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Implement LWG-4293: span::subspan/first/last chooses wrong constructor when T is const-qualified bool #6437

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Implement LWG-4293: span::subspan/first/last chooses wrong constructor when T is const-qualified bool #6437

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c4008ea
Implement LWG-4293 span::subspan/first/last chooses wrong constructor…
tycket033-tech Sep 4, 2026
7804b6e
Address review feedback: remove LWG-4293 comment, use same_as decltyp…
tycket033-tech Sep 5, 2026
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12 changes: 6 additions & 6 deletions stl/inc/span
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Original file line number Diff line number Diff line change
Expand Up @@ -375,15 +375,15 @@ public:
}
#endif // _MSVC_STL_HARDENING_SPAN || _ITERATOR_DEBUG_LEVEL != 0

return span<element_type, _Count>{_Mydata, _Count};
return span<element_type, _Count>(_Mydata, _Count);
}

_NODISCARD constexpr auto first(const size_type _Count) const noexcept /* strengthened */ {
#if _MSVC_STL_HARDENING_SPAN || _ITERATOR_DEBUG_LEVEL != 0
_STL_VERIFY(_Count <= _Mysize, "Count out of range in span::first(count)");
#endif

return span<element_type, dynamic_extent>{_Mydata, _Count};
return span<element_type, dynamic_extent>(_Mydata, _Count);
}

template <size_t _Count>
Expand All @@ -397,15 +397,15 @@ public:
}
#endif // _MSVC_STL_HARDENING_SPAN || _ITERATOR_DEBUG_LEVEL != 0

return span<element_type, _Count>{_Mydata + (_Mysize - _Count), _Count};
return span<element_type, _Count>(_Mydata + (_Mysize - _Count), _Count);
}

_NODISCARD constexpr auto last(const size_type _Count) const noexcept /* strengthened */ {
#if _MSVC_STL_HARDENING_SPAN || _ITERATOR_DEBUG_LEVEL != 0
_STL_VERIFY(_Count <= _Mysize, "Count out of range in span::last(count)");
#endif

return span<element_type, dynamic_extent>{_Mydata + (_Mysize - _Count), _Count};
return span<element_type, dynamic_extent>(_Mydata + (_Mysize - _Count), _Count);
}

template <size_t _Offset, size_t _Count = dynamic_extent>
Expand All @@ -427,7 +427,7 @@ public:

using _ReturnType = span<element_type,
_Count != dynamic_extent ? _Count : (_Extent != dynamic_extent ? _Extent - _Offset : dynamic_extent)>;
return _ReturnType{_Mydata + _Offset, _Count == dynamic_extent ? _Mysize - _Offset : _Count};
return _ReturnType(_Mydata + _Offset, _Count == dynamic_extent ? _Mysize - _Offset : _Count);
}

_NODISCARD constexpr auto subspan(const size_type _Offset, const size_type _Count = dynamic_extent) const noexcept
Expand All @@ -439,7 +439,7 @@ public:
#endif // _MSVC_STL_HARDENING_SPAN || _ITERATOR_DEBUG_LEVEL != 0

using _ReturnType = span<element_type, dynamic_extent>;
return _ReturnType{_Mydata + _Offset, _Count == dynamic_extent ? _Mysize - _Offset : _Count};
return _ReturnType(_Mydata + _Offset, _Count == dynamic_extent ? _Mysize - _Offset : _Count);
}

// [span.obs] Observers
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51 changes: 51 additions & 0 deletions tests/std/tests/P0122R7_span/test.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -3,6 +3,7 @@

#include <array>
#include <cassert>
#include <concepts>
#include <cstddef>
#include <cstdint>
#include <iterator>
Expand Down Expand Up @@ -996,6 +997,56 @@ constexpr bool test() {
static_assert(is_same_v<decltype(sp_nine.rend()), span<int, 9>::reverse_iterator>);
}

{
bool sequence[9]{true, false, true, false, true, false, true, false, true};

const span<const bool> sp_dyn(sequence);
const span<const bool, 9> sp_nine(sequence);

same_as<span<const bool>> decltype(auto) first_1 = sp_dyn.first(4);
same_as<span<const bool, 4>> decltype(auto) first_2 = sp_nine.first<4>();
static_assert(noexcept(sp_dyn.first(4))); // strengthened
static_assert(noexcept(sp_nine.first<4>())); // strengthened
assert(first_1.data() == begin(sequence));
assert(first_2.data() == begin(sequence));
assert(first_1.size() == 4);
assert(first_2.size() == 4);
assert(first_1[3] == false);
assert(first_2[3] == false);

same_as<span<const bool>> decltype(auto) last_1 = sp_dyn.last(4);
same_as<span<const bool, 4>> decltype(auto) last_2 = sp_nine.last<4>();
static_assert(noexcept(sp_dyn.last(4))); // strengthened
static_assert(noexcept(sp_nine.last<4>())); // strengthened
assert(last_1.data() == begin(sequence) + 5);
assert(last_2.data() == begin(sequence) + 5);
assert(last_1.size() == 4);
assert(last_2.size() == 4);
assert(last_1[0] == false);
assert(last_2[3] == true);

same_as<span<const bool>> decltype(auto) subspan_1 = sp_dyn.subspan(3, 4);
same_as<span<const bool>> decltype(auto) subspan_2 = sp_dyn.subspan(3);
same_as<span<const bool, 4>> decltype(auto) subspan_3 = sp_nine.subspan<3, 4>();
same_as<span<const bool, 6>> decltype(auto) subspan_4 = sp_nine.subspan<3>();
static_assert(noexcept(sp_dyn.subspan(3, 4))); // strengthened
static_assert(noexcept(sp_dyn.subspan(3))); // strengthened
static_assert(noexcept(sp_nine.subspan<3, 4>())); // strengthened
static_assert(noexcept(sp_nine.subspan<3>())); // strengthened
assert(subspan_1.data() == begin(sequence) + 3);
assert(subspan_2.data() == begin(sequence) + 3);
assert(subspan_3.data() == begin(sequence) + 3);
assert(subspan_4.data() == begin(sequence) + 3);
assert(subspan_1.size() == 4);
assert(subspan_2.size() == 6);
assert(subspan_3.size() == 4);
assert(subspan_4.size() == 6);
assert(subspan_1[0] == false);
assert(subspan_2[5] == true);
assert(subspan_3[0] == false);
assert(subspan_4[5] == true);
}

return true;
}

Expand Down