Side effects return a Task

I was curious about whether there are any side effects when you do this:

public Task FooAsync()
{
   //do some synchronous stuff
   return Task.CompletedTask;
} 

Versus:

public void FooSync()
{
  //do some synchronous stuff
}

The thing is that I have abstraction and the concrete implementation is synchronous for now. But in the future, it might use async await syntax. So it's nice from the beginning your abstraction to support both sync and async scenarios and you won't need to change the signature of the method (return type from void to Task) and vise versa. Are there any drawbacks returning a Task and running synchronous code?