https://github.com/scala/scala/pull/8177

build.properties

sbt.version=0.13.18

build.sbt

scalaVersion := "2.12.8"

plugins.sbt

addSbtPlugin("pl.project13.sbt" % "sbt-jol" % "0.1.3")

result

 > ;jol:internals foo.B2 ; jol:internals foo.B1
[info] Resolving net.sf.jopt-simple#jopt-simple;4.6 ...
[info] # WARNING: Unable to attach Serviceability Agent. You can try again with escalated privileges. Two options: a) use -Djol.tryWithSudo=true to try with sudo; b) echo 0 | sudo tee /proc/sys/kernel/yama/ptrace_scope
[info] # Running 64-bit HotSpot VM.
[info] # Using compressed oop with 3-bit shift.
[info] # Using compressed klass with 3-bit shift.
[info] # WARNING | Compressed references base/shifts are guessed by the experiment!
[info] # WARNING | Therefore, computed addresses are just guesses, and ARE NOT RELIABLE.
[info] # WARNING | Make sure to attach Serviceability Agent to get the reliable addresses.
[info] # Objects are 8 bytes aligned.
[info] # Field sizes by type: 4, 1, 1, 2, 2, 4, 4, 8, 8 [bytes]
[info] # Array element sizes: 4, 1, 1, 2, 2, 4, 4, 8, 8 [bytes]
[info] 
[info] VM fails to invoke the default constructor, falling back to class-only introspection.
[info] 
[info] foo.B2 object internals:
[info]  OFFSET  SIZE   TYPE DESCRIPTION                    VALUE
[info]       0    12        (object header)                N/A
[info]      12     4 String A2.x                           N/A
[info] Instance size: 16 bytes
[info] Space losses: 0 bytes internal + 0 bytes external = 0 bytes total
[success] Total time: 1 s, completed 2019/06/26 16:23:42
[info] Resolving net.sf.jopt-simple#jopt-simple;4.6 ...
[info] # WARNING: Unable to attach Serviceability Agent. You can try again with escalated privileges. Two options: a) use -Djol.tryWithSudo=true to try with sudo; b) echo 0 | sudo tee /proc/sys/kernel/yama/ptrace_scope
[info] # Running 64-bit HotSpot VM.
[info] # Using compressed oop with 3-bit shift.
[info] # Using compressed klass with 3-bit shift.
[info] # WARNING | Compressed references base/shifts are guessed by the experiment!
[info] # WARNING | Therefore, computed addresses are just guesses, and ARE NOT RELIABLE.
[info] # WARNING | Make sure to attach Serviceability Agent to get the reliable addresses.
[info] # Objects are 8 bytes aligned.
[info] # Field sizes by type: 4, 1, 1, 2, 2, 4, 4, 8, 8 [bytes]
[info] # Array element sizes: 4, 1, 1, 2, 2, 4, 4, 8, 8 [bytes]
[info] 
[info] VM fails to invoke the default constructor, falling back to class-only introspection.
[info] 
[info] foo.B1 object internals:
[info]  OFFSET  SIZE   TYPE DESCRIPTION                    VALUE
[info]       0    12        (object header)                N/A
[info]      12     4 String A1.x                           N/A
[info]      16     4 String B1.x                           N/A
[info]      20     4        (loss due to the next object alignment)
[info] Instance size: 24 bytes
[info] Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
[success] Total time: 1 s, completed 2019/06/26 16:23:42

Scala8177.scala

package foo

abstract class A1(x: String) {
  def y = x
}
final class B1(x: String) extends A1(x) {
  def z = x
}

abstract class A2(protected val x: String) {
  def y = x
}
final class B2(x: String) extends A2(x) {
  def z = x
}