[swift-evolution] Feasibility of "T1 & ¬T2"

[Dave DeLong]

Hi SE,

I’m pondering some esoteric type stuff as I’m sketching out an improved date/time library (https://github.com/davedelong/Chronology <https://github.com/davedelong/Chronology>), and I’m wondering two things:

1️⃣ Is there a way to express a type that does *not* conform to a protocol? 

For example, let’s say I have three protocols:

protocol YearHaving { var year: Int }
protocol MonthHaving { var month: Int }
protocol DayHaving { var day: Int }
typealias DateHaving = YearHaving & MonthHaving & DayHaving

Now, I want to allow for some adjustments to values that have these types:

extension DayHaving {
    func removingDay() → Self & ¬DayHaving // whatever type “self” is, except it does not conform to “DayHaving”, because you just removed the day value
}

Is this possible?

2️⃣ If this is not possible, would it be hard to add something like this in?

Dave
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