False positive for "Unreachable code detected" for code following `switch`/`case` with `allowUnreachableCode: false`

🔎 Search Terms

"false positive allowUnreachableCode"
"allowUnreachableCode case statement"
"Unreachable code detected case statement"

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about switch/case

⏯ Playground Link

https://www.typescriptlang.org/play/?#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-aDRGQWHEs2Vc+KOoX-qWc3nPQWyEA

💻 Code

type Foo = "bar" | "baz"

const fn = (value: Foo): number => {
    switch(value) {
        case "bar":
            return 1;
        case "baz":
            return 2;
    }

    return assertNever("nope!");
}

🙁 Actual behavior

return assertNever("nope!");

is highlighted as "Unreachable code detected". The docs, however, state:

This does not affect errors on the basis of code which appears to be unreachable due to type analysis.

🙂 Expected behavior

No error. This should be the same as the other case form, the if form, and various similar constructs where even though it's allegedly exhaustive, that's just based off of type information (I would like to have a runtime assertion in this case).

Additional information about the issue

Possibly related to: https://github.com/microsoft/TypeScript/issues/18882

Start with the linked TypeScript Playground reproduction and compare the switch/case example with the other case forms described in the report. Review the possibly related issue #18882, then verify that the runtime assertion after the exhaustive switch is no longer reported as unreachable while the existing unreachable-code behavior remains intact.