Alt. SemigroupK for List.scala

import cats.SemigroupK
import cats._
import cats.syntax.all._
import cats.instances.option._
import org.scalacheck.Arbitrary._
import cats.laws.discipline.SemigroupKTests

implicit val semigroupByLongest = new SemigroupK[List] {
  override def combineK[A](x: List[A], y: List[A]): List[A] =
    x.map(_.some).zipAll(y.map(_.some), none[A], none[A])
      .map { case (a, b) => a <+> b }
      .map { _.get }
}

List(1, 2, 3) <+> List(4, 5, 6, 7)

implicit val eq = Eq.fromUniversalEquals[List[Int]]
val rs = SemigroupKTests[List].semigroupK[Int]
rs.all.check