What's wrong with my Bool Awaiter?

So I was trying to make a simple awaitable struct that allow my to return a bool:

Code (CSharp):

1.     // define a special container for returning results
2.     public struct BoolResult {
3.         public bool Value;
4.         public BoolAwaiter GetAwaiter () {
5.             return new BoolAwaiter(Value);
6.         }
7.     }
8.  
9.     // make the interface task-like
10.     public readonly struct BoolAwaiter : INotifyCompletion {
11.         private readonly bool _Input;
12.  
13.         // wrap the async operation
14.         public BoolAwaiter (bool value) {
15.             _Input = value;
16.         }
17.  
18.         // is task already done (yes)
19.         public bool IsCompleted {
20.             get { return true; }
21.         }
22.  
23.         // wait until task is done (never called)
24.         public void OnCompleted (Action continuation) => continuation?.Invoke();
25.  
26.         // return the result
27.         public bool GetResult () {
28.             return _Input;
29.         }
30.     }

And I was using it like:

Code (CSharp):

1. private async BoolResult LoadAssets (string label) { ... return new BoolResult { Value = true }; }

I don't get why C# tells me:

Didn't my BoolResult already meet the "Task-like" requirement?

 

So people told me I need to declare AsyncMethodBuilder, because Task-like only makes it await-able.

But to return this thing in an async function, is a different matter.

https://github.com/dotnet/roslyn/blob/master/docs/features/task-types.md

Problem is, I need AsyncMethodBuilder attribute, and I don't think it's available in Unity 2018.3?

With

Code (CSharp):

1. using System.Runtime.CompilerServices;

AsyncMethodBuilder is still not found... (I think Unity 2018.3 has C# 7 support, but perhaps it was introduced at later version?)

 

I am now starting to understand why all those Async library for Unity ships their own CompileServices.dll...