Microsoft/TypeScript

`satisfies` changes the generic type, "resolves" it immediately

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#52,394 opened on Jan 24, 2023

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 (12 comments) (0 reactions) (0 assignees)TypeScript (48,455 stars) (6,726 forks)batch import
BugDomain: check: Type InferenceHelp Wanted

Description

Bug Report

It seems that using satisfies on a value that is bound to a generic type, in some way resolves the generic type at that place.

const foo = <T extends 1 | 2>(bar: T) => {
  // PROBLEM: a and b should have the same type.
  const a = bar satisfies any; // using any type that doesn't use the generic T here creates this problem
  //    ^? 1 | 2
  const b = bar;
  //    ^? T extends 1 | 2
}

Playground link

🔎 Search Terms

satisfies changes type, generic, widening

🕗 Version & Regression Information

Using 4.9.4 now with satisfies and noticed this.

  • This changed between versions 4.9 and 4.8 when satisfies was introduced
  • This is the behavior in every version I tried, and I reviewed the FAQ
  • I was unable to test this on prior versions because satisfies didn't exist

⏯ Playground Link

Playground link

🙁 Actual behavior

a and b have different types as seen in the playground

🙂 Expected behavior

satisfies should be a no-op

Contributor guide

Tech stack
typescript
Domain
backend
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.
1-2 days
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 genericssatisfies operator
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
35
Research direction
Examine the type checker's handling of satisfies expressions in the presence of generic type parameters. Investigate the code path that resolves type variables when satisfies is used, likely in checker.ts or binder.ts. Compare the behavior of satisfies with other type assertions. Look at existing tests for satisfies to understand expected behavior.