[sealed] Runtime checking of PermittedSubtypes

[Alex Buckley]

On 4/22/2020 4:32 PM, Dan Smith wrote:
> Another module system sanity check: is mutual recursion allowed between unnamed modules of different loaders? (Pre-9, mutual recursion between different loaders was certainly possible...)

Yes. Anything that was possible with class loaders on JDK 8 is still 
possible on JDK 9+. Class loaders in JDK 9+ are free to mutually 
delegate for classes that are in unnamed modules rather than in 
layer-defined run-time modules. We even arrange a complete readability 
graph between those unnamed modules -- "Every unnamed module reads every 
run-time module." -- in case class C in one unnamed module attempts to 
access (possibly via a `Class` object, so no need for c.p. resolution) 
class D in another unnamed module, without any concern or distress over 
the higher-level story that C refers to D and D refers to C.

Alex