         return _Access_impl(static_cast<index_type>(_STD as_const(_Indices[_Seq]))...);
+#endif // ^^^ _CONTAINER_DEBUG_LEVEL <= 0 ^^^
     }
 
 public:

diff --git a/tests/std/tests/P0009R18_mdspan_mdspan_death/test.cpp b/tests/std/tests/P0009R18_mdspan_mdspan_death/test.cpp
@@ -25,6 +25,11 @@ void test_access_with_invalid_multidimensional_index_1() {
     // I must be a multidimensional index in extents()
     (void) mds[3, 4];
 }
+
+void test_access_with_nonrepresentable_index_1() {
+    mdspan mds{some_ints.data(), dextents<unsigned char, 2>{2, 3}};
+    (void) mds[256u, -255];
+}
 #endif // __cpp_multidimensional_subscript
 
 void test_access_with_invalid_multidimensional_index_2() {
@@ -33,6 +38,11 @@ void test_access_with_invalid_multidimensional_index_2() {
     (void) mds[array{4, 5}];
 }
 
+void test_access_with_nonrepresentable_index_2() {
+    mdspan mds{some_ints.data(), dextents<unsigned char, 2>{2, 3}};
+    (void) mds[array{256, -255}];
+}
+
 void test_size_when_index_type_is_signed() {
     mdspan mds{some_ints.data(), dextents<signed char, 3>{8, 8, 4}};
     // The size of the multidimensional index space extents() must be representable as a value of type size_type
@@ -51,8 +61,10 @@ int main(int argc, char* argv[]) {
         test_construction_from_other_mdspan,
 #ifdef __cpp_multidimensional_subscript // TRANSITION, P2128R6
         test_access_with_invalid_multidimensional_index_1,
+        test_access_with_nonrepresentable_index_1,
 #endif // __cpp_multidimensional_subscript
         test_access_with_invalid_multidimensional_index_2,
+        test_access_with_nonrepresentable_index_2,
         test_size_when_index_type_is_signed,
         test_size_when_index_type_is_unsigned,
     });