Add let-bang tests for corner cases

Author: abelbraaksma

src/FSharp.Control.TaskSeq.Test/TaskSeq.Let.Tests.fs

@@ -0,0 +1,82 @@
+module TaskSeq.Tests.Let
+
+open System
+open System.Threading.Tasks
+open FsUnit
+open Xunit
+
+open FSharp.Control
+
+[<Fact>]
+let ``CE taskSeq: use 'let'`` () =
+    let mutable value = 0
+
+    taskSeq {
+        let value1 = value + 1
+        let value2 = value1 + 1
+        yield value2
+    }
+    |> TaskSeq.exactlyOne
+    |> Task.map (should equal 2)
+
+[<Fact>]
+let ``CE taskSeq: use 'let!' with a task<unit>`` () =
+    let mutable value = 0
+
+    taskSeq {
+        let! unit' = task { do value <- value + 1 }
+        do unit'
+    }
+    |> verifyEmpty
+    |> Task.map (fun _ -> value |> should equal 1)
+
+[<Fact>]
+let ``CE taskSeq: use 'let!' with a task<string>`` () =
+    taskSeq {
+        let! test = task { return "test" }
+        yield test
+    }
+    |> TaskSeq.exactlyOne
+    |> Task.map (should equal "test")
+
+[<Fact>]
+let ``CE taskSeq: use 'let!' with a valuetask<unit>`` () =
+    let mutable value = 0
+
+    taskSeq {
+        let! unit' = ValueTask.ofTask (task { do value <- value + 1 })
+        do unit'
+    }
+    |> verifyEmpty
+    |> Task.map (fun _ -> value |> should equal 1)
+
+[<Fact>]
+let ``CE taskSeq: use 'let!' with a valuetask<string>`` () =
+    taskSeq {
+        let! test = ValueTask.ofTask (task { return "test" })
+        yield test
+    }
+    |> TaskSeq.exactlyOne
+    |> Task.map (should equal "test")
+
+[<Fact>]
+let ``CE taskSeq: use 'let!' with a non-generic valuetask`` () =
+    let mutable value = 0
+
+    taskSeq {
+        let! unit' = ValueTask(task { do value <- value + 1 })
+        do unit'
+    }
+    |> verifyEmpty
+    |> Task.map (fun _ -> value |> should equal 1)
+
+[<Fact>]
+let ``CE taskSeq: use 'let!' with a non-generic task`` () =
+    let mutable value = 0
+
+    taskSeq {
+        let! unit' = (task { do value <- value + 1 }) |> Task.ignore
+        do unit'
+    }
+    |> verifyEmpty
+    |> Task.map (fun _ -> value |> should equal 1)

src/FSharp.Control.TaskSeq/TaskSeqBuilder.fs

@@ -635,7 +635,7 @@ module HighPriority =
         //
         member inline _.Bind(task: Task<'TResult1>, continuation: ('TResult1 -> TaskSeqCode<'T>)) : TaskSeqCode<'T> =
             TaskSeqCode<'T>(fun sm ->
-                let mutable awaiter: TaskAwaiter<'TResult1> = task.GetAwaiter()
+                let mutable awaiter = task.GetAwaiter()
                 let mutable __stack_fin = true
 
                 Debug.logInfo "at Bind"