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3940.std::expected<void, E>::value() also needsE to be copy constructible

Section: 22.8.7.6[expected.void.obs]Status:WPSubmitter: Jiang AnOpened: 2023-05-26Last modified: 2023-11-22

Priority:Not Prioritized

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Discussion:

LWG3843⁽ⁱ⁾ addedMandates: tostd::expected::value, but the similar handling is missing forexpected<cv void, E>.

[2023-06-01; Reflector poll]

Set status to Tentatively Ready after seven votes in favour during reflector poll.

[2023-06-17 Approved at June 2023 meeting in Varna. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative toN4950.

1. Modify 22.8.7.6[expected.void.obs] as indicated:

   constexpr void value() const &;

   -?-Mandates:is_copy_constructible_v<E> istrue.

   -3-Throws:bad_expected_access(error()) ifhas_value() isfalse.

   constexpr void value() &&;

   -?-Mandates:is_copy_constructible_v<E> istrue andis_move_constructible_v<E> istrue.

   -4-Throws:bad_expected_access(std::move(error())) ifhas_value() isfalse.