Passing class-constrained type as AnyObject instance behavior is not matched on Darwin and non-Darwin platform

[Kyle-Ye]

Description

Passing class-constrained type as AnyObject instance behavior is not matched on Darwin and non-Darwin platform

On Darwin platform, we can pass it directly.

On non-Darwin platform, a fatalError will be emitted - "error: argument type 'Self.Type' expected to be an instance of a class or class-constrained type". And we can workaround it by manually adding "as AnyObject"

test1(Self.self as AnyObject)

Reproduction

class X {
    func test1(_ o: AnyObject) {}
    func test12() { test1(Self.self) }
}

Expected behavior

The behavior is matched. (The current Darwin platform behavior is preferred.)

Environment

Linux:
swift --version
Swift version 5.9.2 (swift-5.9.2-RELEASE)
Target: aarch64-unknown-linux-gnu

macOS:
swift-driver version: 1.87.1 Apple Swift version 5.9 (swiftlang-5.9.0.128.108 clang-1500.0.40.1)
Target: arm64-apple-macosx14.0

Additional information

Forum Post about the issue: https://forums.swift.org/t/69190

[slavapestov]

The dynamic Self type inside a class is always convertible to AnyObject, regardless of Objective-C interop. This is a type checker bug.

[Kyle-Ye]

The dynamic Self type inside a class is always convertible to AnyObject, regardless of Objective-C interop. This is a type checker bug.

Thanks for the explanation. And I think this should be a "Starter Bug" easy to be fixed.

I have not explored with Swift's "Type Checker" code before, can you help point out the relevant code folder/scope so that I can keep investigating and hopefully fix it myself?

[Kyle-Ye]

expected to be an instance of a class or class-constrained type

Swift have such check here. But the test is marked as "REQUIRES: objc_interop"

https://github.com/apple/swift/blob/f08f86c71617bacbc61f69ce842e284b27036598/test/Parse/metatype_object_conversion.swift#L14

See the 10 years ago commit: cf9e0d2 cc @jckarter

[Kyle-Ye]

The dynamic Self type inside a class is always convertible to AnyObject, regardless of Objective-C interop. This is a type checker bug.

https://github.com/apple/swift/blob/f08f86c71617bacbc61f69ce842e284b27036598/lib/Sema/CSSimplify.cpp#L7556-L7561

The conversion code is under a if check of getASTContext().LangOpts.EnableObjCInterop.

Maybe the solution is we just move this part out of the EnableObjCInterop check

[slavapestov]

I'm sorry, I misunderstood the original bug report. You're not converting an instance of a class to AnyObject -- that's allowed -- you're converting a metatype value to AnyObject. That is indeed not supported without an Objective-C runtime, because metatypes do not have a Swift reference counting header, so allowing the conversion here would lead to a runtime crash.

Can you change your code to traffic in Any.Type instead of AnyObject? Any.Type is also a single pointer, but we don't attempt to retain/release it, hence it can store a metatype even without an Objective-C runtime.

[slavapestov]

And we can workaround it by manually adding "as AnyObject"

I believe this is actually a bug and should not be allowed on non-Objective-C platforms. @jckarter what do you think?

[Kyle-Ye]

I'm also curious about the runtime behavior for this on non-Objective-C platform. I'll give it a try and report later.