* make traits private and sealed to be consistent with typelevel#612

core/src/main/scala/cats/std/function.scala

@@ -6,7 +6,7 @@ import cats.arrow.{Arrow, Choice}
 import cats.data.Xor
 import cats.functor.Contravariant
 
-trait Function0Instances {
+private[std] sealed trait Function0Instances {
   implicit val function0Instance: Bimonad[Function0] =
     new Bimonad[Function0] {
       def extract[A](x: () => A): A = x()
@@ -26,7 +26,7 @@ trait Function0Instances {
     }
 }
 
-trait Function1Instances extends Function1Instances0 {
+private[std] sealed trait Function1Instances extends Function1Instances0 {
   implicit def function1Contravariant[R]: Contravariant[? => R] =
     new Contravariant[? => R] {
       def contramap[T1, T0](fa: T1 => R)(f: T0 => T1): T0 => R =
@@ -78,33 +78,33 @@ trait Function1Instances extends Function1Instances0 {
     new Function1MonoidK {}
 }
 
-trait Function1Instances0 {
+private[std] sealed trait Function1Instances0 {
   implicit def function1Semigroup[A,B](implicit S: Semigroup[B]): Semigroup[A => B] =
     new Function1Semigroup[A, B] { def B: Semigroup[B] = S }
 
   implicit val function1SemigroupK: SemigroupK[Lambda[A => A => A]] =
     new Function1SemigroupK {}
 }
 
-private[std] trait Function1Semigroup[A, B] extends Semigroup[A => B] {
+private[std] sealed trait Function1Semigroup[A, B] extends Semigroup[A => B] {
   implicit def B: Semigroup[B]
 
   override def combine(x: A => B, y: A => B): A => B = { a =>
     B.combine(x(a), y(a))
   }
 }
 
-private[std] trait Function1Monoid[A, B] extends Monoid[A => B] with Function1Semigroup[A, B] {
+private[std] sealed trait Function1Monoid[A, B] extends Monoid[A => B] with Function1Semigroup[A, B] {
   implicit def B: Monoid[B]
 
   override def empty: A => B = _ => B.empty
 }
 
-private[std] trait Function1SemigroupK extends SemigroupK[Lambda[A => A => A]] {
+private[std] sealed trait Function1SemigroupK extends SemigroupK[Lambda[A => A => A]] {
   override def combine[A](x: A => A, y: A => A): A => A = x compose y
 }
 
-private[std] trait Function1MonoidK extends MonoidK[Lambda[A => A => A]] with Function1SemigroupK {
+private[std] sealed trait Function1MonoidK extends MonoidK[Lambda[A => A => A]] with Function1SemigroupK {
   override def empty[A]: A => A = identity[A]
 }