Make sure to override toXxx methods when it’s a no-op

collections/src/main/scala/strawman/collection/View.scala

@@ -308,11 +308,11 @@ trait IndexedView[+A] extends View[A] with ArrayLike[A] with SeqOps[A, View, Ind
   override protected[this] def newSpecificBuilder(): Builder[A, IndexedView[A]] =
     IndexedSeq.newBuilder[A]().mapResult(_.view)
 
-  def iterator(): Iterator[A] = new Iterator[A] {
+  def iterator(): Iterator[A] = new AbstractIterator[A] {
     private var current = 0
     def hasNext = current < self.length
     def next(): A = {
-      val r = apply(current)
+      val r = self.apply(current)
       current += 1
       r
     }

collections/src/main/scala/strawman/collection/immutable/List.scala

@@ -405,6 +405,9 @@ sealed abstract class List[+A]
     }
     loop(null, null, this, this)
   }
+
+  final override def toList: List[A] = this
+
 }
 
 @SerialVersionUID(6493291385232469459L) // value computed for strawman 0.6.0, scala 2.13.0-M2

collections/src/main/scala/strawman/collection/immutable/Map.scala

@@ -5,6 +5,7 @@ package immutable
 import strawman.collection.mutable.Builder
 
 import scala.{Any, Boolean, `inline`, Int, None, NoSuchElementException, Nothing, Option, Some, Serializable, Unit}
+import scala.Predef.<:<
 
 /** Base type of immutable Maps */
 trait Map[K, +V]
@@ -33,6 +34,9 @@ trait Map[K, +V]
     *  @return      a wrapper of the map with a default value
     */
   def withDefaultValue[V1 >: V](d: V1): Map[K, V1] = new Map.WithDefault[K, V1](this, x => d)
+
+  override final def toMap[K2, V2](implicit ev: (K, V) <:< (K2, V2)): Map[K2, V2] = this.asInstanceOf[Map[K2, V2]]
+
 }
 
 /** Base trait of immutable Maps implementations

collections/src/main/scala/strawman/collection/immutable/Seq.scala

@@ -55,7 +55,11 @@ object Seq extends SeqFactory.Delegate[Seq](List)
 /** Base trait for immutable indexed sequences that have efficient `apply` and `length` */
 trait IndexedSeq[+A] extends Seq[A]
                         with collection.IndexedSeq[A]
-                        with IndexedSeqOps[A, IndexedSeq, IndexedSeq[A]]
+                        with IndexedSeqOps[A, IndexedSeq, IndexedSeq[A]] {
+
+  final override def toIndexedSeq: IndexedSeq[A] = this
+
+}
 
 object IndexedSeq extends SeqFactory.Delegate[IndexedSeq](Vector)
 

collections/src/main/scala/strawman/collection/immutable/Set.scala

@@ -7,7 +7,11 @@ import strawman.collection.mutable.Builder
 import scala.{Any, Boolean, Int, deprecatedName, `inline`, None, Option, Serializable, SerialVersionUID, Some, Unit}
 
 /** Base trait for immutable set collections */
-trait Set[A] extends Iterable[A] with collection.Set[A] with SetOps[A, Set, Set[A]]
+trait Set[A] extends Iterable[A] with collection.Set[A] with SetOps[A, Set, Set[A]] {
+
+  override final def toSet[B >: A]: Set[B] = this.asInstanceOf[Set[B]]
+
+}
 
 /** Base trait for immutable set operations
   *

collections/src/main/scala/strawman/collection/immutable/Vector.scala

@@ -572,6 +572,8 @@ final class Vector[+A] private[immutable] (private[collection] val startIndex: I
     s.cleanRightEdge(cutIndex - shift)
     s
   }
+
+  override def toVector: Vector[A] = this
 }
 
 class VectorIterator[+A](_startIndex: Int, endIndex: Int)