Microsoft/TypeScript

Generic constraint is not validated in a recursive type

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#53,895 opened on Apr 18, 2023

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BugDomain: Conditional TypesHelp Wanted

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Description

Bug Report

🕗 Version & Regression Information

  • This is the behavior in every version I tried, starting from 4.1 where recursive conditional types were added

⏯ Playground Link

Playground link with relevant code

💻 Code

export type ComputeRangeLoose<
  N extends number,
  Result extends Array<number> = [],
> = Result['length'] extends N ? Result : ComputeRangeLoose<N, [...Result, Result['length']]>

type A = ComputeRangeLoose<{ foo: true }>

export type ComputeRangeStrict<
  N extends number,
  Result extends Array<number> = [],
> = N extends number
  ? Result['length'] extends N ? Result : ComputeRangeStrict<N, [...Result, Result['length']]>
  : never

type B = ComputeRangeStrict<{ foo: true }>

🙁 Actual behavior

ComputeRangeLoose<{ foo: true }> allows to pass a type that is not related to number, throwing Type instantiation is excessively deep and possibly infinite.

ComputeRangeStrict<{ foo: true }> throws an expected Type '{ foo: true; }' does not satisfy the constraint 'number'

🙂 Expected behavior

Both ComputeRangeLoose and ComputeRangeStrict throw Type '{ foo: true; }' does not satisfy the constraint 'number'

As the issue is reproducible since 4.1, I'm pretty sure it was already mentioned somewhere however I failed to find a suitable issue

Contributor guide

Research direction
Investigate how TypeScript's type checker validates generic constraints in recursive conditional types. Look at the constraint checking logic for type parameters when the conditional type is not used (e.g., ComputeRangeLoose) versus when it is used (e.g., ComputeRangeStrict). Check if the constraint is being bypassed due to the recursive structure.
Tech stack
typescript
Domain
compilers
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.
Active
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
Clear
Prerequisites
Knowledge of TypeScript type systemFamiliarity with recursive conditional types
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
20