WeakSelfSolution.swift

struct Weak<T: AnyObject> {
    private let o: () -> T?

    init(_ o: T) {
        self.o = { [weak o] in o }
    }
}

extension Weak {
    func `do`<U>(execute: (T) -> U?) -> U? {
        guard let o = o() else {
            return nil
        }
        return execute(o)
    }
}

protocol Trait {}
enum Leak: Trait {}
enum NoLeak: Trait {}

final class Sample<T: Trait> {
    private var handler: (() -> String?)?

    deinit {
        print("deinit Called when Trait is \(T.self)")
    }

    private func string(from value: Int) -> String {
        return String(value)
    }
}

extension Sample where T == Leak {
    convenience init(_: Void) {
        self.init()
        self.handler = { self.string(from: 0) }
    }
}

extension Sample where T == NoLeak {
    convenience init(_: Void) {
        self.init()

        let weakSelf = Weak(self)
        self.handler = {
            weakSelf.do { $0.string(from: 0) }
        }
    }
}

var leak: Sample<Leak>? = .init(())
leak = nil

var noLeak: Sample<NoLeak>? = .init(())
noLeak = nil


// deinit Called when Trait is NoLeak