[expected.void.assign]

constexpr expected& operator=(const expected& rhs);

Effects:

(1.1)
If this->has_value() && rhs.has_value() is true, no effects.
(1.2)
Otherwise, if this->has_value() is true, equivalent to: construct_at(addressof(unex), rhs.unex); has_val = false;
(1.3)
Otherwise, if rhs.has_value() is true, destroys unex and sets has_val to true.
(1.4)
Otherwise, equivalent to unex = rhs.error().

Remarks: This operator is defined as deleted unless is_copy_assignable_v<E> is true and is_copy_constructible_v<E> is true.

constexpr expected& operator=(expected&& rhs) noexcept(see below);

Constraints: is_move_constructible_v<E> is true and is_move_assignable_v<E> is true.

Effects:

(5.1)
If this->has_value() && rhs.has_value() is true, no effects.
(5.2)
Otherwise, if this->has_value() is true, equivalent to: construct_at(addressof(unex), std::move(rhs.unex)); has_val = false;
(5.3)
Otherwise, if rhs.has_value() is true, destroys unex and sets has_val to true.
(5.4)
Otherwise, equivalent to unex = std::move(rhs.error().

Remarks: The exception specification is equivalent to is_nothrow_move_constructible_v<E> && is_nothrow_move_assignable_v<E>.

template<class G>
  constexpr expected& operator=(const unexpected<G>& e);
template<class G>
  constexpr expected& operator=(unexpected<G>& e);

Let GF be const G& for the first overload and G for the second overload.

Constraints: is_constructible_v<E, GF> is true and is_assignable_v<E&, GF> is true.

Effects:

(10.1)
If has_value() is true, equivalent to: construct_at(addressof(unex), std::forward<GF>(e.error(); has_val = false;
(10.2)
Otherwise, equivalent to: unex = std::forward<GF>(e.error();

Returns: *this.

constexpr void emplace() noexcept;

Effects: If has_value() is false, destroys unex and sets has_val to true.

22 General utilities library [utilities]