backported #3124 Add SemigroupK sum

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

@@ -95,4 +95,7 @@ trait AllSyntaxBinCompat5 extends ParallelBitraverseSyntax
 
 trait AllSyntaxBinCompat6 extends ParallelUnorderedTraverseSyntax
 
-trait AllSyntaxBinCompat7 extends FunctorSyntaxBinCompat0 with BiFoldableSyntaxBinCompat0
+trait AllSyntaxBinCompat7
+    extends FunctorSyntaxBinCompat0
+    with BiFoldableSyntaxBinCompat0
+    with SemigroupKSyntaxBinCompat0

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

@@ -52,7 +52,7 @@ package object syntax {
   object representable extends RepresentableSyntax
   object semigroup extends SemigroupSyntax
   object semigroupal extends SemigroupalSyntax
-  object semigroupk extends SemigroupKSyntax
+  object semigroupk extends SemigroupKSyntax with SemigroupKSyntaxBinCompat0
   object show extends ShowSyntax
   object strong extends StrongSyntax
   object try_ extends TrySyntax

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

@@ -2,3 +2,27 @@ package cats
 package syntax
 
 trait SemigroupKSyntax extends SemigroupK.ToSemigroupKOps
+
+private[syntax] trait SemigroupKSyntaxBinCompat0 {
+  implicit final def catsSyntaxSemigroupKBinCompat0[F[_]](fa: SemigroupK[F]): SemigroupKOps[F] =
+    new SemigroupKOps(fa)
+}
+
+final private[syntax] class SemigroupKOps[F[_]](val S: SemigroupK[F]) extends AnyVal {
+
+  /**
+   * Combines `F[A]` and `F[B]` into a `F[Either[A,B]]]`.
+   *
+   * Example:
+   * {{{
+   * scala> import cats.SemigroupK
+   * scala> import cats.syntax.semigroupk._
+   * scala> import cats.data.NonEmptyList
+   * scala> SemigroupK[NonEmptyList].sum(NonEmptyList.one("abc"), NonEmptyList.one(2))
+   * res0: NonEmptyList[Either[String,Int]] = NonEmptyList(Left(abc), Right(2))
+   * }}}
+   */
+  def sum[A, B](fa: F[A], fb: F[B])(implicit F: Functor[F]): F[Either[A, B]] =
+    S.combineK(F.map(fa)(Left(_)), F.map(fb)(Right(_)))
+
+}