swift-memberwise-init-macro

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Description

Consider this code:

@MemberwiseInit
public struct A {
    let x: Int

    public struct B {}
}

extension A.B {}

Checklist

• [X] If possible, I've reproduced the issue using the main branch of this package.
• [X] This issue hasn't been addressed in an existing GitHub issue or discussion.

Expected behavior

MemberwiseInit should create a correct init

Actual behavior

Circular reference resolving attached macro 'MemberwiseInit'
Circular reference
'B' is not a member type of struct 'API.A'

swift-memberwise-init-macro version information

main

Destination operating system

iOS 17.2

Xcode version information

Xcode 15.1

Swift Compiler version information

swift-driver version: 1.87.3 Apple Swift version 5.9.2 (swiftlang-5.9.2.2.56 clang-1500.1.0.2.5)
Target: arm64-apple-macosx14.0

Just realized this also happens with the @Observable macro.

Thank you for the report!

And yeah, this is a rather unfortunate bug in Swift: https://github.com/apple/swift/issues/66450. I run into this one a lot. Some circularity issues were addressed in Xcode 15.1, but the cases that I run into are still outstanding, and usually have the same shape as your example.

You can sometimes work around the bug by moving the extension to another file, which can make sense depending on the nature/necessity of the macro. For MemberwiseInit, I personally tend to fall back to a boilerplate init and a FIXME.

This appears fixed in Swift 6! I confirmed using Xcode 16.0 beta 2 (16A5171r).

Closing, Swift 6 has been released with this fix. 🥳