RFR: 8309336: Incorrect switch in enum not reported properly

[Jan Lahoda]

As of JDK 21, enum constants may be qualified in switches. But there's a bug in the way these "constants" are attributed. Having code like:

class EnumLabelMustBeEnumConstant {
    enum E { A };

    void m(E e) {
        switch (e) {
            case EA -> {}
            default -> {}
        }
    }

    static final E EA;
}


javac will attribute `EA`, which will turn out to exist, and have the correct type (`E`), so that it will be accepted (and may crash javac later in the backend).

The proposed fix is simply to check that the given expression is an enum constant.

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

Commit messages:
 - Improving error messages.
 - Fixing tests.
 - Improving error messages for non-enum switch
 - 8309336: Incorrect switch in enum not reported properly

Changes: https://git.openjdk.org/jdk/pull/14284/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14284&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8309336
  Stats: 121 lines in 7 files changed: 112 ins; 1 del; 8 mod
  Patch: https://git.openjdk.org/jdk/pull/14284.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14284/head:pull/14284

PR: https://git.openjdk.org/jdk/pull/14284