Add cats Order->stdlib Ordering implicit summoner to implicits, insta…

…nces.order (#1670)

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

@@ -3,7 +3,7 @@ package instances
 
 import cats.functor.Contravariant
 
-trait OrderInstances {
+trait OrderInstances extends cats.kernel.OrderToOrderingConversion {
 
   implicit val catsFunctorContravariantForOrder: Contravariant[Order] =
     new Contravariant[Order] {
@@ -14,3 +14,4 @@ trait OrderInstances {
       def contramap[A, B](fa: Order[A])(f: B => A): Order[B] = fa.on(f)
     }
 }
+

kernel/src/main/scala/cats/kernel/Order.scala

@@ -139,6 +139,15 @@ abstract class OrderFunctions[O[T] <: Order[T]] extends PartialOrderFunctions[O]
     ev.max(x, y)
 }
 
+trait OrderToOrderingConversion {
+  /**
+   * Implicitly derive a `scala.math.Ordering[A]` from a `Order[A]`
+   * instance.
+   */
+  implicit def catsKernelOrderingForOrder[A](implicit ev: Order[A]): Ordering[A] =
+    ev.toOrdering
+}
+
 object Order extends OrderFunctions[Order] {
   /**
    * Access an implicit `Order[A]`.

tests/src/test/scala/cats/tests/OrderTests.scala

@@ -17,4 +17,18 @@ class OrderTests extends FunSuite {
     Invariant[Order]
     Contravariant[Order]
   }
+
+  // ambiguity test:
+  // the Ordering instance from the Order instance should be trumped
+  // by the one provided in the Ordering companion object
+  {
+    import cats.instances.all._
+    Ordering[String]
+    class C
+    implicit val ording: Ordering[C] = new Ordering[C] {
+      def compare(x: C, y: C) = 0
+    }
+    implicit val ord: Order[C] = Order.allEqual
+    Ordering[C]
+  }
 }