WTF TypeScript

hmm.ts

interface Service {
  update(obj: { id?: string }): void;
  // changing this to a function type `update: (obj: {id?: string}) => void` works
  // and causes the implementation not to type-check
}

class ServiceImpl implements Service {
  // somehow this is a valid implementation
  update(obj: { id: string }): void {
    // `id` can never be undefined or null here according to the type system
    console.log(typeof obj.id); // definitely string surely!
  }
}

function program(service: Service): void {
  return service.update({}); // didn't pass an `id`
}

// prints `undefined` from an implementation where it shouldn't be possible
program(new ServiceImpl());

Related to microsoft/TypeScript#20537