Microsoft/TypeScript

Conditional type eagerly calculated to be invariant breaks referential transparency

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#62798 opened on Nov 23, 2025

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 (5 comments) (0 reactions) (0 assignees)TypeScript (48,455 stars) (6,726 forks)batch import
BugCursed?Domain: check: Variance RelationshipsHelp Wanted

Description

🔎 Search Terms

Conditional types, Invariance, Referential transparency

🕗 Version & Regression Information

Tried with v5.9.3

⏯ Playground Link

https://www.typescriptlang.org/play/?ts=5.9.3#code/C4TwDgpgBAYgPAFQHxQLxQN5QG4EMA2ArhAFxQJQQAewEAdgCYDOUA9gEYBWEAxsFAH4oAawghWAM3JQyFAL4AoBaEhQASgEY0sOBjkpqtRi3hZcZJsABOASzoBzKPsFQARAAtW+Zq5luGrBAsdKz8nt6uCgD0UVBxAHoCSirQMLjapvrK4KnsGbpQ5lCWtg5OSMk56gBMGemG9MyweUIeXj5+rgFBUCFh7ZExcVCJCkA

💻 Code

type F<T> = { value: T extends object ? keyof T : T }

type R1 = F<{}> extends F<{ a: string }> ? "holds" : "does not hold"
//   ^? "does not hold"

type Fa = F<{}>
type Fb = F<{ a: string }>

type R2 = Fa extends Fb ? "holds" : "does not hold"
//   ^? "holds"

🙁 Actual behavior

R1 computes to "does not hold"

🙂 Expected behavior

R1 computes to "holds"

Additional information about the issue

I'm assuming typescript eagerly calculates F to be invariant and then instead of checking the subtype relation on resolved types it calculates subtype relation of the parameters to F. (This would perhaps have made sense if F was explicitly annotated with in out T.)

Contributor guide

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.
4
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
over 1 week
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 type systemvariance in typesconditional types
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
15
Research direction
Investigate the TypeScript type checker logic for conditional types and variance. The issue suggests that eager evaluation leads to invariance, breaking substitution. Focus on files like src/compiler/checker.ts, specifically the parts handling generic conditional types and subtype relations. Review the discussion in the issue for any additional insights.