co_await not working - c++

Question

I attempted to try out co_await as mentioned in this article https://blogs.msdn.microsoft.com/vcblog/2016/04/04/using-c-coroutines-to-simplify-async-uwp-code/ but there are weird compilation errors:
C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\include\experimental\resumable(44,0): Error C2825: '_Ret': must be a class or namespace when followed by '::' (compiling source file MainPage.xaml.cpp)
C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\include\experimental\resumable(44): error C2825: '_Ret': must be a class or namespace when followed by '::' (compiling source file MainPage.xaml.cpp)
MainPage.xaml.cpp(44): note: see reference to class template instantiation 'std::experimental::coroutine_traits<void,::MainPage ^,Windows::UI::Xaml::Navigation::NavigationEventArgs ^>' being compiled
C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\include\experimental\resumable(44,0): Error C2510: '_Ret': left of '::' must be a class/struct/union (compiling source file MainPage.xaml.cpp)
C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\include\experimental\resumable(44): error C2510: '_Ret': left of '::' must be a class/struct/union (compiling source file MainPage.xaml.cpp)
C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\include\experimental\resumable(44,0): Error C2061: syntax error: identifier 'promise_type' (compiling source file MainPage.xaml.cpp)
C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\include\experimental\resumable(44): error C2061: syntax error: identifier 'promise_type' (compiling source file MainPage.xaml.cpp)
C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\include\experimental\resumable(44,0): Error C2238: unexpected token(s) preceding ';' (compiling source file MainPage.xaml.cpp)
Originally, I put the following code in the constructor
#include <experimental\resumable>
#include <pplawait.h>
using namespace concurrency;
MainPage::MainPage()
{
InitializeComponent();
auto my_data_file = co_await Windows::ApplicationModel::Package::Current->InstalledLocation->GetFileAsync("samples.txt");
// Preparing app data structures
}
which does not work. My guess is that this is not possible in constructor (for obvious reason) so I move the co_await line to
void MainPage::OnNavigatedTo(NavigationEventArgs^ e)
{
auto my_data_file = co_await Windows::ApplicationModel::Package::Current->InstalledLocation->GetFileAsync("samples.txt");
}
which result in the aforementioned compilation errors.

Answer 1

My guess is that the first problem is that you can't return void from a resumable function since void does not fill any coroutine traits that co_await expect (like get_return_object , set_result etc.)
if you are already using PPL, return task<void>:
task<void> MainPage::OnNavigatedTo(NavigationEventArgs^ e)
{
auto my_data_file = co_await Windows::ApplicationModel::Package::Current->InstalledLocation->GetFileAsync("samples.txt");
}

Answer 2

I found the answer: co_await can only be used if we are returning a task<> (this is incorrect, see #DavidHaim's answer). I guess it is just a syntactic sugar for create_task. The solution is to extract a method returning task<void> in this case:
#include <experimental\resumable>
#include <pplawait.h>
using namespace concurrency;
MainPage::MainPage()
{
InitializeComponent();
PrepareData();
}
task<void> MainPage::PrepareData()
{
auto my_data_file = co_await Windows::ApplicationModel::Package::Current->InstalledLocation->GetFileAsync("samples.txt");
// Preparing app data structures
}
Some comment: Although the coroutine feature looks nice, it pollutes the language. Observe that the body of PrepareData() does NOT have any return statement but the signature says it returns task<void>, leading to my failure to recognize the requirement in the article.