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[libc++] Implement Resolution of LWG 3886 (#155356)

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Resolves #118336

- Implement the resolution of
[LWG3886](https://cplusplus.github.io/LWG/issue3886) for `optional` and
`expected`
This commit is contained in:
William Tran-Viet 2025-09-23 19:26:40 -04:00 committed by GitHub
parent 59b4621b6a
commit fb72f0ca7d
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GPG Key ID: B5690EEEBB952194
8 changed files with 106 additions and 14 deletions
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2
libcxx/docs/Status/Cxx2cIssues.csv
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@ -78,7 +78,7 @@
"","","","","",""
"`LWG3216 <https://wg21.link/LWG3216>`__","Rebinding the allocator before calling ``construct``/``destroy`` in ``allocate_shared``","2024-11 (Wrocław)","","","`#118332 <https://github.com/llvm/llvm-project/issues/118332>`__",""
"`LWG3436 <https://wg21.link/LWG3436>`__","``std::construct_at`` should support arrays","2024-11 (Wrocław)","","","`#118335 <https://github.com/llvm/llvm-project/issues/118335>`__",""
"`LWG3886 <https://wg21.link/LWG3886>`__","Monad mo' problems","2024-11 (Wrocław)","","","`#118336 <https://github.com/llvm/llvm-project/issues/118336>`__",""
"`LWG3886 <https://wg21.link/LWG3886>`__","Monad mo' problems","2024-11 (Wrocław)","|Complete|","22","`#118336 <https://github.com/llvm/llvm-project/issues/118336>`__",""
"`LWG3899 <https://wg21.link/LWG3899>`__","``co_yield``\ing elements of an lvalue generator is unnecessarily inefficient","2024-11 (Wrocław)","","","`#118337 <https://github.com/llvm/llvm-project/issues/118337>`__",""
"`LWG3900 <https://wg21.link/LWG3900>`__","The ``allocator_arg_t`` overloads of ``generator::promise_type::operator new`` should not be constrained","2024-11 (Wrocław)","","","`#118338 <https://github.com/llvm/llvm-project/issues/118338>`__",""
"`LWG3918 <https://wg21.link/LWG3918>`__","``std::uninitialized_move/_n`` and guaranteed copy elision","2024-11 (Wrocław)","","","`#118339 <https://github.com/llvm/llvm-project/issues/118339>`__",""

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8
libcxx/include/__expected/expected.h
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@ -555,7 +555,7 @@ public:
is_nothrow_constructible_v<_Tp, _Up> && is_nothrow_constructible_v<_Err, _OtherErr>) // strengthened
: __base(__other.__has_val(), std::move(__other.__union())) {}
template <class _Up = _Tp>
template <class _Up = remove_cv_t<_Tp>>
requires(!is_same_v<remove_cvref_t<_Up>, in_place_t> && !is_same_v<expected, remove_cvref_t<_Up>> &&
!is_same_v<remove_cvref_t<_Up>, unexpect_t> && is_constructible_v<_Tp, _Up> &&
!__is_std_unexpected<remove_cvref_t<_Up>>::value &&
@ -669,7 +669,7 @@ public:
return *this;
}
template <class _Up = _Tp>
template <class _Up = remove_cv_t<_Tp>>
_LIBCPP_HIDE_FROM_ABI constexpr expected& operator=(_Up&& __v)
requires(!is_same_v<expected, remove_cvref_t<_Up>> && !__is_std_unexpected<remove_cvref_t<_Up>>::value &&
is_constructible_v<_Tp, _Up> && is_assignable_v<_Tp&, _Up> &&
@ -887,14 +887,14 @@ public:
return std::move(this->__unex());
}
template <class _Up>
template <class _Up = remove_cv_t<_Tp>>
_LIBCPP_HIDE_FROM_ABI constexpr _Tp value_or(_Up&& __v) const& {
static_assert(is_copy_constructible_v<_Tp>, "value_type has to be copy constructible");
static_assert(is_convertible_v<_Up, _Tp>, "argument has to be convertible to value_type");
return this->__has_val() ? this->__val() : static_cast<_Tp>(std::forward<_Up>(__v));
}
template <class _Up>
template <class _Up = remove_cv_t<_Tp>>
_LIBCPP_HIDE_FROM_ABI constexpr _Tp value_or(_Up&& __v) && {
static_assert(is_move_constructible_v<_Tp>, "value_type has to be move constructible");
static_assert(is_convertible_v<_Up, _Tp>, "argument has to be convertible to value_type");

17
libcxx/include/optional
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@ -119,7 +119,7 @@ namespace std {
constexpr explicit optional(in_place_t, Args &&...);
template<class U, class... Args>
constexpr explicit optional(in_place_t, initializer_list<U>, Args &&...);
template<class U = T>
template<class U = remove_cv_t<T>>
constexpr explicit(see-below) optional(U &&);
template<class U>
explicit(see-below) optional(const optional<U> &); // constexpr in C++20
@ -133,7 +133,7 @@ namespace std {
optional &operator=(nullopt_t) noexcept; // constexpr in C++20
constexpr optional &operator=(const optional &);
constexpr optional &operator=(optional &&) noexcept(see below);
template<class U = T> optional &operator=(U &&); // constexpr in C++20
template<class U = remove_cv_t<T>> optional &operator=(U &&); // constexpr in C++20
template<class U> optional &operator=(const optional<U> &); // constexpr in C++20
template<class U> optional &operator=(optional<U> &&); // constexpr in C++20
template<class... Args> T& emplace(Args &&...); // constexpr in C++20
@ -161,8 +161,8 @@ namespace std {
constexpr T &value() &;
constexpr T &&value() &&;
constexpr const T &&value() const &&;
template<class U> constexpr T value_or(U &&) const &;
template<class U> constexpr T value_or(U &&) &&;
template<class U = remove_cv_t<T>> constexpr T value_or(U &&) const &;
template<class U = remove_cv_t<T>> constexpr T value_or(U &&) &&;
// [optional.monadic], monadic operations
template<class F> constexpr auto and_then(F&& f) &; // since C++23
@ -730,7 +730,8 @@ public:
enable_if_t<_CheckOptionalArgsCtor<_Up>::template __enable_implicit<_Up>(), int> = 0>
_LIBCPP_HIDE_FROM_ABI constexpr optional(_Up&& __v) : __base(in_place, std::forward<_Up>(__v)) {}
template <class _Up, enable_if_t<_CheckOptionalArgsCtor<_Up>::template __enable_explicit<_Up>(), int> = 0>
template <class _Up = remove_cv_t<_Tp>,
enable_if_t<_CheckOptionalArgsCtor<_Up>::template __enable_explicit<_Up>(), int> = 0>
_LIBCPP_HIDE_FROM_ABI constexpr explicit optional(_Up&& __v) : __base(in_place, std::forward<_Up>(__v)) {}
// LWG2756: conditionally explicit conversion from const optional<_Up>&
@ -771,7 +772,7 @@ public:
_LIBCPP_HIDE_FROM_ABI constexpr optional& operator=(optional&&) = default;
// LWG2756
template <class _Up = value_type,
template <class _Up = remove_cv_t<value_type>,
enable_if_t<_And<_IsNotSame<__remove_cvref_t<_Up>, optional>,
_Or<_IsNotSame<__remove_cvref_t<_Up>, value_type>, _Not<is_scalar<value_type>>>,
is_constructible<value_type, _Up>,
@ -919,14 +920,14 @@ public:
return std::move(this->__get());
}
template <class _Up>
template <class _Up = remove_cv_t<_Tp>>
_LIBCPP_HIDE_FROM_ABI constexpr value_type value_or(_Up&& __v) const& {
static_assert(is_copy_constructible_v<value_type>, "optional<T>::value_or: T must be copy constructible");
static_assert(is_convertible_v<_Up, value_type>, "optional<T>::value_or: U must be convertible to T");
return this->has_value() ? this->__get() : static_cast<value_type>(std::forward<_Up>(__v));
}
template <class _Up>
template <class _Up = remove_cv_t<_Tp>>
_LIBCPP_HIDE_FROM_ABI constexpr value_type value_or(_Up&& __v) && {
static_assert(is_move_constructible_v<value_type>, "optional<T>::value_or: T must be move constructible");
static_assert(is_convertible_v<_Up, value_type>, "optional<T>::value_or: U must be convertible to T");

38
libcxx/test/std/utilities/expected/expected.expected/assign/assign.U.pass.cpp
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@ -325,6 +325,44 @@ constexpr bool test() {
}
}
// Check move constructor selection
{
struct MoveOnlyMulti {
bool used_move1 = false;
bool used_move2 = false;
constexpr MoveOnlyMulti() = default;
constexpr MoveOnlyMulti(const MoveOnlyMulti&) = delete;
constexpr MoveOnlyMulti& operator=(const MoveOnlyMulti&) = delete;
constexpr MoveOnlyMulti& operator=(MoveOnlyMulti&&) {
used_move1 = true;
return *this;
}
constexpr MoveOnlyMulti& operator=(const MoveOnlyMulti&&) {
used_move2 = true;
return *this;
};
constexpr MoveOnlyMulti(MoveOnlyMulti&&) : used_move1(true) {}
constexpr MoveOnlyMulti(const MoveOnlyMulti&&) : used_move2(true) {}
};
{
MoveOnlyMulti t{};
std::expected<MoveOnlyMulti, int> e1(std::unexpect);
static_assert(std::is_same_v<decltype(std::move(t)), MoveOnlyMulti&&>);
e1 = {std::move(t)};
assert(e1.value().used_move1);
}
{
const MoveOnlyMulti t{};
std::expected<MoveOnlyMulti, int> e1(std::unexpect);
static_assert(std::is_same_v<decltype(std::move(t)), const MoveOnlyMulti&&>);
// _Up = remove_cv_t<const MoveOnlyMulti&&> --> should use MoveOnlyMulti(MoveOnlyMulti&&)
e1 = {std::move(t)};
assert(e1.value().used_move1);
}
}
return true;
}

27
libcxx/test/std/utilities/expected/expected.expected/ctor/ctor.u.pass.cpp
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@ -80,6 +80,17 @@ struct CopyOnly {
friend constexpr bool operator==(const CopyOnly& mi, int ii) { return mi.i == ii; }
};
struct MoveOnly2 {
int j;
bool used_move1 = false;
bool used_move2 = false;
constexpr explicit MoveOnly2(int jj) : j(jj) {}
constexpr MoveOnly2(const MoveOnly2&) = delete;
constexpr MoveOnly2(MoveOnly2&& m) : j(m.j), used_move1(true) {}
constexpr MoveOnly2(const MoveOnly2&& m) : j(m.j), used_move2(true) {}
};
struct BaseError {};
struct DerivedError : BaseError {};
@ -164,6 +175,22 @@ constexpr bool test() {
assert(e2.has_value());
assert(!e2.value()); // yes, e2 holds "false" since LWG3836
}
// Check move constructor selection
{
MoveOnly2 t{1};
std::expected<MoveOnly2, BaseError> e1(std::move(t));
assert(e1.has_value());
assert(e1.value().used_move1 == true);
assert(e1.value().j == 1);
}
{
const MoveOnly2 t2{2};
std::expected<MoveOnly2, BaseError> e1(std::move(t2));
assert(e1.has_value());
assert(e1.value().used_move2 == true);
assert(e1.value().j == 2);
}
return true;
}

8
libcxx/test/std/utilities/optional/optional.object/optional.object.ctor/U.pass.cpp
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@ -59,7 +59,8 @@ constexpr bool explicit_conversion(Input&& in, const Expect& v)
static_assert(!std::is_constructible<O, void*>::value, "");
static_assert(!std::is_constructible<O, Input, int>::value, "");
optional<To> opt(std::forward<Input>(in));
return opt && *opt == static_cast<To>(v);
optional<To> opt2{std::forward<Input>(in)};
return opt && *opt == static_cast<To>(v) && (opt2 && *opt2 == static_cast<To>(v));
}
void test_implicit()
@ -83,6 +84,11 @@ void test_implicit()
using T = TestTypes::TestType;
assert(implicit_conversion<T>(3, T(3)));
}
{
using T = TestTypes::TestType;
optional<T> opt({3});
assert(opt && *opt == static_cast<T>(3));
}
{
using O = optional<ImplicitAny>;
static_assert(!test_convertible<O, std::in_place_t>(), "");

16
libcxx/test/std/utilities/optional/optional.object/optional.object.observe/value_or.pass.cpp
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@ -40,6 +40,12 @@ struct X
{return x.i_ == y.i_;}
};
struct Z {
int i_, j_;
constexpr Z(int i, int j) : i_(i), j_(j) {}
friend constexpr bool operator==(const Z& z1, const Z& z2) { return z1.i_ == z2.i_ && z1.j_ == z2.j_; }
};
constexpr int test()
{
{
@ -64,6 +70,16 @@ constexpr int test()
assert(std::move(opt).value_or(Y(3)) == 4);
assert(!opt);
}
{
optional<X> opt;
assert(std::move(opt).value_or({Y(3)}) == 4);
assert(!opt);
}
{
optional<Z> opt;
assert((std::move(opt).value_or({2, 3}) == Z{2, 3}));
assert(!opt);
}
return 0;
}

4
libcxx/test/std/utilities/optional/optional.object/optional.object.observe/value_or_const.pass.cpp
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@ -75,6 +75,10 @@ int main(int, char**)
const optional<X> opt;
assert(opt.value_or(Y(3)) == 4);
}
{
const optional<X> opt;
assert(opt.value_or({Y(3)}) == 4);
}
return 0;
}