truncate should not throw ArgumentException when count is negative

fixes #453
closes #454

src/fsharp/FSharp.Core.Unittests/FSharp.Core/Microsoft.FSharp.Collections/ArrayModule2.fs

@@ -939,7 +939,8 @@ type ArrayModule2() =
         CheckThrowsArgumentNullException(fun() -> Array.truncate 1 null |> ignore)
 
         // negative count
-        CheckThrowsArgumentException(fun() -> Array.truncate -1 [|1..5|] |> ignore)
+        Assert.AreEqual([| |], Array.truncate -1 [|1..5|])
+        Assert.AreEqual([| |], Array.truncate System.Int32.MinValue [|1..5|])
 
         ()
 

src/fsharp/FSharp.Core.Unittests/FSharp.Core/Microsoft.FSharp.Collections/ListModule2.fs

@@ -815,7 +815,8 @@ type ListModule02() =
         Assert.AreEqual([], List.truncate 1 [])
 
         // negative count
-        CheckThrowsArgumentException(fun() -> List.truncate -1 [1..5] |> ignore)
+        Assert.AreEqual([], List.truncate -1 [1..5])
+        Assert.AreEqual([], List.truncate System.Int32.MinValue [1..5])
 
         ()
 

src/fsharp/FSharp.Core.Unittests/FSharp.Core/Microsoft.FSharp.Collections/SeqModule2.fs

@@ -1533,6 +1533,11 @@ type SeqModule2() =
         
         // null Seq
         CheckThrowsArgumentNullException(fun() -> Seq.truncate 1 null |> ignore)
+
+        // negative count
+        VerifySeqsEqual Seq.empty <| Seq.truncate -1 (seq [1;2;4;5;7])
+        VerifySeqsEqual Seq.empty <| Seq.truncate System.Int32.MinValue (seq [1;2;4;5;7])
+
         ()
         
     [<Test>]

src/fsharp/FSharp.Core/array.fs

@@ -1001,8 +1001,7 @@ namespace Microsoft.FSharp.Collections
         [<CompiledName("Truncate")>]
         let truncate count (array:'T[]) =
             checkNonNull "array" array
-            if count < 0 then invalidArg "count" (SR.GetString(SR.inputMustBeNonNegative))
-            if count = 0 then empty
+            if count <= 0 then empty
             else
                 let len = array.Length
                 let count' = Operators.min count len

src/fsharp/FSharp.Core/array.fsi

@@ -981,7 +981,6 @@ namespace Microsoft.FSharp.Collections
         /// <param name="array">The input array.</param>
         /// <returns>The result array.</returns>
         /// <exception cref="System.ArgumentNullException">Thrown when the input array is null.</exception>
-        /// <exception cref="System.ArgumentException">Thrown when the count is negative.</exception>
         [<CompiledName("Truncate")>]
         val truncate: count:int -> array:'T[] -> 'T[]
 

src/fsharp/FSharp.Core/list.fsi

@@ -778,7 +778,6 @@ namespace Microsoft.FSharp.Collections
         /// <param name="count">The maximum number of items to return.</param>
         /// <param name="array">The input list.</param>
         /// <returns>The result list.</returns>
-        /// <exception cref="System.ArgumentException">Thrown when the count is negative.</exception>
         [<CompiledName("Truncate")>]
         val truncate: count:int -> list:'T list -> 'T list
 

src/fsharp/FSharp.Core/local.fs

@@ -441,17 +441,16 @@ module internal List =
             truncateToFreshConsTail cons2 (count-1) t
 
     let truncate count list =
-        if count < 0 then invalidArg "count" (SR.GetString(SR.inputMustBeNonNegative))
         match list with
         | [] -> list
         | _ :: ([] as nil) -> if count > 0 then list else nil
         | h::t ->
-            if count = 0 then []
+            if count <= 0 then []
             else
                 let cons = freshConsNoTail h
                 truncateToFreshConsTail cons (count-1) t
                 cons
-           
+
     let rec unfoldToFreshConsTail cons f s =
         match f s with
         | None -> setFreshConsTail cons []