Updated TUnit.Assertions.FSharp.Operations check function to support assertions with Throw Type (#2431)

* Updated Check to support assertions with Throw Type

* removed unused open declarations in Extensions.fs

* updated fsharp assertions readme

Author: licon4812

TUnit.Assertions.FSharp/Extensions.fs

@@ -1,18 +1,34 @@
 namespace TUnit.Assertions.FSharp
 
 open TUnit.Assertions.AssertionBuilders
+open TUnit.Assertions.AssertConditions.Throws
 
 module Operations =
     [<CustomOperation(MaintainsVariableSpaceUsingBind = true)>]
-    let check (assertion: IInvokableAssertionBuilder) =
+    let check (assertion: 'T) =
         Async.FromContinuations(fun (cont, econt, ccont) ->
-            let awaiter = assertion.GetAwaiter()
-
-            awaiter.OnCompleted(fun () ->
-                try
-                    if awaiter.IsCompleted then
-                        cont(awaiter.GetResult())
-                    else
-                        ccont (System.OperationCanceledException())
-                with ex ->
-                    econt ex))
+            match box assertion with
+            | :? IInvokableAssertionBuilder as invokable ->
+                let awaiter = invokable.GetAwaiter()
+                awaiter.OnCompleted(fun () ->
+                    try
+                        if awaiter.IsCompleted then
+                            cont(awaiter.GetResult())
+                        else
+                            ccont (System.OperationCanceledException())
+                    with ex ->
+                        econt ex)
+            | :? ThrowsException<obj, exn> as throwsExn ->
+                let awaiter = throwsExn.GetAwaiter()
+                awaiter.OnCompleted(fun () ->
+                    try
+                        if awaiter.IsCompleted then
+                            let _ = awaiter.GetResult() // ignore the exn result
+                            cont ()
+                        else
+                            ccont (System.OperationCanceledException())
+                    with ex ->
+                        econt ex)
+            | _ ->
+                invalidOp $"Unsupported assertion type: We currently don't support Assertion Type {assertion.GetType()}"
+        )

docs/docs/tutorial-assertions/fsharp.md

@@ -17,4 +17,30 @@ member this.CheckPositive() = async {
             let result = 1 + 1
             do! check (Assert.That(result).IsPositive())
         }
-```
+```
+
+F# is a lot more strict with type resolution when it comes to extension methods and method overloads. Because of that you may need to annotate the type for the assertions.
+
+For example,
+
+```fsharp
+    [<Test>]
+    [<Category("Pass")>]
+    member _.Test3() = async {
+        let value = "1"
+        do! check (Assert.That<string>(value).IsEqualTo("1"))
+    }
+
+    [<Test>]
+    [<Category("Fail")>]
+    member _.Throws1() = async {
+        do! check (Assert.That<string>(fun () -> task { return new string([||]) }).ThrowsException())
+    }
+
+    [<Test>]
+    [<Category("Pass")>]
+    member _.Throws4() = async {
+        do! check (Assert.That<bool>(fun () -> Task.FromResult(true)).ThrowsNothing())
+    }
+
+```