The code contains this line:

let result = (try? body(self)) as? Result

where body(self) returns Result? already. The expected behavior here is to try to unwrap multiple layers of Optional, I guess? @jckarter, did something change here?

Tianyong, you can work around this issue by using (try? body(self)) ?? nil instead.

@swift-ci create

Comment by Tianyong (JIRA)

The expected behavior here is to try to unwrap multiple layers of Optional, I guess?

Yes, I want to unwrap multiple layers of Optional here. Thank you for the workaround.

Nothing changed in the runtime implementation of casting AFAIK. Is it type checking the expression differently?

Comment by Tianyong (JIRA)

When I cast `(try? body(self))` to `String` instead of the generic type `Result`, the value can be unwrapped out.

Contrast the following two expressions:

(try? body(self)) as? String // not nil

(try? body(self)) as? Result // nil

The `Result.self == String.self` is true, but the value of later expression is nil. I think it's an unexpected behavior.

This does not reproduce for me using a build from the swift-4.2-branch.

I think this is the same issue as SR-7664, right @mikeash?

Yes, looks like that to me. The underlying problem there was that the runtime function called for the generic version of the cast wouldn't unwrap more than one level of optional. This PR is the fix on the 4.2 branch: #16800