Indexing TypeVars / need a workaround for higher-kindedness

I'm using mypy 0.630.

I have a container, which can contain various types, some of which contain further types. I want the root of the container to be able to return grandchild types, or failing that to be able to return fully-specified child types. My first thought doesn't work:

from typing import TypeVar, Generic, List

ContainedType = TypeVar('ContainedType', bound='ContainedCls')
SubContained = TypeVar('SubContained')
T = TypeVar('T')

class Root_wrong(Generic[ContainedType[SubContained]]):
    def __init__(self, ct):
        # type: (ContainedType[SubContained]) -> None
        self.contained = ct

    def iter_subcontained(self):
        # type: () -> List[SubContained]
        return self.contained.items

class ContainedCls(Generic[T]):
    def __init__(self, items):
        # type: (List[T]) -> None
        self.items = items

Because TypeVars can't be indexed.

This also doesn't work, because iter_subcontained can't be typed:

class Root(Generic[ContainedType]):
    def __init__(self, ct):
        # type: (ContainedType) -> None
        # I know because of the bound that ContainedType will have a parameter of its own, but I
        # can't refer to it in the `class Root` declaration because TypeVars can't be indexed.
        # and the index to Generic has to be a typevar
        self.contained = ct

    def iter_subcontained(self):
        # what goes here?
        return self.contained.items


reveal_type(Root(ContainedCls([1])).iter_subcontained())  # Any

Alas, this also doesn't work:

class Root2(Generic[ContainedType, SubContained]):
    def __init__(self, ct):
        # type: (ContainedType) -> None
        # I know because of the bound that ContainedType will have a parameter of its own, but I
        # can't refer to it in the `class Root` declaration because TypeVars can't be indexed.
        # and the index to Generic has to be a typevar
        self.contained = ct

    def iter_subcontained(self):
        # type: () -> List[SubContained]
        return self.contained.items


reveal_type(Root2(ContainedCls([1])).iter_subcontained())  # builtins.list[<nothing>]

# not matching is not an error
reveal_type(Root2[ContainedType[int], str](ContainedCls([1])).iter_subcontained())  # builtins.list[str]

This almost works:

class Root3(Generic[T]):
    def __init__(self, ct1, ct2):
        # type: (ContainedCls[T], ContainedCls[T]) -> None
        self.contained = ct1
        self.contained2 = ct2

    def iter_subcontained(self):
        # type: () -> List[T]
        return self.contained.items


class ContainedClsPrime(ContainedCls):
    pass


# no way to ensure that ct1 and ct2 are the same container type
# in fact, they aren't even constrained to have the same T!
x = Root3[int](ContainedCls([1]), ContainedClsPrime(['definitely not an int']))
reveal_type(x.iter_subcontained())  # builtins.list[str]
reveal_type(x.contained2.items[0])  # builtins.int* !!!

But it doesn't constrain the two ContainedCls[T]s to be the same ContainedCls subtype, and it doesn't (this really surprised me) even constrain them to be containing the same T! It also seems wrong to be able to make Root3 generic only in the leaf types, and not in any of the intermediate types.

Inizia riproducendo i quattro esempi dell’issue con mypy e confronta i tipi rivelati con le aspettative dichiarate. Leggi la gestione dei generici e di TypeVar rilevante per i TypeVars indicizzati e i tipi di ordine superiore. Il lavoro è completo quando viene stabilito un workaround supportato o un percorso di implementazione chiaramente delimitato per preservare i tipi di contenitore annidati.