`noUncheckedIndexedAccess` should forbid unsound `Record<string, string>` → `Record<"k", string>` coercion

🔎 Search Terms

coercion, noUncheckedIndexedAccess, Record, unsound

🕗 Version & Regression Information

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

⏯ Playground Link

https://www.typescriptlang.org/play/?noUncheckedIndexedAccess=true&ts=5.9.3#code/DYUwLgBAhgXBBKIDGB7ATgEwDwGcxoEsA7AcwBoI9DSA+CAXggG8BfAbgChRIAjORVJiwAiANbCKVYiTqMobCAHpFEAK5EcKdRi7gIPUXCmkG+gHSjOBs2BQAZFAHcQaAMJQcIABQBKBcogAMygCYBxoSDR1MAIAWxAOIA

💻 Code

let a: Record<string, string> = {};
let b: Record<"k", string> = a; // unsound
let bk: string = b.k;
bk.toLowerCase(); // fails at runtime

🙁 Actual behavior

No TypeScript errors.

🙂 Expected behavior

By enabling noUncheckedIndexedAccess, I’ve opted in to stricter errors that prevent mistakes where a potentially undefined record element is assumed to be defined. The coercion from a: Record<string, string> to Record<"k", string> is such a mistake, since it assumes that a.k is defined. So TypeScript should forbid this coercion when noUncheckedIndexedAccess is enabled.

Additional information about the issue

noUncheckedIndexedAccess already causes TypeScript to correctly reject the equivalent unsound coercion Record<string, string> → {k: string}.

Reproduce el ejemplo del TypeScript Playground enlazado con noUncheckedIndexedAccess habilitado y, a continuación, compara el caso Record<string, string> con Record<"k", string> con la coerción equivalente a { k: string }, que ya se rechaza. Se considera terminado cuando la coerción insegura de Record se rechaza con noUncheckedIndexedAccess sin cambiar el comportamiento esperado descrito en el issue.