Creating subset of an interface that returns itself

Getting Help
1

I currently have an interface X with methods that return itself.

type X interface { 
	RetSelf(arg string) X 
	uselessFunc() 
	uselessFunc2() 
	usefulFunc() 
}

I want to break this out into 2 minimal subsets

type A interface { 
	RetSelf(arg string) B 
}
 
type B interface { 
	usefulFunc() 
}

It seems to me like structs which fulfil interface X should also fulfill interface B, and therefore interface A. I suspect that I can just change the definition of legacy structs that use X, to use A instead of X, while keeping the same underlying objects.

type user struct {
    problemInterface X
}
func NewUser(problemInterface X) {
    return user{X}
}

becomes

type user struct {
    problemInterface X
}
func NewUser(problemInterface X) {
    return user{X}
}

With the actual invocations and underlying structs not changing otherwise.

However, instead of trying this I am wondering if there is some elegant solution to typecast interface X into interface A. I can’t do it trivially because A.RetSelf returns B, which does not have all the methods of X so A.RetSelf != X.RetSelf Is there something I’m missing here or am I just SoL?