Bindable ViewModel.swift

protocol ViewModelType {
  associatedtype Model
  associatedtype Input

  init(model: Model, input: Input)
}

class ViewModel: ViewModelType {

  let model: Model
  required init(model: Model, input: Input) {
    self.model = model
  }

  struct Model {
    let userId: String
  }

  struct Input {
    //let buttonTap: Observable<Void>
  }

}

enum Bindable<T: ViewModelType> {

  case unbound(model: T.Model)
  case bound(model: T.Model, viewModel: T)

  mutating func bind(input: T.Input) -> T {
    switch self {
    case let .unbound(model), let .bound(model, _): // <- using the same name for both values of a generic type
      let viewModel = T(model: model, input: input) // <- EXC_BAD_ACCESS here prior to Swift 3.1
      self = .bound(model: model, viewModel: viewModel)
      return viewModel
    }
  }

}

let input = ViewModel.Input()
let model = ViewModel.Model(userId: "123")
var b: Bindable<ViewModel> = .unbound(model: model)

let viewModel = b.bind(input: input)

print(viewModel.model)