RFR: JDK-8246704: --release => "unknown enum constant PreviewFeature$Feature.TEXT_BLOCKS"

[Jan Lahoda]

Hi,

When compiling with --release 14, warnings like this may be printed:
warning: unknown enum constant PreviewFeature$Feature.TEXT_BLOCKS
   reason: class file for jdk.internal.PreviewFeature$Feature not found


The cause is fairly simple - the APIs associated with preview features 
(like new methods in j.l.String) are annotated with a JDK-internal 
annotation, which allows javac to detect them a provide an error/warning 
as appropriate. The issues is that the internal annotation is not inside 
the ct.sym, and loading an element annotated with it leads to resolution 
of the annotation, and to the warning above.

Unfortunately, this is not very trivial to fix, as javac needs the 
annotation to detect the API elements associated with preview features 
(and so the annotation needs to be in ct.sym). And, when compiling 
without --release, not resolving an annotation that is part of the 
classfile would provide inconsistent results.

So, the proposal is to use a different, synthetic, "annotation" (or 
rather name) inside ct.sym, and use that name only for detection of APIs 
associated with preview features and not resolve that annotation.

Proposed webrev:
http://cr.openjdk.java.net/~jlahoda/8246704/webrev.00/

JBS:
https://bugs.openjdk.java.net/browse/JDK-8246704

Any comments on this?

Thanks,
     Jan