Microsoft/TypeScript

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

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#63,724 opened on Aug 6, 2026

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 (6 comments) (1 reaction) (0 assignees)TypeScript (13,395 forks)batch import
BugDomain: check: Control FlowHelp Wanted

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Description

🔎 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

Contributor guide

Research direction
Investigate how TypeScript's control flow analyzer handles type narrowing for intrinsic string manipulation types like Uppercase<string> and Lowercase<string> when compared to string literal values. Look into the checker's code (likely in src/compiler/checker.ts) for narrowing logic and see why the type is not being narrowed. Examine existing tests for Uppercase/Lowercase to understand expected behavior and add a test case.
Tech stack
typescript
Domain
compilers
Issue type
Bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
4
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
3-5 days
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
Active
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
Clear
Prerequisites
TypeScript
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
20