fix: limit TypePattern's VariableDeclaratorList to a single VariableDeclarator

[jtkiesel]

What changed with this PR:

There exists some ambiguity in the parser within comma-separated TypePatterns, because TypePattern contains a VariableDeclaratorList, which is comma-separated VariableDeclarators. The parser cannot currently determine where the VariableDeclaratorList ends and the next TypePattern begins. This was previously realized in #707, within ComponentPatternList, and is now experienced in #733, within SwitchLabel.

I realized that the JLS specifically mentions semantic restrictions that make it easy to resolve this ambiguity. From 14.30.1. Kinds of Patterns (emphasis mine):

The rules for a local variable declared in a type pattern are specified in §14.4. In addition, all of the following must be true, or a compile-time error occurs:

• The LocalVariableType in a top level type pattern denotes a reference type (and furthermore is not var).
• The VariableDeclaratorList consists of a single VariableDeclarator.
• The VariableDeclarator has no initializer.
• The VariableDeclaratorId has no bracket pairs.

As such, this PR limits TypePattern's VariableDeclaratorList to a single VariableDeclarator in our parser. That change also allowed me to remove the patch that was added in #708, as this fix also resolves the ambiguity for the issue seen in #707.

Example

Input

public class Fail {

  private Function<ShootResult, ShootResult.Miss> missed() {
    return lastResult -> switch (lastResult) {
      case ShootResult.Miss(int fails) -> new ShootResult.Miss(fails + 1);
      case ShootResult.Many _ ,ShootResult.One _ -> new ShootResult.Miss(0); // crash on this line
             
    };
  }

  private sealed interface ShootResult {
    record One(Pin pin) implements ShootResult {}
    record Many(int number) implements ShootResult {}
    record Miss(int timesInRow) implements ShootResult {}
  }
}

Output

public class Fail {

  private Function<ShootResult, ShootResult.Miss> missed() {
    return lastResult ->
      switch (lastResult) {
        case ShootResult.Miss(int fails) -> new ShootResult.Miss(fails + 1);
        case ShootResult.Many _, ShootResult.One _ -> new ShootResult.Miss(0); // crash on this line
      };
  }

  private sealed interface ShootResult {
    record One(Pin pin) implements ShootResult {}

    record Many(int number) implements ShootResult {}

    record Miss(int timesInRow) implements ShootResult {}
  }
}

Relative issues or prs:

Closes #733

[clementdessoude]

Great work ! And well spotted 😀

Just a small suggestion but we also could merge at is

[lanwen]

@clementdessoude, @jtkiesel would you be so kind as of releasing this in the next version? Or maybe it's possible to grab that some other way?

[jtkiesel]

@lanwen Sorry for the delay. I'd like to get this project to auto-publish new versions of the package for each meaningful commit pushed to main, but for now releases are manual. Thanks for your patience! I've published version 2.6.8, so please give that a try and let me know if you experience any issues!