Move assignment.

Requires: that value_type and error_type both implement move assignment.

Complexity: If the value_type for both is present, uses value_type’s move assignment operator, else either destructs or move constructs value_type as appropriate. error_type’s move assignment operator is always used. Constexpr, triviality and noexcept of underlying operations is propagated.

Guarantees: If an exception is thrown during the operation, the object is left in a partially completed state, as per the normal rules for the same operation on a struct.