widenDealias before extracting namedTupleElementTypes (#20505)

@odersky perhaps you have a better suggestion?

fixes #20439

Author: odersky

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

@@ -756,7 +756,7 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
       return typedSelect(tree, pt, qual)
 
     // Otherwise, try to expand a named tuple selection
-    val namedTupleElems = qual.tpe.widen.namedTupleElementTypes
+    val namedTupleElems = qual.tpe.widenDealias.namedTupleElementTypes
     val nameIdx = namedTupleElems.indexWhere(_._1 == selName)
     if nameIdx >= 0 && Feature.enabled(Feature.namedTuples) then
       return typed(

tests/pos/named-tuple-selections.scala

@@ -0,0 +1,12 @@
+import scala.language.experimental.namedTuples
+
+object Test1:
+  // original code from issue https://github.com/scala/scala3/issues/20439
+  val bar = (a = 1, b = 2)
+
+  type ThatBar = bar.type
+  val thatBar: ThatBar = bar
+  val thatBar2: bar.type = bar
+
+  def test2 = thatBar.a // error
+  def test3 = thatBar2.a // ok