通用协议Swift 4错误

Question

The following code use to work for me at Swift 3.2, but with the latest release of Swift 4 I am getting an strange error I can not get to understand.

I am trying to create a generic protocol like so:

public protocol FactoryComponent {
  associatedtype Argument
  associatedtype Service
  static var factory: (Resolver) -> (Argument) -> Service { get }
}

public extension FactoryComponent {
    public typealias Factory = (Argument) -> Service
}

And using it here:

public extension Container {
  @discardableResult
  public func register<Component: FactoryComponent>(
    factory componentType: Component.Type
  ) -> ServiceEntry<Component.Factory> { // On this line the error shows
    return self.register(componentType.Factory.self) { resolver in
      componentType.factory(resolver)
    }
  }
}

Error:

'Component' does not have a member type named 'Factory'; did you mean 'Factory'?

And of course, the auto-fix does not help since the error is useless...

I checked out Swift 4 breaking changes and did not see anything involving generic protocols.

Can somone please help me understand what does this mean?

Answer 1

It seems that a concrete class of FactoryComponent is missing. The Factory type alias can be called only by protocol's concrete classes.

1. Try create an FactoryComponent concrete class that implements its protocol
2. The Argument and Service still generic on your implementation and needs an concrete type. My english is very bad, but I hope I've been help you. Check the code bellow if I'm wasn't clear on my answer.

```

class Resolver {}

protocol FactoryComponent {
  associatedtype Argument
  associatedtype Service
  static var factory: (Resolver) -> (Argument) -> Service { get }
}

extension FactoryComponent {
  typealias Factory = (Argument) -> Service
}

class ConcreteFactoryComponent: FactoryComponent {
  static var factory: (Resolver) -> Factory {
    return foo
  }

  static func foo(_ resolver: Resolver) -> Factory {
    let factory: Factory = { argument in return "" }
    return factory
  }

  typealias Argument = String
  typealias Service = String
}

let factory: ConcreteFactoryComponent.Factory = { argument in return "" }

```

Answer 2

I believe the error is not really on the line with the return type. The processing of generic signature resolution has changed slightly in Swift 4 due to the new default for @escaping. The fact that the function is calling a self.register function with an identical signature as the function itself is likely the source of the issue.

From other similar instances, I would suggest looking at the signature of the Container.register. If it happens to have an @escaping closure, then the signature of the generic function in the extension should also have one so that it is recognized as an overload.

See this post: https://stackoverflow.com/a/43081214/5237560

[EDIT] I just realized this change was between Swift 2 and 3. Leaving the answer here in case it can provide some inspiration.