block8.ts

function executeLater(callback: () => void, timeout: PositiveNumber) {
   setTimeout(callback, timeout); //no need to unwrap, timeout is still a `number`
}

const timeout: number = 100;
const callback = () => console.log('Execution!')

executeLater(callback, timeout) //Argument of type 'number' is not assignable to parameter of type 'PositiveNumber'. 
// TypeScript doesn't know that `100` is a `PositiveNumber`

if (isPositiveNumber(timeout)) {
   executeLater(callback, timeout) //compile and works !
}