Add CommutativeMonoid[Option[A]] instance (#2725)

Author: Valy Dia

kernel/src/main/scala/cats/kernel/instances/option/package.scala

@@ -6,13 +6,15 @@ package object option extends OptionInstances
 trait OptionInstances extends OptionInstances0 {
   implicit def catsKernelStdOrderForOption[A: Order]: Order[Option[A]] =
     new OptionOrder[A]
-  implicit def catsKernelStdMonoidForOption[A: Semigroup]: Monoid[Option[A]] =
-    new OptionMonoid[A]
+  implicit def catsKernelStdCommutativeMonoidForOption[A: CommutativeSemigroup]: CommutativeMonoid[Option[A]] =
+    new OptionCommutativeMonoid[A]
 }
 
 trait OptionInstances0 extends OptionInstances1 {
   implicit def catsKernelStdPartialOrderForOption[A: PartialOrder]: PartialOrder[Option[A]] =
     new OptionPartialOrder[A]
+  implicit def catsKernelStdMonoidForOption[A: Semigroup]: Monoid[Option[A]] =
+    new OptionMonoid[A]
 }
 
 trait OptionInstances1 extends OptionInstances2 {
@@ -70,6 +72,10 @@ class OptionEq[A](implicit A: Eq[A]) extends Eq[Option[A]] {
     }
 }
 
+class OptionCommutativeMonoid[A](implicit A: CommutativeSemigroup[A])
+  extends OptionMonoid[A]()(A)
+  with CommutativeMonoid[Option[A]]
+
 class OptionMonoid[A](implicit A: Semigroup[A]) extends Monoid[Option[A]] {
   def empty: Option[A] = None
   def combine(x: Option[A], y: Option[A]): Option[A] =