Widen singleton types when computing fields from .Fields

Fixes scala#22018

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

@@ -868,7 +868,7 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
           && pt != LhsProto
       then
         val pre = if !TypeOps.isLegalPrefix(qual.tpe) then SkolemType(qual.tpe) else qual.tpe
-        val fieldsType = pre.select(tpnme.Fields).dealias.simplified
+        val fieldsType = pre.select(tpnme.Fields).widenDealias.simplified
         val fields = fieldsType.namedTupleElementTypes
         typr.println(i"try dyn select $qual, $selName, $fields")
         fields.find(_._1 == selName) match

tests/pos/i22018.scala

@@ -0,0 +1,18 @@
+import scala.language.experimental.namedTuples
+
+class SelectableNT[A <: NamedTuple.AnyNamedTuple](val nt: A) extends Selectable:
+  type Fields = A
+  def selectDynamic(x: String) = ???
+
+object Test:
+
+  val a = (name = "foo", age = 1)
+
+  val sa = SelectableNT(a)
+  sa.name   // ok
+
+  type B = a.type
+  val b: B = a
+
+  val sb = SelectableNT(b)
+  sb.name   // fails