Microsoft/TypeScript

4.7 regression: Array.flat(infinity) leads to "Type instantiation is excessively deep and possibly infinite." error

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#49,280 opened on May 27, 2022

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Description

Bug Report

🔎 Search Terms

  • Array.flat
  • Type instantiation is excessively deep and possibly infinite.
  • Nested arrays

🕗 Version & Regression Information

  • This changed between versions 4.6 and 4.7

⏯ Playground Link

TypeScript Workbench with relevant code

💻 Code

interface Config {
  prop?: string;
}

interface ExtendedConfig extends Config {}

type NestedConfigs = Array<ExtendedConfig | NestedConfigs>;

const configs: NestedConfigs[] = [];

const flattened = configs.flat(Infinity);

🙁 Actual behavior

TypeScript raises the following error for the configs.flat(Infinity) call:

Type instantiation is excessively deep and possibly infinite. ts(2589)

🙂 Expected behavior

No error should be raised.

Contributor guide

Research direction
Investigate the changes in TypeScript's handling of recursive array types between versions 4.6 and 4.7 that caused this regression with Array.flat(Infinity). Compare the type inference logic for Array.flat when called with Infinity on a recursive array type.
Tech stack
typescript
Domain
developer experience
Issue type
Bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
2
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.
Active
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
Clear
Prerequisites
TypeScript
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
75