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.

[ 2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Immediate → WP. ]

[ Issaquah 2023-02-09; LWG ]

Move to Immediate for C++23

[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 [except.throw] p5.