☂️ 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

• [C++-Interop] C++ struct passed by value to an objc_msgSend is passed as a pointer type of ABIArgInfo::Direct instead of a Indirect byval #61929
• [interop] C++ function returning NonCopyable type returns Never in Swift and is callable #64401
• [cxx-interop] Name lookup failures in C++20 mode on Linux #65710
• [C++-Interop][Swift 5.9] Invoking a manually defaulted non-noexcept C++ constructor from Swift results in a compiler crash in IRGen #65891
• [cxx-interop] std::optional should provide nullable access to the wrapped object #65918
• [cxx-interop] Add a SWIFT_MUTATING macro to <swift/bridging> header #66322
• [cxx-interop] Finalize Swift 5.9 rules that determine when inherited members that shadow other members can be introduced to the Swift type that represents the C++ structure or class  #66323
• [interop] Tracking issue to track support for immutable noncopyable borrows for C++ types in Swift #61454
• [cxx-interop] Enabling C++ interop leads to linking errors with URL.bookmark() #66501
• Xcode-beta (15.0) C++ interop option generates erroneous 'let' property initialization code #66523
• std::vector<std::string> fails to conform to CxxRandomAccessCollection #67410

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

• [cxx-interop] ensure that copy-assignment operator works for Swift value types in C++ #66324
• Cxx to Swift interop - issue with Array and Int type - "type cannot be used in a Swift generic context"  #63452

Build / platform-specific fixes

• Windows toolchain - swiftToCxx directory missing  #63446
• [cxx-interop] fix the C++ methods dispatch for the Windows target #66326
• windows: test C++ stdlib support
• Swift cannot use -enable-experimental-cxx-interop on Linux with Foundation #64457 (linux & libstdc++)

SwiftPM usability

Compiler support for @_implementationOnly imports:

• [cxx-interop] a Swift module that enables interop should not force client Swift module to fail to import CxxShim #65831

Improved diagnostics when importing things requiring C++ interop:

• [cxx-import] a Swift module that imports a C++ module and forces the client to enable interop produces unclear diagnostic #65833
• [cxx-import] a Swift module that imports 'CxxStdlib' and forces the client to enable interop produces unclear diagnostic #65832

SwiftPM fixes:

• [C++ interop] c++ language standard is not passed to Swift's clang importer when C++ interop is enabled swift-package-manager#6565

Interop ecosystem - IDE, tooling support, debugging

• indexing-related: Assertion `member->getDeclContext() == nominal && "Added member to the wrong context"' failed when using CXX interop #65646

[hyp]

We're done with 5.9, closing.
I will open a follow-up issue for tracking 5.9.X fixes.