Should an adapter have access to protected fields of its adapted type?

[zygoloid]

Should this be valid:

class A {
  protected var a: i32;
}
class B {
  extend adapt A;
  fn F[self: Self]() -> i32 { return self.a; }
}

On the one hand, adapting a class is somewhat like inheritance, so perhaps an adapter should be able to access protected fields. On the other hand, adapting a class is not inheritance in various ways. Note that A is a final class; if it inherited a from one of its base classes, it may not expect that any code outside A and its base class can access that field. (Maybe we might even want to reject protected members in final classes.)

If we don't allow this, one could still use friend declarations to allow specific adapters to access non-public class members. The design currently doesn't permit adapters to access protected members of the adapted type; this came up because the toolchain currently does (though we plan to change it to match the current design).