[libcxx-commits] [libcxx] [libc++] Implement Resolution of LWG 3886 (PR #155356)
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Mon Aug 25 22:18:50 PDT 2025
llvmbot wrote:
<!--LLVM PR SUMMARY COMMENT-->
@llvm/pr-subscribers-libcxx
Author: William Tran-Viet (smallp-o-p)
<details>
<summary>Changes</summary>
- Implement the resolution of [LWG3886](https://cplusplus.github.io/LWG/issue3886) for `optional` and `expected`
---
Full diff: https://github.com/llvm/llvm-project/pull/155356.diff
2 Files Affected:
- (modified) libcxx/include/__expected/expected.h (+4-4)
- (modified) libcxx/include/optional (+9-8)
``````````diff
diff --git a/libcxx/include/__expected/expected.h b/libcxx/include/__expected/expected.h
index 38a34121040f6..8b3eeebd38ae7 100644
--- a/libcxx/include/__expected/expected.h
+++ b/libcxx/include/__expected/expected.h
@@ -555,7 +555,7 @@ class expected : private __expected_base<_Tp, _Err> {
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 @@ class expected : private __expected_base<_Tp, _Err> {
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 @@ class expected : private __expected_base<_Tp, _Err> {
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");
diff --git a/libcxx/include/optional b/libcxx/include/optional
index 39fcaa2c2ec18..ef1bfd3ec44c0 100644
--- a/libcxx/include/optional
+++ b/libcxx/include/optional
@@ -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");
``````````
</details>
https://github.com/llvm/llvm-project/pull/155356
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