We don't need no stinkin' Q descriptors

[John Rose]

On 30 Jun 2023, at 18:38, Brian Goetz wrote:

…

>> Here is a complementary principle: In the VM, we should choose to support
>> exactly and only those refinement types that support Valhalla’s prime goals,
>> which are data structure improvement (flattening). Since |String!| doesn’t
>> (yet) have a flattening story, |String!| should not be a (VM) representable
>> type.
>>
>
> I'd agree, but only if I can add "yet".

Good!  There was a “yet” in there, and I meant yet another “yet”.
(It was a silent “yet”.)

…

> Spoiler: I much prefer to check on write, and ultimately, push that check into the VM using the same value set restriction machinery as we do for B3!.

(I teed that up for you.  You might be right.)

>> I don’t think there is a rift-healing move we could do with field
>> declarations, since flat |int| fields are already fully supported.
>>
>
> We could be generous in linkage, treating "I" and "LInteger;" with a type restriction as the same type, but it doesn't seem worth it.

Yes!

>> Although it is technically an incompatibility, we might consider
>> allowing legacy |int[]| arrays to interoperate with |Object[]|,
>> so that more generic code can be written. That would be close to
>> the spirit of allowing |B3![]| arrays be viewed transparently as
>> possibly-null-tolerant |B3[]| arrays.
>>
>
> I think we concluded that this was probably a forced move at some point, but we should revisit that analysis.

Looking at Arrays.java and ArrayList.java, and thinking about
T=int in those algorithms and containers and others, made it clear
the choice is between |int[] <: Object[]| or replicated bytecodes,
either by hand (like we do in Arrays.java) or somehow else
(circa Model-2).

>> I mean that the call |int.class.cast(x)| does not work, and lifting
>> that non-behavior up to |RepresentableType| will make a new and
>> unwelcome distinction between |B3!| and |int|: The mirror for |B3!|
>>
>
> (not actually a mirror)

Sorta-mirror.  Mirror of a different class.  Mirror-oid.