Fix fallout in other tests

Author: odersky

compiler/src/dotty/tools/dotc/ast/TreeTypeMap.scala

@@ -56,7 +56,7 @@ class TreeTypeMap(
   /** Replace occurrences of `This(oldOwner)` in some prefix of a type
    *  by the corresponding `This(newOwner)`.
    */
-  private val mapOwnerThis = new TypeMap {
+  private val mapOwnerThis = new TypeMap with cc.CaptureSet.IdempotentCaptRefMap {
     private def mapPrefix(from: List[Symbol], to: List[Symbol], tp: Type): Type = from match {
       case Nil => tp
       case (cls: ClassSymbol) :: from1 => mapPrefix(from1, to.tail, tp.substThis(cls, to.head.thisType))

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

@@ -523,7 +523,7 @@ object CaptureSet:
 
     private def mapIsIdempotent = tm.isInstanceOf[IdempotentCaptRefMap]
 
-    assert(ccAllowUnsoundMaps || mapIsIdempotent)
+    assert(ccAllowUnsoundMaps || mapIsIdempotent, tm.getClass)
 
     private def whereCreated(using Context): String =
       if stack == null then ""

compiler/test/dotc/pos-test-pickling.blacklist

@@ -19,6 +19,7 @@ i6507b.scala
 i12299a.scala
 i13871.scala
 i15181.scala
+i15922.scala
 
 # Tree is huge and blows stack for printing Text
 i7034.scala