"Should not extend type you don't own with protocol you don't own" is the advice, but

Using Swift
4

That's the cost, yeah. It might be possible to solve this with nested property wrappers; a quick simplified attempt seems to work.

(code) 
import SwiftUI

@propertyWrapper
struct Wrap {
	var wrappedValue: Int
}
extension Wrap: RawRepresentable {
	var rawValue: String {
		wrappedValue.description
	}
	init?(rawValue: String) {
		guard let value = Int(rawValue) else {
			return nil
		}
		wrappedValue = value
	}
}

struct Test {
	@AppStorage("x") @Wrap var x: Int = 5
}

Test().x = 6
print(UserDefaults.standard.object(forKey: "x"))

I meant "…SinceReferenceDate" but once again the fact that it's ambiguous sort of points out why you shouldn't do this: someone else might have different opinions.

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