add SeqOps#concatNeSeq

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

@@ -8,5 +8,37 @@ trait SeqSyntax {
 }
 
 final class SeqOps[A](private val va: Seq[A]) extends AnyVal {
+
+  /**
+   * Returns an `Option` of `NonEmptySeq` from a `Seq`
+   *
+   * Example:
+   * {{{
+   * scala> import cats.data.NonEmptySeq
+   * scala> import cats.implicits._
+   *
+   * scala> val result1: Seq[Int] = Seq(1, 2)
+   * scala> result1.toNeSeq
+   * res0: Option[NonEmptySeq[Int]] = Some(NonEmptySeq(1, 2))
+   *
+   * scala> val result2: Seq[Int] = Seq.empty[Int]
+   * scala> result2.toNeSeq
+   * res1: Option[NonEmptySeq[Int]] = None
+   * }}}
+   */
   def toNeSeq: Option[NonEmptySeq[A]] = NonEmptySeq.fromSeq(va)
+
+  /**
+   * Concatenates this `Seq` with a `NonEmptySeq` producing a new `NonEmptySeq`.
+   *
+   * Example:
+   * {{{
+   * scala> import cats.data.NonEmptySeq
+   * scala> import cats.implicits._
+   *
+   * scala> Seq(1, 2, 3).concatNeSeq(NonEmptySeq.of(4, 5, 6))
+   * res0: NonEmptySeq[Int] = NonEmptySeq(1, 2, 3, 4, 5, 6)
+   * }}}
+   */
+  def concatNeSeq[AA >: A](neseq: NonEmptySeq[AA]): NonEmptySeq[AA] = neseq.prependSeq(va)
 }

tests/src/test/scala/cats/tests/SeqSuite.scala

@@ -64,6 +64,13 @@ class SeqSuite extends CatsSuite {
     assert(Seq.empty[Int].toNeSeq == None)
   }
 
+  test("concatNeSeq should be consistent with Seq#`++`") {
+    forAll { (fa: Seq[Int], neseq: NonEmptySeq[Int]) =>
+      // Note: Scala 2.12.x does not have `Seq#concat`.
+      assert(fa.concatNeSeq(neseq).toSeq === (fa ++ neseq.toSeq))
+    }
+  }
+
   test("traverse is stack-safe") {
     val seq = (0 until 100000).toSeq
     val sumAll = Traverse[Seq]