Variable should be nullable outside try block if assigned nullable value inside try block

[cston]

s.Length should result in a warning:

[System.Runtime.CompilerServices.NonNullTypes]
class C
{
    static void MayThrow()
    {
        throw null;
    }
    static int F()
    {
        string? s = "";
        try
        {
            s = null;
            MayThrow();
            s = "";
        }
        catch (System.Exception)
        {
        }
        return s.Length; // warning: possibly null
    }
}

Reported by @gafter.

[gafter]

Here is a much simpler example demonstrating the shortcomings of our analysis of try-finally:

[module: System.Runtime.CompilerServices.NonNullTypes(true)]

public static class Program {
    public static void Main() {
        string? s = "";
        try
        {
        }
        finally
        {
            s = null;
        }

        _ = s.Length; // warning missing
    }
}

[gafter]

Fixed in #31082