Merge pull request scala#4504 from ruippeixotog/mutable-treemap

SI-4147 Add an implementation of `mutable.TreeMap`

Author: adriaanm

src/library/scala/collection/generic/MutableSortedMapFactory.scala

@@ -0,0 +1,24 @@
+package scala
+package collection
+package generic
+
+import scala.language.higherKinds
+
+/**
+ * A template for companion objects of `SortedMap` and subclasses thereof.
+ *
+ * @tparam CC the type of the collection.
+ *
+ * @author Rui Gonçalves
+ * @since 2.12
+ * @version 2.12
+ *
+ * @define Coll `mutable.SortedMap`
+ * @define coll mutable sorted map
+ * @define factoryInfo
+ *         This object provides a set of operations needed to create sorted maps of type `$Coll`.
+ * @define sortedMapCanBuildFromInfo
+ *         The standard `CanBuildFrom` instance for sorted maps
+ */
+abstract class MutableSortedMapFactory[CC[A, B] <: mutable.SortedMap[A, B] with SortedMapLike[A, B, CC[A, B]]]
+  extends SortedMapFactory[CC]