From 73d180b75f1f43c9d5728841d3efd819931ec74e Mon Sep 17 00:00:00 2001
From: Edmund Noble <edmundnoble@users.noreply.github.com>
Date: Wed, 5 Jul 2017 23:04:18 -0400
Subject: [PATCH] Add cats Order->stdlib Ordering implicit summoner to
 implicits, instances.order (#1670)

---
 core/src/main/scala/cats/instances/order.scala   |  3 ++-
 kernel/src/main/scala/cats/kernel/Order.scala    |  9 +++++++++
 tests/src/test/scala/cats/tests/OrderTests.scala | 14 ++++++++++++++
 3 files changed, 25 insertions(+), 1 deletion(-)

diff --git a/core/src/main/scala/cats/instances/order.scala b/core/src/main/scala/cats/instances/order.scala
index ac05fd87bc..8dba09b790 100644
--- a/core/src/main/scala/cats/instances/order.scala
+++ b/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)
     }
 }
+
diff --git a/kernel/src/main/scala/cats/kernel/Order.scala b/kernel/src/main/scala/cats/kernel/Order.scala
index bf31262d93..0292866718 100644
--- a/kernel/src/main/scala/cats/kernel/Order.scala
+++ b/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]`.
diff --git a/tests/src/test/scala/cats/tests/OrderTests.scala b/tests/src/test/scala/cats/tests/OrderTests.scala
index e319a4b922..324c4a089e 100644
--- a/tests/src/test/scala/cats/tests/OrderTests.scala
+++ b/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]
+  }
 }