diff --git a/src/fsharp/FSharp.Core.Unittests/FSharp.Core/Microsoft.FSharp.Core/OptionModule.fs b/src/fsharp/FSharp.Core.Unittests/FSharp.Core/Microsoft.FSharp.Core/OptionModule.fs
index 595eb7457cc..8c7c48c542a 100644
--- a/src/fsharp/FSharp.Core.Unittests/FSharp.Core/Microsoft.FSharp.Core/OptionModule.fs
+++ b/src/fsharp/FSharp.Core.Unittests/FSharp.Core/Microsoft.FSharp.Core/OptionModule.fs
@@ -215,4 +215,16 @@ type OptionModule() =
     member this.MapBindEquivalenceProperties () =
         let fn x = x + 3
         Assert.AreEqual(Option.map fn None, Option.bind (fn >> Some) None)
-        Assert.AreEqual(Option.map fn (Some 5), Option.bind (fn >> Some) (Some 5))
\ No newline at end of file
+        Assert.AreEqual(Option.map fn (Some 5), Option.bind (fn >> Some) (Some 5))
+
+    [<Test>]
+    member this.ToSeq() =
+        Assert.IsFalse(Some 5 |> Option.toSeq |> Seq.isEmpty)
+        Assert.AreEqual(Some 5 |> Option.toSeq |> Seq.length, 1)
+        Assert.AreEqual(Some 5 |> Option.toSeq |> Seq.tryHead, Some 5)
+        Assert.IsFalse(Some "" |> Option.toSeq |> Seq.isEmpty)
+        Assert.AreEqual(Some "" |> Option.toSeq |> Seq.length, 1)
+        Assert.AreEqual(Some "" |> Option.toSeq |> Seq.tryHead, Some "")
+        Assert.IsTrue(None |> Option.toSeq |> Seq.isEmpty)
+        Assert.AreEqual(None |> Option.toSeq |> Seq.length, 0)
+        Assert.AreEqual(None |> Option.toSeq |> Seq.tryHead, None)
\ No newline at end of file
diff --git a/src/fsharp/FSharp.Core.Unittests/SurfaceArea.coreclr.fs b/src/fsharp/FSharp.Core.Unittests/SurfaceArea.coreclr.fs
index f2f634cab1f..afe19270481 100644
--- a/src/fsharp/FSharp.Core.Unittests/SurfaceArea.coreclr.fs
+++ b/src/fsharp/FSharp.Core.Unittests/SurfaceArea.coreclr.fs
@@ -2732,6 +2732,7 @@ Microsoft.FSharp.Core.OptionModule: Microsoft.FSharp.Core.FSharpOption`1[T] OfNu
 Microsoft.FSharp.Core.OptionModule: Microsoft.FSharp.Core.FSharpOption`1[T] OfObj[T](T)
 Microsoft.FSharp.Core.OptionModule: Microsoft.FSharp.Core.FSharpOption`1[T] OrElseWith[T](Microsoft.FSharp.Core.FSharpFunc`2[Microsoft.FSharp.Core.Unit,Microsoft.FSharp.Core.FSharpOption`1[T]], Microsoft.FSharp.Core.FSharpOption`1[T])
 Microsoft.FSharp.Core.OptionModule: Microsoft.FSharp.Core.FSharpOption`1[T] OrElse[T](Microsoft.FSharp.Core.FSharpOption`1[T], Microsoft.FSharp.Core.FSharpOption`1[T])
+Microsoft.FSharp.Core.OptionModule: System.Collections.Generic.IEnumerable`1[T] ToSeq[T](Microsoft.FSharp.Core.FSharpOption`1[T])
 Microsoft.FSharp.Core.OptionModule: System.Nullable`1[T] ToNullable[T](Microsoft.FSharp.Core.FSharpOption`1[T])
 Microsoft.FSharp.Core.OptionModule: System.String ToString()
 Microsoft.FSharp.Core.OptionModule: System.Type GetType()
diff --git a/src/fsharp/FSharp.Core.Unittests/SurfaceArea.net40.fs b/src/fsharp/FSharp.Core.Unittests/SurfaceArea.net40.fs
index 99d38a69f95..25cdc6ff9a4 100644
--- a/src/fsharp/FSharp.Core.Unittests/SurfaceArea.net40.fs
+++ b/src/fsharp/FSharp.Core.Unittests/SurfaceArea.net40.fs
@@ -2766,6 +2766,7 @@ Microsoft.FSharp.Core.OptionModule: Microsoft.FSharp.Core.FSharpOption`1[T] OfNu
 Microsoft.FSharp.Core.OptionModule: Microsoft.FSharp.Core.FSharpOption`1[T] OfObj[T](T)
 Microsoft.FSharp.Core.OptionModule: Microsoft.FSharp.Core.FSharpOption`1[T] OrElseWith[T](Microsoft.FSharp.Core.FSharpFunc`2[Microsoft.FSharp.Core.Unit,Microsoft.FSharp.Core.FSharpOption`1[T]], Microsoft.FSharp.Core.FSharpOption`1[T])
 Microsoft.FSharp.Core.OptionModule: Microsoft.FSharp.Core.FSharpOption`1[T] OrElse[T](Microsoft.FSharp.Core.FSharpOption`1[T], Microsoft.FSharp.Core.FSharpOption`1[T])
+Microsoft.FSharp.Core.OptionModule: System.Collections.Generic.IEnumerable`1[T] ToSeq[T](Microsoft.FSharp.Core.FSharpOption`1[T])
 Microsoft.FSharp.Core.OptionModule: System.Nullable`1[T] ToNullable[T](Microsoft.FSharp.Core.FSharpOption`1[T])
 Microsoft.FSharp.Core.OptionModule: System.String ToString()
 Microsoft.FSharp.Core.OptionModule: System.Type GetType()
diff --git a/src/fsharp/FSharp.Core.Unittests/SurfaceArea.portable259.fs b/src/fsharp/FSharp.Core.Unittests/SurfaceArea.portable259.fs
index d799117f863..dfd3167fdf0 100644
--- a/src/fsharp/FSharp.Core.Unittests/SurfaceArea.portable259.fs
+++ b/src/fsharp/FSharp.Core.Unittests/SurfaceArea.portable259.fs
@@ -2737,6 +2737,7 @@ Microsoft.FSharp.Core.OptionModule: Microsoft.FSharp.Core.FSharpOption`1[T] OfNu
 Microsoft.FSharp.Core.OptionModule: Microsoft.FSharp.Core.FSharpOption`1[T] OfObj[T](T)
 Microsoft.FSharp.Core.OptionModule: Microsoft.FSharp.Core.FSharpOption`1[T] OrElseWith[T](Microsoft.FSharp.Core.FSharpFunc`2[Microsoft.FSharp.Core.Unit,Microsoft.FSharp.Core.FSharpOption`1[T]], Microsoft.FSharp.Core.FSharpOption`1[T])
 Microsoft.FSharp.Core.OptionModule: Microsoft.FSharp.Core.FSharpOption`1[T] OrElse[T](Microsoft.FSharp.Core.FSharpOption`1[T], Microsoft.FSharp.Core.FSharpOption`1[T])
+Microsoft.FSharp.Core.OptionModule: System.Collections.Generic.IEnumerable`1[T] ToSeq[T](Microsoft.FSharp.Core.FSharpOption`1[T])
 Microsoft.FSharp.Core.OptionModule: System.Nullable`1[T] ToNullable[T](Microsoft.FSharp.Core.FSharpOption`1[T])
 Microsoft.FSharp.Core.OptionModule: System.String ToString()
 Microsoft.FSharp.Core.OptionModule: System.Type GetType()
diff --git a/src/fsharp/FSharp.Core.Unittests/SurfaceArea.portable47.fs b/src/fsharp/FSharp.Core.Unittests/SurfaceArea.portable47.fs
index 9c0f389633d..8bf07402cbc 100644
--- a/src/fsharp/FSharp.Core.Unittests/SurfaceArea.portable47.fs
+++ b/src/fsharp/FSharp.Core.Unittests/SurfaceArea.portable47.fs
@@ -2738,6 +2738,7 @@ Microsoft.FSharp.Core.OptionModule: Microsoft.FSharp.Core.FSharpOption`1[T] OfNu
 Microsoft.FSharp.Core.OptionModule: Microsoft.FSharp.Core.FSharpOption`1[T] OfObj[T](T)
 Microsoft.FSharp.Core.OptionModule: Microsoft.FSharp.Core.FSharpOption`1[T] OrElseWith[T](Microsoft.FSharp.Core.FSharpFunc`2[Microsoft.FSharp.Core.Unit,Microsoft.FSharp.Core.FSharpOption`1[T]], Microsoft.FSharp.Core.FSharpOption`1[T])
 Microsoft.FSharp.Core.OptionModule: Microsoft.FSharp.Core.FSharpOption`1[T] OrElse[T](Microsoft.FSharp.Core.FSharpOption`1[T], Microsoft.FSharp.Core.FSharpOption`1[T])
+Microsoft.FSharp.Core.OptionModule: System.Collections.Generic.IEnumerable`1[T] ToSeq[T](Microsoft.FSharp.Core.FSharpOption`1[T])
 Microsoft.FSharp.Core.OptionModule: System.Nullable`1[T] ToNullable[T](Microsoft.FSharp.Core.FSharpOption`1[T])
 Microsoft.FSharp.Core.OptionModule: System.String ToString()
 Microsoft.FSharp.Core.OptionModule: System.Type GetType()
diff --git a/src/fsharp/FSharp.Core.Unittests/SurfaceArea.portable7.fs b/src/fsharp/FSharp.Core.Unittests/SurfaceArea.portable7.fs
index b9de41ca1eb..20a66a5b6da 100644
--- a/src/fsharp/FSharp.Core.Unittests/SurfaceArea.portable7.fs
+++ b/src/fsharp/FSharp.Core.Unittests/SurfaceArea.portable7.fs
@@ -2750,6 +2750,7 @@ Microsoft.FSharp.Core.OptionModule: Microsoft.FSharp.Core.FSharpOption`1[T] OfNu
 Microsoft.FSharp.Core.OptionModule: Microsoft.FSharp.Core.FSharpOption`1[T] OfObj[T](T)
 Microsoft.FSharp.Core.OptionModule: Microsoft.FSharp.Core.FSharpOption`1[T] OrElseWith[T](Microsoft.FSharp.Core.FSharpFunc`2[Microsoft.FSharp.Core.Unit,Microsoft.FSharp.Core.FSharpOption`1[T]], Microsoft.FSharp.Core.FSharpOption`1[T])
 Microsoft.FSharp.Core.OptionModule: Microsoft.FSharp.Core.FSharpOption`1[T] OrElse[T](Microsoft.FSharp.Core.FSharpOption`1[T], Microsoft.FSharp.Core.FSharpOption`1[T])
+Microsoft.FSharp.Core.OptionModule: System.Collections.Generic.IEnumerable`1[T] ToSeq[T](Microsoft.FSharp.Core.FSharpOption`1[T])
 Microsoft.FSharp.Core.OptionModule: System.Nullable`1[T] ToNullable[T](Microsoft.FSharp.Core.FSharpOption`1[T])
 Microsoft.FSharp.Core.OptionModule: System.String ToString()
 Microsoft.FSharp.Core.OptionModule: System.Type GetType()
diff --git a/src/fsharp/FSharp.Core.Unittests/SurfaceArea.portable78.fs b/src/fsharp/FSharp.Core.Unittests/SurfaceArea.portable78.fs
index aedaca4bff3..50dfb1b011e 100644
--- a/src/fsharp/FSharp.Core.Unittests/SurfaceArea.portable78.fs
+++ b/src/fsharp/FSharp.Core.Unittests/SurfaceArea.portable78.fs
@@ -2737,6 +2737,7 @@ Microsoft.FSharp.Core.OptionModule: Microsoft.FSharp.Core.FSharpOption`1[T] OfNu
 Microsoft.FSharp.Core.OptionModule: Microsoft.FSharp.Core.FSharpOption`1[T] OfObj[T](T)
 Microsoft.FSharp.Core.OptionModule: Microsoft.FSharp.Core.FSharpOption`1[T] OrElseWith[T](Microsoft.FSharp.Core.FSharpFunc`2[Microsoft.FSharp.Core.Unit,Microsoft.FSharp.Core.FSharpOption`1[T]], Microsoft.FSharp.Core.FSharpOption`1[T])
 Microsoft.FSharp.Core.OptionModule: Microsoft.FSharp.Core.FSharpOption`1[T] OrElse[T](Microsoft.FSharp.Core.FSharpOption`1[T], Microsoft.FSharp.Core.FSharpOption`1[T])
+Microsoft.FSharp.Core.OptionModule: System.Collections.Generic.IEnumerable`1[T] ToSeq[T](Microsoft.FSharp.Core.FSharpOption`1[T])
 Microsoft.FSharp.Core.OptionModule: System.Nullable`1[T] ToNullable[T](Microsoft.FSharp.Core.FSharpOption`1[T])
 Microsoft.FSharp.Core.OptionModule: System.String ToString()
 Microsoft.FSharp.Core.OptionModule: System.Type GetType()
diff --git a/src/fsharp/FSharp.Core/local.fs b/src/fsharp/FSharp.Core/local.fs
index 7df2e049e8f..c402e6377b4 100644
--- a/src/fsharp/FSharp.Core/local.fs
+++ b/src/fsharp/FSharp.Core/local.fs
@@ -84,6 +84,62 @@ open Microsoft.FSharp.Core.ICloneableExtensions
 #else
 #endif  
 
+module internal IEnumerator =
+    let noReset() = raise (new System.NotSupportedException(SR.GetString(SR.resetNotSupported)))
+    let notStarted() = raise (new System.InvalidOperationException(SR.GetString(SR.enumerationNotStarted)))
+    let alreadyFinished() = raise (new System.InvalidOperationException(SR.GetString(SR.enumerationAlreadyFinished)))
+    let check started = if not started then notStarted()
+
+    let mkSeq f =
+        { new IEnumerable<'U> with
+              member x.GetEnumerator() = f()
+          interface System.Collections.IEnumerable with
+              member x.GetEnumerator() = (f() :> System.Collections.IEnumerator) }
+
+    /// A concrete implementation of an enumerator that returns no values
+    [<Sealed>]
+    type EmptyEnumerator<'T>() =
+        let mutable started = false
+        interface IEnumerator<'T> with
+            member x.Current =
+                check started
+                (alreadyFinished() : 'T)
+
+        interface System.Collections.IEnumerator with
+            member x.Current =
+                check started
+                (alreadyFinished() : obj)
+            member x.MoveNext() =
+                if not started then started <- true
+                false
+            member x.Reset() = noReset()
+        interface System.IDisposable with
+            member x.Dispose() = ()
+
+    let empty<'T> () = (new EmptyEnumerator<'T>() :> IEnumerator<'T>)
+    
+    [<NoEquality; NoComparison>]
+    type EmptyEnumerable<'T> =
+        | EmptyEnumerable
+        interface IEnumerable<'T> with
+            member x.GetEnumerator() = empty<'T>()
+        interface System.Collections.IEnumerable with
+            member x.GetEnumerator() = (empty<'T>() :> System.Collections.IEnumerator)
+
+    [<Sealed>]
+    type Singleton<'T>(v:'T) =
+        let mutable started = false
+        interface IEnumerator<'T> with
+            member x.Current = v
+        interface System.Collections.IEnumerator with
+            member x.Current = box v
+            member x.MoveNext() = if started then false else (started <- true; true)
+            member x.Reset() = noReset()
+        interface System.IDisposable with
+            member x.Dispose() = ()
+
+    let singleton x = new Singleton<'T>(x) :> IEnumerator<'T>
+
 
 module internal List = 
 
diff --git a/src/fsharp/FSharp.Core/local.fsi b/src/fsharp/FSharp.Core/local.fsi
index dcf8f2a6e15..d61c367ba0c 100644
--- a/src/fsharp/FSharp.Core/local.fsi
+++ b/src/fsharp/FSharp.Core/local.fsi
@@ -18,11 +18,28 @@ module internal DetailedExceptions =
     val invalidArg3ArraysDifferent: arg1:string -> arg2:string -> arg3:string -> len1:int -> len2:int -> len3:int -> _
 
 /// Definitions internal for this library.
-namespace Microsoft.FSharp.Primitives.Basics 
+namespace Microsoft.FSharp.Primitives.Basics
 
 open Microsoft.FSharp.Core
 open Microsoft.FSharp.Collections
 
+module internal IEnumerator =
+    val noReset : unit -> 'T
+    val notStarted : unit -> 'T
+    val alreadyFinished : unit -> 'T
+    val check : bool -> unit
+    val mkSeq : (unit -> System.Collections.Generic.IEnumerator<'T>) -> System.Collections.Generic.IEnumerable<'T>
+
+    [<NoEquality; NoComparison>]
+    type EmptyEnumerable<'T> =
+        | EmptyEnumerable
+        interface System.Collections.Generic.IEnumerable<'T>
+        interface System.Collections.IEnumerable
+
+    val Empty : unit -> System.Collections.Generic.IEnumerator<'T>
+    val Singleton : 'T -> System.Collections.Generic.IEnumerator<'T>
+
+
 module internal List =
     val allPairs : 'T1 list -> 'T2 list -> ('T1 * 'T2) list
     val choose: ('T -> 'U option) -> 'T list -> 'U list
diff --git a/src/fsharp/FSharp.Core/option.fs b/src/fsharp/FSharp.Core/option.fs
index 429cfb7abd6..dd91a262d3c 100644
--- a/src/fsharp/FSharp.Core/option.fs
+++ b/src/fsharp/FSharp.Core/option.fs
@@ -2,6 +2,7 @@
 
 namespace Microsoft.FSharp.Core
 
+    open Microsoft.FSharp.Primitives.Basics
     open Microsoft.FSharp.Core.Operators
 
     [<CompilationRepresentation(CompilationRepresentationFlags.ModuleSuffix)>]
@@ -79,6 +80,9 @@ namespace Microsoft.FSharp.Core
         [<CompiledName("ToList")>]
         let toList option = match option with  None -> [ ] | Some x -> [ x ]
 
+        [<CompiledName("ToSeq")>]
+        let toSeq option = match option with Some x -> IEnumerator.mkSeq (fun () -> IEnumerator.Singleton x) | None -> upcast IEnumerator.EmptyEnumerable
+
         [<CompiledName("ToNullable")>]
         let toNullable option = match option with None -> System.Nullable() | Some v -> System.Nullable(v)
 
diff --git a/src/fsharp/FSharp.Core/option.fsi b/src/fsharp/FSharp.Core/option.fsi
index ecb49808ae4..f9a3e8c16ec 100644
--- a/src/fsharp/FSharp.Core/option.fsi
+++ b/src/fsharp/FSharp.Core/option.fsi
@@ -174,6 +174,11 @@ namespace Microsoft.FSharp.Core
         [<CompiledName("ToList")>]
         val toList: option:'T option -> 'T list
 
+        /// <summary>Convert the option to a sequence of length 0 or 1.</summary>
+        /// <param name="option">The input option.</param>
+        /// <returns>The result sequence.</returns>
+        [<CompiledName("ToSeq")>]
+        val toSeq: option: 'T option -> seq<'T>
 
         /// <summary>Convert the option to a Nullable value.</summary>
         /// <param name="option">The input option.</param>
diff --git a/src/fsharp/FSharp.Core/seq.fs b/src/fsharp/FSharp.Core/seq.fs
index 31c70f57ee6..344924dda74 100644
--- a/src/fsharp/FSharp.Core/seq.fs
+++ b/src/fsharp/FSharp.Core/seq.fs
@@ -15,11 +15,8 @@ namespace Microsoft.FSharp.Collections
 
     module IEnumerator =
 
+      open Microsoft.FSharp.Primitives.Basics.IEnumerator
 
-      let noReset() = raise (new System.NotSupportedException(SR.GetString(SR.resetNotSupported)))
-      let notStarted() = raise (new System.InvalidOperationException(SR.GetString(SR.enumerationNotStarted)))
-      let alreadyFinished() = raise (new System.InvalidOperationException(SR.GetString(SR.enumerationAlreadyFinished)))
-      let check started = if not started then notStarted()
       let dispose (r : System.IDisposable) = r.Dispose()
 
       let cast (e : IEnumerator) : IEnumerator<'T> =
@@ -35,28 +32,6 @@ namespace Microsoft.FSharp.Collections
                     | :? System.IDisposable as e -> e.Dispose()
                     | _ -> ()   }
 
-      /// A concrete implementation of an enumerator that returns no values
-      [<Sealed>]
-      type EmptyEnumerator<'T>() =
-          let mutable started = false
-          interface IEnumerator<'T> with
-                member x.Current =
-                  check started
-                  (alreadyFinished() : 'T)
-
-          interface System.Collections.IEnumerator with
-              member x.Current =
-                  check started
-                  (alreadyFinished() : obj)
-              member x.MoveNext() =
-                  if not started then started <- true
-                  false
-              member x.Reset() = noReset()
-          interface System.IDisposable with
-                member x.Dispose() = ()
-
-      let Empty<'T> () = (new EmptyEnumerator<'T>() :> IEnumerator<'T>)
-
       let rec tryItem index (e : IEnumerator<'T>) =
           if not (e.MoveNext()) then None
           elif index = 0 then Some(e.Current)
@@ -352,20 +327,6 @@ namespace Microsoft.FSharp.Collections
 
       let ofArray arr = (new ArrayEnumerator<'T>(arr) :> IEnumerator<'T>)
 
-      [<Sealed>]
-      type Singleton<'T>(v:'T) =
-          let mutable started = false
-          interface IEnumerator<'T> with
-                member x.Current = v
-          interface IEnumerator with
-              member x.Current = box v
-              member x.MoveNext() = if started then false else (started <- true; true)
-              member x.Reset() = noReset()
-          interface System.IDisposable with
-              member x.Dispose() = ()
-
-      let Singleton x = (new Singleton<'T>(x) :> IEnumerator<'T>)
-
       let EnumerateThenFinally f (e : IEnumerator<'T>) =
           { new IEnumerator<'T> with
                 member x.Current = e.Current
@@ -385,6 +346,7 @@ namespace Microsoft.FSharp.Collections
     //
     module Generator =
 
+        open Microsoft.FSharp.Primitives.Basics
         open System.Collections
         open System.Collections.Generic
 
@@ -554,22 +516,8 @@ namespace Microsoft.FSharp.Core.CompilerServices
             | null -> nullArg argName
             | _ -> ()
 
-        let mkSeq f =
-            { new IEnumerable<'U> with
-                member x.GetEnumerator() = f()
-              interface IEnumerable with
-                member x.GetEnumerator() = (f() :> IEnumerator) }
-
-        [<NoEquality; NoComparison>]
-        type EmptyEnumerable<'T> =
-            | EmptyEnumerable
-            interface IEnumerable<'T> with
-                member x.GetEnumerator() = IEnumerator.Empty<'T>()
-            interface IEnumerable with
-                member x.GetEnumerator() = (IEnumerator.Empty<'T>() :> IEnumerator)
-
         let Generate openf compute closef =
-            mkSeq (fun () -> IEnumerator.generateWhileSome openf compute closef)
+            IEnumerator.mkSeq (fun () -> IEnumerator.generateWhileSome openf compute closef)
 
         let GenerateUsing (openf : unit -> ('U :> System.IDisposable)) compute =
             Generate openf compute (fun (s:'U) -> s.Dispose())
@@ -684,7 +632,7 @@ namespace Microsoft.FSharp.Core.CompilerServices
                                     let ie = outerEnum.Current
                                     // Optimization to detect the statically-allocated empty IEnumerables
                                     match box ie with
-                                    | :? EmptyEnumerable<'T> ->
+                                    | :? IEnumerator.EmptyEnumerable<'T> ->
                                          // This one is empty, just skip, don't call GetEnumerator, try again
                                          takeOuter()
                                     | _ ->
@@ -712,7 +660,7 @@ namespace Microsoft.FSharp.Core.CompilerServices
                                (fun () -> rest resource :> seq<_>)) :> seq<_>)
 
         let mkConcatSeq (sources: seq<'U :> seq<'T>>) =
-            mkSeq (fun () -> new ConcatEnumerator<_,_>(sources) :> IEnumerator<'T>)
+            IEnumerator.mkSeq (fun () -> new ConcatEnumerator<_,_>(sources) :> IEnumerator<'T>)
 
         let EnumerateWhile (g : unit -> bool) (b: seq<'T>) : seq<'T> =
             let started = ref false
@@ -724,7 +672,7 @@ namespace Microsoft.FSharp.Core.CompilerServices
 
             let finish() = (curr := None)
             mkConcatSeq
-               (mkSeq (fun () ->
+               (IEnumerator.mkSeq (fun () ->
                     { new IEnumerator<_> with
                           member x.Current = getCurr()
                        interface IEnumerator with
@@ -843,6 +791,8 @@ namespace Microsoft.FSharp.Collections
     [<CompilationRepresentation(CompilationRepresentationFlags.ModuleSuffix)>]
     module Seq =
 
+        open Microsoft.FSharp.Primitives.Basics.IEnumerator
+
 #if FX_NO_ICLONEABLE
         open Microsoft.FSharp.Core.ICloneableExtensions
 #else
@@ -1238,7 +1188,7 @@ namespace Microsoft.FSharp.Collections
                 foldArraySubRight f arr 0 (len - 2) arr.[len - 1]
 
         [<CompiledName("Singleton")>]
-        let singleton x = mkSeq (fun () -> IEnumerator.Singleton x)
+        let singleton x = mkSeq (fun () -> Singleton x)
 
 
         [<CompiledName("Truncate")>]