Indicate name of classfile where leaky mapping of existential type happens

Fixes #13873

Author: odersky

compiler/src/dotty/tools/dotc/core/unpickleScala2/Scala2Unpickler.scala

@@ -739,7 +739,7 @@ class Scala2Unpickler(bytes: Array[Byte], classRoot: ClassDenotation, moduleClas
       val anyTypes = boundSyms map (_ => defn.AnyType)
       val boundBounds = boundSyms map (_.info.bounds.hi)
       val tp2 = tp1.subst(boundSyms, boundBounds).subst(boundSyms, anyTypes)
-      report.warning(FailureToEliminateExistential(tp, tp1, tp2, boundSyms))
+      report.warning(FailureToEliminateExistential(tp, tp1, tp2, boundSyms, classRoot.symbol))
       tp2
     }
     else tp1

compiler/src/dotty/tools/dotc/reporting/messages.scala

@@ -1765,13 +1765,13 @@ import transform.SymUtils._
     def explain = ""
   }
 
-  class FailureToEliminateExistential(tp: Type, tp1: Type, tp2: Type, boundSyms: List[Symbol])(using Context)
+  class FailureToEliminateExistential(tp: Type, tp1: Type, tp2: Type, boundSyms: List[Symbol], classRoot: Symbol)(using Context)
     extends Message(FailureToEliminateExistentialID) {
     def kind: String = "Compatibility"
     def msg =
       val originalType = ctx.printer.dclsText(boundSyms, "; ").show
-      em"""An existential type that came from a Scala-2 classfile cannot be
-          |mapped accurately to to a Scala-3 equivalent.
+      em"""An existential type that came from a Scala-2 classfile for $classRoot
+          |cannot be mapped accurately to a Scala-3 equivalent.
           |original type    : $tp forSome ${originalType}
           |reduces to       : $tp1
           |type used instead: $tp2