Handle TypeVar in addConstraint

Author: dwijnand

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

@@ -864,6 +864,8 @@ trait ConstraintHandling {
       try bound match
         case bound: TypeParamRef if constraint contains bound =>
           addParamBound(bound)
+        case bound: TypeVar if constraint.contains(bound) =>
+          addParamBound(bound.origin)
         case _ =>
           val pbound = avoidLambdaParams(bound)
           kindCompatible(param, pbound) && addBoundTransitively(param, pbound, !fromBelow)