microsoft / microsoft/TypeScript

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

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Bug Domain: check: Type Inference Help Wanted
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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

Open the contributing guide

First steps

  1. Read the whole issue, then the project's contributing guide.
  2. Comment on the issue to say you are picking it up — it saves two people doing the same work.
  3. Fork the repository and make your change on a branch.
  4. Open a pull request that references the issue number.

Research direction

Start by reproducing the generic satisfies example in the linked TypeScript Playground and compare the inferred types of a and b. Trace the compiler's handling of satisfies expressions involving generic values; done means both variables retain the same generic type rather than resolving a to 1 | 2, with a regression test covering the example.

Written by the indexing model from the issue text.

Assessment

Tech stack
typescript
Domain
compilers
Issue type
Bug
Difficulty
4/5
Estimated time
3-5 days
Activity status
Stale
Clarity
Clearly specified
Newbie friendliness
35/100

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