Inline Record vs JLS / Reflection

[John Rose]

On Dec 16, 2020, at 11:07 AM, Dan Smith <daniel.smith at oracle.com> wrote:
> 
> I don't think we have a good answer right now, but it's something we will want to address at some point. A solution would have to look like one of:
> 
> - Ask clients (e.g., "is this a record?" code) to adapt to the presence of the '$ref' class
> - Modify reflection to hide the '$ref' superclass somehow
> - Change the translation strategy to not disrupt the superclass hierarchy

The last is cleanest; the cost is resolving some technical
debt in Valhalla, which is allowing more kinds of supers
for primitive classes.  There’s no firm reason, IMO, why
Record could not be a super of both primitive and identity
classes, all of which are proper records.  Basically we need
to make interfaces and abstract classes look a little more
similar, with respect to the requirements of primitive
classes.

Spoiler alert:  I think the final solution will endow
abstract classes with *both* abstract and concrete
constructors.  The former will serve primitive
classes and the latter will serve identity classes.
Record will be such an abstract class.

(Alternatively, and more clumsily, Record could
be refactored as a proper interface, but sealed
to PrimitiveRecord and IdentityRecord, and
javac would translate to one or the other.  The
methods on JLO would not be defaults on
record but would be duplicated on the two
sealed subtypes.)

— John