Why would Error always be NSError?

Question

I have the following class defined on a Playground with Swift 3:

class MyError: Error {

}

Then, I create an instance of such class and check if it is a NSError

let firstError = MyError()
firstError is NSError // Output: false

The output is as expected, and I also get a warning which indicates Cast from 'MyError' to unrelated type 'NSError' always fails. This makes total sense for me, but if I change the code a little bit and declare the variable as an Error, I get a strange result:

var secondError: Error
secondError = MyError()
secondError is NSError // Output: true

And in this case I get a warning in the last line that says 'is' test is always true. I don't get why an Error would always be an NSError, when the model is defined the other way round (NSError: Error). Any idea what is going on here?

Answer 1

This is intentional behavior to allow Swift Error types to interop with Objective-C.

The compiler will only do the coercion when bridging Swift errors to Objective-C, or in your case where all you have is the Error existential that could contain anything... remember it could just as well have come from a throws function written in Objective-C. This also gives you an out to get the coercion if you need to pass an NSError directly to some Objective-C method as a parameter (for whatever reason).