`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}.

リンク先の TypeScript Playground の例を noUncheckedIndexedAccess を有効にして再現し、次に Record<string, string> から Record<"k", string> へのケースを、すでに拒否されている { k: string } への同等の coercion と比較します。noUncheckedIndexedAccess の下で安全でない Record の coercion が拒否され、issue に記載された期待される動作が変更されなければ完了です。