TypeScript Version: 2.4.2 / Typescript playground

Code

export interface Storage {
    getItem<T>(onComplete?: OnComplete<T>): Promise<T>;
}

export type OnComplete<T> = (err?: any, result?: T) => any;

let storage: Storage = {
    getItem: (onComplete: OnComplete<string>) => {
        return Promise.resolve("Yay");
    }
};

Typescript playground link:
https://www.typescriptlang.org/play/#src=export%20interface%20Storage%20%7B%0D%0A%20%20%20%20getItem%3CT%3E(onComplete%3F%3A%20OnComplete%3CT%3E)%3A%20Promise%3CT%3E%3B%0D%0A%7D%0D%0A%0D%0Aexport%20type%20OnComplete%3CT%3E%20%3D%20(err%3F%3A%20any%2C%20result%3F%3A%20T)%20%3D%3E%20any%3B%0D%0A%0D%0Alet%20storage%3A%20Storage%20%3D%20%7B%0D%0A%20%20%20%20getItem%3A%20(onComplete%3A%20OnComplete%3Cstring%3E)%20%3D%3E%20%7B%0D%0A%20%20%20%20%20%20%20%20return%20Promise.resolve(%22Yay%22)%3B%0D%0A%20%20%20%20%7D%0D%0A%7D%3B%0D%0A

Expected behavior:
Code snippet above works

Actual behavior:
Ultimately, string is not assignable to type T on Typescript playground. On 2.4.2, I get string is not assignable to T | undefined. Both makes no sense...