RFR: JDK-8315458 Implement JEP 463: Implicitly Declared Classes and Instance Main Method (Second Preview) [v21]

[Jim Laskey]

> Address changes from JEP 445 to JEP 463.
> 
> - Move from a SYNTHETIC unnamed class to a MANDATED implicit class.
> 
> - Don't mark class on read.
> 
> - Remove reflection and annotation processing related to unnamed classes.
> 
> - Simplify main method search.

Jim Laskey has updated the pull request incrementally with one additional commit since the last revision:

  Allow nested class of same name as file
  
  Since it doesn't create a conflicting symbol and would produce a confusing error otherwise.

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Changes:
  - all: https://git.openjdk.org/jdk/pull/16461/files
  - new: https://git.openjdk.org/jdk/pull/16461/files/598553f8..889f3ecc

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=16461&range=20
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=16461&range=19-20

  Stats: 8 lines in 4 files changed: 5 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/16461.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16461/head:pull/16461

PR: https://git.openjdk.org/jdk/pull/16461