Introduce IterableOnceOps as common abstraction for Iterable & Iterator

- Unlike the other Ops types, IterableOnceOps is *not* implemented by
  IterableOnce but only by Iterator and Iterable.

- Fix the hierarchy of MultiSets and SortedMultiSets: The refactoring
  caused soundness problems to show up that were not reported before.
  These types now follow the usual diamond pattern where
  mutable/immutable extends generic and sorted extends unsorted.

Author: szeiger

src/main/scala/strawman/collection/immutable/MultiSet.scala

@@ -10,10 +10,38 @@ import strawman.collection.decorators.ImmutableMapDecorator
   * An immutable multiset
   * @tparam A the element type of the collection
   */
-class MultiSet[A] private (elems: Map[A, Int])
+trait MultiSet[A]
   extends collection.MultiSet[A]
     with Iterable[A]
-    with collection.MultiSetOps[A, MultiSet, MultiSet[A]] {
+    with MultiSetOps[A, MultiSet, MultiSet[A]] {
+
+  override def iterableFactory: IterableFactory[MultiSet] = MultiSet
+}
+
+trait MultiSetOps[A, +CC[X] <: MultiSet[X], +C <: MultiSet[A]] extends collection.MultiSetOps[A, CC, C] {
+  /**
+    * @return an immutable multiset containing all the elements of this multiset
+    *         and one more occurrence of `elem`
+    * @param elem the element to add
+    */
+  def incl(elem: A): C
+
+  /** Alias for `incl` */
+  @`inline` final def + (elem: A): C = incl(elem)
+
+  /**
+    * @return an immutable multiset containing all the elements of this multiset
+    *         and one occurrence less of `elem`
+    *
+    * @param elem the element to remove
+    */
+  def excl(elem: A): C
+
+  /** Alias for `excl` */
+  @`inline` final def - (elem: A): C = excl(elem)
+}
+
+class MultiSetImpl[A] private[immutable] (elems: Map[A, Int]) extends MultiSet[A] {
 
   def occurrences: Map[A, Int] = elems
 
@@ -25,28 +53,22 @@ class MultiSet[A] private (elems: Map[A, Int])
     * @param elem the element to add
     */
   def incl(elem: A): MultiSet[A] =
-    new MultiSet(elems.updatedWith(elem) {
+    new MultiSetImpl(elems.updatedWith(elem) {
       case None    => Some(1)
       case Some(n) => Some(n + 1)
     })
 
-  /** Alias for `incl` */
-  @`inline` final def + (elem: A): MultiSet[A] = incl(elem)
-
   /**
     * @return an immutable multiset containing all the elements of this multiset
     *         and one occurrence less of `elem`
     *
     * @param elem the element to remove
     */
   def excl(elem: A): MultiSet[A] =
-    new MultiSet(elems.updatedWith(elem) {
+    new MultiSetImpl(elems.updatedWith(elem) {
       case Some(n) => if (n > 1) Some(n - 1) else None
     })
 
-  /** Alias for `excl` */
-  @`inline` final def - (elem: A): MultiSet[A] = excl(elem)
-
 }
 
 object MultiSet extends IterableFactory[MultiSet] {
@@ -57,7 +79,7 @@ object MultiSet extends IterableFactory[MultiSet] {
       case _ => (newBuilder[A]() ++= source).result()
     }
 
-  def empty[A] = new MultiSet[A](Map.empty)
+  def empty[A] = new MultiSetImpl[A](Map.empty)
 
   def newBuilder[A](): Builder[A, MultiSet[A]] =
     new ImmutableBuilder[A, MultiSet[A]](empty[A]) {

src/main/scala/strawman/collection/immutable/SortedMultiSet.scala

@@ -10,8 +10,9 @@ import strawman.collection.mutable.{Builder, ImmutableBuilder}
   * @tparam A Type of elements
   */
 class SortedMultiSet[A] private (elems: SortedMap[A, Int])(implicit val ordering: Ordering[A])
-  extends collection.SortedMultiSet[A]
-    with Iterable[A]
+  extends MultiSet[A]
+    with collection.SortedMultiSet[A]
+    with MultiSetOps[A, MultiSet, SortedMultiSet[A]]
     with collection.SortedMultiSetOps[A, SortedMultiSet, SortedMultiSet[A]]
     with collection.IterableOps[A, MultiSet, SortedMultiSet[A]] {
 
@@ -34,9 +35,6 @@ class SortedMultiSet[A] private (elems: SortedMap[A, Int])(implicit val ordering
       case Some(n) => Some(n + 1)
     })
 
-  /** Alias for `incl` */
-  @`inline` final def + (elem: A): SortedMultiSet[A] = incl(elem)
-
   /**
     * @return an immutable sorted multiset containing all the elements of
     *         this multiset and one occurrence less of `elem`
@@ -47,10 +45,6 @@ class SortedMultiSet[A] private (elems: SortedMap[A, Int])(implicit val ordering
     new SortedMultiSet(elems.updatedWith(elem) {
       case Some(n) => if (n > 1) Some(n - 1) else None
     })
-
-  /** Alias for `excl` */
-  @`inline` final def - (elem: A): SortedMultiSet[A] = excl(elem)
-
 }
 
 object SortedMultiSet extends SortedIterableFactory[SortedMultiSet] {

src/main/scala/strawman/collection/mutable/MultiSet.scala

@@ -7,13 +7,16 @@ import strawman.collection.decorators.MutableMapDecorator
 /**
   * A mutable multiset.
   */
-class MultiSet[A] private (val elems: Map[A, Int])
+trait MultiSet[A]
   extends collection.MultiSet[A]
     with collection.MultiSetOps[A, MultiSet, MultiSet[A]]
     with Growable[A]
     with Shrinkable [A] {
 
   override def iterableFactory: IterableFactory[MultiSet] = MultiSet
+}
+
+class MultiSetImpl[A] private[mutable] (val elems: Map[A, Int]) extends MultiSet[A] {
 
   def occurrences: collection.Map[A, Int] = elems
 
@@ -39,7 +42,7 @@ object MultiSet extends IterableFactory[MultiSet] {
 
   def from[A](source: IterableOnce[A]): MultiSet[A] = (newBuilder[A]() ++= source).result()
 
-  def empty[A]: MultiSet[A] = new MultiSet[A](Map.empty)
+  def empty[A]: MultiSet[A] = new MultiSetImpl[A](Map.empty)
 
   def newBuilder[A](): Builder[A, MultiSet[A]] = new GrowableBuilder[A, MultiSet[A]](empty)
 

src/main/scala/strawman/collection/mutable/SortedMultiSet.scala

@@ -10,8 +10,10 @@ import strawman.collection.decorators.MutableMapDecorator
   * @tparam A Type of elements
   */
 class SortedMultiSet[A] private (elems: SortedMap[A, Int])(implicit val ordering: Ordering[A])
-  extends collection.SortedMultiSet[A]
+  extends MultiSet[A]
+    with collection.SortedMultiSet[A]
     with collection.SortedMultiSetOps[A, SortedMultiSet, SortedMultiSet[A]]
+    with MultiSetOps[A, MultiSet, SortedMultiSet[A]]
     with Growable[A]
     with Shrinkable[A] {