add -Xsource:3

build.sbt

@@ -91,6 +91,14 @@ val commonSettings = Def.settings(
         Nil
     }
   },
+  scalacOptions ++= {
+    CrossVersion.partialVersion(scalaVersion.value) match {
+      case Some((2, _)) =>
+        Seq("-Xsource:3")
+      case _ =>
+        Nil
+    }
+  },
   scalacOptions ++= unusedWarnings,
   scalaVersion := scala212,
   crossScalaVersions := scala212 :: "2.13.12" :: "3.3.1" :: Nil,

jvm/src/test/scala/msgpack4z/JavaSpec.scala

@@ -1,6 +1,6 @@
 package msgpack4z
 
 object JavaSpec extends SpecBase {
-  override protected[this] def packer() = new MsgpackJavaPacker()
+  override protected[this] def packer(): MsgPacker = new MsgpackJavaPacker()
   override protected[this] def unpacker(bytes: Array[Byte]) = MsgpackJavaUnpacker.defaultUnpacker(bytes)
 }

src/test/scala/msgpack4z/NativeSpec.scala

@@ -1,6 +1,6 @@
 package msgpack4z
 
 object NativeSpec extends SpecBase {
-  override protected[this] def packer() = MsgOutBuffer.create()
-  override protected[this] def unpacker(bytes: Array[Byte]) = MsgInBuffer(bytes)
+  override protected[this] def packer(): MsgPacker = MsgOutBuffer.create()
+  override protected[this] def unpacker(bytes: Array[Byte]): MsgUnpacker = MsgInBuffer(bytes)
 }