simplify unusableForInterface

Author: olhotak

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

@@ -368,7 +368,7 @@ object Types {
       case AppliedType(tycon, args) => tycon.unusableForInference || args.exists(_.unusableForInference)
       case RefinedType(parent, _, rinfo) => parent.unusableForInference || rinfo.unusableForInference
       case TypeBounds(lo, hi) => lo.unusableForInference || hi.unusableForInference
-      case tp: FlexibleType => tp.underlying.unusableForInference || tp.lo.unusableForInference
+      case FlexibleType(underlying) => underlying.unusableForInference
       case tp: AndOrType => tp.tp1.unusableForInference || tp.tp2.unusableForInference
       case tp: LambdaType => tp.resultType.unusableForInference || tp.paramInfos.exists(_.unusableForInference)
       case WildcardType(optBounds) => optBounds.unusableForInference