protocol A {
    associatedtype T
}


protocol B: A where T: B, T.U == U {
    associatedtype U
}


protocol C: B where T: C, U: Numeric {
    
}


protocol D: C where T: D, U == Int {
    
}


struct X <U>: B {
    typealias T = X<U>
}


extension X: C where U: Numeric {
    
}


extension X: D where U == Int {
    
}


struct Y: C {
    typealias U = Double
    typealias T = X<U>
}

extension X: D where U == Int {
    
}