using System.Threading.Tasks;

interface I1 
{
    int this[int i] {get; set;}
}

struct S1 : I1
{
    public int F1;

    public int this[int i]
    {
        get
        {
            System.Console.Write(F1);
            return 0;
        }
        set
        {
            System.Console.Write(F1);
        }
    }
}

class Program
{
    static async Task Main()
    {
        await Test3<S1>();
    }

    static T GetT<T>() where T : I1 => (T)(object)new S1 { F1 = 123 };

    static async Task Test3<T>() where T : I1
    {
       GetT<T>()[0] += await Get1Async();
    }

    static async Task<int> Get1Async()
    {
        await Task.Yield();
        return 1;
    }
}

// /Program.cs(37,8): error CS8178: A reference returned by a call to 'Program.GetT<T>()' cannot be preserved across 'await' or 'yield' boundary.
//        GetT<T>()[0] += await Get1Async();
Diagnostic(ErrorCode.ERR_RefReturningCallAndAwait, "GetT<T>()").WithArguments("Program.GetT<T>()").WithLocation(37, 8)