async enumerable should not error if it throws without yield statement

[JeffreyMercado]

I'm stubbing out an application making all methods throw a NotImplementedException. But it seems I cannot do this for an async generator, I will get a CS8420 error.

error CS8420: The body of an async-iterator method must contain a 'yield' statement. Consider removing 'async' from the method declaration or adding a 'yield' statement.

static async System.Collections.Generic.IAsyncEnumerable<object> TestAsyncEnumerable() // error CS8420
{
    throw new System.NotImplementedException();
}

To get around it, I'm throwing in a useless yield break in the body or omit async from the declaration.

static async System.Collections.Generic.IAsyncEnumerable<object> TestAsyncEnumerable() // ok
{
    throw new System.NotImplementedException();
    yield break;
}
static System.Collections.Generic.IAsyncEnumerable<object> TestAsyncEnumerable() // also ok
{
    throw new System.NotImplementedException();
}

The compiler allows this fine for synchronous generators (which I understand is treated as a regular method that returns an enumerable).

static System.Collections.Generic.IEnumerable<object> TestSyncEnumerable() // ok
{
    throw new System.NotImplementedException();
}

It does warn for regular asynchronous methods with CS1998.

warning CS1998: This async method lacks 'await' operators and will run synchronously. Consider using the 'await' operator to await non-blocking API calls, or 'await Task.Run(...)' to do CPU-bound work on a background thread.

static async System.Threading.Tasks.Task<object> TestAsync() // warning CS1998
{
    throw new System.NotImplementedException();
}

I think it should be allowed and not error for the async generator case.

[HaloFour]

You can omit the async keyword until you're ready to implement those methods.