Drop deep toCap when checking closures

It seems it's not necessary and it is problematic for soundness.

Author: odersky

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

@@ -939,8 +939,8 @@ class CheckCaptures extends Recheck, SymTransformer:
           // which are less intelligible. An example is the line `a = x` in
           // neg-custom-args/captures/vars.scala. That's why this code is conditioned.
           // to apply only to closures that are not eta expansions.
-          val res1 = Existential.toCap(res, deep = true) // TODO: why deep = true?
-          val pt1 = Existential.toCap(pt, deep = true)
+          val res1 = Existential.toCap(res) // TODO: why deep = true?
+          val pt1 = Existential.toCap(pt)
             // We need to open existentials here in order not to get vars mixed up in them
             // We do the proper check with existentials when we are finished with the closure block.
           capt.println(i"pre-check closure $expr of type $res1 against $pt1")

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

@@ -226,7 +226,7 @@ object Existential:
       case _ => None
 
   /** Map top-level free existential variables one-to-one to Fresh instances */
-  def toCap(tp: Type, deep: Boolean = false)(using Context): Type =
+  def toCap(tp: Type)(using Context): Type =
     val subst = new IdempotentCaptRefMap:
       val seen = EqHashMap[Annotation, CaptureRef]()
       var localBinders: SimpleIdentitySet[MethodType] = SimpleIdentitySet.empty
@@ -238,7 +238,7 @@ object Existential:
         case t: MethodType =>
           // skip parameters
           val saved = localBinders
-          if t.isFreshBinder && !deep then localBinders = localBinders + t
+          if t.isFreshBinder then localBinders = localBinders + t
           try t.derivedLambdaType(resType = this(t.resType))
           finally localBinders = saved
         case t: PolyType =>