expandedtypemismatch.scala

// Foo can be extended, and `step` should return the extended type
trait Foo[S,A] {
  type Self = Foo[S,A]
  def step(s: S): Self
}

// Bar can be extended, fixing a more specific F
trait Bar {
  type S
  type A
  type F <: Foo[S, A]
}

// Should run on any subtype of Bar, and operate with its derived Foos
case class Sim[G<:Bar with Singleton]( f: G#F, s: G#S) {
  def step: Sim[G] = {
    def newF = f.step(s): G#F // fail
    copy(f = newF)
  }
}
  
/*
type mismatch;
[error]  found   : Sim.this.f.Self
[error]     (which expands to)  bugs.ExpandedTypeMismatch2.Foo[G#S,G#A]
[error]  required: G#F
[error]       def newF = f.step(s): G#F
[error]                        ^
*/