Nullable state isn't propagated from out parameters if the return value is stored in local

[tmat]

Version Used:

Version 17.6.0 Preview 2.0 [33407.517.main]

Steps to Reproduce:

using System.Diagnostics.CodeAnalysis;

var success = TryGet(out var x);
if (success)
{
    Console.WriteLine(x.ToString());
}

bool TryGet([NotNullWhen(true)] out object? x)
{
    x = new object();
    return true;
}

Expected Behavior:

No warnings.

Actual Behavior:

Program.cs(6,23,6,24): warning CS8602: Dereference of a possibly null reference.

This works as expected:

if (TryGet(out var x))
{
    Console.WriteLine(x.ToString());
}

[RikkiGibson]

Root cause is same as #27011. See also #66693 (comment).

It's not clear whether a tractable design exists which handles cases like these. Some fairly deep investigation would be needed to determine that which is better done on the csharplang side.

(In other words, I don't know for sure that it's impossible, I just know that I don't know how it would work within the practical constraints of the compiler's static analysis.)