Empty shortcuts (#5370)

* Empty shortcuts

* Empty shortcuts

* Empty shortcuts

Author: forki

src/fsharp/FSharp.Core/array.fs

@@ -163,6 +163,9 @@ namespace Microsoft.FSharp.Collections
                 Microsoft.FSharp.Primitives.Basics.Array.subUnchecked 0 count array
 
         let inline countByImpl (comparer:IEqualityComparer<'SafeKey>) (projection:'T->'SafeKey) (getKey:'SafeKey->'Key) (array:'T[]) =
+            let length = array.Length
+            if length = 0 then Microsoft.FSharp.Primitives.Basics.Array.zeroCreateUnchecked 0 else
+
             let dict = Dictionary comparer
 
             // Build the groupings
@@ -279,6 +282,9 @@ namespace Microsoft.FSharp.Collections
         [<CompiledName("DistinctBy")>]
         let distinctBy projection (array:'T[]) =
             checkNonNull "array" array
+            let length = array.Length
+            if length = 0 then Microsoft.FSharp.Primitives.Basics.Array.zeroCreateUnchecked 0 else
+
             let temp = Microsoft.FSharp.Primitives.Basics.Array.zeroCreateUnchecked array.Length
             let mutable i = 0 
             let hashSet = HashSet<_>(HashIdentity.Structural<_>)

src/fsharp/FSharp.Core/list.fs

@@ -48,6 +48,10 @@ namespace Microsoft.FSharp.Collections
         let concat lists = Microsoft.FSharp.Primitives.Basics.List.concat lists
 
         let inline countByImpl (comparer:IEqualityComparer<'SafeKey>) (projection:'T->'SafeKey) (getKey:'SafeKey->'Key) (list:'T list) =
+            match list with
+            | [] -> []
+            | _ ->
+
             let dict = Dictionary comparer
             let rec loop srcList  =
                 match srcList with

src/fsharp/FSharp.Core/seq.fs

@@ -1087,6 +1087,7 @@ namespace Microsoft.FSharp.Collections
         [<CompiledName("Distinct")>]
         let distinct source =
             checkNonNull "source" source
+            if isEmpty source then empty else
             seq { let hashSet = HashSet<'T>(HashIdentity.Structural<'T>)
                   for v in source do
                       if hashSet.Add(v) then
@@ -1095,6 +1096,7 @@ namespace Microsoft.FSharp.Collections
         [<CompiledName("DistinctBy")>]
         let distinctBy projection source =
             checkNonNull "source" source
+            if isEmpty source then empty else
             seq { let hashSet = HashSet<_>(HashIdentity.Structural<_>)
                   for v in source do
                     if hashSet.Add(projection v) then
@@ -1138,6 +1140,7 @@ namespace Microsoft.FSharp.Collections
 
         let inline countByImpl (comparer:IEqualityComparer<'SafeKey>) (keyf:'T->'SafeKey) (getKey:'SafeKey->'Key) (source:seq<'T>) =
             checkNonNull "source" source
+            if isEmpty source then empty else
 
             let dict = Dictionary comparer