Add filterKeys extension method to MapView

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

@@ -246,7 +246,12 @@ class TraversableExtensionMethods[A](private val self: c.Traversable[A]) extends
 class MapViewExtensionMethods[K, V, C <: scala.collection.Map[K, V]](
     private val self: IterableView[(K, V), C])
     extends AnyVal {
+
   def mapValues[W, That](f: V => W)(
       implicit bf: CanBuildFrom[IterableView[(K, V), C], (K, W), That]): That =
     self.map[(K, W), That] { case (k, v) => (k, f(v)) }
+
+  def filterKeys[That](p: K => Boolean)(
+      implicit bf: CanBuildFrom[IterableView[(K, V), C], (K, V), That]): That =
+    self.collect[(K, V), That] { case (k, v) if p(k) => (k, v) }
 }

compat/src/test/scala/test/scala/collection/ViewTest.scala

@@ -27,4 +27,12 @@ class ViewTest {
     assertEquals(oldStyle.toMap, newStyle.toMap)
   }
 
+  @Test
+  def filterKeys: Unit = {
+    val m        = Map("a" -> 1, "b" -> 2, "c" -> 3)
+    val oldStyle = m.filterKeys(_ > "a")
+    val newStyle = m.view.filterKeys(_ > "a")
+    assertEquals(oldStyle.toMap, newStyle.toMap)
+  }
+
 }