final
class C 
{
    init() 
    {
    }
}
struct S
{
    var c:C

    init()
    {
        self.c = .init()
    }
    mutating 
    func foo()
    {
        print(isKnownUniquelyReferenced(&self.c))
    }
}

enum E
{
    case empty 
    case s(S) 
}

func foo()  
{
    var e:E = .s(.init())
    switch e 
    {
    case .s(var s):
        e = .empty
        s.foo()
    default:
        break
    }
}

foo()