Add an empty operation to all collection instances

This will allow us to experiment with a different desugaring of `for`
expressions, as described in scala#2573.

library/src/dotty/DottyPredef.scala

@@ -1,6 +1,7 @@
 package dotty
 
 import scala.forceInline
+import scala.collection.generic.IsTraversableLike
 
 object DottyPredef {
 
@@ -55,4 +56,14 @@ object DottyPredef {
    * @group utilities
    */
   @forceInline def valueOf[T](implicit vt: ValueOf[T]): T = vt.value
+
+  /**
+    * Add an `empty` operation to all collection instances, returning an empty collection
+    * of the same type.
+    */
+  implicit class EmptyOperation[C](c: C)(implicit isTraversable: IsTraversableLike[C]) {
+    // Ideally we would use `withFilter`, but it is not defined on `GenTraversableLike`.
+    def empty: C = isTraversable.conversion(c).filter(_ => false)
+  }
+
 }

tests/pos/collectionsEmpty.scala

@@ -0,0 +1,40 @@
+object collectionsEmpty {
+
+  locally {
+    val xs1 = List(1, 2, 3)
+    val xs2 = xs1.empty
+    val xs3: List[Int] = xs2
+  }
+
+  locally {
+    val xs1 = Array(1, 2, 3)
+    val xs2 = xs1.empty
+    val xs3: Array[Int] = xs2
+  }
+
+  locally {
+    val xs1 = Set(1, 2, 3)
+    val xs2 = xs1.empty
+    val xs3: Set[Int] = xs2
+  }
+
+  locally {
+    val xs1 = "foo"
+    val xs2 = xs1.empty
+    val xs3: String = xs2
+  }
+
+  // Commented because there is no IsTraversableLike[Range] instance
+  // locally {
+  //   val xs1 = 1 to 3
+  //   val xs2 = xs1.empty
+  //   val xs3: IndexedSeq[Int] = xs2
+  // }
+
+  locally {
+    val xs1 = Iterable(1, 2, 3)
+    val xs2 = xs1.empty
+    val xs3: Iterable[Int] = xs2
+  }
+
+}