Nullable inference in conditional expressions

[RikkiGibson]

#nullable enable

public class Program
{
    static void M(bool flag, string? x)
    {
        if (flag ? x?.Length == 2 : x?.Length == 3)
        {
            x.ToString(); // warns today, but perhaps shouldn't warn
        }
    }
}

SharpLab

Note that if flag is changed to the constant true, the warning is not given.

The suggestion here is that if either conditional expression arm has a split state, we join the StateWhenTrue on each arm, and join the StateWhenFalse on each arm, and use the resulting split state as the resulting state of the conditional expression.

[gafter]

This behavior is the same as our other flow analyses:

using System;
public class C {
    public void M(bool b1, bool b2) {
        int x;
        if (b1 ? b2 && M(out x) : b2 && M(out x))
        {
            Console.WriteLine(x); // error CS0165: Use of unassigned local variable 'x'
        }
    }
    private static bool M(out int x)
    {
        x = 1;
        return true;
    }
}

[RikkiGibson]

I simplified the reproducer and added a description of what the implementation change would probably be.

[RikkiGibson]

This will be fixed as part of the "improved definite assignment" feature. #51463