Wrapped property’s `didSet` is not really useful

a.swift

import Foundation

@propertyWrapper
struct Wrapper<T> {
    class Box<T> {
        var value: T

        init(_ initial: T) {
            value = initial
        }
    }

    var projectedValue: Box<T>

    var wrappedValue: T {
        get {
            return projectedValue.value
        }
        nonmutating set {
            projectedValue.value = newValue
        }
    }

    init(wrappedValue: T) {
        projectedValue = Box<T>(wrappedValue)
    }
}

struct S {
    @Wrapper
    var a: Int = 1 {
        didSet {
            print("didSet")
        }
    }
}

let s = S()
print(s.a) // 1

s.a = 2 // `didSet` is called.
print(s.a) // 2

s.$a.value = 3 // We can bypass `didSet` by changing wrappedValue via projectedValue.
print(s.a) // 3