(1 comment) (3 reactions) (0 assignees)TypeScript (48,455 stars) (6,726 forks)batch import

BugDomain: IntersectionHelp Wanted

説明

Bug Report

🔎 Search Terms

tuple intersection index type

⏯ 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.

コントリビューターガイド

技術スタック: typescript
領域: tooling
Issue 種別: bug
難度: 3
推定時間: 1-3 hours
活動状況: stale
明確さ: clear
前提条件: Understanding of TypeScript type systemFamiliarity with tuple and intersection types
初心者向け度: 25
調査方針: 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.