Accessing parent properties of `CoreStoreObject` entity is prohibited?

[vovsyannikov]

I have a structure in may app where I have a parent object:

open class BaseContact: CoreStoreObject {
    @Field.Stored("externalId")
    open var externalId: Int?

    @Field.Stored("isUser")
    open var isUser: Bool = false    

    // Other props
}

A child and another entity:

final public class Contact: BaseContact {
    // Other props

    @Field.Relationship("userDL", inverse: \DBS.UserDL.$contact)
    public var userDL: DBS.UserDL?

    // Other props
}

final public class UserDL: CoreStoreObject {
    // Other props

    @Field.Relationship("contact")
    public var contact: DBS.Contact?

    // Other props
}

When trying to fetch Contact specifically by isUser property I get an error:

func contactIsUser() -> DBS.Contact? {
    try? fetchOne(where: \DBS.Contact.$isUser == true)
}

Binary operator '==' cannot be applied to operands of type 'KeyPath<DBS.Contact, FieldContainer<DBS.BaseContact>.Stored<Bool>>' and 'Bool'

Same thing with:

func currentUserDL(withContactID externalId: Int?) -> DBS.UserDL? {
    try? fetchOne(where: \.$contact ~ \.$externalId == externalId)
}

There's a different error but in the same vein:
Operator function '~' requires the types 'Optional<DBS.Contact>.DestinationObjectType' (aka 'DBS.Contact') and 'FieldContainer<DBS.BaseContact>.Stored<Int?>.ObjectType' (aka 'DBS.BaseContact') be equivalent