diff --git a/src/fsharp/FSharp.Core/seq.fs b/src/fsharp/FSharp.Core/seq.fs
index e5608348bb4..2c1a765272b 100644
--- a/src/fsharp/FSharp.Core/seq.fs
+++ b/src/fsharp/FSharp.Core/seq.fs
@@ -564,9 +564,13 @@ namespace Microsoft.FSharp.Collections
                 inherit SeqComponentFactory<'T,'U> ()
                 override __.Create<'V> (_result:Result<'V>) (next:SeqComponent<'U,'V>) : SeqComponent<'T,'V> = next.CreateMap map
 
-            and Map2Factory<'First,'Second,'U> (map:'First->'Second->'U, input2:IEnumerable<'Second>) =
+            and Map2FirstFactory<'First,'Second,'U> (map:'First->'Second->'U, input2:IEnumerable<'Second>) =
                 inherit SeqComponentFactory<'First,'U> ()
-                override __.Create<'V> (result:Result<'V>) (next:SeqComponent<'U,'V>) : SeqComponent<'First,'V> = upcast Map2 (map, input2, result, next)
+                override __.Create<'V> (result:Result<'V>) (next:SeqComponent<'U,'V>) : SeqComponent<'First,'V> = upcast Map2First (map, input2, result, next)
+
+            and Map2SecondFactory<'First,'Second,'U> (map:'First->'Second->'U, input1:IEnumerable<'First>) =
+                inherit SeqComponentFactory<'Second,'U> ()
+                override __.Create<'V> (result:Result<'V>) (next:SeqComponent<'U,'V>) : SeqComponent<'Second,'V> = upcast Map2Second (map, input1, result, next)
 
             and Map3Factory<'First,'Second,'Third,'U> (map:'First->'Second->'Third->'U, input2:IEnumerable<'Second>, input3:IEnumerable<'Third>) =
                 inherit SeqComponentFactory<'First,'U> ()
@@ -692,7 +696,7 @@ namespace Microsoft.FSharp.Collections
                 override __.ProcessNext (input:'T) : bool = 
                     Helpers.avoidTailCall (next.ProcessNext (map input))
 
-            and Map2<'First,'Second,'U,'V> (map:'First->'Second->'U, enumerable2:IEnumerable<'Second>, result:Result<'V>, next:SeqComponent<'U,'V>) =
+            and Map2First<'First,'Second,'U,'V> (map:'First->'Second->'U, enumerable2:IEnumerable<'Second>, result:Result<'V>, next:SeqComponent<'U,'V>) =
                 inherit SeqComponent<'First,'V>(next)
 
                 let input2 = enumerable2.GetEnumerator ()
@@ -712,6 +716,26 @@ namespace Microsoft.FSharp.Collections
                         finally
                             (Helpers.upcastISeqComponent next).OnDispose ()
 
+            and Map2Second<'First,'Second,'U,'V> (map:'First->'Second->'U, enumerable1:IEnumerable<'First>, result:Result<'V>, next:SeqComponent<'U,'V>) =
+                inherit SeqComponent<'Second,'V>(next)
+
+                let input1 = enumerable1.GetEnumerator ()
+                let map' = OptimizedClosures.FSharpFunc<_,_,_>.Adapt map
+
+                override __.ProcessNext (input:'Second) : bool =
+                    if input1.MoveNext () then
+                        Helpers.avoidTailCall (next.ProcessNext (map'.Invoke (input1.Current, input)))
+                    else
+                        result.StopFurtherProcessing ()
+                        false
+
+                interface ISeqComponent with
+                    override __.OnDispose () =
+                        try
+                            input1.Dispose ()
+                        finally
+                            (Helpers.upcastISeqComponent next).OnDispose ()
+
             and Map3<'First,'Second,'Third,'U,'V> (map:'First->'Second->'Third->'U, enumerable2:IEnumerable<'Second>, enumerable3:IEnumerable<'Third>, result:Result<'V>, next:SeqComponent<'U,'V>) =
                 inherit SeqComponent<'First,'V>(next)
 
@@ -1496,19 +1520,18 @@ namespace Microsoft.FSharp.Collections
 
         [<CompiledName("MapIndexed2")>]
         let mapi2 f source1 source2 =
-            checkNonNull "source1" source1
             checkNonNull "source2" source2
             source1 |> seqFactory (SeqComposer.Mapi2Factory (f, source2))
 
         [<CompiledName("Map2")>]
         let map2<'T,'U,'V> (f:'T->'U->'V) (source1:seq<'T>) (source2:seq<'U>) : seq<'V> =
-            checkNonNull "source1" source1
             checkNonNull "source2" source2
-            source1 |> seqFactory (SeqComposer.Map2Factory (f, source2))
+            match source1 with
+            | :? SeqComposer.Enumerable.EnumerableBase<'T> as s -> s.Compose (SeqComposer.Map2FirstFactory (f, source2))
+            | _ -> source2 |> seqFactory (SeqComposer.Map2SecondFactory (f, source1))
 
         [<CompiledName("Map3")>]
         let map3 f source1 source2 source3 =
-            checkNonNull "source1" source1
             checkNonNull "source2" source2
             checkNonNull "source3" source3
             source1 |> seqFactory (SeqComposer.Map3Factory (f, source2, source3))