Make Show contravariant without breaking things (#1649)

core/src/main/scala/cats/Show.scala

@@ -10,11 +10,16 @@ import cats.functor.Contravariant
  * made a toString method, a Show instance will only exist if someone
  * explicitly provided one.
  */
-@typeclass trait Show[T] {
+@typeclass trait Show[T] extends Show.ContravariantShow[T] {
+  // duplicated so simulacrum knows it requires an instance of this trait
   def show(t: T): String
 }
 
 object Show {
+  trait ContravariantShow[-T] {
+    def show(t: T): String
+  }
+
   /** creates an instance of [[Show]] using the provided function */
   def show[A](f: A => String): Show[A] = new Show[A] {
     def show(a: A): String = f(a)
@@ -27,7 +32,7 @@ object Show {
 
   final case class Shown(override val toString: String) extends AnyVal
   object Shown {
-    implicit def mat[A](x: A)(implicit z: Show[A]): Shown = Shown(z show x)
+    implicit def mat[A](x: A)(implicit z: ContravariantShow[A]): Shown = Shown(z show x)
   }
 
   final case class ShowInterpolator(_sc: StringContext) extends AnyVal {

tests/src/test/scala/cats/tests/ShowTests.scala

@@ -10,23 +10,28 @@ class ShowTests extends CatsSuite {
   checkAll("Contravariant[Show]", ContravariantTests[Show].contravariant[Int, Int, Int])
   checkAll("Contravariant[Show]", SerializableTests.serializable(Contravariant[Show]))
 
+  sealed trait TimeOfDay
+  case object Morning extends TimeOfDay
+  object TimeOfDay {
+    implicit val showTimeOfDay: Show[TimeOfDay] = Show.show { case Morning => "morning" }
+  }
+
   test("show string interpolator") {
     case class Cat(name: String)
     object Cat {
       implicit val showCat: Show[Cat] = Show.show(_.name)
     }
-
-    sealed trait TimeOfDay
-    case object Morning extends TimeOfDay
-    object TimeOfDay {
-      implicit val showTimeOfDay: Show[TimeOfDay] = Show.show { case Morning => "morning" }
-    }
-
     val tod: TimeOfDay = Morning
     val cat = Cat("Whiskers")
 
     assertResult("Good morning, Whiskers!")(show"Good $tod, $cat!")
 
     assertResult("Good morning, Whiskers!")(show"Good $tod, ${List(cat).head}!")
   }
+
+  test("show string interpolator and contravariance") {
+    val tod: Morning.type = Morning
+
+    assertResult("Good morning")(show"Good $tod")
+  }
 }