MergeSort.scala

object MergeSort {

  def sort(l1: List[Int], l2: List[Int]): List[Int] = (l1, l2) match {
    case (List(), List())  => List()
    case (x :: xs, List()) => l1
    case (List(), y :: ys) => l2
    case (x :: xs, y :: ys) =>
      if (x < y) x :: sort(xs, l2) else y :: sort(ys, l1)
  }
}

MergeSortSpec.scala

import org.scalatest.funspec.AnyFunSpec
import org.scalatest.diagrams.Diagrams
class MergeSortSpec extends AnyFunSpec with Diagrams {
  describe("MergeSort") {
    it("returns merged list") {
      assert(MergeSort.sort(List(), List()) == List())
      assert(MergeSort.sort(List(), List(1, 2)) == List(1, 2))
      assert(MergeSort.sort(List(1, 2), List()) == List(1, 2))
      assert(MergeSort.sort(List(1, 3), List(2, 4)) == List(1, 2, 3, 4))
      assert(MergeSort.sort(List(2, 4), List(1, 3)) == List(1, 2, 3, 4))
      assert(MergeSort.sort(List(1, 4), List(1, 3)) == List(1, 1, 3, 4))
    }
  }
}