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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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
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