[RuntimeAsync] Add a testcase for Task-returning methods that use __arglst

[VSadov]

We need to add a testcase under src\tests\async\varargs .
The testcase should contain scenarios validating that both direct calls and await operator work correctly with methods that use __arglist feature in cases when the methods return:

• Task
• ValueTask<int>

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[jkotas]

The testcase should contain scenarios validating that both direct calls and await operator work correctly with methods that use __arglist feature in cases when the methods return

I do not think we want to spend time on implementing __arglist support for runtime async. It gets very complex to handle all corner-cases.

It should be ok to leave it unsupported.

[VSadov]

I am aiming for "not crashing on scenarios that used to work" here.

[jkotas]

__arglist is not supported by Roslyn for async v1 either. It produces:

CS4006: __arglist is not allowed in the parameter list of async methods

[VSadov]

It does not need to be async to be awaited. Can just return Task.

[VSadov]

We can have these kinds of support:

1. fully support, generate variants/thunks - I do not think so.
2. treat as ordinary method, if easy to detect, so things continue to work the old way. Probably the best option, if vararg can be detected early enough. Otherwise:
3. throw some kind of typeload error when convenient, document as a case where user should opt out the awaiting method from runtime async.

I suspect right now we may go on a path of creating a broken thunk leading to a crash or UB.
Maybe we already have implemented #3 indirectly. Then I'll just have a test.

[VSadov]

It is fairly easy to implement #2. Like here: 835bb64
I think it might not hurt to have.

Also it turned out a hard to test scenario.
Roslyn simply forbids uses of __arglist inside async methods if runtime async codegen is enabled. That makes the scenario completely unreachable in C#.

E:\dotnet008\runtime\src\tests\async\varargs\varargs.cs(26,35): error CS9328: Method 'Async2Varargs.TestTaskAwaitAsync()' uses a feature that is not supported by runtime async currently. Opt the method out of runtime async by attributing it with 'System.Runtime.CompilerServices.RuntimeAsyncMethodGenerationAttribute(false)'.

Even manually calling via AsyncHelpers.Await cannot bypass this block as it is not keyed off the await, it simply does not like any use of __arglist in a runtime async method.

Thanks @333fred (dotnet/roslyn@42bc8f1) !

This is perfect from UX point of view.

But now I wonder if I want to bother with a testcase for this as it would need to be in IL.