How to resolve error C3986: signature of member contains native type 'Concurrency::task<void>'

Question

Hi all,

I want add a task<void> to .h file.
Following these action, it will get the 「error C3986:  signature of member contains native type 'Concurrency::task<void>'」

1. Create new project, File -> New -> Project -> Blank Application
2. Add "#include <ppltasks.h>" and "using namespace Concurrency" to .h file
3. Add "task<void> testLogin();" to public: scope
4. Compile
5. Show the error code "error C3986: 'testLogin': signature of member contains native type 'Concurrency::task<void>' "

#include "pch.h"
#include "BlankPage.g.h"
#include <ppltasks.h>
using namespace Concurrency;

namespace Application1
{
	public ref class BlankPage sealed
	{
	public:
		BlankPage();
		task<void> testLogin();
	protected:
		virtual void OnNavigatedTo(Windows::UI::Xaml::Navigation::NavigationEventArgs^ e) override;
	};
}

But, if move "task<void> testLogin();" to private: scope
It will fine.

Why?
Is there limitation for declare "task<void>" or I have mistake?

Thanks