microsoft / microsoft/TypeScript
Function argument is not infered correctly
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Description
### 🔎 Search Terms
function, argument, parameter, generic, callback, infer
### 🕗 Version & Regression Information
By trying out different versions in Playground, it looks like it works correctly in version 4.7.4 and lower
### ⏯ Playground Link
https://www.typescriptlang.org/play/?#code/CYUwxgNghgTiAEAzArgOzAFwJYHtXzCgggB4AVAPgApFUAueKqBgZwxi1QHMAaeAIwZkAlPAC8FeGgDWqHAHdUwhlUHwR4yTLmKA3ACh9oSLAQp02PElQkAgjC4t4IAB4YQqYE6ioAngG0AXWp9eGsVADoo2EcGe0dRCSlUWQVUHn1lRiiImJY4hxZErRSdVAN9MDw2eDgWZAgMcQIiCBpUGjRMXHwmPjVUZABbfhAYUQBvAF9hYV0gA
### 💻 Code
```ts
declare function call(fn: (a: string, b: T) => unknown): (b: T) => unknown;
declare function fn(
fn: (...args: Args) => unknown,
): (...args: Args) => unknown;
const result = call(fn(function (a, b: number) {}));
```
### 🙁 Actual behavior
The `a` parameter's type is `any`.
### 🙂 Expected behavior
The `a` paramater's type should be `string`
### Additional information about the issue
I encountered this while trying to create a helper function in a codebase that uses Effect:
```ts
import { Effect } from "effect"
function call(fn: (a: string, b: T) => Effect.Effect) {
return (b: T) => fn("value", b)
}
const fnA = call((a, b: number) => // a is `string`
Effect.gen(function* () {
return yield* Effect.succeed(`${a}: ${b}`)
}),
)
const fnB = call(
Effect.fn(function* (a, b: number) { // a is `any`?
return yield* Effect.succeed(`${a}: ${b}`)
}),
)
```
Contributor guide
Research direction
Start by running the linked TypeScript Playground example and compare the inferred types of `a` in the direct callback and the `fn`-wrapped callback. Investigate generic callback argument inference around `call` and `fn`; done means the wrapped callback infers `a` as `string` rather than `any`, while preserving the existing `b: number` inference.
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
- 45/100