RFR: 8232765: NullPointerException at Types.eraseNotNeeded() when compiling a class [v2]

[Guoxiong Li]

> Hi all,
> 
> The method `types::asSuper` may return `null` so that `types.erasure(types.asSuper(iterator.type.getReturnType(), syms.iteratorType.tsym))` throws `NPE`.
> 
> This patch sets the return type and outputs the corresponding error message if `types::asSuper` returns `null`.
> 
> Thank you for taking the time to review.
> 
> Best Regards.

Guoxiong Li 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 three additional commits since the last revision:

 - Move the check to Attr. Use meaningful class name. Fix failed tests.
 - Merge branch 'master' into JDK-8232765
 - 8232765: NullPointerException at Types.eraseNotNeeded() when compiling a class

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/2099/files
  - new: https://git.openjdk.java.net/jdk/pull/2099/files/f2fafbde..cbbaeddc

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=2099&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=2099&range=00-01

  Stats: 342070 lines in 8019 files changed: 185380 ins; 111856 del; 44834 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2099.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2099/head:pull/2099

PR: https://git.openjdk.java.net/jdk/pull/2099