Scala 2.12 transient lazy val

build.sbt

lazy val core = project.in(file("."))
  .settings(commonSettings)

lazy val commonSettings = Seq[Setting[_]](
  scalaVersion := "2.11.8",
  crossScalaVersions := Seq("2.11.8", "2.12.0"),
  scalacOptions ++= Seq(
    "-feature",
    "-deprecation",
    "-unchecked",
    "-encoding", "UTF-8",
    "-Xlint:_",
    "-Xfuture"
  )
)

javap_session.txt

> ++ 2.11.8
[info] Setting version to 2.11.8
[info] Reapplying settings...
[info] Set current project to core (in build file:/home/.../transient/)
> console
[info] Starting scala interpreter...
[info] 
Welcome to Scala 2.11.8 (OpenJDK 64-Bit Server VM, Java 1.8.0_111).
Type in expressions for evaluation. Or try :help.

scala> :javap TestClass -p
Compiled from "test.scala"
public class TestClass implements scala.Serializable {
  private transient MyClass foo;
  private volatile transient boolean bitmap$trans$0;
  private MyClass foo$lzycompute();
  public MyClass foo();
  public TestClass();
}

scala> :quit

[success] Total time: 8 s, completed Nov 22, 2016 8:47:04 PM
> ++ 2.12.0
[info] Setting version to 2.12.0
[info] Reapplying settings...
[info] Set current project to core (in build file:/home/.../transient/)
> console
[info] Starting scala interpreter...
[info] 
Welcome to Scala 2.12.0 (OpenJDK 64-Bit Server VM, Java 1.8.0_111).
Type in expressions for evaluation. Or try :help.

scala> :javap TestClass -p
Compiled from "test.scala"
public class TestClass implements scala.Serializable {
  private MyClass foo;
  private volatile boolean bitmap$0;
  private MyClass foo$lzycompute();
  public MyClass foo();
  public TestClass();
}

scala> :quit

[success] Total time: 17 s, completed Nov 22, 2016 8:47:30 PM
>

sbt_session.txt

[info] Set current project to core (in build file:/home/.../transient/)
> run
[info] Running Main 
MyClass@335f5fe6
[success] Total time: 0 s, completed Nov 22, 2016 8:41:56 PM
> ++ 2.12.0
[info] Setting version to 2.12.0
[info] Reapplying settings...
[info] Set current project to core (in build file:/home/.../transient/)
> run
[info] Running Main 
MyClass@403b81fd
[error] (run-main-1) java.io.NotSerializableException: MyClass
[error]         - field (class "TestClass", name: "foo", type: "class MyClass")
[error]         - root object (class "TestClass", TestClass@7ceaba54)
java.io.NotSerializableException: MyClass
        - field (class "TestClass", name: "foo", type: "class MyClass")
        - root object (class "TestClass", TestClass@7ceaba54)
        at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1182)
        at java.io.ObjectOutputStream.defaultWriteFields(ObjectOutputStream.java:1548)
        at java.io.ObjectOutputStream.writeSerialData(ObjectOutputStream.java:1509)
        at java.io.ObjectOutputStream.writeOrdinaryObject(ObjectOutputStream.java:1432)
        at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1178)
        at java.io.ObjectOutputStream.writeObject(ObjectOutputStream.java:348)
        at Main$.main(test.scala:17)
        at Main.main(test.scala)
        at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
        at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
        at java.lang.reflect.Method.invoke(Method.java:498)
[trace] Stack trace suppressed: run last compile:run for the full output.
java.lang.RuntimeException: Nonzero exit code: 1
        at scala.sys.package$.error(package.scala:27)
[trace] Stack trace suppressed: run last compile:run for the full output.
[error] (compile:run) Nonzero exit code: 1
[error] Total time: 0 s, completed Nov 22, 2016 8:42:01 PM
>

test.scala

class MyClass // not Serializable

class TestClass extends Serializable {

  @transient
  lazy val foo: MyClass =
    new MyClass
}

object Main {

  def main(args: Array[String]): Unit = {
    val obj = new TestClass
    println(obj.foo)
    val os = new java.io.ObjectOutputStream(new java.io.ByteArrayOutputStream)
    os.writeObject(obj)
  }
}