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

Section: [expected.object.obs] Status: WP Submitter: Jonathan Wakely Opened: 2022-12-20 Last modified: 2023-02-13

Priority: Not Prioritized

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Discussion: [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 Status changed: Immediate → WP.]

Proposed resolution:

  1. Modify [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.