When using lambda coroutines as delegates, care must be taken that the lifetime of captured objects may be shorter than that of the coroutine, see #766 (comment). This issue has a better solution in C++23, which is to use explicit object parameters to copy the captures into the coroutine frame. However, this feature is currently not available in C++/WinRT.

The following code fails to compile with an error that the copy constructor has been deleted.

#include <winrt/Windows.Foundation.Collections.h>
using namespace winrt::Windows::Foundation::Collections;

int main()
{
	int called{};
	int sum{};

	auto vector = winrt::single_threaded_observable_vector<int>();
	auto revoker = vector.VectorChanged(winrt::auto_revoke, 
		[called, sum](this auto, IObservableVector<int>, IVectorChangedEventArgs)
		{
		});
}

The reason is that the final type delegate inherits from the handler (lambda), and code_writers.h#L2619 uses the type delegate (*this in the Invoke member function of delegate) to call operator(), causing this (explicit parameter of lambda) to be deduced as delegate. Since delegate is non-copyable, operator() cannot be called. This change makes Invoke use the original lambda's type to call operator(), so this is deduced as the lambda's type, resolving the issue.

Because of the purpose is to copy the captured object into the coroutine frame, the slicing that occurs here is intentional, and there is no need to copy the complete delegate object.