Approach for JDK-8225377: type annotations are not visible to javac plugins across compilation boundaries

[Liam Miller-Cushon]

Hello,

I'm interested in trying to help with
https://bugs.openjdk.java.net/browse/JDK-8225377 (type annotations are not
visible to javac plugins across compilation boundaries).

The issue described in the bug is that type annotations are not currently
associated with some TypeMirrors loaded from the classpath. For example,
TypeMirror#getAnnotationMirrors for the return type of a method does not
report type annotations if the method was loaded from the classpath, but it
does work for elements compiled from source in the current compilation.

What I understand so far is:

* the ClassReader reads in the full *TypeAnnotations attributes, including
type paths
* it creates a TypeAnnotationCompleter that saves the type annotations to
the associated Symbol using Symbol#setTypeAttributes
* the Symbol never associates the type annotations with their corresponding
types, e.g. a MethodSymbol does not attach annotations to its return type

It seems like there's logic missing to interpret the type paths read from
class files and attach them to the appropriate type model element. This
would be somewhat similar to the logic in core reflection that handles type
paths and deals with reporting type annotations on the correct type [1].

I'm wondering if I'm missing something, is there any prior work in javac to
interpret type paths read from class files? If not, would there be interest
in accepting that as a contribution? It would add some complexity and
potential overhead, but I'm not seeing a good alternative to fix
TypeMirror#getAnnotationMirrors for types read from class files.

[1]
https://github.com/openjdk/jdk/blob/d983d108c565654e717e2811d88aa94d982da2f5/src/java.base/share/classes/sun/reflect/annotation/AnnotatedTypeFactory.java#L194
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