Fix #6998: Make sure to traverse all kinds of untyped trees

Author: nicolasstucki

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

@@ -1758,7 +1758,7 @@ object desugar {
       case Block(Nil, expr) =>
         collect(expr)
       case Quote(expr) =>
-        new TreeTraverser {
+        new UntypedTreeTraverser {
           def traverse(tree: untpd.Tree)(implicit ctx: Context): Unit = tree match {
             case Splice(expr) => collect(expr)
             case TypSplice(expr) =>

tests/pos/i6998.scala

@@ -0,0 +1,5 @@
+import scala.quoted._
+
+def foo(given QuoteContext) : Unit = {
+  val '{ $f : (Int => Double) } = ???
+}