type FAILURE = "FAILURE";
const FAILURE = "FAILURE";

type Result<T> = T | FAILURE;

function doWork<T>(): Result<T> {
  return FAILURE;
}

function isSuccess<T>(result: Result<T>): result is T {
  return !isFailure(result);
}

function isFailure<T>(result: Result<T>): result is FAILURE {
  return result === FAILURE;
}

function increment(x: number): number {
  return x + 1;
}

let result = doWork<number>();

if (isSuccess(result)) {
  increment(result);
//          ~~~~~~
//          Argument of type 'string | number' is not assignable to parameter of type 'number'.
//            Type 'string' is not assignable to type 'number'.
}