ref/out parameters in async methods with AsyncMethodBuilderAttribute

[ufcpp]

Version Used:

VS 15.1.2
.NET SDK 6.0.201
C# Tools 4.1.0-5.22165.10+e555772db77ca828b02b4bd547c318387f11d01f

Steps to Reproduce:

using System.Runtime.CompilerServices;

int x = 1;
await m0(ref x, out var y0);
await m1(ref x, out var y1);

static async ValueTask m0(ref int x, out int y) // Error CS1988
{
    await Task.Delay(1000);
    y = x * x;
    x = y * y;
}

[AsyncMethodBuilder(typeof(PoolingAsyncValueTaskMethodBuilder))]
static async ValueTask m1(ref int x, out int y) // No error
{
    await Task.Delay(1000);
    y = x * x;
    x = y * y;
}

Thanks to @neuecc
https://twitter.com/neuecc/status/1506262798946701314

Expected Behavior:

CS 1988 Error on m1.

Actual Behavior:

No error on m1.

As a side note, not only PoolingAsyncValueTaskMethodBuilder but also any builders cause the same behavior.

[AsyncMethodBuilder(typeof(MyAsyncMethodBuilder))]
static async ValueTask m2(ref int x, out int y) // No error
{
    await Task.Delay(1000);
    y = x * x;
    x = y * y;
}

public struct MyAsyncMethodBuilder
{
    public ValueTask Task { get; }
    public static MyAsyncMethodBuilder Create() => default;
    public void AwaitOnCompleted<TAwaiter, TStateMachine>(ref TAwaiter awaiter, ref TStateMachine stateMachine)
        where TAwaiter : INotifyCompletion
        where TStateMachine : IAsyncStateMachine
    { }
    public void AwaitUnsafeOnCompleted<TAwaiter, TStateMachine>(ref TAwaiter awaiter, ref TStateMachine stateMachine)
        where TAwaiter : ICriticalNotifyCompletion
        where TStateMachine : IAsyncStateMachine
    { }
    public void SetException(Exception exception) { }
    public void SetResult() { }
    public void SetStateMachine(IAsyncStateMachine stateMachine) { }
    public void Start<TStateMachine>(ref TStateMachine stateMachine) where TStateMachine : IAsyncStateMachine { }
}

[jcouv]

Thanks @neuecc @ufcpp for reporting this! Fix is pending review