Optional is intended as a substitute for non-nullable values. Ideally, it should have a generic type of T extends Object in order to prevent non-sensical declarations like Optional<Foo?>. Such declarations are unlikely to happen intentionally, but one could imagine the case of an Optional used in another generic class, where T is a nullable type.

Ideally, we should prevent such uses by declaring Optional as Optional<T extends Object>.

This is a potentially significant breaking change, since currently T implicitly extends dynamic and therefore checks like the following don't produce any analysis errors:

if (obj is Optional) {
  return obj.value.someMethod();
}

where someMethod is a method defined on T. All such call sites would need to be fixed up to obj is Optional<AppropriateType>.

A survey of the Google codebase suggests this would be a fairly extensive breaking change. Given that Optional makes little sense once null-safety is enabled in the language by default, the simpler change may simply be to leave as-is and deprecate the class once null-safety lands in the language.