Support .lazy version of doubly reversed collection (with ReversedCollectionProtocol)

Author: Moximillian

stdlib/public/core/Reverse.swift

@@ -35,6 +35,17 @@ extension MutableCollection where Self: BidirectionalCollection {
   }
 }
 
+/// A collection that presents the elements of its base collection
+/// in reverse order.
+///
+/// - See also: `ReversedCollection` below.
+public protocol ReversedCollectionProtocol: BidirectionalCollection {
+  /// A `Collection` that can contain the same elements as this one,
+  /// just in reverse order.
+  associatedtype Elements: Collection where Elements.Iterator.Element == Iterator.Element
+  var elements: Elements { get }
+}
+
 /// A collection that presents the elements of its base collection
 /// in reverse order.
 ///
@@ -84,7 +95,16 @@ extension ReversedCollection {
     }
   }
 }
- 
+
+extension ReversedCollection: ReversedCollectionProtocol {
+  /// The type of the underlying collection.
+  public typealias Elements = Base
+
+  /// The underlying collection.
+  @inlinable
+  public var elements: Elements { return _base }
+}
+
 extension ReversedCollection.Iterator: IteratorProtocol, Sequence {
   public typealias Element = Base.Element
 
@@ -254,13 +274,8 @@ extension ReversedCollection: BidirectionalCollection {
 extension ReversedCollection: RandomAccessCollection where Base: RandomAccessCollection { }
 
 extension ReversedCollection {
-  /// This is optimization to return identity of doubly reversed collection
+  /// This is optimization to return original collection of doubly reversed collection
   /// For example [1,2].reversed().reversed() => [1,2]
-  ///
-  /// Returns a view presenting the elements of the collection in reverse
-  /// order.
-  ///
-  /// - Complexity: O(1)
   public func reversed() -> Base {
     return _base
   }
@@ -298,6 +313,14 @@ extension BidirectionalCollection {
   }
 }
 
+extension LazyCollection where Base: ReversedCollectionProtocol {
+  /// This is optimization to return original collection of doubly reversed lazy collection
+  /// For example [1,2].lazy.reversed().reversed() => [1,2].lazy
+  public func reversed() -> LazyCollection<Elements.Elements> {
+    return elements.elements.lazy
+  }
+}
+
 extension LazyCollectionProtocol
   where
   Self: BidirectionalCollection,