SI-9072 Vector ++ concatenation of parallel collection cause inconsistent results

Author: mzitnik

src/library/scala/collection/immutable/Vector.scala

@@ -215,7 +215,7 @@ override def companion: GenericCompanion[Vector] = Vector
       import Vector.{Log2ConcatFaster, TinyAppendFaster}
       if (that.isEmpty) this.asInstanceOf[That]
       else {
-        val again = if (!that.isTraversableAgain) that.toVector else that
+        val again = if (!that.isTraversableAgain) that.toVector else that.seq
         again.size match {
           // Often it's better to append small numbers of elements (or prepend if RHS is a vector)
           case n if n <= TinyAppendFaster || n < (this.size >> Log2ConcatFaster) => 

test/junit/scala/collection/immutable/VectorTest.scala

@@ -0,0 +1,20 @@
+package scala.collection.immutable
+
+import org.junit.{Assert, Test}
+import org.junit.runner.RunWith
+import org.junit.runners.JUnit4
+
+@RunWith(classOf[JUnit4])
+class VectorTest {
+  /**
+   * Test Vector ++ with a small parallel collection concatenation (SI-9072).
+   *
+   */
+  @Test
+  def testPlusPlus(): Unit = {
+    val smallVec = (0 to 1)
+    val smallParVec = smallVec.par
+    val testElementsSize = (0 to 1000).map( _ => Vector.empty ++ smallParVec )
+    Assert.assertTrue(testElementsSize.forall( v => v.size == 2 ))
+  }
+}