Interop: Use of c++20 type char8_t confuses the swift compiler and Xcode

[ADKaster]

Description

An imported cxx module (clang module) with a struct that uses the type char8_t has its char8_t member dropped on the floor.

Steps to reproduce

In this fork of the swift-cmake-examples:

https://github.com/ADKaster/swift-cmake-examples/tree/xcode-15/Interop

I have a CMake project that uses c++20, sets the interop flag to true, has a C++20-only feature in the C++ component (a char8_t member).

When I generate an Xcode project

cmake -S Interop -B Xcode -GXcode

and open that Xcode project in Xcode Version 15.0 (15A240d), I can set the interop mode to C++, and set the C++ language standard to C++20 (or gnu++20), and the project builds.

cmake --build Xcode

or

Build in Xcode GUI

Expected behavior

Building and running the project gives an executable E that when run, prints Version(major: 1, minor: 0, patch: 0)

However, there are two bugs instead:

1. the major version is dropped on the floor

swift-cmake-examples/Interop$ ./Xcode/Sources/Swift/Executable/Debug/E
Version(minor: 0, patch: 0)

2. Xcode cannot parse the native module when ctrl+clicking on the module:

Environment

• Swift compiler version info: 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-macosx13.0

• Xcode version info:

Xcode 15.0
Build version 15A240d

• Deployment target: macOS 13.0 (running 13.6)

I reported this on apple developer a few months ago but didn't get a reply that actually helped.

https://developer.apple.com/forums/thread/735186