Generate oneofs as sealed abstract class

Fixes #1694

Author: thesamet

CHANGELOG.md

@@ -3,6 +3,7 @@
 - Bump protobuf and protoc to 4.26.0. Currently files must specify proto2 and
   proto3 syntax. Editions are not supported yet.
 - References to descriptors are fully qualified (#1724)
+- Oneofs are now generated as sealed abstract class instead of sealed traits (#1694)
 
 ## [0.11.18] (Unreleased)
 - References to descriptors are fully qualified (#1724)

compiler-plugin/src/main/scala/scalapb/compiler/ProtobufGenerator.scala

@@ -115,7 +115,9 @@ class ProtobufGenerator(
 
   def printOneof(printer: FunctionalPrinter, e: OneofDescriptor): FunctionalPrinter = {
     printer
-      .add(s"sealed trait ${e.scalaType.nameSymbol} extends ${e.baseClasses.mkString(" with ")} {")
+      .add(
+        s"sealed abstract class ${e.scalaType.nameSymbol} extends ${e.baseClasses.mkString(" with ")} {"
+      )
       .indent
       .add(s"def isEmpty: _root_.scala.Boolean = false")
       .add(s"def isDefined: _root_.scala.Boolean = true")

scalapb-runtime/src/main/js-native/com/google/protobuf/struct/Value.scala

@@ -195,7 +195,7 @@ object Value extends scalapb.GeneratedMessageCompanion[com.google.protobuf.struc
   lazy val defaultInstance = com.google.protobuf.struct.Value(
     kind = com.google.protobuf.struct.Value.Kind.Empty
   )
-  sealed trait Kind extends _root_.scalapb.GeneratedOneof {
+  sealed abstract class Kind extends _root_.scalapb.GeneratedOneof {
     def isEmpty: _root_.scala.Boolean = false
     def isDefined: _root_.scala.Boolean = true
     def isNullValue: _root_.scala.Boolean = false

scalapb-runtime/src/main/scalajvm/com/google/protobuf/struct/Value.scala

@@ -216,7 +216,7 @@ object Value extends scalapb.GeneratedMessageCompanion[com.google.protobuf.struc
   lazy val defaultInstance = com.google.protobuf.struct.Value(
     kind = com.google.protobuf.struct.Value.Kind.Empty
   )
-  sealed trait Kind extends _root_.scalapb.GeneratedOneof {
+  sealed abstract class Kind extends _root_.scalapb.GeneratedOneof {
     def isEmpty: _root_.scala.Boolean = false
     def isDefined: _root_.scala.Boolean = true
     def isNullValue: _root_.scala.Boolean = false