Imported From: https://issues.scala-lang.org/browse/SI-9337?orig=1
Reporter: Robin Green (ST_robin_green)
Affected Versions: 2.11.6

@adriaanm said:
That's some fancy naming for Validator[T] -- never seen that trick before!

Both definitions should be accepted. Workaround is to duplicate the type alias, or to factor our the type constructor used for the bound on T's X.

This compiles:

class Bar[U] extends Foo[U] {
  type VU[This <: Validator[U, This]] = Validator[U, This]
  val g: TypePreservingFn[VU] = ???  // does not compile
}

@retronym said (edited on Jun 2, 2015 3:26:49 AM UTC):
@adriaanm: here's a patch that makes this work: scala/scala@2.11.x...retronym:ticket/9337

But I fear my head might explode trying to write the commit comment. Perhaps you have a better chance at that :)

@retronym said:
Another test case from #10186 for the same problem. This is also fixed by my patch.

trait Foo {
  type A
  type F[P <: A]
}
 
object Test {
  def noop[A1, F1[P <: A1]]: Unit = ()

  def f(foo: Foo): Unit = {
    noop[foo.A, foo.F] // does not compile
  }
}

But I fear my head might explode trying to write the commit comment. Perhaps you have a better chance at that :)

This is what I came up with: scala/scala@1f4611d
Higher-kinded bounds have found their way again.
Unfortunately it doesn't fix this bug but it does fix #10186

Here is the issue:

  trait AliasTypeRef extends NonClassTypeRef {
    require(sym.isAliasType, sym)

    override def dealias    = if (typeParamsMatchArgs) betaReduce.dealias else super.dealias
    override def narrow     = normalize.narrow
    override def prefix     = if (this ne normalize) normalize.prefix else pre
    override def termSymbol = if (this ne normalize) normalize.termSymbol else super.termSymbol
    override def typeSymbol = if (this ne normalize) normalize.typeSymbol else sym

the prefix of Validator[T] is the prefix of Validator instead of Bar