squash! Fix implementation of filterKeys

add implicit cbf for chaining

Author: NthPortal

compat/src/main/scala-2.11/scala/collection/compat/package.scala

@@ -12,11 +12,22 @@
 
 package scala.collection
 
-import scala.collection.generic.IsTraversableLike
+import scala.collection.generic.{CanBuildFrom, IsTraversableLike}
 
 package object compat extends compat.PackageShared {
   implicit def toTraversableLikeExtensionMethods[Repr](self: Repr)(
     implicit traversable: IsTraversableLike[Repr])
   : TraversableLikeExtensionMethods[traversable.A, Repr] =
     new TraversableLikeExtensionMethods[traversable.A, Repr](traversable.conversion(self))
+
+  // CanBuildFrom instances for `IterableView[(K, V), Map[K, V]]` that preserve
+  // the strict type of the view to be `Map` instead of `Iterable`
+  // Instances produced by this method are used to chain `filterKeys` after `mapValues`
+  implicit def canBuildFromIterableViewMapLike[K, V, L, W, CC[X, Y] <: Map[X, Y]]: CanBuildFrom[IterableView[(K, V), CC[K, V]], (L, W), IterableView[(L, W), CC[L, W]]] =
+    new CanBuildFrom[IterableView[(K, V), CC[K, V]], (L, W), IterableView[(L, W), CC[L, W]]] {
+      // `CanBuildFrom` parameters are used as type constraints, they are not used
+      // at run-time, hence the dummy builder implementations
+      def apply(from: IterableView[(K, V), CC[K, V]]) = new TraversableView.NoBuilder
+      def apply() = new TraversableView.NoBuilder
+    }
 }

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

@@ -284,8 +284,9 @@ class MapViewExtensionMethods[K, V, C <: scala.collection.Map[K, V]](
       implicit bf: CanBuildFrom[IterableView[(K, V), C], (K, W), That]): That =
     self.map[(K, W), That] { case (k, v) => (k, f(v)) }
 
-  // TODO: replace the current implementation of `mapValues` with this
-  //       after major version bump when bincompat can be broken
+  // TODO: Replace the current implementation of `mapValues` with this
+  //       after major version bump when bincompat can be broken.
+  //       At the same time, remove `canBuildFromIterableViewMapLike`
   /*
   def mapValues[W](f: V => W): IterableView[(K, W), C] =
     // the implementation of `self.map` also casts the result

compat/src/main/scala-2.12/scala/collection/compat/package.scala

@@ -12,7 +12,7 @@
 
 package scala.collection
 
-import scala.collection.generic.IsTraversableLike
+import scala.collection.generic.{CanBuildFrom, IsTraversableLike}
 import scala.collection.{mutable => m}
 
 package object compat extends compat.PackageShared {
@@ -30,4 +30,15 @@ package object compat extends compat.PackageShared {
     implicit traversable: IsTraversableLike[Repr])
   : TraversableLikeExtensionMethods[traversable.A, Repr] =
     new TraversableLikeExtensionMethods[traversable.A, Repr](traversable.conversion(self))
+
+  // CanBuildFrom instances for `IterableView[(K, V), Map[K, V]]` that preserve
+  // the strict type of the view to be `Map` instead of `Iterable`
+  // Instances produced by this method are used to chain `filterKeys` after `mapValues`
+  implicit def canBuildFromIterableViewMapLike[K, V, L, W, CC[X, Y] <: Map[X, Y]]: CanBuildFrom[IterableView[(K, V), CC[K, V]], (L, W), IterableView[(L, W), CC[L, W]]] =
+    new CanBuildFrom[IterableView[(K, V), CC[K, V]], (L, W), IterableView[(L, W), CC[L, W]]] {
+      // `CanBuildFrom` parameters are used as type constraints, they are not used
+      // at run-time, hence the dummy builder implementations
+      def apply(from: IterableView[(K, V), CC[K, V]]) = new TraversableView.NoBuilder
+      def apply() = new TraversableView.NoBuilder
+    }
 }

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

@@ -39,8 +39,7 @@ class ViewTest {
   def filterKeysMapValues(): Unit = {
     val m = Map("a" -> 1, "b" -> 2, "c" -> 3)
     assertEquals(Map(), m.view.filterKeys(_.length > 1).mapValues(_ + 1).toMap)
-    // TODO: uncomment after major version bump when `mapValues` can be fixed
-    //assertEquals(Map(), m.view.mapValues(_ + 1).filterKeys(_.length > 1).toMap)
+    assertEquals(Map(), m.view.mapValues(_ + 1).filterKeys(_.length > 1).toMap)
   }
 
 }