Fix unchecked warning in scaladoc

It is possible that a subtype of MdNode extends Seq.

Author: liufengyun

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

@@ -133,7 +133,7 @@ object TypeTestsCasts {
           case _                   => recur(defn.AnyType, tpT)
         }
       case tpe: AppliedType     =>
-        X.widen match {
+        X.widenDealias match {
           case OrType(tp1, tp2) =>
             // This case is required to retrofit type inference,
             // which cut constraints in the following two cases:

scaladoc/src/dotty/tools/scaladoc/renderers/WikiDocRenderer.scala

@@ -13,7 +13,7 @@ class DocRender(signatureRenderer: SignatureRenderer)(using DocContext):
     case md: MdNode => renderMarkdown(md)
     case Nil => raw("")
     case Seq(elem: WikiDocElement) => renderElement(elem)
-    case list: Seq[WikiDocElement] => div(list.map(renderElement))
+    case list: Seq[WikiDocElement @unchecked] => div(list.map(renderElement))
 
   private def renderMarkdown(el: MdNode): AppliedTag =
     raw(DocFlexmarkRenderer.render(el)( (link,name) =>