cs4014-interfaces.cs

public interface IA
{
    Task F();
}

public class A : IA
{
    public async Task F() => await Task.Delay(0).ConfigureAwait(false);
}

static class Program
{
    static void Main()
    {
        A a = new A();
        IA ia = a;
        a.F();          // CS4014 triggered here...
        ia.F();         // ... but not here :(
    }
}