RFR: 8132984: incorrect type for Reference.discovered [v4]

[Kim Barrett]

> Please review this change which fixes the type of the private
> Reference.discovered field.  It was Reference<T>, but that's wrong because
> it can be any Reference object.
> 
> I've changed it to Reference<?> and let that flow through, updating some
> other variables that were previously somewhat incorrectly typed (usually
> with an Object type parameter). The interesting change is to the
> ReferenceQueue.enqueue parameter, which is now also Reference<?>.
> 
> This ultimately end up with a provably safe and correct, but uncheckable,
> cast in ReferenceQueue.enqueue.
> 
> An alternative might be to use a raw type for the discovered field, but I
> think that ends up with more @SuppressWarnings of various flavors.  I think
> the unbounded wildcard approach is clearer and cleaner.
> 
> Note that all of the pending list handling, including the discovered field,
> could be moved into a non-public, non-generic, sealed(?) base class of
> Reference<T>.  The pending list handling has nothing to do with the generic
> parameter T.
> 
> Testing:
> mach5 tier1 and tier4 (tier4 is where vmTestbase_vm_gc_ref tests are run)

Kim Barrett has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains five additional commits since the last revision:

 - Merge branch 'master' into fix_discovered_type
 - plevart improvement
 - update copyrights
 - Merge branch 'master' into fix_discovered_type
 - Use unbounded wildcard placeholders and final safe but unchecked cast

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/1897/files
  - new: https://git.openjdk.java.net/jdk/pull/1897/files/b95f5140..cd66bff1

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=1897&range=03
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=1897&range=02-03

  Stats: 3433 lines in 92 files changed: 381 ins; 2791 del; 261 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1897.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1897/head:pull/1897

PR: https://git.openjdk.java.net/jdk/pull/1897