Bind Async within task{} should start on the same thread

Author: T-Gro

src/FSharp.Core/tasks.fs

@@ -459,7 +459,7 @@ module MediumPriority =
                 computation: Async<'TResult1>,
                 continuation: ('TResult1 -> TaskCode<'TOverall, 'TResult2>)
             ) : TaskCode<'TOverall, 'TResult2> =
-            this.Bind(Async.StartAsTask computation, continuation)
+            this.Bind(Async.StartImmediateAsTask computation, continuation)
 
         member inline this.ReturnFrom(computation: Async<'T>) : TaskCode<'T, 'T> =
-            this.ReturnFrom(Async.StartAsTask computation)
+            this.ReturnFrom(Async.StartImmediateAsTask computation)

tests/FSharp.Core.UnitTests/FSharp.Core/Microsoft.FSharp.Control/Tasks.fs

@@ -1160,6 +1160,22 @@ type Basics() =
         let result = t.Result
         require (result = 8) "something weird happened"
 
+    [<Fact>]
+    member _.testAsyncsMixedWithTasks_ShouldNotSwitchContext() =
+        let t = task {
+            let a = Thread.CurrentThread.ManagedThreadId
+            let! b = async {
+                return Thread.CurrentThread.ManagedThreadId
+            }
+            let c = Thread.CurrentThread.ManagedThreadId
+            return $"Before: {a}, in async: {b}, after async: {c}"
+        }
+        let d = Thread.CurrentThread.ManagedThreadId
+        let actual = $"{t.Result}, after task: {d}"
+
+        require (actual = $"Before: {d}, in async: {d}, after async: {d}, after task: {d}") actual
+        
+
     [<Fact>]
     // no need to call this, we just want to check that it compiles w/o warnings
     member _.testDefaultInferenceForReturnFrom() =
@@ -1390,5 +1406,4 @@ module Issue12184f =
             // The overload resolution for Bind commits to 'Task' via SRTP pattern since the type annotation is available
             let! result = t
             return result
-        }
-
+        }