Code analysis IDE0039 quick fix does not respect nullability of async return value

[vsfeedback]

This issue has been moved from a ticket on Developer Community.

---

var testTask = async () =>
{
await Task.Delay(1);
if (Random.Shared.Next(2) == 0)
{
return "test";
}
return null;
};

Quick fix result:

static async Task<string> testTask()
{
await Task.Delay(1);
if (Random.Shared.Next(2) == 0)
{
return "test";
}
return null;
}

It should be Task<string?> because it has a nullable return value "return null"

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Original Comments

Feedback Bot on 12/16/2024, 00:24 AM:

We have directed your feedback to the appropriate engineering team for further evaluation. The team will review the feedback and notify you about the next steps.

[CyrusNajmabadi]

I've debugged through. When we ask the compiler what the delegate type of the lambda is (using semanticModel.GetTypeInfo(anonymousFunction, cancellationToken) it returns Func<Task<string>>, which can also be seen here in quick info:

As such, when we make the local function, we make it have a return type of Task<string>, not Task<string?>. I do not know why the compiler thinks the return type here is that.

Note: you can repro this without the feature, just with no compiler warnings like so:

    void M()
    {
        var testTask = async () =>
        {
            await Task.Delay(1);
            if (Random.Shared.Next(2) == 0)
            {
                return "test";
            }
            return null;
        };

        Func<Task<string>> s = testTask;
    }

This assignment should warn. If you try to write:

    void M()
    {
        Func<Task<string>> testTask = async () =>
        {
            await Task.Delay(1);
            if (Random.Shared.Next(2) == 0)
            {
                return "test";
            }
            return null;
        };
    }

You do get a warning, as hte compiler clearly can tell this body doesn't support that specified return type. But something about the 'var' case breaks it.