Type narrowing in one case statement does not propagate to the next case

🔎 Search Terms

• Type narrowing
• Switch statement

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about switch statements and type narrowing.

⏯ Playground Link

https://www.typescriptlang.org/play?ts=5.3.0-beta#code/GYVwdgxgLglg9mABMOcAUBDATgcwFyIDOUWMYOiAPouACYCmwZ9tAlIgN4BQivRA7jCgQAFojQkQ9dtz5zEEDIXqJsFALyaaYBkzAs8Pecagiscfon2WAolnNY0rANxcjxxctW4AdMWxQhADqQiJoAEQAHuGsBAD0cYj09nBYiAAqAMoAjAAcAAwALLkEAORqpYgwhIgADnCEhDAARgA2AJ6IpXSMzLSlPu7GfFj0UCBYSOEAguGuQ3K6GCCtUIbD8qPjk4jhAEJzQwC+XCdAA

💻 Code

function foo(arg: string | undefined) {
    switch (true) {
        case arg === undefined:
            throw new Error();

        case arg.startsWith("x"): // error TS18048: 'arg' is possibly 'undefined'.
            return "A";

        default:
            return "B";
    }
}

🙁 Actual behavior

error TS18048: 'arg' is possibly 'undefined'. in the second case.

🙂 Expected behavior

I expect the type to be narrowed to string in the second case statement.

Additional information about the issue

See also #56352. Ping @Andarist.

Start with the linked TypeScript Playground reproduction and compare the related issue #56352. Trace how the switch cases are type-checked, then verify that the second case narrows arg to string without producing TS18048.