microsoft / microsoft/TypeScript
Inference breaks when a function returns a tuple with another function
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Bug
Domain: check: Type Inference
- Dominant language
- Go
- Stars
- 111k
- Forks
- 14.4k
- Avg merge
- 1d 19h
- Merged PRs (30d)
- 117
Description
Bug Report
🔎 Search Terms
Inference, tuple, generic, callback, higher-order function
🕗 Version & Regression Information
v4.3.2 and below (tested down to v3.3.3)
⏯ Playground Link
Playground link with relevant code
💻 Code
declare function a(): [
<T>(arg: T) => T
]
declare function b<T, U>(a1: T, a2: [(a: T) => U]): U
b(1, [a()[0]]) // returns number, as expected
b(1, a()) // returns unknown (expected 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 with the linked TypeScript Playground and reproduce the difference between b(1, [a()[0]]) and b(1, a()). Trace the inference behavior for the generic callback returned inside the tuple. Done means the second call infers number rather than unknown, with coverage for the shown regression case.
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