[libcxx-commits] [libcxx] 4aeb7e3 - [libc++] LWG4266: `layout_stride::mapping` should treat empty mappings as exhaustive (#191629)
via libcxx-commits
libcxx-commits at lists.llvm.org
Fri May 15 00:46:49 PDT 2026
Author: eiytoq
Date: 2026-05-15T15:46:44+08:00
New Revision: 4aeb7e386bc6d6f3d9a9343f5d2c26a66bb097cd
URL: https://github.com/llvm/llvm-project/commit/4aeb7e386bc6d6f3d9a9343f5d2c26a66bb097cd
DIFF: https://github.com/llvm/llvm-project/commit/4aeb7e386bc6d6f3d9a9343f5d2c26a66bb097cd.diff
LOG: [libc++] LWG4266: `layout_stride::mapping` should treat empty mappings as exhaustive (#191629)
Fixes: #171325
---------
Co-authored-by: S. B. Tam <cpplearner at outlook.com>
Added:
Modified:
libcxx/docs/Status/Cxx2cIssues.csv
libcxx/include/__mdspan/layout_stride.h
libcxx/include/mdspan
libcxx/test/std/containers/views/mdspan/layout_stride/is_exhaustive_corner_case.pass.cpp
libcxx/test/std/containers/views/mdspan/layout_stride/properties.pass.cpp
Removed:
################################################################################
diff --git a/libcxx/docs/Status/Cxx2cIssues.csv b/libcxx/docs/Status/Cxx2cIssues.csv
index 42ab96ef5657a..e329e14f70ab3 100644
--- a/libcxx/docs/Status/Cxx2cIssues.csv
+++ b/libcxx/docs/Status/Cxx2cIssues.csv
@@ -165,7 +165,7 @@
"`LWG4257 <https://wg21.link/LWG4257>`__","Stream insertion for ``chrono::local_time`` should be constrained","2025-11 (Kona)","","","`#171322 <https://github.com/llvm/llvm-project/issues/171322>`__",""
"`LWG4260 <https://wg21.link/LWG4260>`__","Query objects must be default constructible","2025-11 (Kona)","","","`#171323 <https://github.com/llvm/llvm-project/issues/171323>`__",""
"`LWG4265 <https://wg21.link/LWG4265>`__","``std::midpoint`` should not accept ``const bool``","2025-11 (Kona)","|Complete|","22","`#171324 <https://github.com/llvm/llvm-project/issues/171324>`__",""
-"`LWG4266 <https://wg21.link/LWG4266>`__","``layout_stride::mapping`` should treat empty mappings as exhaustive","2025-11 (Kona)","","","`#171325 <https://github.com/llvm/llvm-project/issues/171325>`__",""
+"`LWG4266 <https://wg21.link/LWG4266>`__","``layout_stride::mapping`` should treat empty mappings as exhaustive","2025-11 (Kona)","|Complete|","23","`#171325 <https://github.com/llvm/llvm-project/issues/171325>`__",""
"`LWG4269 <https://wg21.link/LWG4269>`__","``unique_copy`` passes arguments to its predicate backwards","2025-11 (Kona)","","","`#171326 <https://github.com/llvm/llvm-project/issues/171326>`__",""
"`LWG4272 <https://wg21.link/LWG4272>`__","For ``rank == 0``, ``layout_stride`` is atypically convertible","2025-11 (Kona)","","","`#171327 <https://github.com/llvm/llvm-project/issues/171327>`__",""
"`LWG4274 <https://wg21.link/LWG4274>`__","The ``chrono::hh_mm_ss`` constructor is ill-formed for unsigned durations","2025-11 (Kona)","","","`#171328 <https://github.com/llvm/llvm-project/issues/171328>`__",""
diff --git a/libcxx/include/__mdspan/layout_stride.h b/libcxx/include/__mdspan/layout_stride.h
index 66d5a7f499e4c..5ee4672e0d1cf 100644
--- a/libcxx/include/__mdspan/layout_stride.h
+++ b/libcxx/include/__mdspan/layout_stride.h
@@ -296,38 +296,26 @@ class layout_stride::mapping {
}
_LIBCPP_HIDE_FROM_ABI static constexpr bool is_always_unique() noexcept { return true; }
- _LIBCPP_HIDE_FROM_ABI static constexpr bool is_always_exhaustive() noexcept { return false; }
+ _LIBCPP_HIDE_FROM_ABI static constexpr bool is_always_exhaustive() noexcept {
+ if constexpr (__rank_ == 0)
+ return true;
+ for (size_t __r = 0; __r < __rank_; ++__r)
+ if (extents_type::static_extent(__r) == 0)
+ return true;
+ return false;
+ }
_LIBCPP_HIDE_FROM_ABI static constexpr bool is_always_strided() noexcept { return true; }
_LIBCPP_HIDE_FROM_ABI static constexpr bool is_unique() noexcept { return true; }
- // The answer of this function is fairly complex in the case where one or more
- // extents are zero.
- // Technically it is meaningless to query is_exhaustive() in that case, but unfortunately
- // the way the standard defines this function, we can't give a simple true or false then.
_LIBCPP_HIDE_FROM_ABI constexpr bool is_exhaustive() const noexcept {
- if constexpr (__rank_ == 0)
+ if constexpr (is_always_exhaustive())
return true;
- else {
- index_type __span_size = required_span_size();
- if (__span_size == static_cast<index_type>(0)) {
- if constexpr (__rank_ == 1)
- return __strides_[0] == 1;
- else {
- rank_type __r_largest = 0;
- for (rank_type __r = 1; __r < __rank_; __r++)
- if (__strides_[__r] > __strides_[__r_largest])
- __r_largest = __r;
- for (rank_type __r = 0; __r < __rank_; __r++)
- if (__extents_.extent(__r) == 0 && __r != __r_largest)
- return false;
- return true;
- }
- } else {
- return required_span_size() == [&]<size_t... _Pos>(index_sequence<_Pos...>) {
- return (__extents_.extent(_Pos) * ... * static_cast<index_type>(1));
- }(make_index_sequence<__rank_>());
- }
- }
+ index_type __span_size = required_span_size();
+ if (__span_size == static_cast<index_type>(0))
+ return true;
+ return __span_size == [&]<size_t... _Pos>(index_sequence<_Pos...>) {
+ return (__extents_.extent(_Pos) * ... * static_cast<index_type>(1));
+ }(make_index_sequence<__rank_>());
}
_LIBCPP_HIDE_FROM_ABI static constexpr bool is_strided() noexcept { return true; }
diff --git a/libcxx/include/mdspan b/libcxx/include/mdspan
index d46f87cb0f014..30a17e4299068 100644
--- a/libcxx/include/mdspan
+++ b/libcxx/include/mdspan
@@ -237,7 +237,7 @@ namespace std {
constexpr index_type operator()(Indices...) const noexcept;
static constexpr bool is_always_unique() noexcept { return true; }
- static constexpr bool is_always_exhaustive() noexcept { return false; }
+ static constexpr bool is_always_exhaustive() noexcept;
static constexpr bool is_always_strided() noexcept { return true; }
static constexpr bool is_unique() noexcept { return true; }
diff --git a/libcxx/test/std/containers/views/mdspan/layout_stride/is_exhaustive_corner_case.pass.cpp b/libcxx/test/std/containers/views/mdspan/layout_stride/is_exhaustive_corner_case.pass.cpp
index 900adb42096a7..c139d4241ed3b 100644
--- a/libcxx/test/std/containers/views/mdspan/layout_stride/is_exhaustive_corner_case.pass.cpp
+++ b/libcxx/test/std/containers/views/mdspan/layout_stride/is_exhaustive_corner_case.pass.cpp
@@ -13,7 +13,7 @@
// constexpr bool is_exhaustive() const noexcept;
//
// Returns:
-// - true if rank_ is 0.
+// - true if rank_ or the size of the multidimensional index space m.extents() is 0.
// - Otherwise, true if there is a permutation P of the integers in the range [0, rank_) such that
// stride(p0) equals 1, and stride(pi) equals stride(pi_1) * extents().extent(pi_1) for i in the
// range [1, rank_), where pi is the ith element of P.
@@ -35,13 +35,13 @@ test_layout_mapping_stride(E ext, std::array<typename E::index_type, E::rank()>
constexpr bool test() {
constexpr size_t D = std::dynamic_extent;
test_layout_mapping_stride(std::extents<int, 0>(), std::array<int, 1>{1}, true);
- test_layout_mapping_stride(std::extents<unsigned, D>(0), std::array<unsigned, 1>{3}, false);
+ test_layout_mapping_stride(std::extents<unsigned, D>(0), std::array<unsigned, 1>{3}, true);
test_layout_mapping_stride(std::extents<int, 0, 3>(), std::array<int, 2>{6, 2}, true);
- test_layout_mapping_stride(std::extents<int, D, D>(3, 0), std::array<int, 2>{6, 2}, false);
- test_layout_mapping_stride(std::extents<int, D, D>(0, 0), std::array<int, 2>{6, 2}, false);
+ test_layout_mapping_stride(std::extents<int, D, D>(3, 0), std::array<int, 2>{6, 2}, true);
+ test_layout_mapping_stride(std::extents<int, D, D>(0, 0), std::array<int, 2>{6, 2}, true);
test_layout_mapping_stride(
std::extents<unsigned, D, D, D, D>(3, 3, 0, 3), std::array<unsigned, 4>{3, 1, 27, 9}, true);
- test_layout_mapping_stride(std::extents<int, D, D, D, D>(0, 3, 3, 3), std::array<int, 4>{3, 1, 27, 9}, false);
+ test_layout_mapping_stride(std::extents<int, D, D, D, D>(0, 3, 3, 3), std::array<int, 4>{3, 1, 27, 9}, true);
return true;
}
diff --git a/libcxx/test/std/containers/views/mdspan/layout_stride/properties.pass.cpp b/libcxx/test/std/containers/views/mdspan/layout_stride/properties.pass.cpp
index 21ce56fcc65f1..c3f472e790ba5 100644
--- a/libcxx/test/std/containers/views/mdspan/layout_stride/properties.pass.cpp
+++ b/libcxx/test/std/containers/views/mdspan/layout_stride/properties.pass.cpp
@@ -16,11 +16,11 @@
//
// ...
// static constexpr bool is_always_unique() noexcept { return true; }
-// static constexpr bool is_always_exhaustive() noexcept { return false; }
+// static constexpr bool is_always_exhaustive() noexcept;
// static constexpr bool is_always_strided() noexcept { return true; }
//
// static constexpr bool is_unique() noexcept { return true; }
-// static constexpr bool is_exhaustive() noexcept;
+// constexpr bool is_exhaustive() noexcept;
// static constexpr bool is_strided() noexcept { return true; }
// ...
// };
@@ -29,6 +29,12 @@
//
// layout_stride::mapping<E> is a trivially copyable type that models regular for each E.
//
+// static constexpr bool is_always_exhaustive() noexcept;
+//
+// Returns:
+// - true if rank_ is 0 or if there is a rank index r of extents() such that
+// extents_type::static_extent(r) is 0, otherwise false.
+//
// constexpr bool is_exhaustive() const noexcept;
//
// Returns:
@@ -48,8 +54,8 @@
#include "test_macros.h"
template <class E>
-constexpr void
-test_layout_mapping_stride(E ext, std::array<typename E::index_type, E::rank()> strides, bool exhaustive) {
+constexpr void test_layout_mapping_stride(
+ E ext, std::array<typename E::index_type, E::rank()> strides, bool exhaustive, bool always_exhaustive = false) {
using M = std::layout_stride::mapping<E>;
M m(ext, strides);
const M c_m = m;
@@ -62,7 +68,7 @@ test_layout_mapping_stride(E ext, std::array<typename E::index_type, E::rank()>
assert(c_m.is_exhaustive() == exhaustive);
assert(M::is_strided() == true);
assert(M::is_always_unique() == true);
- assert(M::is_always_exhaustive() == false);
+ assert(M::is_always_exhaustive() == always_exhaustive);
assert(M::is_always_strided() == true);
ASSERT_NOEXCEPT(m.strides());
@@ -103,7 +109,8 @@ test_layout_mapping_stride(E ext, std::array<typename E::index_type, E::rank()>
constexpr bool test() {
constexpr size_t D = std::dynamic_extent;
- test_layout_mapping_stride(std::extents<int>(), std::array<int, 0>{}, true);
+ test_layout_mapping_stride(std::extents<int>(), std::array<int, 0>{}, true, true);
+ test_layout_mapping_stride(std::extents<int, 2, 0>(), std::array<int, 2>{1, 2}, true, true);
test_layout_mapping_stride(std::extents<signed char, 4, 5>(), std::array<signed char, 2>{1, 4}, true);
test_layout_mapping_stride(std::extents<signed char, 4, 5>(), std::array<signed char, 2>{1, 5}, false);
test_layout_mapping_stride(std::extents<unsigned, D, 4>(7), std::array<unsigned, 2>{20, 2}, false);
More information about the libcxx-commits
mailing list