Roslyn: Consider representing nullable flow state as: MaybeNull, NotNull, or T

[cston]

Consider adding a distinct nullable flow state, T, to allow distinguishing "maybe null when the type allows null" from "maybe null even when type disallows null".
C# internal enum NullableFlowState : byte { NotNull, // not null T, // maybe null when type allows MaybeNull // maybe null even when type disallows }

[jcouv]

FYI for @jasonmalinowski

[jcouv]

Even if we just add the third state, without adding a corresponding type/annotation for locals, some scenarios would be improved.
Here's one reported by Sam:

#nullable enable

using System;
using System.Diagnostics.CodeAnalysis;

public class C<T> {
    [MaybeNull] T value;
    public void M() {
        if (value is null)
            return;
    }
}

[saucecontrol]

This example just started giving me warnings in VS 16.4 Preview 2

C# [return: MaybeNull] public static TValue GetValueOrDefault<TKey, TValue>(this IDictionary<TKey, TValue> dic, TKey key, TValue defaultValue = default) where TKey : notnull => dic.TryGetValue(key, out var value) ? value : defaultValue;

Results in warning CS8717: A member returning a [MaybeNull] value introduces a null value when 'TValue' is a non-nullable reference type

Would this allow the [MaybeNull] from TryGetValue to flow through?

[jnm2]

CS8717 feels subpar to me. The warning should follow the (mis)usage, not the declaration. As it stands, CS8717 only shows up on valid (even ideal) code.

void M<T>()
{
    var x = new Dictionary<int, T>();

    // CS8717 A member returning a [MaybeNull] value introduces a null value when 'TResult' is a
    // non-nullable reference type.
    //                        ↓
    if (x.TryGetValue(42, out var someValue))
    {
    }
}

[CyrusNajmabadi]

FYI, this is super unpleasant. I honestly don't know how to write code in some cases that is actually NRT correct. I thought MaybeNull was intended for this, but @jcouv informs me that for the actual uninstantiated cases (i.e. while you're writing generic code), you're just SOL here :-/