Mypy type narrowing with recursive generic types

Let's say I make a generic class whose objects only contain one value (of type T).

T = TypeVar('T')class Contains(Generic[T]):    val: T    def __init__(self, val: T):        self.val = val

Note that self.val can itself be a Contains object, so a recursive structure is possible. I want to define a function that will reduce such a structure to a single non-Contains object.

def flatten(x):    while isinstance(x, Contains):        x = x.val    return x

What should the type signature of 'flatten' be?

I tried to make a recursive Nested type

Nested = T | Contains['Nested[T]']

but it confuses the type checker as T can also mean a Contains object.

def flatten(x: Nested[T]) -> T:    while isinstance(x, Contains):        reveal_type(x) # reveals Contains[Unknown] | Contains[Nested]        x = x.val    reveal_type(x) # reveals object* | Unknown    return x

Another approach was to make a separate class

class Base(Generic[T]):    self.val: T    def __init__(self, val):        self.val = valNested = Base[T] | Contains['Nested[T]']def flatten(x: Nested[T]) -> T:    while isinstance(x, Contains):        x = x.val    return x.val

This works, but you have to wrap the argument in a Base object every time, which is cumbersome. Furthermore, Base has the same behaviour as Contains, it's the same thing written twice! I tried to use NewType instead, but it isn't subscriptable.

Is there any nice (or, at least not too ugly) way to do it?