Section: 22.8.6.6 [expected.object.obs] Status: C++23 Submitter: Jonathan Wakely Opened: 2022-12-20 Last modified: 2023-11-22 15:57:15 UTC
Priority: Not Prioritized
View all issues with C++23 status.
Discussion:
22.8.6.6 [expected.object.obs] p9 says:
Throws: bad_expected_access(error()) if has_value() is false.
But if error() returns a reference to a move-only type then it can't be copied and the function body is ill-formed. Should it be constrained with is_copy_constructible_v<E>? Or just mandate it?
Similarly, the value()&& and value() const&& overloads require is_move_constructible_v<E> to be true for bad_expected_access(std::move(error())) to be valid. Casey Carter pointed out they also require it to be copyable so that the exception can be thrown, as per 14.2 [except.throw] p5.
[Issaquah 2023-02-09; LWG]
Move to Immediate for C++23
[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Immediate → WP.]
Proposed resolution:
Modify 22.8.6.6 [expected.object.obs] as indicated:
constexpr const T& value() const &; constexpr T& value() &;-?- Mandates: is_copy_constructible_v<E> is true.
-8- Returns: val, if has_value() is true.
-9- Throws: bad_expected_access(as_const(error())) if has_value() is false.
constexpr T&& value() &&; constexpr const T&& value() const &&;-?- Mandates: is_copy_constructible_v<E> is true and is_constructible_v<E, decltype(std::move(error()))> is true.
-10- Returns: std::move(val), if has_value() is true.
-11- Throws: bad_expected_access(std::move(error())) if has_value() is false.