Make Show contravariant without breaking things

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

@@ -11,7 +11,6 @@ class ShowTests extends CatsSuite {
   checkAll("Contravariant[Show]", SerializableTests.serializable(Contravariant[Show]))
 
   test("show string interpolator") {
-    import cats.syntax.show._
 
     case class Cat(name: String)
     object Cat {
@@ -31,4 +30,26 @@ class ShowTests extends CatsSuite {
 
     assertResult("Good morning, Whiskers!")(show"Good $tod, ${List(cat).head}!")
   }
+
+  test("show string interpolator and contravariance") {
+
+    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" }
+    }
+
+    // only difference: no type annotation here
+    val tod = Morning
+    val cat = Cat("Whiskers")
+
+    assertResult("Good morning, Whiskers!")(show"Good $tod, $cat!")
+
+    assertResult("Good morning, Whiskers!")(show"Good $tod, ${List(cat).head}!")
+  }
 }