AnyObject not working in Xcode8 beta6?

问题

In Xcode8 beta6, the following code will cause a warning: \'is\' test is always true. But it won\'t print pass.

    struct TestStruct {
    }

    //warning: \'is\' test is always true
    if TestStruct() is AnyObject {
        print(\"pass\")
    }

And the following code will cause a warning: Conditional cast from \'T\' to \'AnyObject\' always succeeds

public static func register<T>(_ protocolType: T.Type, observer: T) {

    //Warning: Conditional cast from \'T\' to \'AnyObject\' always succeeds
    guard let object = observer as? AnyObject else {
        fatalError(\"expecting reference type but found value type: \\(observer)\")
    }
    //...
}

回答1:

The warning works as intended: the false return of TestStruct() is AnyObject, however, does not

The prior version of this answer perceived the warning,

'is' test is always true

as the bug, and contained some discussion as to why this perceived buggy warning would manifest itself. That TestStruct() is AnyObject evaluated to false at runtime, however, was perceived as expected behaviour.

Given the comments to the bug report filed by the OP (SR-2420), it seems the situation is the reverse: since Xcode 8/beta 6, the is test should always evaluate to true, and the bug the OP:s post is the fact that TestStruct() is AnyObject evaluates to false during runtime.

Joe Groff writes:

This is correct, because everything bridges to AnyObject now.

...

is/as AnyObject always succeed for all types now. It's behaving as intended.

The new SwiftValue box for conversion from Swift values to Obj-C objects

(for additional details, see discussion in the comments below, thanks @MartinR)

It seems as if Swift values that are not explicitly implemented to be bridgeable to Obj-C objects via e.g. conformance to _ObjectiveCBridgeable (see e.g. the following Q&A for details regarding _ObjectiveCBridgeable), will instead automatically make use of the new SwiftValue box to allow conversion to Obj-C objects.

The initial commit message for swift/stdlib/public/runtime/SwiftValue.mm reads:

Runtime: Implement an opaque 'SwiftValue' ObjC class to hold bridged values

If there's no better mapping for a Swift value into an Objective-C object for bridging purposes, we can fall back to boxing the value in a class. This class doesn't have any public interface beyond being NSObject-conforming in Objective-C, but is recognized by the Swift runtime so that it can be dynamically cast back to the boxed type.

回答2:

Long story short.

To check if value has a reference type:

if type(of: value) is AnyClass {
    // ...
}

To check if type is a reference type:

if SomeType.self is AnyClass {
   // ...
}