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).