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

🔎 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

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

interface API {
  works<T extends Obj>(
    obj: T,
  ): void;

  alsoWorks<T extends Obj>(
    obj: Pick<T, 'type' | 'id'>,
  ): void;

  alsoAlsoWorks<T extends Obj>(
    obj: Partial<T>
  ): void;

  doesntWork<T extends Obj>(
    obj: Pick<T, 'type' | 'id'> & Partial<T>
  ): void;
}

type WorksParams = Parameters<API['works']>;                 // [obj: Obj]
type AlsoWorksParams = Parameters<API['alsoWorks']>;         // [obj: Pick<Obj, "type" | "id">]
type AlsoAlsoWorksParams = Parameters<API['alsoAlsoWorks']>; // [obj: Partial<Obj>]
type DoesntWorkParams = Parameters<API['doesntWork']>;       // never

🙁 Actual behavior

Parameters<API['doesntWork']> returns never

🙂 Expected behavior

The Parameters<API['doesntWork']> case behaves like the other cases and returns [obj: Pick<Obj, "type" | "id"> & Partial<Obj>].

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 method. If anyone knows of a workaround for this Parameters problem, I'd love to hear it.

从链接的 TypeScript Playground 开始，缩减涉及 Pick<T, 'type' | 'id'> & Partial 的 API 示例。比较四种方法的 Parameters 结果，并追踪相关的类型推断行为。当 Pick 和 Partial 的组合情况产生预期的参数元组，且其他情况没有回归时，即视为完成。