Use summonFrom to prioritize DerivedEmpty instances (#355)

core/src/main/scala-3/cats/derived/empty.scala

@@ -2,29 +2,34 @@ package cats.derived
 
 import alleycats.Empty
 import shapeless3.deriving.K0
+import scala.compiletime.summonFrom
 
 object empty extends EmptyDerivation
 
 trait DerivedEmpty[A] extends Empty[A]:
   protected def emptyValue(): A
   lazy val empty: A = emptyValue()
 
-object DerivedEmpty extends DerivedEmptyLowPriority:
-  given delegated[A](using A: => Empty[A]): DerivedEmpty[A] =
+object DerivedEmpty:
+  inline given [A]: DerivedEmpty[A] = summonFrom {
+    case given Empty[A] => delegated
+    case given K0.Generic[A] => derived
+  }
+
+  def delegated[A](using A: => Empty[A]): DerivedEmpty[A] =
     () => A.empty
 
+  inline def derived[A](using gen: K0.Generic[A]): DerivedEmpty[A] =
+    inline gen match
+      case given K0.ProductGeneric[A] => DerivedEmpty.product
+      case given K0.CoproductGeneric[A] => DerivedEmpty.coproduct
+
   def product[A](using inst: K0.ProductInstances[DerivedEmpty, A]): DerivedEmpty[A] =
     () => inst.construct([A] => (A: DerivedEmpty[A]) => A.empty)
 
   inline def coproduct[A](using gen: K0.CoproductGeneric[A]): DerivedEmpty[A] =
     K0.summonFirst[DerivedEmpty, gen.MirroredElemTypes, A]
 
-private[derived] sealed abstract class DerivedEmptyLowPriority:
-  inline given derived[A](using gen: K0.Generic[A]): DerivedEmpty[A] =
-    inline gen match
-      case given K0.ProductGeneric[A] => DerivedEmpty.product
-      case given K0.CoproductGeneric[A] => DerivedEmpty.coproduct
-
 trait EmptyDerivation:
   extension (E: Empty.type)
     def derived[A](using instance: DerivedEmpty[A]): Empty[A] = instance