unexpected error: private class escapes its defining scope as part of a self type

[scabug]

adriaanm/scala@c5c0b136d9

private abstract class TreeBuilderStrategy {
  // private commented out since it cause "error: private class TreeBuilderStrategy escapes its defining scope as part of type TreeBuilder.this.TreeBuilderStrategy"
+  /*private <-- BUG*/ abstract class TreeBuilderStrategy {

this used to compile

[scabug]

Imported From: https://issues.scala-lang.org/browse/SI-6232?orig=1
Reporter: @adriaanm

[SethTisue]

if this is still reproducible in current Sclaa, can someone prove it by showing a self-contained reproduction for it...?

[som-snytt]

Example is not current source.