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I write a protocol (in Xcode playground),and want it be conform by specified class , eg: Root.

Because I used Swift 4.1 , So there are new syntax for this : 

 protocol Delegate where Self:Root{
 var titleImage:UIImage? {get}
 }

(PS:If you want to know more about this , please follow the link : https://stackoverflow.com/questions/50092984/how-to-make-swift-protocol-conformed-by-a-specific-kind-of-class)

And I also write class Root and Foo :

 class Root:Delegate{
 var titleImage:UIImage?
 
 func willDo(){
 let foo = Foo()
 foo.delegate = self
 foo.invoke()
 }
 }

 class Foo{
 var delegate:Delegate!
 
 func invoke(){
 if let image = delegate.titleImage{
 print("have a image [\(image)")
 }else{
 print("not have a image")
 }
 }
 }

Problem is that when I call :

 let r = Root()
 r.willDo()

Here are runtime error :

[![enter image description here][1]][1]


 [1]: https://i.sstatic.net/LKGSx.png

But when I changed Delegate code to this :

 protocol Delegate{
 var titleImage:UIImage? {get}
 }

That's all right!!! But it can't limited Delegate to be conform by class Root anymore!

Could anyone can tell me Why is it? Why when add " Self:Root " to Delegate will trigger a BAD_ACCESS Runtime error???

Is this a Bug in Swift or I misunderstand something???


Thanks a lot ;)

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