Generalize compareHkApply to types other than PolyParams

This seems to be required to make Typer compile, and it is logical
that we should do this. We still need to study how it fits in the
general context.

Also, it required neg test to be fixed. With the change, the error is
now reported earlier.

Author: odersky

src/dotty/tools/dotc/core/TypeComparer.scala

@@ -656,8 +656,19 @@ class TypeComparer(initctx: Context) extends DotClass with ConstraintHandling {
           subtyping.println(i"type params = $hkTypeParams")
           if (inOrder) unifyWith(other)
           else testLifted(other, app, hkTypeParams, unifyWith)
-        case _ =>
-          false
+
+        case tp => // TODO check this is the right way to do it.
+          other.typeConstructor.widen match {
+            case tycon: TypeRef =>
+              tycon.isEtaExpandable &&
+              tycon.typeParams.length == tp.typeParams.length &&
+              !ctx.mode.is(Mode.TypevarsMissContext) && {
+                val app1 = EtaExpansion(tycon).appliedTo(app.argInfos)
+                if (inOrder) isSubType(app1, other) else isSubType(other, app1)
+              }
+            case _ =>
+              false
+          }
       }
     }
     Config.newHK && app.isHKApply && !other.isHKApply && {

tests/neg/i1240a.scala

@@ -10,8 +10,8 @@ abstract class A {
 class B extends A {
   type C[X] = List[X]
   override def give[X] = Nil
-  override def foo[B](x: C[B]): C[B] =  {println("B.C"); x} // error: merge error during erasure
+  override def foo[B](x: C[B]): C[B] =  {println("B.C"); x}
   val a: A = this
-  a.foo(a.give[Int]) // what method should be called here in runtime?
+  a.foo(a.give[Int]) // what method should be called here in runtime? // error: ambiguous
 }