#nullable enable
using System;

namespace N {
    public class C {
        public int X;
    }
}

public static bool operator ==(C? left, C? right) => EqualityComparer<C?>.Default.Equals(left, right);
public static bool operator !=(C? left, C? right) => !(left == right);

public static bool operator ==(C left, C right) => global::System.Collections.Generic.EqualityComparer<global::N.C>.Default.Equals(left, right);
public static bool operator !=(C left, C right) => !(left == right);

C? c = null;
if (c == null) { // Possible null reference argument for parameter 'left' in 'bool C.operator ==(C left, C right)'.
}