`@objc` async method has `_Nonnull` completion handler parameter instead of `_Nullable`

[fumoboy007]

Description
SE-0297 says that

@objc func perform(operation: String) async -> Int { ... }

will be translated into

- (void)performWithOperation:(NSString * _Nonnull)operation
           completionHandler:(void (^ _Nullable)(NSInteger))completionHandler;

However, when I tried it, the completion handler is _Nonnull, resulting in the following warning when I try to pass nil:

Null passed to a callee that requires a non-null argument

Steps to reproduce
See the above sample code.

Expected behavior
Passing nil to the completion handler parameter should not produce a warning.

Environment

• Swift compiler version info:

swift-driver version: 1.62.15 Apple Swift version 5.7.2 (swiftlang-5.7.2.135.5 clang-1400.0.29.51)
Target: x86_64-apple-macosx12.0

• Xcode version info:

Xcode 14.2
Build version 14C18

• Deployment target: iOS 16.2

[fumoboy007]

Here’s a sample project in case it helps: AsyncMethodTranslatedToObjCBug.zip.

[fumoboy007]

Passing nil to the completion handler parameter should not produce a warning.

The method also crashes at runtime if nil is passed to it.

[AnthonyLatsis]

cc @DougGregor