Unexpected diagnostic on an overriding member, whose base member implements an annotated interface member

[bkoelman]

A diagnostic is reported on N.C.M in the next code snippet:

namespace N
{
    public interface I
    {
        [NotNull]
        string M();
    }

    public class B : I
    {
        public virtual string M() { throw new NotImplementedException(); }
    }

    public class C : B
    {
        public override string M() { throw new NotImplementedException(); }
    }
}

But when applying a nullability attribute on it, Resharper then grays it out as unneeded, because the annotation from N.I.M applies.