CS8604 warning when using MaybeNullWhen and return value is assigned

[benjamin-hodgson]

Version Used: 8.0

Steps to Reproduce:

When calling a method with an out parameter annotated with MaybeNullWhen, the flow analyser seems to fail when the return value is assigned to a variable before being scrutinised.

Compile this code with <Nullable>enable</Nullable>:

using System.Diagnostics.CodeAnalysis;

class Example
{
    void OK()
    {
        if (TryParse("foo", out var result))
        {
            DoSomething(result);
        }
    }
    void Warning()
    {
        var success = TryParse("foo", out var result);
        if (success)
        {
            DoSomething(result);
        }
    }
    bool TryParse(string s, [MaybeNullWhen(false)] out object result)
    {
        // dummy implementation
        result = default;
        return false;
    }
    void DoSomething(object item)
    {
    }
}

Expected Behavior:

No warning in either the OK or the Warning methods. (They self-evidently behave identically.)

Actual Behavior:

CS8604 warning ("Possible null reference argument for parameter 'item' in 'void Example.DoSomething(object item)'") inside the Warning method.

[RikkiGibson]

Duplicate of #27011