Add .nested syntax. (#2262)

* Add `.nested` syntax.

* Add test for new `.nested` syntax.

* Fix binary compatibility issue.

* Change mixin in SyntaxSuite.

core/src/main/scala/cats/syntax/all.scala

@@ -59,4 +59,6 @@ trait AllSyntaxBinCompat0
     with ApplicativeErrorExtension
     with TrySyntax
 
-trait AllSyntaxBinCompat1 extends FlatMapOptionSyntax
+trait AllSyntaxBinCompat1
+    extends FlatMapOptionSyntax
+    with NestedSyntax

core/src/main/scala/cats/syntax/nested.scala

@@ -0,0 +1,27 @@
+package cats
+package syntax
+
+import cats.data.Nested
+
+trait NestedSyntax {
+  implicit final def catsSyntaxNestedId[F[_], G[_], A](value: F[G[A]]): NestedIdOps[F, G, A] =
+    new NestedIdOps[F, G, A](value)
+}
+
+final class NestedIdOps[F[_], G[_], A](val value: F[G[A]]) extends AnyVal {
+  /**
+    * Wrap a value in `Nested`.
+    *
+    * `x.nested` is equivalent to `Nested(x)`.
+    *
+    * Example:
+    * {{{
+    * scala> import cats.implicits._
+    * scala> List(Some(3), None).nested.map(_+1).value
+    * res0: List[Option[Int]] = List(Some(4), None)
+    * }}}
+    */
+  def nested: Nested[F, G, A] = Nested[F, G, A](value)
+}
+
+

core/src/main/scala/cats/syntax/package.scala

@@ -33,6 +33,7 @@ package object syntax {
   object monad extends MonadSyntax
   object monadError extends MonadErrorSyntax
   object monoid extends MonoidSyntax
+  object nested extends NestedSyntax
   object option extends OptionSyntax
   object order extends OrderSyntax
   object parallel extends ParallelSyntax

tests/src/test/scala/cats/tests/SyntaxSuite.scala

@@ -2,8 +2,9 @@ package cats
 package tests
 
 import cats.arrow.Compose
+import cats.data.Nested
 import cats.instances.AllInstances
-import cats.syntax.AllSyntax
+import cats.syntax.{AllSyntax, AllSyntaxBinCompat1}
 
 
 /**
@@ -24,7 +25,7 @@ import cats.syntax.AllSyntax
  *
  * None of these tests should ever run, or do any runtime checks.
  */
-object SyntaxSuite extends AllInstances with AllSyntax {
+object SyntaxSuite extends AllInstances with AllSyntax with AllSyntaxBinCompat1 {
 
   // pretend we have a value of type A
   def mock[A]: A = ???
@@ -338,5 +339,11 @@ object SyntaxSuite extends AllInstances with AllSyntax {
     val gea4 = ga.recoverWith(pfegea)
   }
 
+  def testNested[F[_], G[_], A]: Unit = {
+    val fga: F[G[A]] = mock[F[G[A]]]
+
+    val nested: Nested[F, G, A] = fga.nested
+  }
+
 }