A customer using C++/WinRT found that their attempt to perform a co_await on an IAsyncAction failed to compile with the error

promise_type: is not a member of coroutine_traits<void>

What does this error mean? “We included <winrt/Windows.Foundation.h>, which is allegedly how to fix this weird error message, but that didn’t fix it.”

This is a case of skipping the reading the explanation part of looking for a solution to your problem and skipping directly to the how to fix it part, without checking that the how to fix it applies to your specific situation. “Somebody called in to a car repair show because their car didn’t start, and the answer was that they had loose battery cables. My car won’t start, and I tightened the battery cables, but it didn’t fix it.” Well yeah, because you skipped over the part of the call where they did the troubleshooting.

As I noted in the earlier article, the co_await keyword triggers the coroutine transformation which rewrites the function performing the co_await into a state machine, and one of the pieces of the state machine is the promise_type associated with the coroutine’s return type and parameters.

The coroutine_traits specialization in the error message is coroutine_traits<void>, so the return type is void and there are no parameters. In other words, the function performing the co_await looks something like this:

void DoSomething()
{
    co_await MyFunctionAsync();
}

The C++/WinRT library provides support for producing coroutines returning winrt::Windows::Foundation::IAsyncAction and related interfaces, as well as the special type winrt::fire_and_forget. It does not, however, add support for coroutines returning void.

The winrt/Windows.Foundation.h header file defines the Windows Runtime coroutines, but void is not a Windows Runtime coroutine type, so including Windows Runtime headers isn’t going to help. The code probably meant for Do­Something() to return a type that can be used for coroutines, most likely IAsyncAction or fire_and_forget.

Now, you could always write your own support for producing coroutines returning void, say, by having it behave roughly the same as fire_and_forget. It’s not hard.

But probably not advisable.