When the initial element of the tuple is a rest element, it is not possible to correctly infer the type of the initial element.

🔍 Search Terms

initial element of the tuple is a rest element

✅ Viability Checklist

• This wouldn't be a breaking change in existing TypeScript/JavaScript code
• This wouldn't change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, new syntax sugar for JS, etc.)
• This isn't a request to add a new utility type: https://github.com/microsoft/TypeScript/wiki/No-New-Utility-Types
• This feature would agree with the rest of our Design Goals: https://github.com/Microsoft/TypeScript/wiki/TypeScript-Design-Goals

⭐ Suggestion

type T1 = [...string[], number, boolean]

declare const head: T1[0]
         // Actual: string | number | boolean
         // Expected: string | number

📃 Motivating Example

💻 Use Cases

https://github.com/microsoft/TypeScript/issues/56883

Inizia con l’esempio minimo di tipo tupla nell’issue e segui come il type checker di TypeScript risolve l’accesso indicizzato T1[0] quando la tupla inizia con un elemento rest. Il lavoro è completo quando il tipo inferito è string | number invece di string | number | boolean e i test esistenti del type checker sono stati estesi per coprire questo caso.