☂️ Tracking issue for C++ interoperability fixes we want for Swift-5.9

[hyp]

Must-fixes for Swift-5.9

This section tracks all of the C++ interoperability fixes we would like to have in the Swift-5.9 release. They're categorized by different groups presented below:

"Forward" interop - using C++ APIs in Swift

• [ ] https://github.com/apple/swift/issues/61929
• [ ] https://github.com/apple/swift/issues/64401: Land fixed https://github.com/apple/swift/pull/65105 (@hyp)
• [x] https://github.com/apple/swift/issues/65710
• [ ] https://github.com/apple/swift/issues/65876
• [x] https://github.com/apple/swift/issues/65891
• [x] https://github.com/apple/swift/issues/65918
• [x] https://github.com/apple/swift/issues/66322
• [ ] https://github.com/apple/swift/issues/66323
• [ ] https://github.com/apple/swift/issues/61454
• [ ] https://github.com/apple/swift/issues/66501

"Reverse" interop - using Swift APIs in C++

• [x] https://github.com/apple/swift/issues/66324

Build / platform-specific fixes

• [x] https://github.com/apple/swift/issues/63446
• [ ] windows: calling C++ methods miscompilations: https://github.com/apple/swift/issues/66326
• [ ] windows: test C++ stdlib support
• [ ] linux & libstdc++: https://github.com/apple/swift/issues/64457

SwiftPM usability

Compiler support for @_implementationOnly imports:

• [x] https://github.com/apple/swift/issues/65831

Improved diagnostics when importing things requiring C++ interop:

• [ ] https://github.com/apple/swift/issues/65833
• [ ] https://github.com/apple/swift/issues/65832

SwiftPM fixes:

• [ ] https://github.com/apple/swift-package-manager/issues/6565

Interop ecosystem - IDE, tooling support, debugging

• [ ] indexing-related: https://github.com/apple/swift/issues/65646