Make Show contravariant

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

@@ -10,8 +10,8 @@ import cats.functor.Contravariant
  * made a toString method, a Show instance will only exist if someone
  * explicitly provided one.
  */
-@typeclass trait Show[T] {
-  def show(f: T): String
+@typeclass trait Show[-T] {
+  def show(t: T): String
 }
 
 object Show {

core/src/main/scala/cats/instances/bitSet.scala

@@ -1,8 +1,3 @@
 package cats.instances
 
-import scala.collection.immutable.BitSet
-import cats.Show
-
-trait BitSetInstances extends cats.kernel.instances.BitSetInstances {
-  implicit def catsStdShowForBitSet: Show[BitSet] = Show.fromToString[BitSet]
-}
+trait BitSetInstances extends cats.kernel.instances.BitSetInstances

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 +30,19 @@ class ShowTests extends CatsSuite {
 
     assertResult("Good morning, Whiskers!")(show"Good $tod, ${List(cat).head}!")
   }
+
+  test("show string interpolator and contravariance") {
+    trait Animal {
+      val name: String
+    }
+    object Animal {
+      implicit val showAnimal: Show[Animal] =
+        Show.show(_.name)
+    }
+    class Cat(override val name: String, val breed: String) extends Animal
+
+    val cat = new Cat("Whiskers", "Calico")
+
+    assertResult("Good morning, Whiskers!")(show"Good morning, $cat!")
+  }
 }