IntelliSense cannot detect IAsyncEnumerable's yield return type when that parameter is used

[datvm]

Version Used: 4.5.2-3.23171.7 (d17f741)

Steps to Reproduce:

Put in the following code (in this example it's in the top-level Program.cs):

Console.WriteLine("Hello");
_ = BarAsync();

static async IAsyncEnumerable<Foo> BarAsync()
{
    var tmp = new string[10];
    foreach (var item in tmp)
    {
        yield return new(async s =>
        {
            // Happen here
            await Task.CompletedTask;
            throw new NotImplementedException();
        });
    }
}

record Foo(Func<string, Task<string>> func); // Happen to both Func<> and delegate

The code above compiles but at the // Happen here line, any attempt to access s results in s being unknown ? even though without it, the compiler and IntelliSense correctly detect s as string.

Diagnostic Id: N/A

Expected Behavior:

s should maintain its type as string while typing.

Actual Behavior:

Whenever there is an attempt to access s, it becomes ?. This happens to both Visual Studio and VS Code (latest version). If you put in valid code without IntelliSense, it still compiles, for example:

yield return new(async s =>
{
    return s.ToUpper(); // Have to manually type it
    await Task.CompletedTask;
    throw new NotImplementedException();            
});

Correct behavior when s is not being typed:

In both VS and VS Code, s becomes unknown when any attempt to access it is made:

[datvm]

Attempt to "navigate" around the issue by declaring a variable does not work as well (note that s2 now has type of ?? instead of ?):

However this issue does not happen if s2 is explicitly typed, maybe because at this time IntelliSense trusts that the error would be thrown earlier and s2 doesn't need to be guessed:

[datvm]

Hi any update on this? This is quite annoying to work with because I keep losing code suggestion.

Thanks.

[CyrusNajmabadi]

@datvm There is no update on this. We would take a community contribution to fix this though, if you were interested :)

[datvm]

@CyrusNajmabadi thanks. I hope someone can take a look at this. Unfortunately I am quite unfamiliar with Roslyn so I cannot really help :(

[CyrusNajmabadi]

Moving to compiler. We call into their apis for semantic information.

[jcouv]

Fix was merged into 17.13p2. Thanks for reporting this issue @datvm !

[datvm]

@jcouv Thank you and congrats, that's awesome! I assume this issue is not related to #72571 and it's not fixed as a result of this fix then?

[jcouv]

I assume this issue is not related to #72571 and it's not fixed as a result of this fix then?

That issue is not related. The issue here, which was fixed, relates to a lambda in a yield return.