Loose inferred return type of generator function

[purepear]

Description

The inferred type of the generator allows for extra properties(variant 1).
In the case of class extension it also requires narrowing the literal types(variant 3).
It seems to work well when the return type is set explicitly(variant 2, variant 4).

Looking at the source code(the generic types), I would expect the generator return type to be inferred properly 🤔

I'm not sure if this is a bug or a feature 😄

To Reproduce

https://stackblitz.com/edit/fishery-test?file=Post.ts

Additional context
Fishery version: 2.2.2

[pangolingo]

I have observed this behavior too.

This is a Typescript bug/feature (I also am not sure which to call it).
Here's some details:

• StackOverflow post explaining why Typescript lets this happen
• GitHub issue on the Typescript repo that talks about this limitation

I don't know if we can fix this without getting it resolved in Typescript itself. We're open to suggestions/PRs if you have an idea!

Usually that extra parameter doesn't cause any trouble. If it does, add the explicit return type from your examples:

Factory.define<Post>(
  (): Post => ({ ... })
)

[pangolingo]

I don't know why Typescript requires you to narrow your type (status: 'published' as const) for the class extension but not the factory. They're both using the same define function internally. Probably because the Factory type definition (Factory<Post>) happens a little bit further away from the function definition. I'm not sure what we could do for a fix there.