[Pitch] Generic associatedtype subtype constraints

Currently you can constrain a type parameter via a generic using equality

protocol A {
    associatedtype T where T == BKind.V
    associatedtype BKind: B

protocol B {
    associatedtype V

However, you cannot do the following

    associatedtype T where T: BKind.V

Could this be added to the language? Is there a reason something like this
would not be sound?

My use case would look like the following

protocol Mapping {
    associatedtype MappedObject: DatabaseAdapterKind.BaseType
    associatedtype DatabaseAdapterKind: DatabaseAdapter


protocol DatabaseAdapter {
    associatedtype BaseType


For Realm Obj-C fulfillment would look like the following

// User is an RLMObject subtype

protocol UserMapping: Mapping {
    typealias MappedObject = User
    typealias DatabaseAdapterKind = RealmAdapter

    // ... functions

protocol RealmAdapter: DatabaseAdapter {
    typealias BaseType = RLMObject

    // ... functions

For CoreData replace RLMObject with NSManagedObject

The mapping system allows the developer to write a mapping from JSON to a
type (MappedObject). Given that the type of MappedObject is a subtype of
the BaseType of the DatabaseAdapter the system will write an object of type
MappedObject to the database. This setup works with Realm Obj-C,
RealmSwift, and Core Data all of which have different BaseTypes but use
subtyping from a BaseType to specify the Model/MappedObject that is stored
in the db.

I can't use MappedObject == DatabaseAdapterKind.BaseType because this needs
to be a subtyping relation. E.g. a User: NSManagedObject is true but User
== NSManagedObject is not.

Since I can't specify this subtype relation in the protocol I end up having
to do runtime type checking and as! casts in quite a few places which seems
unnecessary and error prone during refactoring.




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