Unfortunately the <experimental/coroutine> and/or <experimental/resumable> headers exist in c++/WinRT libraries which I am using. I am currently using the latest windows SDK that’s compatible with win10 so unless I forgo win10 support (assuming later version of windows SDK and c++/WinRT switched to using <coroutine>), my only option is to compile with /await flag if I want to use c++20 compilier.