`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 the example in the linked TypeScript Playground with noUncheckedIndexedAccess enabled, then compare the Record<string, string> to Record<"k", string> case with the equivalent coercion to { k: string }, which is already rejected. Done means the unsound Record coercion is rejected under noUncheckedIndexedAccess without changing the expected behavior described in the issue.