Handle flexible types in isConcrete

Author: noti0na1

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

@@ -3441,6 +3441,8 @@ class MatchReducer(initctx: Context) extends TypeComparer(initctx) {
             isConcrete(tp1.underlying)
           case tp1: AndOrType =>
             isConcrete(tp1.tp1) && isConcrete(tp1.tp2)
+          case tp: FlexibleType =>
+            isConcrete(tp.hi)
           case _ =>
             val tp2 = tp1.stripped.stripLazyRef
             (tp2 ne tp) && isConcrete(tp2)