Arithmetic is defined on range types. So day 30 + day 4 is 34, which if you then try to coerce into a day will throw an exception.
And... So? It seems you are saying Day_type + Integer is Integer, and Integer (in general) cannot safely be coerced into Day_type and... that’s logically correct?
`Day_type + Day_type` is also defined, and it overflows. That's like normal integers, and unlike enums.