switch-true case-true narrowing not equivalent to default case.

🔎 Search Terms

"switch true" "switch-true" switch true narrowing. Most issues that turned up were from 2023 which I think precedes the release of the switch-true type narrowing feature?

🕗 Version & Regression Information

• This is a gap

⏯ Playground Link

https://www.typescriptlang.org/play/?ts=5.6.0-dev.20240703&ssl=22&ssc=1&pln=12&pc=1#code/GYVwdgxgLglg9mABHA1gQwJ4AoZgA4hQBciAzlAE64DmAlIiQEQCCjAPowELuMDCjiAN4AoRGLIB3GFAgALRFkogApvRHiNiCGlLLEAejT6AdFGXkc+QrSKjN9isqggKSFowDcd+9t0GARiZmFrgEUDbe9uKOzq6IXJ6RGgAmysBoIAA2xElRMS5u-F4aAL7CZcKgkLAIiCCycLKWYSTkVGB0DPGsHNwc-EKRpFIy8ooUKmq5Yr56hkHmUM3WtlGa+XHuxVGzAQshVuGra9FOBfHc2z46ekrKxyeIG4WJpeXCQA

💻 Code

function uhoh(input: string) : "A"|"B"|"C" {
    switch (true) {
        case /a/.test(input):
            return "A";
        case /b/.test(input):
            return "B";
        case true: // <--- default: works fine here
            return "C";
    }
}

🙁 Actual behavior

Complains that the result of uhoh could be undefined.

🙂 Expected behavior

switch(true) { case true: } should behave like switch(true) { default: }, probably?

Additional information about the issue

Use the linked TypeScript Playground example as the starting repro. Trace how switch(true) handles the final case true versus default, then verify that the function no longer permits undefined while preserving the A/B/C results.