Strengthen isMultiSingleton

We missed some cases before.

Author: odersky

compiler/src/dotty/tools/dotc/core/ConstraintHandling.scala

@@ -253,6 +253,9 @@ trait ConstraintHandling {
       case tp: SingletonType => true
       case AndType(tp1, tp2) => isMultiSingleton(tp1) | isMultiSingleton(tp2)
       case OrType(tp1, tp2) => isMultiSingleton(tp1) & isMultiSingleton(tp2)
+      case tp: TypeRef => isMultiSingleton(tp.info.hiBound)
+      case tp: TypeVar => isMultiSingleton(tp.underlying)
+      case tp: TypeParamRef => isMultiSingleton(bounds(tp).hi)
       case _ => false
     }
     def isFullyDefined(tp: Type): Boolean = tp match {