"Model 2" prototype status

[Brian Goetz]

> Placing a sort on a type variable in, for example, a class declaration
> can have consequences in the declaration body. If you declare a class
> `Pop<any T>` you are declaring it *for all* instantiations of T,
> including primitives. Hence in the body of declaration, you shouldn’t be
> able to assign null to an instance of the type variable, for example. It
> breaks the contract in the declaration. This is no different to Java 8,
> where we check in the body that we respect the bounds contract on type
> variables.

Don't forget nullability is just an example here of the category of 
things that "classical" generics allow based on the assumption that type 
variables only quantify over reference types.

A similar restriction is that you can't use an avar as the lock object 
for a synchronized block.  Why?  Because the lock object has to be an 
_Object_.  And if T is an any-var, it might be an int -- which means you 
can't use it as a lock object.

Same for:
  - Object methods that are not going to be lifted to values (it seems 
likely that equals/hashCode/toString/getClass will find their way to 
values, and that wait/notify will not.  (But note this has little to do 
with anyfied generics directly -- this has to do with the properties of 
value types when we add them to the type system.  Of course, the goal 
here is that generics interact nicely with value types, but the two are 
separate problems.)
  - Assumptions that T[] <: Object[]; this is true for reference T but 
not for value T.

Again, these restrictions are simply consequences of quantifying over a 
heterogeneous sort of types; if there's at least one type (e.g., int) in 
the sort "any T", then things like "synchronized (T)" or "T = null" 
can't be allowed because we can't be confident they mean something.

This is no different from restrictions we impose now.  For example:

class Foo<T> {
     T t;

     ... I can call t.toString() ...

     ... But I can't call t.intValue() because I have no reason to 
believe this method exists on T ...
}