value type hygiene

[Brian Goetz]

As promised, here's the part I'm not so comfortable with.  My concern 
has specifically to do with migration.  Suppose V is a value-based class 
(as per 
https://docs.oracle.com/javase/8/docs/api/java/lang/doc-files/ValueBased.html), 
which is later migrated to a value type.  D has been compiled before 
this transition, so its view of V is still that it is an object type.

Here's where I think this approach gets too optimistic:

> Well, yes, but
> remember also that if `Q` started its career as an object type, it was
> a value-based class, and such classes are documented as being
> null-hostile.  The null-passers were in a fool's paradise.

Objection, Your Honor, assumes facts not in evidence!  The letters 
"n-u-l-l" do not appear in the definition of value-based linked above.  
Users have no reasons to believe any of he following are bad for a VBC, 
among others:

     V v = null;

     if (v == null) { ... }

     List<V> list = new ArrayList<>();
     list.add(null);

In Q world, these uses of V were compiled to LV rather than QV, so these 
idioms mapped to a natural and sensible translation.  Our story was that 
when you went to recompile, then the stricter requirements of value-ness 
would be enforced, and you'd have to fix your code.  (That is: making V 
a value is binary compatible but not necessarily source compatible.  
This was a pretty valuable outcome.)

One of the possible coping strategies in Q world for such code is to 
allow the box type LV to be denoted at source level in a possibly-ugly 
way, so that code like the above could continue to work by saying 
"V.BOX" instead of "V".  IOW, you can opt into the old behavior, and 
even mix and match between int and Integer.  So users who wanted to 
fight progress had some escape hatches.

While I don't have a specific answer here, I do think we have to back up 
and reconsider the assumption that all uses of V were well informed 
about V's null-hostility, and have a more nuanced notion of the 
boundaries between V-as-value-world and V-as-ref-world.