Upper Bound not unified

[scabug]

In the following code, H1 and H0 are both Int, but the types are not unified.

object TypeParam {
  trait GC[K[_ <: H0], H0]

  trait PA[H1] {
    type Apply[A <: H1] = Any
  }

  // error:
  // kinds of the type arguments ([A <: H1]Any,Int) do not conform to the expected kinds of the type parameters (type K,type H0) in trait GC.
  // [A <: H1]Any's type parameters do not match type K's expected parameters:
  // type A's bounds >: Nothing <: H1 are stricter than type _'s declared bounds >: Nothing <: H0
  type a = GC[PA[Int]#Apply, Int]
}

Substituting H1 to Int manually allows compilation:

  trait PA[H1] {
    type Apply[A <: Int] = Any
  }

What versions of the following are you using?
Scala: 2.8.1, 2.9.0-SNAPSHOT

See also r23383

[scabug]

Imported From: https://issues.scala-lang.org/browse/SI-4043?orig=1
Reporter: @retronym

[scabug]

@retronym said:
Motivation:

https://gist.github.com/719247

https://gist.github.com/719352

The workaround is to express the abstract types as members, rather than parameters. this is a bit more verbose.

[scabug]

@retronym said:
This is the "as seen from" operation that loses track of the fact that H1 = Int

Welcome to Scala version 2.11.6 (Java HotSpot(TM) 64-Bit Server VM, Java 1.8.0_51).
Type in expressions to have them evaluated.
Type :help for more information.

scala> :power
** Power User mode enabled - BEEP WHIR GYVE **
** :phase has been set to 'typer'.          **
** scala.tools.nsc._ has been imported      **
** global._, definitions._ also imported    **
** Try  :help, :vals, power.<tab>           **

scala> :paste -raw
// Entering paste mode (ctrl-D to finish)

object TypeParam {
  trait GC[K[_ <: H0], H0]

  trait PA[H1] {
    type Apply[A <: H1] = Any
  }
 }

// Exiting paste mode, now interpreting.

warning: there was one feature warning; re-run with -feature for details

scala> val pInt = typeOf[TypeParam.PA[Int]]
pInt: $r.intp.global.Type = TypeParam.PA[Int]

scala> val Apply = pInt member TypeName("Apply")
Apply: $r.intp.global.Symbol = type Apply

scala> pInt memberType Apply
res0: $r.intp.global.Type = [A <: H1]Any

[scabug]

@retronym said:
I believe this is an oversight in eta expansion of type aliases. Here's an attempt at a fix: retronym/scala#16

[scabug]

@retronym said:
Darn, the fix doesn't bootstrap.

Related discussion: https://groups.google.com/d/msg/scala-internals/45XNa6WqvlU/hSWHFF66CQAJ

[diesalbla]

This still fails in Scala 2.12.8 (using REPL), but works in the current Scala 2.13.x branch.