Fix UninitializedFieldError in MacroCodecs under -Xcheckinit

SCALA-542

Author: avdv

bson/src/main/scala/org/mongodb/scala/bson/codecs/macrocodecs/CaseClassCodec.scala

@@ -307,11 +307,12 @@ private[codecs] object CaseClassCodec {
         import org.bson.codecs.configuration.CodecRegistry
         import org.mongodb.scala.bson.codecs.macrocodecs.MacroCodec
 
-        case class $codecName(codecRegistry: CodecRegistry) extends MacroCodec[$classTypeName] {
+        final case class $codecName(codecRegistry: CodecRegistry) extends {
+          val encoderClass = classOf[$classTypeName]
+        } with MacroCodec[$classTypeName] {
           val caseClassesMap = $caseClassesMap
           val classToCaseClassMap = $classToCaseClassMap
           val classFieldTypeArgsMap = $createClassFieldTypeArgsMap
-          val encoderClass = classOf[$classTypeName]
           def getInstance(className: String, fieldData: Map[String, Any]) = $getInstance
           def writeCaseClassData(className: String, writer: BsonWriter, value: $mainType, encoderContext: EncoderContext) = $writeValue
         }

docs/reference/content/changelog.md

@@ -15,6 +15,7 @@ Changes between released versions
   * Updated DEFAULT_CODEC_REGISTRY order, as upstream BsonCodec captures all Bson instances and priority needs to be lower.
   * Support $merge pipeline stage in aggregations [SCALA-473](https://jira.mongodb.org/browse/SCALA-473)
   * Deprecated BsonArray.apply(Iterable[BsonValue]) added BsonArray.fromIterable [SCALA-531](https://jira.mongodb.org/browse/SCALA-531)
+  * Fix `UninitializedFieldError` in MacroCodecs under `-Xcheckinit` [SCALA-542](https://jira.mongodb.org/browse/SCALA-542)
 
 ### 2.6.0
   * Updated MongoDB Driver Async to 3.10.0

project/MongoScalaBuild.scala

@@ -47,7 +47,7 @@ object MongoScalaBuild extends Build {
     scalacOptions in IntegrationTest := scalacOptionsTest
   )
 
-  val scalacOptionsTest: Seq[String] = Seq( "-unchecked", "-deprecation", "-feature", "-Xlint:-missing-interpolator,_")
+  val scalacOptionsTest: Seq[String] = Seq( "-unchecked", "-deprecation", "-feature", "-Xlint:-missing-interpolator,_", "-Xcheckinit")
 
   def scalacOptionsVersion(scalaVersion: String): Seq[String] = {
     Seq( "-unchecked", "-deprecation", "-feature", "-Ywarn-dead-code"