charlypoly · November 29, 2021 11:21
diff --git a/retool-typescript-control-flow-analysis-13.ts b/retool-typescript-control-flow-analysis-13.ts
 function isPremiumUser(user: User | PremiumUser): user is PremiumUser {
  return user.plan === "premium"
 }

 let user: PremiumUser | User | undefined

 // the line below should raise a TypeScript error,
 //  but it doesn't!
 if (isPremiumUser(user)) {
  // ...
 } else {
  // ...
 }
	function isPremiumUser(user: User \| PremiumUser): user is PremiumUser {
	return user.plan === "premium"
	}

	let user: PremiumUser \| User \| undefined

	// the line below should raise a TypeScript error,
	// but it doesn't!
	if (isPremiumUser(user)) {
	// ...
	} else {
	// ...
	}