Warning about null value while value is set as not null

[NN---]

Version Used:
Visual Studio 17.4.0

Steps to Reproduce:

using System;
using System.Diagnostics.CodeAnalysis;

#nullable enable

public class A
{
    private readonly string _s;

    public required string S
    {
        get
        {
            return _s;
        }
        [MemberNotNull(nameof(_s))]
        init
        {
            if (_s is null) throw new ArgumentException("Must not be null or empty");
            _s = value;
        }
    }
}

public class C
{
    public static void Main()
    {
        new A { S = "" };
    }
}

Diagnostic Id:

warning CS8618: Non-nullable field '_s' must contain a non-null value when exiting constructor. Consider declaring the field as nullable.

Expected Behavior:
No warning.
The field _s is not null after init block which specifies it using MemberNotNullAttribute.

Actual Behavior:
Warning about making _s to be nullable.

[NN---]

Probably the same as #43587

[jaredpar]

@333fred to confirm dupe

[333fred]

Unrelated. I'm not sure whether we really support this scenario, however. We generally don't process property bodies when considering nullability of fields, but we might make an exception for MemberNotNull of required properties. We'll need to run it by LDM.

[333fred]

dotnet/csharplang#6754 will be discussed by LDM on Wednesday. There are some subtleties to this request: chained constructors, in particular, are more difficult than they appear on the surface, as we would need to use MemberNotNull attributes to infer a starting state for the mentioned members, not just an ending state.