Implement Fields as a Selectable type member

Subclasses of Selectable can instantiate Fields to a named tuple type
that provides possible selection names and their types on instances of the type.

See: https://contributors.scala-lang.org/t/expanding-changing-selectable-based-on-upcoming-named-tuples-feature/6395/5

Author: odersky

compiler/src/dotty/tools/dotc/core/StdNames.scala

@@ -362,6 +362,7 @@ object StdNames {
     val EnumValue: N            = "EnumValue"
     val ExistentialTypeTree: N  = "ExistentialTypeTree"
     val Flag : N                = "Flag"
+    val Fields: N               = "Fields"
     val From: N                 = "From"
     val Ident: N                = "Ident"
     val Import: N               = "Import"

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

@@ -782,6 +782,19 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
           else
             typedDynamicSelect(tree2, Nil, pt)
         else
+          if qual.tpe.derivesFrom(defn.SelectableClass)
+            && selName.isTermName && !isDynamicExpansion(tree)
+            && !pt.isInstanceOf[FunOrPolyProto] && pt != LhsProto
+          then
+            val fieldsType = qual.tpe.select(tpnme.Fields).dealias.simplified
+            val fields = fieldsType.namedTupleElementTypes
+            typr.println(i"try dyn select $qual, $selName, $fields")
+            fields.find(_._1 == selName) match
+              case Some((fieldName, fieldType)) =>
+                val tree2 = cpy.Select(tree0)(untpd.TypedSplice(qual), selName)
+                val sel = typedDynamicSelect(tree2, Nil, pt)
+                return sel.cast(fieldType)
+              case _ =>
           assignType(tree,
             rawType match
               case rawType: NamedType =>

library/src/scala/Selectable.scala

@@ -22,7 +22,8 @@ package scala
  *  In this case the call will synthesize `Class` arguments for the erasure of
  *  all formal parameter types of the method in the structural type.
  */
-trait Selectable extends Any
+trait Selectable extends Any:
+  type Fields // TODO: add <: NamedTyple.AnyNamedTuple when NamedTuple is no longer experimental
 
 object Selectable:
   /* Scala 2 compat + allowing for cross-compilation: