Allow apply adaptation on Selectable with Fields (#20489)

Just like when using a regular Selectable with refinements, this change
allows:

    fromFlds.xs(0)

to be expanded to:

    fromFlds.xs.apply(0)

Author: odersky

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

@@ -821,10 +821,10 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
 
     // Otherwise, if the qualifier derives from class Selectable,
     // and the selector name matches one of the element of the `Fields` type member,
-    // and the selector is neither applied nor assigned to,
+    // and the selector is not assigned to,
     // expand to a typed dynamic dispatch using selectDynamic wrapped in a cast
     if qual.tpe.derivesFrom(defn.SelectableClass) && !isDynamicExpansion(tree)
-        && !pt.isInstanceOf[FunOrPolyProto] && pt != LhsProto
+        && 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

tests/neg/named-tuple-selectable.scala

@@ -0,0 +1,29 @@
+import scala.language.experimental.namedTuples
+
+class FromFields extends Selectable:
+  type Fields = (i: Int)
+  def selectDynamic(key: String) =
+    List(1, 2, 3)
+
+trait FromRefs extends Selectable:
+  def selectDynamic(key: String) =
+    List(1, 2, 3)
+
+def test(
+    fromFlds: FromFields,
+    fromRefs: FromRefs { val i: Int }
+): Unit =
+  fromFlds.i(0) // error
+  fromRefs.i(0) // error
+
+  fromFlds.i.apply(0) // error
+  fromRefs.i.apply(0) // error
+
+  fromFlds.i[Int](List(1)) // error
+  fromRefs.i[Int](List(1)) // error
+
+  fromFlds.i(List(1)) // error
+  fromRefs.i(List(1)) // error
+
+  fromFlds.i.apply(List(1)) // error
+  fromRefs.i.apply(List(1)) // error

tests/pos/named-tuple-selectable.scala

@@ -0,0 +1,29 @@
+import scala.language.experimental.namedTuples
+
+class FromFields extends Selectable:
+  type Fields = (xs: List[Int], poly: [T] => (x: List[T]) => Option[T])
+  def selectDynamic(key: String) =
+    List(1, 2, 3)
+
+trait FromRefs extends Selectable:
+  def selectDynamic(key: String) =
+    List(1, 2, 3)
+
+def test(
+    fromFlds: FromFields,
+    fromRefs: FromRefs { val xs: List[Int]; val poly: [T] => (x: List[T]) => Option[T] }
+): Unit =
+  fromFlds.xs(0)
+  fromRefs.xs(0)
+
+  fromFlds.xs.apply(0)
+  fromRefs.xs.apply(0)
+
+  fromFlds.poly[Int](List(1)): Option[Int]
+  fromRefs.poly[Int](List(1)): Option[Int]
+
+  fromFlds.poly(List(1))
+  fromRefs.poly(List(1))
+
+  fromFlds.poly.apply(List(1))
+  fromRefs.poly.apply(List(1))