Defining a function with a closure as its parameter, how to use Self as the type of the closure's parameter?

Using Swift
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1

I'm having issue with conforming UIView to one of my protocols. Here's a demo of the protocol:

protocol MyProtocol {
    func through(_ modifier: (Self) -> Void) -> Self
}

extension MyProtocol {
    @discardableResult
    func through(modifier: (Self) -> Void) -> Self {
        modifier(self)
        return self
    }
}

As you can see, I'm using Self in the parameter of a parameter, which is a closure.

Before this Swift 5.8.1 release, I was able to extend UIView to conform to it:

extension UIView: MyProtocol {  }

Now the extension UIView: MyProtocol code won't compile:

Protocol 'MyProtocol' requirement 'through(modifier:)' cannot be satisfied by a non-final class ('UIView') because it uses 'Self' in a non-parameter, non-result type position

The workaround here is simple. Just remove the requirement in the protocol definition while keep the implementation in the extension:

protocol MyProtocol {
    // remove this line:
    // func through(_ modifier: (Self) -> Void) -> Self
}

extension MyProtocol {
    // and keep this:
    @discardableResult
    func through(modifier: (Self) -> Void) -> Self {
        modifier(self)
        return self
    }
}

And it compiles.

Any reason why this happens?

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