SI-9359 Fix InnerClass entry flags for nested Java enums

The access flags in InnerClass entries for nested Java enums were
basically completely off.

A first step is to use the recently introduced backend method
`javaClassfileFlags`, which is now moved to BCodeAsmCommon.
See its doc for an explanation.

Then the flags of the enum class symbol were off. An enum is
  - final if none of its values has a class body
  - abstract if it has an abstract method
(https://docs.oracle.com/javase/specs/jls/se7/html/jls-8.html#jls-8.9)

When using the ClassfileParser:
  - ENUM was never added. I guess that's just an oversight.
  - ABSTRACT (together with SEALED) was always added. This is to
    enable exhaustiveness checking, see 3f7b8b5. This is a hack and we
    have to go through the class members in the backend to find out if
    the enum actually has the `ACC_ABSTRACT` flag or not.

When using the JavaParser:
  - FINAL was never added.
  - ABSTRACT was never added.

This commit fixes all of the above and tests cases (Java enum read
from the classfile and from source).

1 files changed, 2 insertions, 0 deletions

diff --git a/src/compiler/scala/tools/nsc/symtab/classfile/ClassfileParser.scala b/src/compiler/scala/tools/nsc/symtab/classfile/ClassfileParser.scala
index 518a402230..660028eab8 100644
--- a/src/compiler/scala/tools/nsc/symtab/classfile/ClassfileParser.scala
+++ b/src/compiler/scala/tools/nsc/symtab/classfile/ClassfileParser.scala
@@ -539,6 +539,8 @@ abstract class ClassfileParser {
             devWarning(s"no linked class for java enum $sym in ${sym.owner}. A referencing class file might be missing an InnerClasses entry.")
           case linked =>
             if (!linked.isSealed)
+              // Marking the enum class SEALED | ABSTRACT enables exhaustiveness checking.
+              // This is a bit of a hack and requires excluding the ABSTRACT flag in the backend, see method javaClassfileFlags.
               linked setFlag (SEALED | ABSTRACT)
             linked addChild sym
         }