findIndexBack and findIndex work in reverse

src/fsharp/FSharp.Core.PropertyTests/CollectionModulesConsistency.fs

@@ -238,10 +238,10 @@ let ``findBack is consistent`` () =
     Check.QuickThrowOnFailure findBack<string>
     Check.QuickThrowOnFailure findBack<NormalFloat>
 
-let findIndex<'a when 'a : equality> (xs : 'a []) index =
-    let s = run (fun () -> xs |> Seq.findIndex index)
-    let l = run (fun () -> xs |> List.ofArray |> List.findIndex index)
-    let a = run (fun () -> xs |> Array.findIndex index)
+let findIndex<'a when 'a : equality> (xs : 'a []) predicate =
+    let s = run (fun () -> xs |> Seq.findIndex predicate)
+    let l = run (fun () -> xs |> List.ofArray |> List.findIndex predicate)
+    let a = run (fun () -> xs |> Array.findIndex predicate)
     s = a && l = a
 
 [<Test>]
@@ -250,10 +250,10 @@ let ``findIndex is consistent`` () =
     Check.QuickThrowOnFailure findIndex<string>
     Check.QuickThrowOnFailure findIndex<NormalFloat>
 
-let findIndexBack<'a when 'a : equality> (xs : 'a []) index =
-    let s = run (fun () -> xs |> Seq.findIndexBack index)
-    let l = run (fun () -> xs |> List.ofArray |> List.findIndexBack index)
-    let a = run (fun () -> xs |> Array.findIndexBack index)
+let findIndexBack<'a when 'a : equality> (xs : 'a []) predicate =
+    let s = run (fun () -> xs |> Seq.findIndexBack predicate)
+    let l = run (fun () -> xs |> List.ofArray |> List.findIndexBack predicate)
+    let a = run (fun () -> xs |> Array.findIndexBack predicate)
     s = a && l = a
 
 [<Test>]

src/fsharp/FSharp.Core.PropertyTests/ListProperties.fs

@@ -137,6 +137,20 @@ let ``findBack and find work in reverse`` () =
     Check.QuickThrowOnFailure findBack_and_find<string>
     Check.QuickThrowOnFailure findBack_and_find<NormalFloat>
 
+let findIndexBack_and_findIndex<'a when 'a : comparison>  (xs : list<'a>) (F (_, predicate)) =
+    let a = run (fun () -> xs |> List.findIndex predicate)
+    let b = run (fun () -> xs |> List.rev |> List.findIndexBack predicate)
+    match a,b with
+    | Success a, Success b -> a = (xs.Length - b - 1)
+    | _ -> a = b
+
+[<Test>]
+let ``findIndexBack and findIndex work in reverse`` () =
+    Check.QuickThrowOnFailure findIndexBack_and_findIndex<int>
+    Check.QuickThrowOnFailure findIndexBack_and_findIndex<string>
+    Check.QuickThrowOnFailure findIndexBack_and_findIndex<NormalFloat>
+
+
 let distinct_works_like_set<'a when 'a : comparison> (xs : 'a list) =
     let a = List.distinct xs
     let b = Set.ofList xs