Fix code for proper results before/after enumerating singleton

Author: abelbraaksma

src/FSharp.Control.TaskSeq/TaskSeqInternal.fs

@@ -11,6 +11,12 @@ module ExtraTaskSeqOperators =
     /// A TaskSeq workflow for IAsyncEnumerable<'T> types.
     let taskSeq = TaskSeqBuilder()
 
+[<Struct>]
+type AsyncEnumStatus =
+    | BeforeAll
+    | WithCurrent
+    | AfterAll
+
 [<Struct>]
 type Action<'T, 'U, 'TaskU when 'TaskU :> Task<'U>> =
     | CountableAction of countable_action: (int -> 'T -> 'U)
@@ -64,15 +70,24 @@ module internal TaskSeqInternal =
     let singleton (source: 'T) =
         { new IAsyncEnumerable<'T> with
             member _.GetAsyncEnumerator(_) =
-                let mutable ended = false
+                let mutable status = BeforeAll
 
                 { new IAsyncEnumerator<'T> with
                     member _.MoveNextAsync() =
-                        let vt = ValueTask.FromResult(not ended)
-                        ended <- true
-                        vt
+                        match status with
+                        | BeforeAll ->
+                            status <- WithCurrent
+                            ValueTask.True
+                        | WithCurrent ->
+                            status <- AfterAll
+                            ValueTask.False
+                        | AfterAll -> ValueTask.False
+
+                    member _.Current: 'T =
+                        match status with
+                        | WithCurrent -> source
+                        | _ -> Unchecked.defaultof<'T>
 
-                    member _.Current: 'T = if ended then Unchecked.defaultof<'T> else source
                     member _.DisposeAsync() = ValueTask.CompletedTask
                 }
         }