interface Data {
  fn(arg: string): Array<string>
  fn(arg: number): null
}
let data: Data;
const a = data.fn(42); // null
const b = data.fn("str"); // Array<string>


interface API {
  "/users": { params: [], response: User[]}
  "/users/:id": { params: [number], response: User}
}

let users: API['/users'] // { params: [], response: User[]}
let user: API['/users/:id'] // params: [number], response: User


const fn = (arg: 0 | 1) => 42;

fn(0); // OK
fn(1); // OK
fn(2); // ERROR Argument of type '2' is not assignable to parameter of type '0 | 1'.
(num: number) => fn(num); // ERROR Argument of type 'number' is not assignable to parameter of type '0 | 1'.