Sealed classes - error: expected token: 'sealed'; generated class instead

[ZacSweers]

When trying to use GJF with JDK 16, I get the following error

Caused by: com.google.googlejavaformat.java.FormatterException: 8:52: error: expected token: 'sealed'; generated class instead
        at com.google.googlejavaformat.java.Formatter.getFormatReplacements(Formatter.java:293)
        at com.google.googlejavaformat.java.Formatter.formatSource(Formatter.java:267)
        at com.google.googlejavaformat.java.Formatter.formatSource(Formatter.java:233)
        ... 135 more

With this source code

package dev.zacsweers.moshix.sealed.sample.java;

import com.squareup.moshi.JsonClass;

import dev.zacsweers.moshix.sealed.annotations.TypeLabel;

public interface FailureTestCases {
  @JsonClass(generateAdapter = false, generator = "sealed:type")
  sealed class DuplicateLabels permits DuplicateLabels.TypeA, DuplicateLabels.TypeB {
    @TypeLabel(label = "a")
    static final class TypeA extends DuplicateLabels {}

    @TypeLabel(label = "a")
    static final class TypeB extends DuplicateLabels {}
  }

  @JsonClass(generateAdapter = false, generator = "sealed:type")
  sealed class DuplicateAlternateLabels permits DuplicateAlternateLabels.TypeA, DuplicateAlternateLabels.TypeB {
    @TypeLabel(label = "a", alternateLabels = {"aa"})
    static final class TypeA extends DuplicateAlternateLabels {}

    @TypeLabel(label = "b", alternateLabels = {"aa"})
    static final class TypeB extends DuplicateAlternateLabels {}
  }

  @JsonClass(generateAdapter = false, generator = "sealed:type")
  sealed class GenericSubtypes<T> permits GenericSubtypes.TypeA, GenericSubtypes.TypeB {
    // This form is ok
    @TypeLabel(label = "a", alternateLabels = {"aa"})
    static final class TypeA extends GenericSubtypes<String> {}

    // This form is not ok
    @TypeLabel(label = "b", alternateLabels = {"aa"})
    static final class TypeB<T> extends GenericSubtypes<T> {}
  }
}

I know sealed classes are still in preview though, so understand if this isn't expected to be supported yet. The error message is a little surprising though

[kevinb9n]

We'd like to support preview features -- if not by formatting them well, that at least through the magic of not crashing.

If for some reason this is a nontrivial fix (?), we might at least fix the error message first.