// @target: es2015
// @strict: false
class Chain<T extends A> {
    constructor(public value: T) { }
    then<S extends T>(cb: (x: T) => S): Chain<S> {
        return null;
    }
}

class A {
    x;
}
class B extends A {
    y;
}
class C extends B {
    z;
}

// Ok to go down the chain, but error to try to climb back up
(new Chain(new A)).then(a => new B).then(b => new C).then(c => new B).then(b => new A);