Slightly improve the error message after the previous commit

Author: smarter

compiler/src/dotty/tools/dotc/ast/tpd.scala

@@ -480,7 +480,15 @@ object tpd extends Trees.Instance[Type] with TypedTreeInfo {
     else if tp.symbol.hasAnnotation(defn.ScalaStaticAnnot) then
       Ident(tp)
     else
+      // Throw an error here if we detect a skolem to improve the error message in tests/neg/i8623.scala
+      def checkNoSkolemInPrefix(pre: Type): Unit = pre.dealias match
+        case pre: SkolemType =>
+          throw TypeError(em"cannot construct a tree referring to $tp because of skolem prefix $pre")
+        case pre: TermRef => checkNoSkolemInPrefix(pre.prefix)
+        case _ =>
+
       val pre = tp.prefix
+      checkNoSkolemInPrefix(tp)
       if pre.isSingleton then followOuterLinks(singleton(pre.dealias, needLoad)).select(tp)
       else
         val res = Select(TypeTree(pre), tp)

tests/neg/i8623.check

@@ -4,7 +4,7 @@
    |  value pos is not a member of ?1.tasty.Tree.
    |  Extension methods were tried, but the search failed with:
    |
-   |      cannot construct a tree referring to skolem (?1 : QC)
+   |      cannot construct a tree referring to ?1.tasty.type because of skolem prefix (?1 : QC)
    |      
    |      where:    ?1 is an unknown value of type QC
    |