https://gcc.gnu.org/g:1725567dfce807e6395d726057ccc85b47a7f3c1
commit r16-5497-g1725567dfce807e6395d726057ccc85b47a7f3c1 Author: Jonathan Wakely <[email protected]> Date: Wed Nov 19 19:04:05 2025 +0000 libstdc++: Implement LWG 4406 and LWG 3424 for std::optional and std::expected This adjusts the return statements of optional::value_or and expected::value_or to not perform explicit conversions, so that the actual conversion performed matches the requirements expressed in the Mandates: elements (LWG 4406). Also adjust the return types to remove cv-qualifiers (LWG 3424). libstdc++-v3/ChangeLog: * include/std/expected (expected::value_or): Use remove_cv_t for the return type. Do not use static_cast for return statement. Adjust static_assert conditions to match return statements. * include/std/optional (optional::value_or): Likewise. (optional<T&>::value_or): Likewise. Reviewed-by: Tomasz Kamiński <[email protected]> Diff: --- libstdc++-v3/include/std/expected | 18 +++++++++++------- libstdc++-v3/include/std/optional | 28 ++++++++++++++++------------ 2 files changed, 27 insertions(+), 19 deletions(-) diff --git a/libstdc++-v3/include/std/expected b/libstdc++-v3/include/std/expected index 4eaaab693e1e..591fc72a4388 100644 --- a/libstdc++-v3/include/std/expected +++ b/libstdc++-v3/include/std/expected @@ -822,32 +822,36 @@ namespace __expected return std::move(_M_unex); } + // _GLIBCXX_RESOLVE_LIB_DEFECTS + // 4406. value_or return statement is inconsistent with Mandates template<typename _Up = remove_cv_t<_Tp>> - constexpr _Tp + constexpr remove_cv_t<_Tp> value_or(_Up&& __v) const & noexcept(__and_v<is_nothrow_copy_constructible<_Tp>, is_nothrow_convertible<_Up, _Tp>>) { - static_assert( is_copy_constructible_v<_Tp> ); + using _Xp = remove_cv_t<_Tp>; + static_assert( is_convertible_v<const _Tp&, _Xp> ); static_assert( is_convertible_v<_Up, _Tp> ); if (_M_has_value) return _M_val; - return static_cast<_Tp>(std::forward<_Up>(__v)); + return std::forward<_Up>(__v); } template<typename _Up = remove_cv_t<_Tp>> - constexpr _Tp + constexpr remove_cv_t<_Tp> value_or(_Up&& __v) && noexcept(__and_v<is_nothrow_move_constructible<_Tp>, is_nothrow_convertible<_Up, _Tp>>) { - static_assert( is_move_constructible_v<_Tp> ); - static_assert( is_convertible_v<_Up, _Tp> ); + using _Xp = remove_cv_t<_Tp>; + static_assert( is_convertible_v<_Tp, _Xp> ); + static_assert( is_convertible_v<_Up, _Xp> ); if (_M_has_value) return std::move(_M_val); - return static_cast<_Tp>(std::forward<_Up>(__v)); + return std::forward<_Up>(__v); } template<typename _Gr = _Er> diff --git a/libstdc++-v3/include/std/optional b/libstdc++-v3/include/std/optional index 41c04b10720e..f54dda6cd237 100644 --- a/libstdc++-v3/include/std/optional +++ b/libstdc++-v3/include/std/optional @@ -1285,30 +1285,32 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION __throw_bad_optional_access(); } + // _GLIBCXX_RESOLVE_LIB_DEFECTS + // 4406. value_or return statement is inconsistent with Mandates template<typename _Up = remove_cv_t<_Tp>> - constexpr _Tp + constexpr remove_cv_t<_Tp> value_or(_Up&& __u) const& { - static_assert(is_copy_constructible_v<_Tp>); - static_assert(is_convertible_v<_Up&&, _Tp>); + using _Xp = remove_cv_t<_Tp>; + static_assert(is_convertible_v<const _Tp&, _Xp>); + static_assert(is_convertible_v<_Up, _Xp>); if (this->_M_is_engaged()) return this->_M_get(); - else - return static_cast<_Tp>(std::forward<_Up>(__u)); + return std::forward<_Up>(__u); } template<typename _Up = remove_cv_t<_Tp>> - constexpr _Tp + constexpr remove_cv_t<_Tp> value_or(_Up&& __u) && { - static_assert(is_move_constructible_v<_Tp>); - static_assert(is_convertible_v<_Up&&, _Tp>); + using _Xp = remove_cv_t<_Tp>; + static_assert(is_convertible_v<_Tp, _Xp>); + static_assert(is_convertible_v<_Up, _Xp>); if (this->_M_is_engaged()) return std::move(this->_M_get()); - else - return static_cast<_Tp>(std::forward<_Up>(__u)); + return std::forward<_Up>(__u); } #if __cpp_lib_optional >= 202110L // C++23 @@ -1726,9 +1728,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION value_or(_Up&& __u) const { using _Xp = remove_cv_t<_Tp>; - static_assert(is_constructible_v<_Xp, _Tp&>); + static_assert(is_convertible_v<_Tp&, _Xp>); static_assert(is_convertible_v<_Up, _Xp>); - return _M_val ? *_M_val : static_cast<_Xp>(std::forward<_Up>(__u)); + if (_M_val) + return *_M_val; + return std::forward<_Up>(__u); } // Monadic operations.
