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[libc++] Implement LWG4222 'expected' constructor from a single value missing a constraint #152676

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Merged
merged 1 commit into from
Aug 9, 2025
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[libc++] Implement LWG4222 'expected' constructor from a single value missing a constraint #152676

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2 changes: 1 addition & 1 deletion libcxx/docs/Status/Cxx2cIssues.csv
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Original file line number Diff line number Diff line change
Expand Up @@ -132,7 +132,7 @@
"`LWG4200 <https://wg21.link/LWG4200>`__","The ``operation_state`` concept can be simplified","2025-06 (Sofia)","","",""
"`LWG4201 <https://wg21.link/LWG4201>`__","``with-await-transform::await_transform`` should not use a deduced return type","2025-06 (Sofia)","","",""
"`LWG4217 <https://wg21.link/LWG4217>`__","Clarify ``mdspan`` layout mapping requirements for ``rank == 0``","2025-06 (Sofia)","","",""
"`LWG4222 <https://wg21.link/LWG4222>`__","``expected`` constructor from a single value missing a constraint","2025-06 (Sofia)","","",""
"`LWG4222 <https://wg21.link/LWG4222>`__","``expected`` constructor from a single value missing a constraint","2025-06 (Sofia)","|Complete|","22",""
"`LWG4224 <https://wg21.link/LWG4224>`__","Philox engines should be freestanding","2025-06 (Sofia)","","",""
"`LWG4227 <https://wg21.link/LWG4227>`__","Missing ``noexcept`` operator in [exec.when.all]","2025-06 (Sofia)","","",""
"`LWG4231 <https://wg21.link/LWG4231>`__","``datapar::chunk<N>`` should use ``simd-size-type`` instead of ``size_t``","2025-06 (Sofia)","","",""
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3 changes: 2 additions & 1 deletion libcxx/include/__expected/expected.h
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Expand Up @@ -557,7 +557,8 @@ class expected : private __expected_base<_Tp, _Err> {

template <class _Up = _Tp>
requires(!is_same_v<remove_cvref_t<_Up>, in_place_t> && !is_same_v<expected, remove_cvref_t<_Up>> &&
is_constructible_v<_Tp, _Up> && !__is_std_unexpected<remove_cvref_t<_Up>>::value &&
!is_same_v<remove_cvref_t<_Up>, unexpect_t> && is_constructible_v<_Tp, _Up> &&
!__is_std_unexpected<remove_cvref_t<_Up>>::value &&
(!is_same_v<remove_cv_t<_Tp>, bool> || !__is_std_expected<remove_cvref_t<_Up>>::value))
_LIBCPP_HIDE_FROM_ABI constexpr explicit(!is_convertible_v<_Up, _Tp>)
expected(_Up&& __u) noexcept(is_nothrow_constructible_v<_Tp, _Up>) // strengthened
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11 changes: 11 additions & 0 deletions libcxx/test/std/utilities/expected/expected.expected/ctor/ctor.u.pass.cpp
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Expand Up @@ -14,6 +14,7 @@
// Constraints:
// - is_same_v<remove_cvref_t<U>, in_place_t> is false; and
// - is_same_v<expected, remove_cvref_t<U>> is false; and
// - is_same_v<remove_cvref_t<U>, unexpect_t> is false; and
// - remove_cvref_t<U> is not a specialization of unexpected; and
// - is_constructible_v<T, U> is true.
//
Expand Down Expand Up @@ -46,6 +47,16 @@ static_assert(!std::is_constructible_v<std::expected<FromJustInplace, int>, std:
// Note that result is true because it is covered by the constructors that take expected
static_assert(std::is_constructible_v<std::expected<int, int>, std::expected<int, int>&>);

struct T {
explicit T(auto) {}
};
struct E {
E(int) {}
};

// is_same_v<remove_cvref_t<U>, unexpect_t> is false;
static_assert(!std::is_constructible_v<std::expected<T, E>, std::unexpect_t>);

// remove_cvref_t<U> is a specialization of unexpected
// Note that result is true because it is covered by the constructors that take unexpected
static_assert(std::is_constructible_v<std::expected<int, int>, std::unexpected<int>&>);
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