`<mdspan>`: Fixes signed overflow in `_Fwd_prod_of_extents` (#4037)

Co-authored-by: Stephan T. Lavavej <stl@nuwen.net>

stl/inc/mdspan

@@ -348,11 +348,13 @@ public:
         if constexpr (_Extents::rank() == 0) {
             return 1;
         } else {
-            typename _Extents::index_type _Result = 1;
+            // Use an unsigned type to prevent overflow when called from mdspan::size.
+            typename _Extents::size_type _Result = 1;
             for (size_t _Dim = 0; _Dim < _Idx; ++_Dim) {
-                _Result *= _Exts.extent(_Dim);
+                _Result *= static_cast<_Extents::size_type>(_Exts.extent(_Dim));
             }
-            return _Result;
+
+            return static_cast<_Extents::index_type>(_Result);
         }
     }
 };

tests/std/test.lst

@@ -231,6 +231,7 @@ tests\GH_003676_format_large_hh_mm_ss_values
 tests\GH_003735_char_traits_signatures
 tests\GH_003840_tellg_when_reading_lf_file_in_text_mode
 tests\GH_003867_output_nan
+tests\GH_004023_mdspan_fwd_prod_overflow
 tests\LWG2381_num_get_floating_point
 tests\LWG2597_complex_branch_cut
 tests\LWG3018_shared_ptr_function

tests/std/tests/GH_004023_mdspan_fwd_prod_overflow/env.lst

@@ -0,0 +1,4 @@
+# Copyright (c) Microsoft Corporation.
+# SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+RUNALL_INCLUDE ..\concepts_latest_matrix.lst

tests/std/tests/GH_004023_mdspan_fwd_prod_overflow/test.compile.pass.cpp

@@ -0,0 +1,79 @@
+// Copyright (c) Microsoft Corporation.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#include <cassert>
+#include <cstdint>
+#include <mdspan>
+#include <type_traits>
+
+using namespace std;
+
+struct layout_packed_upper { // LAPACK packed storage, VERY INCOMPLETE
+    template <class Extents>
+    class mapping {
+    public:
+        using extents_type = Extents;
+        using index_type   = extents_type::index_type;
+        using size_type    = extents_type::size_type;
+        using rank_type    = extents_type::rank_type;
+        using layout_type  = layout_packed_upper;
+
+        constexpr mapping() noexcept = default;
+        constexpr mapping(const extents_type& ex_) : ex{ex_} {}
+
+        constexpr const extents_type& extents() const {
+            return ex;
+        }
+
+        constexpr index_type required_span_size() const {
+            if (dim % 2 == 0) {
+                return (dim / 2) * (dim + 1);
+            } else {
+                return ((dim + 1) / 2) * dim;
+            }
+        }
+
+    private:
+        extents_type ex = extents_type();
+        index_type dim  = ex.extent(0);
+    };
+};
+
+int main() {
+    constexpr int32_t dim        = 47'000; // sqrt(2^32) > dim > sqrt(2^31), dim assumed even below
+    constexpr auto expected_size = [] {
+        int32_t unused;
+        bool overflow = _Mul_overflow(dim, dim, unused);
+        assert(overflow);
+
+        using UType     = remove_cv_t<make_unsigned_t<decltype(dim)>>;
+        const auto udim = static_cast<UType>(dim);
+        UType result{};
+        overflow = _Mul_overflow(udim, udim, result);
+        assert(!overflow);
+        return result;
+    }();
+
+    constexpr auto expected_req_size = [] {
+        int32_t result{};
+        const bool overflow = _Mul_overflow(dim / 2, dim + 1, result);
+        assert(!overflow);
+        return result;
+    }();
+
+    {
+        using E = extents<int32_t, dim, dim>;
+        constexpr mdspan<double, E, layout_packed_upper> m(nullptr);
+        static_assert(m.size() == expected_size);
+        static_assert(m.mapping().required_span_size() == expected_req_size);
+    }
+
+    {
+        using E = dextents<int32_t, 2>;
+        constexpr mdspan<double, E, layout_packed_upper> m(nullptr, dim, dim);
+        static_assert(m.size() == expected_size);
+        static_assert(m.mapping().required_span_size() == expected_req_size);
+    }
+
+    return 0;
+}