[Types] Adjusments to ApplyBasicQueryCasting (ff2bf3e8b3abe0e09a523465e5db8743083702b9) break FilterQueries for generic types

[ghost91-]

Prerequisites

• I have written a descriptive issue title
• I have searched existing issues to ensure the bug has not already been reported

Mongoose version

6.4.0

Node.js version

16.x

MongoDB server version

5.x

Description

The changes from ff2bf3e break FilterQuerys for generic types that have some know properties. With this change, it's not possible anymore to assign values for the known properties.

It seems like the issue is the T[0] & in

  export type ApplyBasicQueryCasting<T, defaultT = T | T[] | {}> = T extends any[] ? T[0] & defaultT: defaultT;

What exactly is the purpose of having T[0] & defaultT? For arrays, that would mean the property needs to be of the element type, and the array type or an array of the array type or any non nullable type, at the same time. That doesn't make a lot of sense to me.

Also, I doubt {} is the correct type here. It means "any non nullable value", not "empty object". In particular, you can do stuff like const foo: {} = true. null and undefined are the only values that can not be assigned to it. I doubt that's what's meant here.

From what I can see, the second generic parameter (defaultT) isn't ever set explicitly, so what is the purpose of it?

And lastly, that commit also introduces some tests, but it is in no way clear how the tests are related to the changes. And it seems like they don't cover all parts of the changes. So I think the tests should be expanded a bit here.

Steps to Reproduce

import { ApplyBasicQueryCasting, FilterQuery, QuerySelector } from 'mongoose';

type Condition<T> = ApplyBasicQueryCasting<T> | QuerySelector<ApplyBasicQueryCasting<T>>; // redefined here because it's not exported by mongoose

type WithId<T extends object> = T & { id: string };

class Repository<T extends object> {
  /* ... */

  find(id: string) {
    const idCondition: Condition<WithId<T>>['id'] = id; // error :(
    const filter: FilterQuery<WithId<T>> = { id }; // error :(

    /* ... */
  }
}

Expected Behavior

Using FilterQuery with generic types with know properties works as it has previously.

[vkarpov15]

Thanks for the detailed issue report, you're right that the new implementation of ApplyBasicQueryCasting in v6.4.0 is incorrect. We'll have a fix in v6.4.1.