About Equals Operator Overloading in Interface: What Constraint Does CS9046 Need?

[Orange23333]

Problem

public interface IPathNode<TValue, TCoordinate> where TValue : IBinaryNumber<TValue> where TCoordinate : struct
{
    Guid NId { get; }

    public virtual static bool operator ==<TSelf>(TSelf left, TSelf right) where TSelf : IPathNode<TValue, TCoordinate>
    {
        return left.NId == right.NId;
    }	
    public virtual static bool operator !=<TSelf>(TSelf left, TSelf right) where TSelf : IPathNode<TValue, TCoordinate>
    {
        return left.NId != right.NId;
    }
}

According to the picture above. It said the type parameter needs constraint. But how?

• This is an operator overriding method in interface, but not a generic method.
• I think really can't constrain anything here with where any more.

Blob

• I tried to overload +, > and <. No error comes out. Yes, the CS9046 mentions only == and !=. No loss is highlighted. So special (with an I don't know why error) that really confused me.
• No such document about CS9046 on MSDN so far.
• I tried to search C# operator overloading needs constraint in interface on Bing. There are many pages about operator overloading, generics constraint, operator overloading of generics with +, an issue with similar title but for + as well. But there is nothing about ==.

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I need help!

[CyrusNajmabadi]

the pattern this is looking for is something like:

interface I<X> where X : I<X>
{
     public virtual static operator ==(X x, ...)
}

here, one of the parameters of the equality operator is a type parameter on I, (specifically X) and that type parameter itself is constrained to I<X>.

[Orange23333]

Thanks!
Looks like we can't escape TSelf in this version. So, we have to enjoy the advantages that it brings.

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And I just found this Type arguments implement declared interface after your answer was given out. A TSelf really gives me many conveniences.
According to my test and the column("Type arguments implement declared interface"), public virtual static I operator +(I left, ...)is allowed (although it needs an explicit interface implementation). But I still haven't understood why == is not allowed.