microsoft / microsoft/TypeScript

`Parameters` returns `never` when used with a generic function that combines `Pick` and `Partial` on the generic type argument

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Bug Domain: check: Type Inference Help Wanted
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Description

### 🔎 Search Terms

partial pick generic parameters never

### 🕗 Version & Regression Information

- This is the behavior in every version I tried, and I reviewed the FAQ for entries about Parameters

### ⏯ Playground Link

https://www.typescriptlang.org/play/?ts=5.9.0-dev.20250331#code/C4TwDgpgBA8gRgKygXgFBSgHygbyqSALigHIBDEgGigEsATYgZ2ACcaA7Ac2rKdY85QAvuiy584CMRJwqtBlGZsu1OMXYBXALZwILYQG5UqDsD0AzMgGNoAQQAKASVyiA7gHsWAa0YAeACpQEAAeZux0jLCIAHwAFKIY7ojE-pSiAJTEAG7u9EaiZAA2jO4A6p4+AUGhEOGR8AhxCVBJCMT2NFZeAdQkBBAkYiT0JNFpGJlQOXnGGEUltsVlFX6BIWERUY3xGInJUPZkLMA0RQHRGdm5dPkYdO4QjOzA5d5V67WbDU27LfsdXR6pH6g2wwzooygADIDkcTmd-BcJlcZiJUP0oK8fIcWGQtJFkLDcVoIGYWH4HI4ANokDzeRgkAC60QMvzZ7IwAHpOVAqa1iA1GejJFBFiUsYwcXiCUS8aS9BSnDT5st6UyWRyuTy+f9Ot0GtQAET9Q1iQ30Q3RIUYsXuW0SqX4lCyklkxXU8hLe0rdWs7m8-lE+GFXzfa0igAiDyeLwqjpljvl5N8lJp90ezyxvo5-vYECyeiAA

### 💻 Code

```ts
type Obj =
| { type: 'a', id: string, a: string }
| { type: 'b', id: string, b: number };

interface API {
works(
obj: T,
): void;

alsoWorks(
obj: Pick,
): void;

alsoAlsoWorks(
obj: Partial
): void;

doesntWork(
obj: Pick & Partial
): void;
}

type WorksParams = Parameters; // [obj: Obj]
type AlsoWorksParams = Parameters; // [obj: Pick]
type AlsoAlsoWorksParams = Parameters; // [obj: Partial]
type DoesntWorkParams = Parameters; // never
```

### 🙁 Actual behavior

`Parameters` returns `never`

### 🙂 Expected behavior

The `Parameters` case behaves like the other cases and returns `[obj: Pick & Partial]`.

### Additional information about the issue

I'm not sure why `Parameters` works for all cases except when `Pick` and `Partial` are used together. I'm having the same problem when using `extends` and `infer` to get the parameters.

My use case is to prepend an argument `event: IpcMainInvokeEvent` to several generic functions to match the signature expected by the `listener` argument of Electron's [`ipcMain.handle`](https://www.electronjs.org/docs/latest/api/ipc-main#ipcmainhandlechannel-listener) method. If anyone knows of a workaround for this `Parameters` problem, I'd love to hear it.

Contributor guide

Open the contributing guide

Research direction

Start with the linked TypeScript Playground and reduce the API example involving Pick & Partial. Compare the Parameters results for the four methods and trace the relevant type-inference behavior. Done means the combined Pick and Partial case produces the expected parameter tuple without regressing the other cases.

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

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