Remove empty tree after erasure to pass -Ycheck

Author: liufengyun

compiler/src/dotty/tools/dotc/transform/Erasure.scala

@@ -600,7 +600,7 @@ object Erasure extends TypeTestsCasts{
       val stats1 =
         if (takesBridges(ctx.owner)) new Bridges(ctx.owner.asClass).add(stats)
         else stats
-      super.typedStats(stats1, exprOwner)
+      super.typedStats(stats1, exprOwner).filter(!_.isEmpty)
     }
 
     override def adapt(tree: Tree, pt: Type, original: untpd.Tree)(implicit ctx: Context): Tree =

tests/pos/erased-typedef.scala

@@ -0,0 +1,8 @@
+trait Monadless[Monad[_]] {
+
+  type M[T] = Monad[T]
+
+  def lift[T](body: T): Monad[T] =  ???
+
+  def unlift[T](m: M[T]): T = ???
+}