Compiler crash with AssertionError "mixed in lazy value .. from class .. is not param?!"

[scabug]

The code:

object Broken {
  trait Parent
  class Child extends Parent

  trait Test { lazy val x: Child = new Child }
  abstract class Test2 extends Test { override def x: Parent }
}

Causes the compiler to assertion fail:

java.lang.AssertionError: assertion failed:
  mixed in lazy value x from class Broken$Test2 is not param?!?
     while compiling: C:\Users\anixon\source\myproject\src\test\scala\myproject\Broken.scala
        during phase: globalPhase=mixin, enteringPhase=cleanup
     library version: version 2.12.1
    compiler version: version 2.12.1

[scabug]

Imported From: https://issues.scala-lang.org/browse/SI-10223?orig=1
Reporter: Alex Nixon (alexnixon)
Affected Versions: 2.12.0, 2.12.1

[scabug]

@adriaanm said:
Thanks! This shouldn't crash (it should compile). However, you can't make the def x concrete, as a lazy val cannot be overridden by a method.

What's the purpose of the abstract method x?

[scabug]

@adriaanm said:
Dotty cross-ref: scala/scala3#2072 --> scala/scala3#2085