Inconsistency between element access and object destruction

🔎 Search Terms

destruct object

🕗 Version & Regression Information

Before v4.4.4, index expression in element access was also not inferred as const. So this issue starts from v4.4.4. Reproducible in Nightly build.

⏯ Playground Link

https://www.typescriptlang.org/play/#code/DYUwLgBAdhC8EEYIEMDOEDGB7KqwChtdIs4IBvFBALggCIE6V0i8IBffQnNgDzKwBtAAbIAJOSjthAXXwB6eRGUQAegH5uxChBHjJ0mbQCeHAfiA

💻 Code

let n = 1 as const
const o = { a1: "abcd" }

const x = o[`a${n}`]
//    ^? : string
const { [`a${n}`]: y } = o
//           ^? Errors: Type '{ a1: string; }' has no matching index signature for type 'string'.(2537)

🙁 Actual behavior

`a${n}` in object destruction not inferred as const ("a1")

🙂 Expected behavior

`a${n}` in object destruction should be inferred as "a1", just like the one in element access.

Additional information about the issue

A possible workaround is to add as const after `a${n}`. I think the main problem is the inconsistency between element access and object destruction, which is confusing.

Inizia con la riproduzione collegata in TypeScript Playground e confronta il tipo inferito per l'accesso calcolato a un elemento con la chiave calcolata del destructuring di oggetti. Traccia il percorso del controllo dei tipi per queste due forme e fai in modo che il destructuring inferisca "a1" in modo coerente; il lavoro è completato quando l'esempio accetta il destructuring e segnala il tipo di chiave previsto senza il workaround.