Changed based on code-review

Author: Thorium

src/FSharp.Core/array.fs

@@ -1578,7 +1578,7 @@ module Array =
         checkNonNull "array" array
         Microsoft.FSharp.Primitives.Basics.Array.permute indexMap array
 
-    let inline private classicSum (array: ^T array) : ^T =
+    let inline private fsharpSumImpl (array: ^T array) : ^T =
         checkNonNull "array" array
         let mutable acc = LanguagePrimitives.GenericZero< ^T>
 
@@ -1587,26 +1587,28 @@ module Array =
 
         acc
 
+    let isNetFramework = System.Runtime.InteropServices.RuntimeInformation.FrameworkDescription.StartsWith ".NET Framework"
+
     [<CompiledName("Sum")>]
     let inline sum (array: ^T array) : ^T =
-        classicSum array
+        fsharpSumImpl array
         when ^T : float = 
-            if System.Runtime.InteropServices.RuntimeInformation.FrameworkDescription.StartsWith ".NET Framework" then classicSum array
+            if isNetFramework then fsharpSumImpl array
             else 
                 let r = (System.Linq.Enumerable.Sum : IEnumerable<float> -> float) (# "" array : IEnumerable<float> #)
                 (# "" r : 'T #)
         when ^T : float32 = 
-            if System.Runtime.InteropServices.RuntimeInformation.FrameworkDescription.StartsWith ".NET Framework" then classicSum array
+            if isNetFramework then fsharpSumImpl array
             else 
                 let r = (System.Linq.Enumerable.Sum : IEnumerable<float32> -> float32) (# "" array : IEnumerable<float32> #)
                 (# "" r : 'T #)
         when ^T : int = 
-            if System.Runtime.InteropServices.RuntimeInformation.FrameworkDescription.StartsWith ".NET Framework" then classicSum array
+            if isNetFramework then fsharpSumImpl array
             else 
                 let r = (System.Linq.Enumerable.Sum : IEnumerable<int> -> int) (# "" array : IEnumerable<int> #)
                 (# "" r : 'T #)
         when ^T : int64 = 
-            if System.Runtime.InteropServices.RuntimeInformation.FrameworkDescription.StartsWith ".NET Framework" then classicSum array
+            if isNetFramework then fsharpSumImpl array
             else 
                 let r = (System.Linq.Enumerable.Sum : IEnumerable<int64> -> int64) (# "" array : IEnumerable<int64> #)
                 (# "" r : 'T #)

src/FSharp.Core/array.fsi

@@ -2465,6 +2465,11 @@ module Array =
     [<CompiledName("SortByDescending")>]
     val inline sortByDescending: projection: ('T -> 'Key) -> array: 'T array -> 'T array when 'Key: comparison
 
+    /// Internal use of Array.sum to detect if vectorization can be used.
+    /// Due to sum "inline" this can't be private.
+    [<System.ComponentModel.EditorBrowsable(System.ComponentModel.EditorBrowsableState.Never)>]
+    val isNetFramework : bool
+
     /// <summary>Returns the sum of the elements in the array.</summary>
     ///
     /// <param name="array">The input array.</param>

src/FSharp.Core/seq.fs

@@ -1464,7 +1464,7 @@ module Seq =
         else
             mkDelayedSeq (fun () -> countByRefType projection source)
 
-    let inline private classicSum (source: seq< ^a >) : ^a =
+    let inline private fsharpSumImpl (source: seq< ^a >) : ^a =
         use e = source.GetEnumerator()
         let mutable acc = LanguagePrimitives.GenericZero< ^a>
 
@@ -1473,26 +1473,28 @@ module Seq =
 
         acc 
 
+    let isNetFramework = System.Runtime.InteropServices.RuntimeInformation.FrameworkDescription.StartsWith ".NET Framework"
+
     [<CompiledName("Sum")>]
     let inline sum (source: seq< ^a >) : ^a =
-        classicSum source
+        fsharpSumImpl source
         when ^a: int64 = 
-            if System.Runtime.InteropServices.RuntimeInformation.FrameworkDescription.StartsWith ".NET Framework" then classicSum source
+            if isNetFramework then fsharpSumImpl source
             else 
                 let r = (System.Linq.Enumerable.Sum: IEnumerable<int64> -> int64) (# "" source : IEnumerable<int64> #)
                 (# "" r : 'a #)
         when ^a: int = 
-            if System.Runtime.InteropServices.RuntimeInformation.FrameworkDescription.StartsWith ".NET Framework" then classicSum source
+            if isNetFramework then fsharpSumImpl source
             else
                 let r = (System.Linq.Enumerable.Sum: IEnumerable<int> -> int) (# "" source : IEnumerable<int> #)
                 (# "" r : 'a #)
         when ^a: float32 = 
-            if System.Runtime.InteropServices.RuntimeInformation.FrameworkDescription.StartsWith ".NET Framework" then classicSum source
+            if isNetFramework then fsharpSumImpl source
             else 
                 let r = (System.Linq.Enumerable.Sum: IEnumerable<float32> -> float32) (# "" source : IEnumerable<float32> #)
                 (# "" r : 'a #)
         when ^a: float = 
-            if System.Runtime.InteropServices.RuntimeInformation.FrameworkDescription.StartsWith ".NET Framework" then classicSum source
+            if isNetFramework then fsharpSumImpl source
             else
                 let r = (System.Linq.Enumerable.Sum: IEnumerable<float> -> float) (# "" source : IEnumerable<float> #)
                 (# "" r : 'a #)

src/FSharp.Core/seq.fsi

@@ -2328,6 +2328,11 @@ module Seq =
     [<CompiledName("SortByDescending")>]
     val inline sortByDescending: projection: ('T -> 'Key) -> source: seq<'T> -> seq<'T> when 'Key: comparison
 
+    /// Internal use of Seq.sum to detect if vectorization can be used.
+    /// Due to sum "inline" this can't be private.
+    [<System.ComponentModel.EditorBrowsable(System.ComponentModel.EditorBrowsableState.Never)>]
+    val isNetFramework : bool
+
     /// <summary>Returns the sum of the elements in the sequence.</summary>
     ///
     /// <remarks>The elements are summed using the <c>+</c> operator and <c>Zero</c> property associated with the generated type.</remarks>