Nullable + dynamic

[vsfeedback]

If there is defined class A which overloads + operator such that it returns always null then it is possible to write program throwing ArgumentNullException but has no warnings by dynamics.

This issue has been moved from https://developercommunity.visualstudio.com/content/problem/401974/nullable-dynamic.html
VSTS ticketId: 743594
These are the original issue comments:

libor.drozdek on 12/7/2018, 01:58 AM (34 days ago):

I have problems with formatting when I use "insert Code" so I will post the code here:

#nullable enable 
class Program
{
    private static void Main()
    {
        A obj = new A();
        var x = f(obj); 
        // Null reference exception but no warning
        x.ToString();
    }
    
    public static object f(dynamic a)
    {
        if (a == null)
            throw new System.Exception();

        return a + a;
    }
    
    public class A
    {
        public static A? operator +(A fst, A snd) => null;
    }
}

These are the original issue solutions:
(no solutions)

---

[jcouv update:] I referenced this issue in code. Please find those references before closing this issue.

[jcouv]

There's a number of questions around dynamic and nullability analysis. We'll have to confirm with LDM, but my sense is that once you enter the world of dynamic you exit the world of static analysis, including static nullability analysis.
We can track top-level nullability, but nothing about members/methods.

A few scenarios that need discussion:

dynamic d = something;
int y = d.x.y; // ok
d.x = null; // ok
d.M(); // ok
d.M!(); // useless

d = null;
d.M(); // warn
int x = d.x; // warn

var z = d.y!; // unnecessary

[jaredpar]

This behavior appears to be "By Design" today. The return type of an expression that has dynamic input is dynamic (non-null). Hence compiler is acting correctly here by not issuing a warning on the dereference.

[jcouv]

I'm re-opening this issue as I've used this to track questions relative to dynamic and nullable. (see updated I made to OP)

[gafter]

This behavior was confirmed by the LDM on 2019-04-03.