'Failed to emit module' error when compiling usages of a nested tasklike type

[TessenR]

Roslyn doesn't detect errors in the following code snippet but is unable to compile it. Fails with 'failed to emit module' error in VS2017 RC.

public class Program
{
    public static void Main() { }
}

public class TypeParametersProvider<T>
{
    public async ValueTask Method()
    {
        await Task.Delay(5);
        return;
    }

    [AsyncMethodBuilder(typeof(AsyncValueTaskMethodBuilder))]
    public struct ValueTask
    {
    }
}

public class AsyncValueTaskMethodBuilder
{
    public TypeParametersProvider<int>.ValueTask Task { get => default(TypeParametersProvider<int>.ValueTask); }
    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 static AsyncValueTaskMethodBuilder Create() { return default(AsyncValueTaskMethodBuilder); }
    public void SetException(Exception exception) { }
    public void SetResult() { }
    public void SetStateMachine(IAsyncStateMachine stateMachine) { }
    public void Start<TStateMachine>(ref TStateMachine stateMachine) where TStateMachine : IAsyncStateMachine { }
}

namespace System.Runtime.CompilerServices
{
    public class AsyncMethodBuilderAttribute : System.Attribute
    {
        public AsyncMethodBuilderAttribute(Type type) { }
    }
}

[jcouv]

This repros on latest master (TryRoslyn).

[jcouv]

@cston, I couldn't find a feature doc for task-like, so I went back to the feature proposal. From my understanding, TypeParametersProvider<T>.ValueTask cannot be "task-like" because it is neither (1) a concrete type nor (2) a generic type of arity 1.
Can you confirm?
If that is correct, I'm assuming we should just expect an error in the above scenario.

[jcouv]

It looks like my thinking was wrong. There are tests with task-like types nested in generic types, so this is supported.
Looking at this more, it looks like the problem is the mismatch between the return type of Method (returns TypeParametersProvider<T>.ValueTask) and the type created by the AsyncMethodBuilder (creates a TypeParametersProvider<int>.ValueTask).

[jcouv]

Tracked by VSO https://devdiv.visualstudio.com/DevDiv/_workitems?id=369807

[jcouv]

@TessenR Thanks for reporting this issue.
Fixed with #16530. Unfortunately, the fix won't make it into VS 2017 RTM. But it will ship in the first minor release after that.