[bug#12059] Make MapView#values and #keys return Views

Change`MapView#values` and `MapView#keys` to return `View`s instead of `AbstractIterable`s, so that further operations on the returned values remain lazy.

Author: NthPortal

library/src/scala/collection/MapView.scala

@@ -21,6 +21,20 @@ trait MapView[K, +V]
 
  override def view: MapView[K, V] = this
 
+ // Ideally this returns a `View`, but bincompat
+ /** Creates a view over all keys of this map.
+ *
+ * @return the keys of this map as a view.
+ */
+ override def keys: Iterable[K] = new MapView.Keys(this)
+
+ // Ideally this returns a `View`, but bincompat
+ /** Creates a view over all values of this map.
+ *
+ * @return the values of this map as a view.
+ */
+ override def values: Iterable[V] = new MapView.Values(this)
+
  /** Filters this map by retaining only keys satisfying a predicate.
  * @param p the predicate used to test keys
  * @return an immutable map consisting only of those key value pairs of this map where the key satisfies
@@ -82,6 +96,22 @@ object MapView extends MapViewFactory {
  override def isEmpty: Boolean = underlying.isEmpty
  }
 
+ // Ideally this is public, but bincompat
+ @SerialVersionUID(3L)
+ private class Keys[K](underlying: SomeMapOps[K, _]) extends AbstractView[K] {
+ def iterator: Iterator[K] = underlying.keysIterator
+ override def knownSize: Int = underlying.knownSize
+ override def isEmpty: Boolean = underlying.isEmpty
+ }
+
+ // Ideally this is public, but bincompat
+ @SerialVersionUID(3L)
+ private class Values[+V](underlying: SomeMapOps[_, V]) extends AbstractView[V] {
+ def iterator: Iterator[V] = underlying.valuesIterator
+ override def knownSize: Int = underlying.knownSize
+ override def isEmpty: Boolean = underlying.isEmpty
+ }
+
  @SerialVersionUID(3L)
  class MapValues[K, +V, +W](underlying: SomeMapOps[K, V], f: V => W) extends AbstractMapView[K, W] {
  def iterator: Iterator[(K, W)] = underlying.iterator.map(kv => (kv._1, f(kv._2)))