Don't add to hidden sets in mightSubsume

This required another fix to subsumes, which fixed a bug that was
undetected before.

Author: odersky

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

@@ -158,8 +158,9 @@ trait CaptureRef extends TypeProxy, ValueType:
         case _ => false
       case _ => false
 
-    def viaInfo(info: Type)(test: Type => Boolean): Boolean = info.match
+    def viaInfo(info: Type)(test: Type => Boolean): Boolean = info.dealias match
       case info: SingletonCaptureRef => test(info)
+      case CapturingType(parent, _) => viaInfo(parent)(test)
       case info: AndType => viaInfo(info.tp1)(test) || viaInfo(info.tp2)(test)
       case info: OrType => viaInfo(info.tp1)(test) && viaInfo(info.tp2)(test)
       case _ => false

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

@@ -214,7 +214,7 @@ sealed abstract class CaptureSet extends Showable:
    */
   def mightAccountFor(x: CaptureRef)(using Context): Boolean =
     reporting.trace(i"$this mightAccountFor $x, ${x.captureSetOfInfo}?", show = true):
-      elems.exists(_.subsumes(x)(using ctx, FrozenVarState()))
+      elems.exists(_.subsumes(x)(using ctx, FrozenUnrecordedHiddenState))
       || !x.isMaxCapability
         && {
           val elems = x.captureSetOfInfo.elems

tests/neg-custom-args/captures/filevar.check

@@ -0,0 +1,9 @@
+-- Error: tests/neg-custom-args/captures/filevar.scala:8:6 -------------------------------------------------------------
+8 |  var file: File^ = uninitialized  // error, was OK under unsealed
+  |      ^
+  |      Mutable variable file cannot have type File^ since
+  |      that type captures the root capability `cap`.
+-- Warning: tests/neg-custom-args/captures/filevar.scala:11:55 ---------------------------------------------------------
+11 |def withFile[T](op: (l: caps.Capability) ?-> (f: File^{l}) => T): T =
+   |                                                       ^
+   |                                                       redundant capture: File already accounts for l.type