Improve subsumes test

Don't add elements to hidden sets if they were subsumed previously.

Author: odersky

compiler/src/dotty/tools/dotc/cc/CaptureRef.scala

@@ -168,8 +168,7 @@ trait CaptureRef extends TypeProxy, ValueType:
     || this.isCap
     || this.match
       case Fresh.Cap(hidden) =>
-        if hidden.elems.contains(y) then true
-        //if hidden.elems.exists(_.subsumes(y)) then true
+        if vs.ifNotSeen(this)(hidden.elems.exists(_.subsumes(y))) then true
         else if !hidden.recordElemsState() || y.stripReadOnly.isCap then false
         else
           hidden.elems += y

compiler/src/dotty/tools/dotc/cc/CaptureSet.scala

@@ -1070,6 +1070,14 @@ object CaptureSet:
     def rollBack(): Unit =
       elemsMap.keysIterator.foreach(_.resetElems()(using this))
       depsMap.keysIterator.foreach(_.resetDeps()(using this))
+
+    private var seen: util.EqHashSet[CaptureRef] = new util.EqHashSet
+
+    /** Run test `pred` unless `ref` was seen in an enclosing `ifNotSeen` operation */
+    def ifNotSeen(ref: CaptureRef)(pred: => Boolean): Boolean =
+      if seen.add(ref) then
+        try pred finally seen -= ref
+      else false
   end VarState
 
   /** A special state that does not allow to record elements or dependent sets.