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3843.std::expected<T,E>::value() & assumesE is copy constructible

Section: 22.8.6.6[expected.object.obs]Status:C++23Submitter: Jonathan WakelyOpened: 2022-12-20Last modified: 2023-11-22

Priority:Not Prioritized

View all issues withC++23 status.

Discussion:

22.8.6.6[expected.object.obs] p9 says:

Throws:bad_expected_access(error()) ifhas_value() isfalse.

But iferror() returns a reference to a move-only typethen it can't be copied and the function body is ill-formed.Should it be constrained withis_copy_constructible_v<E>?Or just mandate it?

Similarly, thevalue()&& andvalue() const&&overloads requireis_move_constructible_v<E> to be trueforbad_expected_access(std::move(error())) to be valid.Casey Carter pointed out they also require it to be copyable so that theexception 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:

1. Modify 22.8.6.6[expected.object.obs] as indicated:

   constexpr const T& value() const &;constexpr T& value() &;

   -?-Mandates:is_copy_constructible_v<E> istrue.

   -8-Returns:val, ifhas_value() istrue.

   -9-Throws:bad_expected_access(as_const(error()))ifhas_value() isfalse.

   constexpr T&& value() &&;constexpr const T&& value() const &&;

   -?-Mandates:is_copy_constructible_v<E> istrueandis_constructible_v<E, decltype(std::move(error()))> istrue.

   -10-Returns:std::move(val), ifhas_value() istrue.

   -11-Throws:bad_expected_access(std::move(error()))ifhas_value() isfalse.