TypeComparer: delay looking up members of AndTypes

In ParFactory.scala we have checks that look like:
  (Foo { type Bar = X }) & (Foo { type Bar = X }) <:< (Foo { type Bar = X })
where `Foo` is a recursive type.

Before this commit, we would first try to check this by looking up `Bar`
in the `AndType` on the left, which means looking it up in both branches
and then merging the result, but the merge requires more subtyping
checks, which in turn require looking up a member inside an `AndType`,
seemingly ad infinitum.

We now avoid this by checking if either branch of the `AndType` on the left
is a subtype of the `RefinedType` on the right before looking up a member
in the `AndType` itself.

Author: smarter

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

@@ -331,17 +331,23 @@ class TypeComparer(initctx: Context) extends DotClass with ConstraintHandling {
         }
       comparePolyParam
     case tp2: RefinedType =>
+      def compareRefinedSlow: Boolean = {
+        val name2 = tp2.refinedName
+        isSubType(tp1, tp2.parent) &&
+          (name2 == nme.WILDCARD || hasMatchingMember(name2, tp1, tp2))
+      }
       def compareRefined: Boolean = {
         val tp1w = tp1.widen
         val skipped2 = skipMatching(tp1w, tp2)
         if ((skipped2 eq tp2) || !Config.fastPathForRefinedSubtype) {
-          val name2 = tp2.refinedName
-          val normalPath =
-            isSubType(tp1, tp2.parent) &&
-              (  name2 == nme.WILDCARD
-              || hasMatchingMember(name2, tp1, tp2)
-              || fourthTry(tp1, tp2)
-              )
+          val normalPath = tp1 match {
+            case tp1: AndType =>
+              // Delay calling `compareRefinedSlow` because looking up a member
+              // of an `AndType` can lead to a cascade of subtyping checks
+              fourthTry(tp1, tp2) || compareRefinedSlow
+            case _ =>
+              compareRefinedSlow || fourthTry(tp1, tp2)
+          }
           normalPath ||
             needsEtaLift(tp1, tp2) && tp1.testLifted(tp2.typeParams, isSubType(_, tp2), classBounds(tp2))
         }

test/dotc/scala-collections.whitelist

@@ -287,9 +287,7 @@
 ./scala-scala/src/library/scala/collection/generic/SortedMapFactory.scala
 ./scala-scala/src/library/scala/collection/generic/SortedSetFactory.scala
 ./scala-scala/src/library/scala/collection/generic/SetFactory.scala
-
-# deep subtype
-#./scala-scala/src/library/scala/collection/generic/ParFactory.scala
+./scala-scala/src/library/scala/collection/generic/ParFactory.scala
 
 # https://github.com/lampepfl/dotty/issues/974
 #./scala-scala/src/library/scala/collection/generic/MutableSortedSetFactory.scala