The code is structured like this:

```
class S2Polyline {

  class OwningShape {
      std::unique_ptr<S2Polyline> owned_polyline_;
  };
};
```

libcxx 16 complains that S2Polyline is not defined at the point where
the dtor of the unique_ptr is called (which is OwningShape's implicitly
declared default destructor in the same header file) (also see [0]).

As a remedy, declare OwningShape's destructor in the source file where
the full declaration of S2Polyline is visible.

[0] https://stackoverflow.com/a/34073093/22422288)