`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}.

Riproduci l’esempio nel TypeScript Playground collegato con noUncheckedIndexedAccess abilitato, quindi confronta il caso Record<string, string> con Record<"k", string> con la coercizione equivalente a { k: string }, che viene già rifiutata. Il lavoro è completato quando la coercizione non sicura di Record viene rifiutata con noUncheckedIndexedAccess senza modificare il comportamento previsto descritto nell’issue.