Semantics of an empty PermittedSubtypes attribute for the VM

[John Rose]

On Apr 3, 2020, at 1:36 PM, Brian Goetz <brian.goetz at oracle.com> wrote:
> 
> I'm mostly thinking of interfaces that will only be implemented by hidden classes. 

In order to do that we need a way to hook the HCs
into the permits list.  That can be done with a private
abstract class that nobody extends except the desired HCs.
The HCs themselves cannot be placed on the permits list
because they cannot be named, and the permits list contains
names.  (N.B. the experimental constant pool patching
mechanism would also, in principle, allow HCs to be
“named” on permit lists, by direct patching of a relevant
CP entry.  This is one sort of use case I was thinking of
when I joined the CP patching feature to the HC prototype.
Basically, imagine a whole ecosystem of HC hierarchies,
which would require patching the CPs of their various
classfiles.  It would be a powerful mess.  But there are
less powerful ways to get the same effects as CP patching,
for all the use cases we’ve looked at.)