Awaiting custom `Thenable<T>` in TypeScript

awaiting-custom-thenable.ts

interface Thenable<T> {
    then<U>(onResolve: (value: T) => U, onReject: (error: any) => Thenable<U>): Thenable<U>
    then<U>(onResolve: (value: T) => Thenable<U>, onReject: (error: any) => Thenable<U>): Thenable<U>
}

const client: Thenable<string> = {
  async then(resolve: (value: string) => Promise<string>, reject: (error: string) => Promise<string>) {
    return Math.random() > 0.5
      ? resolve('Success!')
      : reject('Failure!')
  }
}

const result = await client;

/* Enable top-level await */
export { }