Microsoft/TypeScript

Missing constraint error in instantiation expression for a nested class

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#61,982 opened on Jul 1, 2025

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 (2 comments) (0 reactions) (0 assignees)TypeScript (48,455 stars) (6,726 forks)batch import
BugDomain: check: Variance RelationshipsHelp Wanted

Description

🔎 Search Terms

instantiation expression caching class expression declaration type arguments type parameter

🕗 Version & Regression Information

⏯ Playground Link

https://www.typescriptlang.org/play/?ts=5.9.0-dev.20250701#code/MYGwhgzhAECCA8AVaBeaA7ArgWwEYFMAnAPmgG8AoaaANzBE3wEIAuaRAbiumAAsBLEABNW0AJLoIAFzDpg+RAE8ADvnhSV+APYAzOADoAQvASJi5jtAD0V6FIEwIvLZmHQihLYQrdpYKfzA0IaoPOBQwUjuAB5S+OhCMLChsKSU1NTKYITxUqKc3AC+XIUUQA

💻 Code

class A<T = number> {
  value!: T;
  child!: InstanceType<typeof A.B<A<T>>>; // this should error

  static B = class B<T extends A = A> {
    parent!: T;
  };
}

🙁 Actual behavior

It doesn't error

🙂 Expected behavior

it should error with:

Type 'A<T>' does not satisfy the constraint 'A<number>'.
  Type 'T' is not assignable to type 'number'.(2344)

like it did in 5.6: TS playground

Additional information about the issue

The problem here is that the situation is a little bit circular and variances worker gets hits in reentrant manner. The nested call returns emptyArray that is a signal for structuredTypeRelatedToWorker to return Ternary.Unknown.

So checkTypeArguments called by getInstantiatedSignatures doesn't report an error. In the old version of the code, this would be re-called after the variance worker would completely exit and the error would be raised.

I see 2 ways to solve this:

  • ignore the introduced caching based inVarianceComputation to avoid spoiling this
  • defer checkTypeArguments in getInstantiatedSignatures (well, call it with /*reportErrors*/ false and defer anoher call with /*reportErrors*/ true)

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.
4
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
3-5 days
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
fresh
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
TypeScript type systemcompiler internalsvariance workerscaching mechanisms
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
15
Research direction
Investigate the caching of instantiation expressions in the TypeScript compiler, specifically around the variance worker. The issue describes a reentrant call to 'structuredTypeRelatedToWorker' returning 'Unknown', suppressing the error. Two solutions are proposed: ignore caching based on 'inVarianceComputation', or defer 'checkTypeArguments' in 'getInstantiatedSignatures'. Review the code in the TypeScript repository, focusing on the variance worker and type argument checking, and consider the impact of the changes from PR #59931.
Missing constraint error in instantiation expression for a nested class · Microsoft/TypeScript#61982 | Good First Issue