[libcxx-commits] [libcxx] [libc++][mdspan] LWG3974: `mdspan::operator[]` should not copy `OtherIndexTypes` (PR #195814)
via libcxx-commits
libcxx-commits at lists.llvm.org
Thu May 7 03:33:57 PDT 2026
https://github.com/eiytoq updated https://github.com/llvm/llvm-project/pull/195814
>From 0dfdb46ac0298a80b2d84e1c4059641ae8b502b4 Mon Sep 17 00:00:00 2001
From: eiytoq <eiytoq at outlook.com>
Date: Tue, 5 May 2026 17:19:34 +0800
Subject: [PATCH 1/2] [libc++][mdspan] LWG3974: mdspan::operator[] should not
copy OtherIndexTypes
---
libcxx/docs/Status/Cxx2cIssues.csv | 2 +-
libcxx/include/__mdspan/mdspan.h | 4 ++--
.../mdspan/mdspan/index_operator.pass.cpp | 21 ++++++++++++++++++-
3 files changed, 23 insertions(+), 4 deletions(-)
diff --git a/libcxx/docs/Status/Cxx2cIssues.csv b/libcxx/docs/Status/Cxx2cIssues.csv
index d7f75380cb93e..45fba58cb3f91 100644
--- a/libcxx/docs/Status/Cxx2cIssues.csv
+++ b/libcxx/docs/Status/Cxx2cIssues.csv
@@ -35,7 +35,7 @@
"`LWG3965 <https://wg21.link/LWG3965>`__","Incorrect example in [format.string.escaped] p3 for formatting of combining characters","2023-11 (Kona)","|Complete|","19","`#105305 <https://github.com/llvm/llvm-project/issues/105305>`__",""
"`LWG3970 <https://wg21.link/LWG3970>`__","[mdspan.syn] Missing definition of ``full_extent_t`` and ``full_extent``","2023-11 (Kona)","","","`#105306 <https://github.com/llvm/llvm-project/issues/105306>`__",""
"`LWG3973 <https://wg21.link/LWG3973>`__","Monadic operations should be ADL-proof","2023-11 (Kona)","","","`#105307 <https://github.com/llvm/llvm-project/issues/105307>`__",""
-"`LWG3974 <https://wg21.link/LWG3974>`__","``mdspan::operator[]`` should not copy ``OtherIndexTypes``","2023-11 (Kona)","","","`#105308 <https://github.com/llvm/llvm-project/issues/105308>`__",""
+"`LWG3974 <https://wg21.link/LWG3974>`__","``mdspan::operator[]`` should not copy ``OtherIndexTypes``","2023-11 (Kona)","|Complete|","23","`#105308 <https://github.com/llvm/llvm-project/issues/105308>`__",""
"`LWG3987 <https://wg21.link/LWG3987>`__","Including ``<flat_foo>`` doesn't provide ``std::begin``/``end``","2023-11 (Kona)","|Complete|","","`#105309 <https://github.com/llvm/llvm-project/issues/105309>`__",""
"`LWG3990 <https://wg21.link/LWG3990>`__","Program-defined specializations of ``std::tuple`` and ``std::variant`` can't be properly supported","2023-11 (Kona)","|Complete|","21","`#105310 <https://github.com/llvm/llvm-project/issues/105310>`__",""
"`LWG4001 <https://wg21.link/LWG4001>`__","``iota_view`` should provide ``empty``","2023-11 (Kona)","|Complete|","19","`#105311 <https://github.com/llvm/llvm-project/issues/105311>`__",""
diff --git a/libcxx/include/__mdspan/mdspan.h b/libcxx/include/__mdspan/mdspan.h
index 7f577953ee7ec..7f71b0750a78d 100644
--- a/libcxx/include/__mdspan/mdspan.h
+++ b/libcxx/include/__mdspan/mdspan.h
@@ -204,7 +204,7 @@ class mdspan {
[[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr reference
operator[](const array< _OtherIndexType, rank()>& __indices) const {
return __acc_.access(__ptr_, [&]<size_t... _Idxs>(index_sequence<_Idxs...>) {
- return __map_(__indices[_Idxs]...);
+ return __map_(extents_type::__index_cast(__indices[_Idxs])...);
}(make_index_sequence<rank()>()));
}
@@ -213,7 +213,7 @@ class mdspan {
is_nothrow_constructible_v<index_type, const _OtherIndexType&>)
[[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr reference operator[](span<_OtherIndexType, rank()> __indices) const {
return __acc_.access(__ptr_, [&]<size_t... _Idxs>(index_sequence<_Idxs...>) {
- return __map_(__indices[_Idxs]...);
+ return __map_(extents_type::__index_cast(__indices[_Idxs])...);
}(make_index_sequence<rank()>()));
}
diff --git a/libcxx/test/std/containers/views/mdspan/mdspan/index_operator.pass.cpp b/libcxx/test/std/containers/views/mdspan/mdspan/index_operator.pass.cpp
index 3f20bc886d106..bcf8d3628dbb0 100644
--- a/libcxx/test/std/containers/views/mdspan/mdspan/index_operator.pass.cpp
+++ b/libcxx/test/std/containers/views/mdspan/mdspan/index_operator.pass.cpp
@@ -196,9 +196,27 @@ constexpr void test_layout_large() {
test_iteration(construct_mapping(Layout(), std::extents<int64_t, D, 4, 1, D>(3, 6)));
}
+struct Index {
+ Index() = default;
+ Index(const Index&) = delete;
+ constexpr operator int() const noexcept { return 0; }
+};
+
// mdspan::operator[] casts to index_type before calling mapping
// mapping requirements only require the index operator to mixed integer types not anything convertible to index_type
-constexpr void test_index_cast_happens() {}
+constexpr void test_index_cast_happens() {
+ int data[1]{};
+ std::mdspan m(data, std::extents<int, 1>{1});
+
+ // Index i;
+ std::array<Index, 1> a{};
+ std::span s(a);
+
+ // LWG3995: Issue with custom index conversion in <mdspan>
+ // TEST_IGNORE_NODISCARD m[i];
+ TEST_IGNORE_NODISCARD m[a];
+ TEST_IGNORE_NODISCARD m[s];
+}
struct RValueInt {
constexpr operator int() && noexcept { return 0; }
@@ -213,6 +231,7 @@ constexpr bool test() {
std::mdspan m(data, std::extents<int, 1>{1});
TEST_IGNORE_NODISCARD m[RValueInt{}];
+ test_index_cast_happens();
return true;
}
>From 8343d665eaa8555762d5e3a8f802431c3f719f37 Mon Sep 17 00:00:00 2001
From: eiytoq <eiytoq at outlook.com>
Date: Thu, 7 May 2026 18:33:38 +0800
Subject: [PATCH 2/2] Improve test coverage
---
.../mdspan/mdspan/index_operator.pass.cpp | 30 ++++++++++---------
1 file changed, 16 insertions(+), 14 deletions(-)
diff --git a/libcxx/test/std/containers/views/mdspan/mdspan/index_operator.pass.cpp b/libcxx/test/std/containers/views/mdspan/mdspan/index_operator.pass.cpp
index bcf8d3628dbb0..656d2475d2588 100644
--- a/libcxx/test/std/containers/views/mdspan/mdspan/index_operator.pass.cpp
+++ b/libcxx/test/std/containers/views/mdspan/mdspan/index_operator.pass.cpp
@@ -196,26 +196,28 @@ constexpr void test_layout_large() {
test_iteration(construct_mapping(Layout(), std::extents<int64_t, D, 4, 1, D>(3, 6)));
}
-struct Index {
- Index() = default;
- Index(const Index&) = delete;
- constexpr operator int() const noexcept { return 0; }
+struct NoCopyIndex {
+ int val = 0;
+ constexpr NoCopyIndex(int v) : val(v) {}
+ constexpr NoCopyIndex(const NoCopyIndex&) = delete;
+ constexpr operator int() const noexcept { return val; }
};
// mdspan::operator[] casts to index_type before calling mapping
// mapping requirements only require the index operator to mixed integer types not anything convertible to index_type
-constexpr void test_index_cast_happens() {
- int data[1]{};
- std::mdspan m(data, std::extents<int, 1>{1});
+constexpr void test_index_cast() {
+ std::array data{0, 1, 2, 3, 4, 5, 6, 7, 8, 9};
+ const std::mdspan m{data.data(), std::dextents<int, 1>{10}};
+
+ std::array indices{NoCopyIndex{3}};
- // Index i;
- std::array<Index, 1> a{};
- std::span s(a);
+ assert(&m[NoCopyIndex{3}] == &data[3]);
+ assert(&m[indices] == &data[3]);
+ assert(&m[std::span{indices}] == &data[3]);
// LWG3995: Issue with custom index conversion in <mdspan>
- // TEST_IGNORE_NODISCARD m[i];
- TEST_IGNORE_NODISCARD m[a];
- TEST_IGNORE_NODISCARD m[s];
+ // NoCopyIndex index(3);
+ // assert(&m[index] == &data[3]);
}
struct RValueInt {
@@ -231,7 +233,7 @@ constexpr bool test() {
std::mdspan m(data, std::extents<int, 1>{1});
TEST_IGNORE_NODISCARD m[RValueInt{}];
- test_index_cast_happens();
+ test_index_cast();
return true;
}
More information about the libcxx-commits
mailing list