Narrowed `const` variables are widened in block-scoped closures

🔎 Search Terms

narrowing, cfa, closure,

🕗 Version & Regression Information

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

⏯ Playground Link

https://www.typescriptlang.org/play/?noUncheckedIndexedAccess=true&target=99&ts=5.8.0-dev.20250209#code/CYUwxgNghgTiAEYD2A7AzgF3mgXNjMAligOYDcAUBYQGbwAUa8AvK-AOQ1JLsCU8AbwrwRiVJmwBBCIShNm2SqPg0ArijAZCqeACMowevyHLlydFi5I8nbuxaLhogL5VlajVp0luh409MxCxVuGyt7BTRpWTQlFwpXChAADwAHJBgsAWcgA

💻 Code

declare const s: string;

if (s === 'foo') {
    function bad() {
        const foo: 'foo' = s;
    }
}

// in non-strict code bad() might be accessible outside of the narrowed branch,
// so make sure we're strict
export {}

🙁 Actual behavior

error trying to use the variable as its narrowed value.

🙂 Expected behavior

No error. bad() is a local function only reachable within the scope in which s has been definitely narrowed to 'foo'.

Additional Information

Found organically in https://github.com/typescript-eslint/typescript-eslint/pull/10182#discussion_r1948337564 (with a narrowed member access corollary)