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Implement LWG-4272: For rank == 0, layout_stride is atypically convertible #5900

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Implement LWG-4272: For rank == 0, layout_stride is atypically convertible #5900

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9d9d6b6
Implement LWG-4272: `layout_left`'s constructor
JMazurkiewicz Nov 18, 2025
781805a
Implement LWG-4272: `layout_right`'s constructor
JMazurkiewicz Nov 18, 2025
cf9fc99
Update libc++ tests
JMazurkiewicz Nov 19, 2025
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5 changes: 3 additions & 2 deletions stl/inc/mdspan
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Original file line number Diff line number Diff line change
Expand Up @@ -538,7 +538,7 @@ public:

template <class _OtherExtents>
requires is_constructible_v<extents_type, _OtherExtents>
constexpr explicit(extents_type::rank() > 0)
constexpr explicit(!(extents_type::rank() == 0 && is_convertible_v<_OtherExtents, extents_type>) )
mapping(const layout_stride::mapping<_OtherExtents>& _Other) noexcept // strengthened
: _Base(_Other.extents()) {
#if _ITERATOR_DEBUG_LEVEL != 0
Expand Down Expand Up @@ -693,7 +693,8 @@ public:

template <class _OtherExtents>
requires is_constructible_v<extents_type, _OtherExtents>
constexpr explicit(extents_type::rank() > 0) mapping(const layout_stride::mapping<_OtherExtents>& _Other) noexcept
constexpr explicit(!(extents_type::rank() == 0 && is_convertible_v<_OtherExtents, extents_type>) )
mapping(const layout_stride::mapping<_OtherExtents>& _Other) noexcept
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@StephanTLavavej StephanTLavavej Nov 23, 2025

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No change requested: layout_left::mapping and layout_right::mapping vary in their noexcept // strengthened vs. noexcept, but this matches the Standard's varying noexcept, despite the fact that they both have preconditions. I think this should be an LWG issue (but am too lazybusy to report it).

: _Base(_Other.extents()) {
#if _ITERATOR_DEBUG_LEVEL != 0
if constexpr (extents_type::rank() > 0) {
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4 changes: 4 additions & 0 deletions tests/libcxx/expected_results.txt
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Expand Up @@ -185,6 +185,10 @@ std/ranges/range.adaptors/range.join/range.join.iterator/arrow.pass.cpp FAIL
std/containers/views/mdspan/layout_stride/is_exhaustive_corner_case.pass.cpp FAIL
std/containers/views/mdspan/layout_stride/properties.pass.cpp FAIL

# libc++ doesn't implement LWG-4272 "For `rank == 0`, `layout_stride` is atypically convertible"
std/containers/views/mdspan/layout_left/ctor.layout_stride.pass.cpp FAIL
std/containers/views/mdspan/layout_right/ctor.layout_stride.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

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2 changes: 2 additions & 0 deletions tests/std/tests/P0009R18_mdspan_layout_left/test.cpp
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Expand Up @@ -293,6 +293,8 @@ constexpr void check_construction_from_other_stride_mapping() {
layout_stride::mapping<extents<long long, 3>>>);
static_assert(NotImplicitlyConstructibleFrom<layout_left::mapping<extents<int, 3>>,
layout_stride::mapping<extents<int, dynamic_extent>>>);
static_assert(NotImplicitlyConstructibleFrom<layout_left::mapping<extents<int>>,
layout_stride::mapping<extents<long long>>>);
}
}

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2 changes: 2 additions & 0 deletions tests/std/tests/P0009R18_mdspan_layout_right/test.cpp
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Expand Up @@ -292,6 +292,8 @@ constexpr void check_construction_from_other_stride_mapping() {
layout_stride::mapping<extents<long long, 3>>>);
static_assert(NotImplicitlyConstructibleFrom<layout_right::mapping<extents<int, 3>>,
layout_stride::mapping<extents<int, dynamic_extent>>>);
static_assert(NotImplicitlyConstructibleFrom<layout_right::mapping<extents<int>>,
layout_stride::mapping<extents<long long>>>);
}
}

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