Dynamic unowned generic value

Unowned.swift

@propertyWrapper
internal final class Unowned<Wrapped> {
  private let wrapper: Wrapper

  fileprivate init(wrapper: Wrapper) {
    self.wrapper = wrapper
  }

  internal var wrappedValue: Wrapped {
    get { wrapper.get() }
    set { wrapper.set(newValue) }
  }

  fileprivate struct Wrapper {
    fileprivate var get: () -> Wrapped
    fileprivate var set: (Wrapped) -> Void
  }
}

extension Unowned where Wrapped: AnyObject {
  internal convenience init(wrappedValue: Wrapped)  {
    self.init(wrapper: fromClass(wrappedValue))
  }
}

extension Unowned {
  internal convenience init(wrappedValue: Wrapped) {
    self.init(wrapper: from(wrappedValue))
  }
}

fileprivate func fromClass<T: AnyObject>(_ value: T) -> Unowned<T>.Wrapper {
  let container = ReferenceContainer<T>(value: value)
  return .init(
    get: { container.value },
    set: { container.value = $0 }
  )
}

fileprivate func from<T: Any>(_ value: T) -> Unowned<T>.Wrapper {
  let container = ValueContainer<T>(value: value)
  return .init(
    get: { container.value },
    set: { container.value = $0 }
  )
}

private final class ReferenceContainer<Value: AnyObject> {
  unowned var value: Value
  init(value: Value) {
    self.value = value
  }
}

private final class ValueContainer<Value> {
  var value: Value
  init(value: Value) {
    self.value = value
  }
}

Sample:

class ClassToTest { init() {} }
struct StructToTest { init() {} }
struct TestUnowned {
  @Unowned var unownedClass: ClassToTest = ClassToTest()
  @Unowned var unownedStruct: StructToTest = StructToTest()
}
---
_ = TestUnowned().unownedStruct // ok
_ = TestUnowned().unownedClass // crash - unowned deallocated