Issue 337 factory.newBuilder should return new instances

.scalafmt.conf

@@ -1,3 +1,4 @@
+version=1.5.1
 align = more
 docstrings = JavaDoc
 assumeStandardLibraryStripMargin = true

compat/src/main/scala-2.11_2.12/scala/collection/compat/PackageShared.scala

@@ -55,7 +55,12 @@ private[compat] trait PackageShared {
 
   implicit def genericCompanionToCBF[A, CC[X] <: GenTraversable[X]](
       fact: GenericCompanion[CC]): CanBuildFrom[Any, A, CC[A]] = {
-    val builder: m.Builder[A, CC[A]] = fact match {
+    /* see https://github.com/scala/scala-library-compat/issues/337
+       `simpleCBF.apply` takes a by-name parameter and relies on
+       repeated references generating new builders, thus this expression
+       must be non-strict
+     */
+    def builder: m.Builder[A, CC[A]] = fact match {
       case c.Seq | i.Seq => new IdentityPreservingBuilder[A, i.Seq](i.Seq.newBuilder[A])
       case c.LinearSeq | i.LinearSeq =>
         new IdentityPreservingBuilder[A, i.LinearSeq](i.LinearSeq.newBuilder[A])

compat/src/test/scala/test/scala/collection/FactoryTest.scala

@@ -42,5 +42,19 @@ class FactoryTest {
     val result  = factory.fromSpecific(source)
     Assert.assertEquals(1, counter) // One element has been evaluated because Stream is not lazy in its head
   }
+  @Test
+  def factoriesAreReusable(): Unit = {
+    def generically[M[X] <: Iterable[X]](in: M[Int], factory: Factory[Int, M[Int]]): Unit = {
+      val l = Iterator(-3, -2, -1).to(factory)
+      val m = in.iterator.to(factory)
+      Assert.assertEquals(in, m)
+    }
+
+    generically[List](List(1, 2, 3), List)
+    generically[Seq](Seq(1, 2, 3), Seq)
+    generically[IndexedSeq](IndexedSeq(1, 2, 3), IndexedSeq)
+    generically[Vector](Vector(1, 2, 3), Vector)
+    generically[Set](Set(1, 2, 3), Set)
+  }
 
 }