Open Radar

 
I’m not sure if Self is allowed in this context, but even if it’s not, this error message is very strange:

import Foundation

@objc protocol TestProtocol {}

@objc class TestClass: TestProtocol {
    func test() -> Self {
        let test = self as TestProtocol // cannot downcast from 'Self' to non-@objc protocol type 'TestProtocol'
        return self
    }
}

If you adjust the signature of test() to return TestClass, the compiler is happy.

Here’s another variant presentation of what seems to be the same issue:

import Foundation

@objc protocol TestProtocol {}

@objc class TestClass: TestProtocol {
    func test() -> Self {
        // Error: Type 'Self' does not conform to protocol 'TestProtocol'
        testFunction(self)
        return self
    }
}