Infinite loop in subtyping when a default parameter has an F-bounded type

[Sciss]

Steps to reproduce:

$ git clone -b dotty git@github.com:Sciss/Lucre.git
$ cd Lucre
$ sbt compile

This crashes with the following trace: https://gist.github.com/smarter/975785e299847135de41

[smarter]

Compiling with -Yno-deep-subtypes -Ylog:all gives some more information:

[log frontend] !!! deep subtype recursion involving S <:< de.sciss.lucre.stm.Sys, constraint = ...
exception occurred while typechecking core/src/main/scala/de/sciss/lucre/event/Map.scala

[smarter]

OK, minimized:

class Sys[S]
case class Foo[T <: Sys[T]](t: T)

[smarter]

Minimized further to not require a case class:

class Sys[S]
class Foo[T <: Sys[T]] {
  val t: T = ???
  def foo[A <: Sys[A]](x: A = t) = x
}