Create custom types using typing module?

[Chirag Dhyani]

I am trying to create a custom type on Python 3.7 typing module. The new
type (say Struct) should be same as type tuple. In Python3.6, I was able to
do the same by taking cue from Typing.GenericMeta and typing.TupleMeta.

With typing module updated in Python3.7, GenericMeta and TupleMeta do not
exist, and the special class that I would like to subclass is not possible.
e.g. _VariadicGenericAlias cannot be subclassed.

What I really want is something similar to:

Struct = _VariadicGenericAlias(tuple, (), , inst=False, special=True)

and

assert _origin(Struct[int, str]) == <class 'Struct'>

Note:

def _origin(typ: Any) -> Any:
    """Get the original (the bare) typing class.
    Get the unsubscripted version of a type. Supports generic types, Union,
    Callable, and Tuple. Returns None for unsupported types. Examples::
        get_origin(int) == None
        get_origin(ClassVar[int]) == None
        get_origin(Generic) == Generic
        get_origin(Generic[T]) == Generic
        get_origin(Union[T, int]) == Union
        get_origin(List[Tuple[T, T]][int]) == list
    """
    if isinstance(typ, _GenericAlias):
        return typ.__origin__ if typ.__origin__ is not ClassVar else None
    if typ is Generic:
        return Generic
    return None

Deeply appreciate your help around this !!

Thanks,

Chirag