CS8602 false positive after TryGetValue

[AArnott]

Version Used: VS 17.11 Preview 1 (34828.26.main)

Steps to Reproduce:

Sharplab

Diagnostic Id: CS8602

Expected Behavior:

No warning

Actual Behavior:

A warning that list is dereferenced when it may be null. When in fact it cannot be null because of the simple check and assignment I do.

The really interesting thing is this only shows up when targeting .NET 6 and 8 for me. .NET Standard 2.0 and 2.1 targets don't mis-fire here.
It looks like .NET 6 added [MaybeNullWhen(false)] to the TryGetValue output parameter. I bet that's what is triggering this, though it shouldn't, since I check the false condition and assign a non-null value.

[jjonescz]

Dictionary<string, List<bool>> d = new();
bool e = d.TryGetValue("hi", out List<bool>? list);
if (!e)
{
    list = new();
}
Console.WriteLine(list.Count);

Nullability info isn't propagated through boolean variables like that. See #27011.

If you "inline" the check, it works:

Dictionary<string, List<bool>> d = new();
if (!d.TryGetValue("hi", out List<bool>? list))
{
    list = new();
}
Console.WriteLine(list.Count);

[AArnott]

Thanks for looking.
In the case I got this from, I need the bool local because it is used in later expressions. So it isn't as simple as inlining it, which is what I normally do.

[jjonescz]

I agree it would be useful to support this, but I think it needs a language proposal - #27011 (comment).

[jaredpar]

The current behavior is indeed "By Design". I agree it's unfortunate and it would be nice if this worked. Getting this to work though is not a trivial undertaking, it's a significant amount of design and implementation work. Rikki wrote up a summary of the problems that this poses.