Microsoft/TypeScript

Indexing tuple intersection type beyond length produces unexpected type

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#42,557 opened on Jan 30, 2021

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

Description

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.

Contributor guide

Tech stack
typescript
Domain
tooling
Issue type
bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-3 hours
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
stale
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
Understanding of TypeScript type systemFamiliarity with tuple and intersection types
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
25
Research direction
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.