Fix the tparam bounds of exported inherited classes

When trying to export M2.F, seeing M1#F from the prefix M2 doesn't
change the bounds of F's T type parameter, which still refers to
M1.this.A, rather than M2.A.  So, we run asSeenFrom against that info,
when eta-expanding the class into a hk type lambda.

Author: dwijnand

compiler/src/dotty/tools/dotc/typer/Namer.scala

@@ -1175,8 +1175,14 @@ class Namer { typer: Typer =>
             if mbr.isType then
               val forwarderName = checkNoConflict(alias.toTypeName, isPrivate = false, span)
               var target = pathType.select(sym)
-              if target.typeParams.nonEmpty then
-                target = target.EtaExpand(target.typeParams)
+              val tparams = target.typeParamSymbols
+              if tparams.nonEmpty then
+                target = HKTypeLambda(tparams.map(_.paramName))(
+                  tl => tparams.map { p =>
+                    val info = p.info.asSeenFrom(pathType, sym.owner)
+                    HKTypeLambda.toPInfo(tl.integrate(tparams, info))
+                  },
+                  tl => tl.integrate(tparams, target.appliedTo(tparams.map(_.paramRef))))
               newSymbol(
                 cls, forwarderName,
                 Exported | Final,

tests/pos/i18569.scala

@@ -0,0 +1,9 @@
+trait M1:
+  trait A
+  trait F[T <: A]
+
+object M2 extends M1
+
+trait Test:
+  export M2.*
+  def y: F[A]