print-certified-mobile.ts

const exhaustionCheck = (e: never) => console.log('this will never be executed')

const printMobileSafe = (mobile: Mobile) => {
  if(mobile.status === 'confirmed') {
    console.log('Your mobile is confirmed!')
  } else if(mobile.status === 'unconfirmed') {
    console.warn('Your mobile is not confirmed!')
  } else {
    exhaustionCheck(mobile) // <-- you'll get a compilation error here!
  }
}

const certified: CertifiedMobile = { status: 'certified', value: '543234531' }

printMobileSafe(certified)