Simplify level fixing scheme

Don't instantiate co- and contravariant inner type variables eagerly.
Lift them instead to the reference level, same as for invariant type variables.

Fixes scala#15934

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

@@ -482,14 +482,9 @@ trait ConstraintHandling {
      *  and computes on the side sets of nested type variables that need
      *  to be instantiated.
      */
-    class NeedsLeveling extends TypeAccumulator[Boolean]:
+    def needsLeveling = new TypeAccumulator[Boolean]:
       if !fromBelow then variance = -1
 
-      /** Nested type variables that should be instiated to theor lower (respoctively
-       *  upper) bounds.
-       */
-      var nestedVarsLo, nestedVarsHi: SimpleIdentitySet[TypeVar] = SimpleIdentitySet.empty
-
       def apply(need: Boolean, tp: Type) =
         need || tp.match
           case tp: NamedType =>
@@ -499,39 +494,30 @@ trait ConstraintHandling {
             val inst = tp.instanceOpt
             if inst.exists then apply(need, inst)
             else if tp.nestingLevel > maxLevel then
-              if variance > 0 then nestedVarsLo += tp
-              else if variance < 0 then nestedVarsHi += tp
-              else
-                // For invariant type variables, we use a different strategy.
-                // Rather than instantiating to a bound and then propagating in an
-                // AvoidMap, change the nesting level of an invariant type
-                // variable to `maxLevel`. This means that the type variable will be
-                // instantiated later to a less nested type. If there are other references
-                // to the same type variable that do not come from the type undergoing
-                // `fixLevels`, this could lead to coarser types. But it has the potential
-                // to give a better approximation for the current type, since it avoids forming
-                // a Range in invariant position, which can lead to very coarse types further out.
-                constr.println(i"widening nesting level of type variable $tp from ${tp.nestingLevel} to $maxLevel")
-                ctx.typerState.setNestingLevel(tp, maxLevel)
+              // Change the nesting level of inner type variable to `maxLevel`.
+              // This means that the type variable will be instantiated later to a
+              // less nested type. If there are other references to the same type variable
+              // that do not come from the type undergoing `fixLevels`, this could lead
+              // to coarser types than intended. An alternative is to instantiate the
+              // type variable right away, but this also loses information. See
+              // i15934.scala for a test where the current strategey works but an early instantiation
+              // of `tp` would fail.
+              constr.println(i"widening nesting level of type variable $tp from ${tp.nestingLevel} to $maxLevel")
+              ctx.typerState.setNestingLevel(tp, maxLevel)
               true
             else false
           case _ =>
             foldOver(need, tp)
-    end NeedsLeveling
+    end needsLeveling
 
-    class LevelAvoidMap extends TypeOps.AvoidMap:
+    def levelAvoid = new TypeOps.AvoidMap:
       if !fromBelow then variance = -1
       def toAvoid(tp: NamedType) = needsFix(tp)
 
-    if !Config.checkLevelsOnInstantiation || ctx.isAfterTyper then tp
-    else
-      val needsLeveling = NeedsLeveling()
-      if needsLeveling(false, tp) then
-        typr.println(i"instance $tp for $param needs leveling to $maxLevel, nested = ${needsLeveling.nestedVarsLo.toList} | ${needsLeveling.nestedVarsHi.toList}")
-        needsLeveling.nestedVarsLo.foreach(_.instantiate(fromBelow = true))
-        needsLeveling.nestedVarsHi.foreach(_.instantiate(fromBelow = false))
-        LevelAvoidMap()(tp)
-      else tp
+    if Config.checkLevelsOnInstantiation && !ctx.isAfterTyper && needsLeveling(false, tp) then
+      typr.println(i"instance $tp for $param needs leveling to $maxLevel")
+      levelAvoid(tp)
+    else tp
   end fixLevels
 
   /** Solve constraint set for given type parameter `param`.

tests/pos/i15934.scala

@@ -0,0 +1,44 @@
+trait ReplicatedData
+trait ActorRef[-T] {
+  def tell(msg: T): Unit = ???
+}
+
+// shared in both domains
+abstract class Key[+T1 <: ReplicatedData]
+
+// domain 1
+object dd {
+  sealed abstract class GetResponse[A1 <: ReplicatedData] {
+    def key: Key[A1]
+  }
+  case class GetSuccess[A2 <: ReplicatedData](key: Key[A2]) extends GetResponse[A2]
+  case class GetFailure[A3 <: ReplicatedData](key: Key[A3]) extends GetResponse[A3]
+}
+
+// domain 2
+object JReplicator {
+  final case class Get[A4 <: ReplicatedData](
+      key: Key[A4],
+      replyTo: ActorRef[GetResponse[A4]]
+  )
+  sealed abstract class GetResponse[A5 <: ReplicatedData] {
+    def key: Key[A5]
+  }
+  case class GetSuccess[A6 <: ReplicatedData](key: Key[A6]) extends GetResponse[A6]
+  case class GetFailure[A7 <: ReplicatedData](key: Key[A7]) extends GetResponse[A7]
+}
+
+val _ = null.asInstanceOf[Any] match {
+  case cmd: JReplicator.Get[d] =>
+    val reply =
+      util
+        .Try[dd.GetResponse[d]](???)
+        .map/*[JReplicator.GetResponse[d]]*/ {
+          // Needs at least 2 cases to triger failure
+          case rsp: dd.GetSuccess[d1]  => JReplicator.GetSuccess(rsp.key)
+          case rsp: dd.GetResponse[d2] => JReplicator.GetFailure(rsp.key)
+        }
+        // needs recover to trigger failure
+        .recover { case _ => new JReplicator.GetFailure(cmd.key) }
+    reply.foreach { cmd.replyTo tell _ } // error
+}