Add regression test: #3783, Mutually recursive computation expression NullReferenceException

tests/FSharp.Compiler.ComponentTests/Conformance/Expressions/ComputationExpressions/ComputationExpressions.fs

@@ -321,3 +321,46 @@ module LetUseBangTests =
             (Error 748, Line 4, Col 13, Line 4, Col 20, 
                 "This construct may only be used within computation expressions. To return a value from an ordinary function simply write the expression without 'return'.")
         ]
+
+// https://github.com/dotnet/fsharp/issues/3783
+[<Fact>]
+let ``Issue 3783 - Mutually recursive computation expression should not raise NullReferenceException`` () =
+    FSharp """
+#nowarn "40"
+#nowarn "21"
+
+let x () =
+    let rec a = seq {
+        yield 0
+        yield! b () }
+    and b () = seq {
+        yield 1
+        yield! a }
+    Seq.take 10 a |> Seq.toList
+
+type A () =
+    let test =
+        let rec a = seq {
+            yield 0
+            yield! b () }
+        and b () = seq {
+            yield 1
+            yield! a }
+        Seq.take 10 a |> Seq.toList
+    member _.Test = test
+
+[<EntryPoint>]
+let main _ =
+    let result1 = x ()
+    if result1 <> [0; 1; 0; 1; 0; 1; 0; 1; 0; 1] then
+        failwithf "Function variant failed: %A" result1
+
+    let a = A()
+    if a.Test <> [0; 1; 0; 1; 0; 1; 0; 1; 0; 1] then
+        failwithf "Type constructor variant failed: %A" a.Test
+
+    0
+    """
+    |> asExe
+    |> compileExeAndRun
+    |> shouldSucceed