error_condition() noexcept;
Postconditions: val_ == 0 and
cat_ == &generic_category(). error_condition(int val, const error_category& cat) noexcept;
Postconditions: val_ == val and
cat_ == &cat. template<class ErrorConditionEnum>
error_condition(ErrorConditionEnum e) noexcept;
Constraints: is_error_condition_enum_v<ErrorConditionEnum> is
true. Postconditions: *this == make_error_condition(e).