microsoft / microsoft/TypeScript
inferred generic incorrectly widened
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Description
Bug Report
🔎 Search Terms
inferred generic widened
🕗 Version & Regression Information
4.3.0-beta
⏯ Playground Link
Playground link with relevant code
💻 Code
type Foo<T extends number> = T extends infer R? Bar<R>: never //compile error: Type 'R' does not satisfy the constraint 'number'
type Bar<T extends number> = never
🙁 Actual behavior
R was incorrectly widened to no longer be a number, even though it was inferred from T which is a number
🙂 Expected behavior
no error, as R is a number
Contributor guide
First steps
- Read the whole issue, then the project's contributing guide.
- Comment on the issue to say you are picking it up — it saves two people doing the same work.
- Fork the repository and make your change on a branch.
- Open a pull request that references the issue number.
Research direction
Start by reproducing the provided TypeScript 4.3.0-beta example in the linked Playground and inspect how conditional types handle the inferred R constraint. Done means the example compiles without an error while preserving the expected number constraint; add regression coverage for this case if the repository's existing test structure identifies a suitable location.
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
- Mostly clear
- Newbie friendliness
- 35/100