Implement upcoming STL C++20 coroutines support

[sylveon]

The Microsoft STL has recently merged non-experimental coroutine support: microsoft/STL#894

This support, once shipped (expected around 16.8 preview 1), will prevent mixing code using the new coroutines and the experimental ones.

Using the /await command line (which is forced in various places by the C++/WinRT project files) will prevent including <coroutine>, telling you to use <experimental/coroutine>. Similarly, using the /std:c++latest command line (my code makes use of many new C++ 20 only features), will prevent including <experimental/coroutine>, telling you to instead use <coroutine>.

This puts me in a weird spot: I can't include <coroutine> because C++/WinRT uses <experimental/coroutine> and /await, but neither can I include <experimental/coroutine> because I use /std:c++latest.

Ideally, the C++/WinRT headers should detect the presence of C++20 coroutines (using __cpp_impl_coroutine) and use the right header. Additionally, the project files should not force /await in VS 16.8 and newer, or provide a way to opt-out from this behavior.

[kennykerr]

Yes, it might soon be time to create a C++17 branch for servicing and then switch master over to C++20. I'd like to avoid supporting both C++17 and C++20 as it can become quite complex especially since Clang and Visual C++ are never quite in line.

[DefaultRyan]

It looks like you're saying VC++ in C++17 mode does not define __cpp_impl_coroutine. Did I read that correctly?

[sylveon]

https://github.com/microsoft/STL/blob/21acd23ad81463628e95cb6c571c56ba7200cdba/stl/inc/yvals_core.h#L1162-L1164

https://github.com/microsoft/STL/blob/21acd23ad81463628e95cb6c571c56ba7200cdba/stl/inc/coroutine#L12-L21

Yes (although it might be better to use __cpp_lib_coroutines to detect it)

[sylveon]

It happened, VS 16.8 Preview 1 is out and my code doesn't compile anymore:

[sylveon]

Looks like __cpp_impl_coroutine isn't defined yet so existing code using <experimental/coroutine> will build fine (once this is defined <experimental/coroutine> will stop working with /std:c++latest), but trying to use <coroutine> anywhere else will fail because of the C++/WinRT props file forcing /await

[AlexBAV]

Now VS 16.8 Preview 3.0 is out with "full C++20 coroutines support".
What is the ETA of WinRT support for conformant coroutines?
What is the recommended (if any) way to switch own code to C++20 coroutines and still use the current version of WinRT?

[kennykerr]

I've been away for a few weeks but should be back in the "office" next week and plan to take a look at 16.8 support.

[DethRaid]

Subscribed to this issue because I'm also blocked by it

[kennykerr]

The C++/WinRT library has been updated to support both C++17 and C++20 coroutines.

[DethRaid]

Can confirm that I'm now un-blocked. Thank you so much!

[mauriceackel]

Hi @kennykerr,

I was just trying to build my C++/WinRT header (which I have created using cppwinrt.exe -input local -output .) with c++20. However, there seems to be an obvious error with this. My base.h header includes the passage:

#ifndef __clang__

#include <experimental/coroutine>

#else
...

Which will try to include <experimental/coroutine> as I compile with MSVC.

My cppwinrt.exe version is C++/WinRT v2.0.200203.5. I'm a bit confused at the moment given that some people reportedly were able to successfully compile with c++20... Do you perhaps see where the issue is?

[sylveon]

That cppwinrt version is super old, it seems to be from around February 2020. Try the NuGet for an up to date version.

[mauriceackel]

Oh well.. Shame on me 😓 I just installed it yesterday via a Nuget package but somehow I have managed to install an old version.. Tried it with the news version of cppwinrt.exe and what do you know, no compile errors anymore 🚀

Thanks for the quick response!