[Scala3] Add support for Selectable Modules (#5087)

Author: adkian-sifive

core/src/main/scala-2/chisel3/SelectableCompat.scala

@@ -2,8 +2,16 @@
 
 package chisel3
 
-/** Does nothing in Scala 2; this stub trait is for maintaining
-  * compatibility with Scala 3's selectable, required to support
-  * structurally-typed Bundles in Scala 3.
+/** Shim for Scala 3's `scala.reflect.Selectable`
+  *
+  * Allows Chisel to cross-compile Scala 2 and Scala 3
+  * while enabling structural typing in Scala 3.
+  */
+trait ReflectSelectable
+
+/** Shim for Scala 3's `scala.Selectable`
+  *
+  * Allows Chisel to cross-compile Scala 2 and Scala 3
+  * while enabling structural typing in Scala 3.
   */
 trait Selectable

core/src/main/scala-3/chisel3/SelectableCompat.scala

@@ -0,0 +1,5 @@
+// SPDX-License-Identifier: Apache-2.0
+
+package chisel3
+
+type ReflectSelectable = scala.reflect.Selectable

core/src/main/scala/chisel3/Module.scala

@@ -413,7 +413,7 @@ package experimental {
   /** Abstract base class for Modules, an instantiable organizational unit for RTL.
     */
   // TODO: seal this?
-  abstract class BaseModule extends HasId with IsInstantiable {
+  abstract class BaseModule extends HasId with IsInstantiable with ReflectSelectable {
     _parent.foreach(_.addId(this))
 
     // Set if the returned top-level module of a nested call to the Chisel Builder, see Definition.apply