Type narrowing not working correctly with Uppercase<string> & Lowercase<string>

🔎 Search Terms

"narrowing lowercase" "narrowing uppercase"

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about Common Bugs and Type System Behavior

⏯ Playground Link

https://www.typescriptlang.org/play/?#code/DYUwLgBAxg9grgOzAJwJ4GEYBMQC4IBEAQgKIEQA+hAMgKrlUEBy1BA3AFAewIDOkAB2QgAZgEsAHhAC8hAEYgCAOjAxaAgSGToAhrxAAKAJQQ9EdZuRQ9IADz9kYhAHMAfJw5iREA0NGSZaVliMkoqP3EpIOC6BnDhSMDglgITAG8OCCzoeCQ0TBwZCAjJTgB6MqyAPQ4KrIAVVE0IAHILLWt9exQnNxaIMV4IBBhIPV4xZwQdOVAIVXmmkFaQhhp6SkIUlqVayohG5paHXv7B4dHTXgmpmbmFsCWV0jWCWM3mVh2OAF8gA

💻 Code

let countryCode: "BE" | "LU" | "NL";

const prefix = "be".toUpperCase() as Uppercase<string>;

if (prefix === "BE" || prefix === "LU" || prefix === "NL") {
    countryCode = prefix;
//  ^
//  Type 'Uppercase<string>' is not assignable to type '"BE" | "LU" | "NL"'.
//  Type 'string' is not assignable to type '"BE" | "LU" | "NL"'.
}

🙁 Actual behavior

TypeScript cannot narrow down an Uppercase<string> or Lowercase<string> type after explicitly comparing against specific values.

🙂 Expected behavior

TypeScript should accept the code above as "BE" | "LU" | "NL" is strictly a sub-type of Uppercase<string>.

Additional information about the issue

No response