prevent crash when extension not found (scala#18830)

found when entering expression `Tuple(1)` into `repl`, if I made
`TastyHeaderUnpickler` always throw an exception, so maybe there are
other situations where this could occur.

compiler/src/dotty/tools/dotc/typer/Implicits.scala

@@ -432,6 +432,7 @@ object Implicits:
 
   /** A failed search */
   case class SearchFailure(tree: Tree) extends SearchResult {
+    require(tree.tpe.isInstanceOf[SearchFailureType], s"unexpected type for ${tree}")
     final def isAmbiguous: Boolean = tree.tpe.isInstanceOf[AmbiguousImplicits | TooUnspecific]
     final def reason: SearchFailureType = tree.tpe.asInstanceOf[SearchFailureType]
   }

compiler/src/dotty/tools/dotc/typer/Typer.scala

@@ -3625,8 +3625,9 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
       val remembered = // report AmbiguousReferences as priority, otherwise last error
         (errs.filter(_.msg.isInstanceOf[AmbiguousReference]) ++ errs).take(1)
       for err <- remembered do
+        val tree = if app.isEmpty then qual else app
         rememberSearchFailure(qual,
-          SearchFailure(app.withType(FailedExtension(app, selectionProto, err.msg))))
+          SearchFailure(tree.withType(FailedExtension(tree, selectionProto, err.msg))))
     catch case ex: TypeError => nestedFailure(ex)
 
     // try an implicit conversion or given extension