Strict derivation for Empty

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

@@ -13,12 +13,22 @@ Make sure that A satisfies one of the following conditions:
 type DerivedEmpty[A] = Derived[Empty[A]]
 object DerivedEmpty:
   type Or[A] = Derived.Or[Empty[A]]
+
   inline def apply[A]: Empty[A] =
     import DerivedEmpty.given
     summonInline[DerivedEmpty[A]].instance
 
+  inline def strict[A]: Empty[A] =
+    import DerivedEmpty.given
+    import Strict.product
+    summonInline[DerivedEmpty[A]].instance
+
   given product[A](using inst: K0.ProductInstances[Or, A]): DerivedEmpty[A] =
-    Empty(inst.unify.construct([a] => (_: Empty[a]).empty))
+    Strict.product(using inst.unify)
 
   inline given coproduct[A](using gen: K0.CoproductGeneric[A]): DerivedEmpty[A] =
-    Empty(gen.withOnly[Or, A]([a <: A] => (_: Or[a]).unify.empty))
+    Empty(gen.withOnly[Or, A]([a <: A] => (A: Or[a]) => A.unify.empty))
+
+  object Strict:
+    given product[A](using inst: K0.ProductInstances[Empty, A]): DerivedEmpty[A] =
+      Empty(inst.construct([a] => (A: Empty[a]) => A.empty))

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

@@ -27,8 +27,7 @@ object DerivedEq:
     Eq.allEqual
 
   given product[A](using inst: => K0.ProductInstances[Or, A]): DerivedEq[A] =
-    given K0.ProductInstances[Eq, A] = inst.unify
-    new Product[Eq, A] {}
+    Strict.product(using inst.unify)
 
   given coproduct[A](using inst: => K0.CoproductInstances[Or, A]): DerivedEq[A] =
     given K0.CoproductInstances[Eq, A] = inst.unify

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

@@ -37,8 +37,7 @@ object DerivedHash:
   given symbol[A <: Symbol]: DerivedHash[A] = Hash.fromUniversalHashCode
 
   given product[A <: scala.Product](using inst: => K0.ProductInstances[Or, A]): DerivedHash[A] =
-    given K0.ProductInstances[Hash, A] = inst.unify
-    new Product[Hash, A] {}
+    Strict.product(using inst.unify)
 
   given coproduct[A](using inst: => K0.CoproductInstances[Or, A]): DerivedHash[A] =
     given K0.CoproductInstances[Hash, A] = inst.unify

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

@@ -60,10 +60,12 @@ object semiauto:
 object strict:
   extension (x: Eq.type) inline def derived[A]: Eq[A] = DerivedEq.strict[A]
   extension (x: Hash.type) inline def derived[A]: Hash[A] = DerivedHash.strict[A]
+  extension (x: Empty.type) inline def derived[A]: Empty[A] = DerivedEmpty.strict[A]
 
   object semiauto:
     inline def eq[A]: Eq[A] = DerivedEq.strict[A]
     inline def hash[A]: Hash[A] = DerivedHash.strict[A]
+    inline def empty[A]: Empty[A] = DerivedEmpty.strict[A]
 
 object auto:
   object eq:

core/src/test/scala-3/cats/derived/EmptySuite.scala

@@ -38,23 +38,30 @@ class EmptySuite extends KittensSuite:
     test(s"$instance respects existing instances")(assert(empty[Box[Mask]] == Box(Mask(0xffffffff))))
     checkAll(s"$instance is Serializable", SerializableTests.serializable(summonInline[Empty[Foo]]))
 
-  locally {
+  locally:
     import auto.empty.given
     validate("auto.empty")
     testNoInstance("Empty", "IList[Int]")
     testNoInstance("Empty", "Snoc[Int]")
     testNoInstance("Empty", "Rgb")
-  }
 
-  locally {
+  locally:
     import semiInstances.given
     validate("semiauto.empty")
-    testNoInstance("Empty", "IList[Int]")
-    testNoInstance("Empty", "Snoc[Int]")
-    testNoInstance("Empty", "Rgb")
-  }
+    testNoInstance("semiauto.empty", "IList[Int]")
+    testNoInstance("semiauto.empty", "Snoc[Int]")
+    testNoInstance("semiauto.empty", "Rgb")
 
-  locally {
+  locally:
+    import strictInstances.given
+    validate("strict.semiauto.empty")
+    testNoInstance("strict.semiauto.empty", "Rgb")
+    testNoInstance("strict.semiauto.empty", "Top")
+    test("No strict.semiauto.empty for IList[Int] or Snoc[Int]"):
+      assertNoInstance(compileErrors("given Empty[IList[Int]] = strict.semiauto.empty"))
+      assertNoInstance(compileErrors("given Empty[Snoc[Int]] = strict.semiauto.empty"))
+
+  locally:
     import derivedInstances.*
     val instance = "derived.empty"
     test(s"$instance[Foo]")(assert(empty[Foo].x == ADTs.Foo(0, None)))
@@ -65,7 +72,6 @@ class EmptySuite extends KittensSuite:
     test(s"$instance[Snoc[Dummy]]")(assert(empty[Snoc[Int]].x == SNil()))
     test(s"$instance respects existing instances")(assert(empty[BoxMask].x == Box(Mask(0xffffffff))))
     checkAll(s"$instance is Serializable", SerializableTests.serializable(Empty[Foo]))
-  }
 
 end EmptySuite
 
@@ -85,6 +91,18 @@ object EmptySuite:
     given Empty[Box[Mask]] = semiauto.empty
     given Empty[Chain] = semiauto.empty
 
+  object strictInstances:
+    given Empty[Foo] = strict.semiauto.empty
+    given Empty[Outer] =
+      given Empty[Inner] = strict.semiauto.empty
+      strict.semiauto.empty
+    given Empty[Interleaved[String]] = strict.semiauto.empty
+    given Empty[Recursive] = strict.semiauto.empty
+    given Empty[IList[Dummy]] = strict.semiauto.empty
+    given Empty[Snoc[Dummy]] = strict.semiauto.empty
+    given Empty[Box[Mask]] = strict.semiauto.empty
+    given Empty[Chain] = strict.semiauto.empty
+
   object derivedInstances:
     case class Foo(x: ADTs.Foo) derives Empty
     case class Outer(x: ADTs.Outer) derives Empty

core/src/test/scala-3/cats/derived/KittensSuite.scala

@@ -61,7 +61,9 @@ object KittensSuite:
     given [A <: Product](using mirror: Mirror.ProductOf[A], via: Cogen[mirror.MirroredElemTypes]): Cogen[A] =
       via.contramap(Tuple.fromProductTyped)
 
-    inline def testNoInstance(inline tc: String, target: String): Unit =
-      val errors = compileErrors(tc + "[" + target + "]")
+    def assertNoInstance(errors: String): Unit =
       val message = "No given instance of type"
-      test(s"No $tc for $target")(assert(errors.contains(message), s"$errors did not contain $message"))
+      assert(errors.contains(message), s"$errors did not contain $message")
+
+    inline def testNoInstance(inline tc: String, target: String): Unit =
+      test(s"No $tc for $target")(assertNoInstance(compileErrors(tc + "[" + target + "]")))