Implementing ResolverInterface breaks typings

[giacomorebonato]

Reporting an example where the usage of ResolverInterface on the resolver break the typings on the property decorated with @FieldResolver.

@Resolver((_of) => Task)
class TaskResolver implements ResolverInterface<Task> {
  async category(@Root() task: Task) {
    return (dataloader: DataLoader<string, Category>) => {
      if (task.categoryId === undefined) return null

      return dataloader.load(task.categoryId)
    }
  }
}

@ObjectType({ description: 'A task is something to do' })
class Task extends TimeStamps {
  @GQLField()
  _id: string = ''

  @MongoField()
  @GQLField()
  name: string = ''

  @MongoField()
  @GQLField()
  completed: boolean = false

  @MongoField()
  categoryId?: string

  @GQLField({ nullable: true })
  category?: Category
}

The error is:

error TS2416: Property 'category' in type 'TaskResolver' is not assignable to the same property in base type 'ResolverInterface<Task>'.
  Type '(task: Task) => Promise<(dataloader: DataLoader<string, Category, string>) => Promise<Category> | null>' is not assignable to type '(root: Task, ...args: any[]) => Category | Promise<Category | undefined> | undefined'.
    Type 'Promise<(dataloader: DataLoader<string, Category, string>) => Promise<Category> | null>' is not assignable to type 'Category | Promise<Category | undefined> | undefined'.
      Type 'Promise<(dataloader: DataLoader<string, Category, string>) => Promise<Category> | null>' is not assignable to type 'Promise<Category | undefined>'.
        Property '_id' is missing in type '(dataloader: DataLoader<string, Category>) => Promise<Category> | null' but required in type 'Category'.

46   async category(@Root() task: Task) {

The solution at the moment is not to implement the interface and the implementation works.
It first look ResolverInterface should be enriched in type-graphql, but I am posting here first for opinion and feedback and I'll close the issue if it doesn't make sense to be here.