RFR: 8357016: Candidate main methods not computed properly

[Jan Lahoda]

A consider class like this:


public class TwoMains {
    private static void main(String... args) {}
    static void main() {
        System.out.println("Should be called, but is not.");
    }
}


The `MethodFinder` will do lookup for the `main(String[])` method, and it finds one, so does not proceed with a lookup for `main()`. But then, it will check the access modifier, and will reject that method, never going back to the `main()` method. This is not what the JLS says about the lookup - the private method is not a candidate, and should be ignored.

Something similar happens if the return type is not `void`.

This PR is fixing that by checking whether the `main(String[])` method is usable early, and falling back to `main()` if it `main(String[])` is not usable.

It also removes the check for the `abstract` method, as that, by itself, is not really backed by JLS, but adds a check for `abstract` class, producing a user-friendly message is trying to invoke an instance `main` method on an `abstract` class (which, obviously, cannot be instantiated).

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

Commit messages:
 - 8357016: Candidate main methods not computed properly

Changes: https://git.openjdk.org/jdk/pull/25256/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=25256&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8357016
  Stats: 129 lines in 8 files changed: 103 ins; 10 del; 16 mod
  Patch: https://git.openjdk.org/jdk/pull/25256.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25256/head:pull/25256

PR: https://git.openjdk.org/jdk/pull/25256