Make MapView#values preserve laziness

build.sbt

@@ -394,6 +394,10 @@ val mimaFilterSettings = Seq {
     ProblemFilters.exclude[MissingTypesProblem]("scala.collection.convert.JavaCollectionWrappers$JPropertiesWrapper"),
     ProblemFilters.exclude[MissingTypesProblem]("scala.collection.convert.JavaCollectionWrappers$JSetWrapper"),
     ProblemFilters.exclude[MissingTypesProblem]("scala.collection.mutable.WeakHashMap"),
+
+    // Fix for scala/bug#12059
+    ProblemFilters.exclude[MissingClassProblem]("scala.collection.MapView$Keys"),
+    ProblemFilters.exclude[MissingClassProblem]("scala.collection.MapView$Values"),
   ),
 }
 

src/library/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)))

test/junit/scala/collection/MapViewTest.scala

@@ -8,6 +8,7 @@ class MapViewTest {
   def _toString(): Unit = {
     assertEquals("MapView(<not computed>)", Map(1 -> 2).view.toString)
   }
+
   @deprecated("Tests deprecated API", since="2.13")
   @Test
   def testStringPrefixToString(): Unit = {
@@ -18,6 +19,7 @@ class MapViewTest {
     }
     assertEquals("FooMapView(<not computed>)", mapView.toString)
   }
+
   @Test
   def testClassNameToString(): Unit = {
     val mapView = new collection.MapView[Int,Int] {
@@ -27,4 +29,34 @@ class MapViewTest {
     }
     assertEquals("FooMapView(<not computed>)", mapView.toString)
   }
+
+  @Test
+  def testKeysIsLazy(): Unit = {
+    var counter = 0
+    def assertLazy(): Unit = assertEquals(0, counter)
+
+    val map = (1 to 10).map(i => i -> i).toMap
+    val mapView = map.view.filterKeys(_ => { counter += 1; true })
+    assertLazy()
+    val keys = mapView.keys
+    assert(keys.isInstanceOf[View[_]])
+    assertLazy()
+    val _ = keys.map(_ + 1)
+    assertLazy()
+  }
+
+  @Test
+  def testValuesIsLazy(): Unit = {
+    var counter = 0
+    def assertLazy(): Unit = assertEquals(0, counter)
+
+    val map = (1 to 10).map(i => i -> i).toMap
+    val mapView = map.view.mapValues(i => { counter += 1; i })
+    assertLazy()
+    val values = mapView.values
+    assert(values.isInstanceOf[View[_]])
+    assertLazy()
+    val _ = values.map(_ + 1)
+    assertLazy()
+  }
 }