K2 plugin API has a limitation that meta annotations from plugin predicates
can't be used on nested annotations because their resolve process includes supertypes
resolve, that can be affected by the plugin itself. Therefore, @MetaSerializable
can't be applied to nested annotation classes in K2, which is reflected by
this diagnostic.
For old FE, diagnostic is lowered to WARNING with deprecation message.
Leonid Startsev
f627d81d53