How to create a typescript custom type error and default type for it?

I've tried to create a PojoDocument<T> generic type to guard from leaking mongoose documents out of our DALs.

My goal was to make sure the returned object is not a mongoose Document, but a POJO(PojoDocument).
I've done it by checking that T can never have the $locals property, as it's found on all documents:

// If `T` has `$locals` property, then resolve the string as the type, which will clash with `T` and show as an error. 
// So when `$locals` property not found, `T` will be returned as the type. 
export type PojoDocument<T> = T & T extends { $locals?: never }
  ? T
  : 'Please convert the document to a POJO via `.toObject()` or .lean()`';

My usage of the type looks like this:

class A {
...
  async create(dto: CreateUserDto): Promise<PojoDocument<User>> {
    const result = await this.userModel.create(dto);
    return result.toObject();
    // ^ Type 'LeanDocument<User & Document<any, any, User>>' is not assignable to type '"Please convert the document to a POJO via `.toObject()` or .lean()`"'.
  }
}

The LeanDocument<User & Document<any, any, User>> type is equals to:

interface ResultToObjectType {
  __v?: any;
  _id?: any;
  name: string;
}

Also, when I check what is returned by the create method where it's used, I get this:

Promise<"Please convert the document to a POJO via `.toObject()` or .lean()`">

This means PojoDocument<T> defaults to the "else" condition instead of the T that was provided for it.

I've tried modifying the type to include __v and _id, thinking that's the reason it doesn't consider the type as being extended by $locals?: never, as these properties don't exist on T, but it didn't change anything:

export type PojoDocument<T> = T extends { $locals?: never; __v?: any; _id?: any }
  ? T
  : 'Please convert the document to a POJO via `.toObject()` or .lean()`';

Any ideas how this check can be achieved?

Edit #1

I've added a minimal repro playground.

The issue is actually with UsersDal.create, where the return type is:

Promise<PojoDocument<User>>

User has only name and it doesn't meet the requirements of PojoDocument, thus you get the string error returned:

// type Test = "Please convert the document to a POJO via `.toObject()` or .lean()`"
type Test = PojoDocument<User>

However, pojoResult does obey the conditions of PojoDocument and obviously, it's not the string that is expected to be returned by the function.

Summarizing the issue:

Your create expects a promise of a literal string to be returned, not the promise object.

I'm not familiar with mongoose, but I can suggest changing the return type of the create to:

Promise<PojoDocument<LeanDocument<User & Document<any, any>>>>

Full code:

class UsersDal {
  constructor(private userModel: Model<User>) {}

  async create(createDto: UserDto): Promise<PojoDocument<LeanDocument<User & Document<any, any>>>> {
    const result = await this.userModel.create(createDto);

    const pojoResult = result.toObject();
    return pojoResult; // no error
  }
}

class UsersService {
  constructor(private usersDal: UsersDal) {}

  async create(createDto: UserDto): Promise<User> {
    const result = await this.usersDal.create(createDto);
    return result; // no error
  }
}

playground