Fix scala#2989: HK type Y not considered a subtype of [T] => Y[T]

Slightly relax the conditions under which we eta-reduce to account for
this case.

Author: smarter

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

@@ -475,14 +475,12 @@ class TypeComparer(initctx: Context) extends DotClass with ConstraintHandling {
             isSubType(tp1.resType, tp2.resType.subst(tp2, tp1))
           finally comparedTypeLambdas = saved
         case _ =>
-          if (!tp1.isHK) {
-            tp2 match {
-              case EtaExpansion(tycon2) if tycon2.symbol.isClass =>
-                return isSubType(tp1, tycon2)
-              case _ =>
-            }
+          tp2 match {
+            case EtaExpansion(tycon2) if (!tp1.isHK && tycon2.symbol.isClass) || tp1.isDirectRef(tycon2.symbol) =>
+              isSubType(tp1, tycon2)
+            case _ =>
+              fourthTry(tp1, tp2)
           }
-          fourthTry(tp1, tp2)
       }
       compareTypeLambda
     case OrType(tp21, tp22) =>

tests/pos/i2989.scala

@@ -0,0 +1,12 @@
+class Foo[+X[_]] {
+  // OK
+  def foo1[Y[_]](right: Foo[Y]): Foo[Y] = right
+  // OK
+  def foo2[Y[_]](right: Foo[[T] => Y[T]]): Foo[Y] = right
+  // OK
+  def foo3[Y[_]](right: Foo[[T] => Y[T]]): Foo[[T] => Y[T]] = right
+  // Error:
+  //   found:    Foo[Y](right)
+  //   required: Foo[Y]
+  def foo4[Y[_]](right: Foo[Y]): Foo[[T] => Y[T]] = right
+}