public protocol P {
    associatedtype Key
    func f <T> () -> T? where T == Key
}

public struct AnyP <Key>: P {
    let _f: () -> Key?
    
    public init <S: P> (_ s: S) where S.Key == Key {
        _f = s.f
    }
    
    public init (_ f: @escaping () -> Key?) {
        _f = f
    }
    
    public func f() -> Key? {
        return _f()
    }
}
let anyP = AnyP<String>( { return nil })
let nestedAnyP = AnyP(anyP)
anyP.f() // This works
nestedAnyP.f() // This crashes