CMake also generates the 'Swift Compiler - Custom Flags' for each debug/release mode, and they all look like so: '-cxx-interoperability-mode=default' -Xcc '-std=c++20' $(inherited) (modulo the -O options for size/none/etc). So I'm confused what flags Xcode 15 Beta is using when showing me the bindings it generates per the WWDC 2023 talk I tagged.

Yeah, I'm using Xcode 15.0 beta 5 (15A5209g). I set the C++ Language Dialect on the Project, on the C++ library target, and on the Swift executable target, and the GUI still complains about the C++-20 feature I'm using. If I make sure they're all C++2b, the error still shows up in the Xcode GUI when trying to parse the auto-generated swift bindings. Asking Xcode to build the project says it succeeds too.

@KTRosenberg I am not using C++20 standard modules. Those are not complete in any version of Clang, and their documentation is here https://clang.llvm.org/docs/StandardCPlusPlusModules.html. The module information I linked is about Clang modules, which is a pre-C++20 feature that Swift/Objective-C use to import modules into Swift projects. As noted on the Clang modules documentation, they are very different https://clang.llvm.org/docs/Modules.html#standard-c-modules