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

Created on 2022-12-20.00:00:00 last changed 1 month ago


Date: 2022-12-20.12:34:38

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(error()) if has_value() is false.

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

    -?- Mandates: is_move_constructible_v<E> 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.

Date: 2022-12-20.00:00:00

[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.

Date User Action Args
2022-12-20 12:34:38adminsetmessages: + msg13164
2022-12-20 00:00:00admincreate