Swift (4) cannot cast to generic base class?

Question

In Swift, if I declare a class based on generic type:

class BasicRequest<T>{
}

class StartRequest: BasicRequest<BasicPayload> {
}

And then try to use generic method:

class SocketClient {

  public func send(request: BasicRequest<Any>){
  }
}

It does not compile:

SocketClient.shared.send(request: StartRequest())

with message

Cannot convert value of type 'StartRequest' to expected argument type 'BasicRequest'

Why actually? The type info should be available across the inheritance chain.

ReconnectRequest/AbstractWebSocketMessage from the error message are nowhere defined in your code. Please post the *real* code/message. — Martin R, Sep 18 '18 at 11:24
Looks like a duplicate of [Swift generic coercion misunderstanding](https://stackoverflow.com/questions/41976844/swift-generic-coercion-misunderstanding): `BasicRequest` and `BasicRequest` are different, unrelated types. — Martin R, Sep 18 '18 at 11:29
@ArnieSchwarzvogel, it's `func send(request: BasicRequest)`, you forgot to declare generic parameter. — user28434'mstep, Sep 18 '18 at 11:50
Recurring question, generics are invariant in Swift, this is why your code doesn't work. — Cristik, Sep 18 '18 at 12:16

score 2 · Answer 1 · answered Sep 18 '18 at 12:23

Generics are invariant in Swift, which in your case means that BasicRequest<Any> is not compatible with BasicRequest<BasicPayload>, even if BasicPayload is compatible with Any. This is how invariants work, there's little you can do in this direction.

What you can do, is to make send generic, thus allowing any kind of BasicRequest instances to be passed:

public func send<T>(request: BasicRequest<T>){
}

Swift (4) cannot cast to generic base class?

1 Answers1