`coroutine_handle<T>::from_promise(T &p)` requires that "`p` is a
reference to a promise object of a coroutine."

OK, but... a reference to a base class subobject is also a reference
to the most derived-object, right? So if you have some type `T` which
you use as a coroutine's promise, you can in theory manufacture a
`coroutine_handle<U>` for that object if `U` is a base class subobject
of `T`:

```
struct T : public U
{
  coroutine_handle<U> make_base_handle()
  {
    return coroutine_handle<U>::from_promise(*this);
  }
}
```

So long as `*this` is a `T` which is a promise object, this ought to
work. But I'm pretty sure it *shouldn't*.

`from_promise(p)` should additionally require that `p` refers to the
most-derived object. IE: not a base class of a promise object.