Microsoft/TypeScript

Indexing tuple intersection type beyond length produces unexpected type

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#42,557 建立於 2021年1月30日

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 (1 留言) (3 反應) (0 負責人)TypeScript (48,455 star) (6,726 fork)batch import
BugDomain: IntersectionHelp Wanted

描述

Bug Report

🔎 Search Terms

tuple intersection index type

⏯ Playground Link

Playground Link

💻 Code

type T = [number, boolean] & { x: string };
declare const t: T;
const a = t[0]; // number
const b = t[1]; // boolean
const c = t[2]; // number | boolean !?

🙁 Actual behavior

c is typed as number | boolean

🙂 Expected behavior

c should be typed as undefined

I'm not sure if this is a bug, or intended behavior. But it was surprising. If the type is just a tuple, indexing beyond its length produced undefined. But if it is part of an intersection, the element type is changed to number | boolean, as if it is no longer a tuple, but an array. However, indexing 0 or 1 still produces the correct type for those element positions.

貢獻者指南

技術棧
typescript
領域
tooling
議題類型
bug
難度面向新貢獻者的預計實作難度，1 表示很小改動，5 表示專家級工作。
3
預計時間有經驗貢獻者完成調查、實作、測試並準備 pull request 的粗略時間範圍。
1-3 hours
活動狀態議題目前的可參與程度：新鮮、活躍、陳舊、阻塞或等待維護者輸入。
stale
清晰度議題是否清楚說明預期改動、驗收標準和下一步。
clear
前置要求
Understanding of TypeScript type systemFamiliarity with tuple and intersection types
新手友善度1-100 的估計分數，表示該議題對首次貢獻者的友善程度。
25
研究方向
Investigate the TypeScript compiler source code, specifically the handling of indexed access types on intersection types. Look at checker.ts for how tuple types are resolved when intersected with object types. The issue shows that indexing beyond a tuple's length should yield 'undefined', but with an intersection, it yields the union of element types. Check the logic for computing element types of intersection types involving tuples. The playground link provides a minimal repro.